Nowadays, most electronic circuits are made using printed circuit boards. Using printed circuit board manufacturing technologies, prefabricated microelectronics components are also produced - hybrid modules that contain components of various functional purposes and degrees of integration. Multilayer printed circuit boards and electronic components with a high degree of integration make it possible to reduce the weight and size characteristics of electronics and computer components. Now the printed circuit board is more than a hundred years old.

Printed circuit board

This (in English PCB - printed circuit board)- a plate made of electrical insulating material (getinax, textolite, fiberglass and other similar dielectrics), on the surface of which thin electrically conductive strips (printed conductors) with contact pads for connecting mounted radio elements, including modules and integrated circuits, are somehow applied. This wording is taken verbatim from the Polytechnic Dictionary.

There is a more universal formulation:

A printed circuit board refers to a design of fixed electrical interconnections on an insulating base.

The main structural elements of a printed circuit board are a dielectric base (rigid or flexible) on the surface of which the conductors are located. The dielectric base and conductors are elements necessary and sufficient for a printed circuit board to be a printed circuit board. To install components and connect them to conductors, additional elements are used: contact pads, metallized transition and mounting holes, connector lamellas, areas for heat removal, shielding and current-carrying surfaces, etc.

The transition to printed circuit boards marked a qualitative leap in the field of electronic equipment design. A printed circuit board combines the functions of a carrier of radioelements and the electrical connection of such elements. The latter function cannot be performed if a sufficient level of insulation resistance is not provided between the conductors and other conductive elements of the printed circuit board. Therefore, the PCB substrate must act as an insulator.

Historical reference

During the 1920s and 1930s, many patents were issued for printed circuit board designs and methods for making them. The first methods of manufacturing printed circuit boards remained predominantly additive (the development of the ideas of Thomas Edison). But in its modern form, the printed circuit board appeared thanks to the use of technologies borrowed from the printing industry. Printed circuit board is a direct translation from the English printing term printing plate (“printing plate” or “matrix”). Therefore, the Austrian engineer Paul Eisler is considered the true “father of printed circuit boards.” He was the first to conclude that printing (subtractive) technologies could be used for mass production of printed circuit boards. In subtractive technologies, an image is formed by removing unnecessary fragments. Paul Eisler developed the technology of galvanic deposition of copper foil and its etching with ferric chloride. Technologies for mass production of printed circuit boards were in demand already during the Second World War. And from the mid-1950s, the formation of printed circuit boards began as a constructive basis for radio equipment not only for military, but also for domestic purposes.

PCB materials

Basic dielectrics for printed circuit boards

Knowledge of the parameters of materials for printed circuit boards, both single-layer and multilayer, is important for everyone involved in their use, especially for printed circuit boards for devices with increased speed and microwaves. When designing MPP, developers are faced with the following tasks:
- calculation of the wave resistance of conductors on the board;
- calculation of the value of interlayer high-voltage insulation;
- selection of the structure of blind and hidden holes.
Available options and thicknesses various materials are given in tables 2–6. It should be taken into account that the tolerance on the thickness of the material is usually up to ±10%, therefore the tolerance on the thickness of the finished multilayer board cannot be less than ±10%.

Tg - glass transition temperature (structure destruction)

Dk - dielectric constant

Basic dielectrics for microwave printed circuit boards

Typical designs of printed circuit boards are based on the use of standard fiberglass type FR4, with an operating temperature from –50 to +110 °C, and a glass transition temperature Tg (softening) of about 135 °C.
If there are increased requirements for heat resistance or when mounting boards in a lead-free technology oven (t up to 260 °C), a high-temperature FR4 High Tg or FR5.
If there are requirements for continuous operation at high temperatures or with sudden temperature changes, it is used polyimide. In addition, polyimide is used for the manufacture of high-reliability circuit boards, for military applications, and also in cases where increased electrical strength is required.
For boards with Microwave circuits(over 2 GHz) separate layers are used microwave material, or the board is entirely made of microwave material. The most well-known suppliers of special materials are Rogers, Arlon, Taconic, Dupont. The cost of these materials is higher than FR4, and is conditionally shown in the penultimate column of the table relative to the cost of FR4.

* Dk - dielectric constant

Dk - dielectric constant

PCB pad coatings

Most often, sites are coated with a tin-lead alloy, or PIC. The method of applying and leveling the surface of solder is called HAL or HASL (from English Hot Air Solder Leveling - leveling solder with hot air). This coating provides the best solderability of the pads. However, it is being replaced by more modern coatings, usually compatible with the requirements of the international RoHS directive.

This directive requires the prohibition of the presence harmful substances, including lead, in products. So far, RoHS does not apply to the territory of our country, but it is useful to remember its existence.

Possible options for covering MPP sites are in Table 7.

HASL is used everywhere unless otherwise required.

Immersion (chemical) gilding used to provide a more even board surface (this is especially important for BGA pads), but has slightly lower solderability. Oven soldering is performed using approximately the same technology as HASL, but hand soldering requires the use of special fluxes. Organic coating, or OSP, protects the copper surface from oxidation. Its disadvantage is the short shelf life of solderability (less than 6 months).

Immersion tin provides flat surface and good solderability, although it also has a limited shelf life for soldering. Lead-free HAL has the same properties as lead-containing HAL, but the solder composition is approximately 99.8% tin and 0.2% additives.

Blade connector contacts that are subject to friction during operation of the board are electroplated with a thicker and more rigid layer of gold. For both types of gilding, a nickel underlayer is used to prevent diffusion of gold.

Note: All coatings except HASL are RoHS compliant and suitable for lead-free soldering.

Protective and other types of printed circuit board coatings

Protective coatings are used to insulate surfaces of conductors not intended for soldering.

To complete the picture, let’s consider the functional purpose and materials of printed circuit board coatings.

1. Solder mask - applied to the surface of the board to protect conductors from accidental short circuits and dirt, as well as to protect fiberglass laminate from thermal shock during soldering. The mask does not carry any other functional load and cannot serve as protection against moisture, mold, breakdown, etc. (except when special types of masks are used).
2. Marking - applied to the board with paint over a mask to simplify identification of the board itself and the components located on it.
3. Peel-off mask - applied to specified areas of the board that need to be temporarily protected, for example, from soldering. It is easy to remove in the future, since it is a rubber-like compound and simply peels off.
4. Carbon contact coating - applied to certain places on the board as contact fields for keyboards. The coating has good conductivity, does not oxidize and is wear-resistant.
5. Graphite resistive elements - can be applied to the surface of the board to perform the function of resistors. Unfortunately, the accuracy of the denominations is low - no more accurate than ±20% (with laser adjustment - up to 5%).
6. Silver contact jumpers - can be applied as additional conductors, creating another conductive layer when there is not enough space for routing. Mainly used for single-layer and double-sided printed circuit boards.

PCB design

The most distant predecessor of printed circuit boards is ordinary wire, most often insulated. He had a significant flaw. In conditions of high vibrations, it required the use of additional mechanical elements to fix it inside the REA. For this purpose, carriers were used on which radioelements were installed, the radioelements themselves and structural elements for intermediate connections and fixing wires. This is a volumetric installation.

Printed circuit boards are free from these shortcomings. Their conductors are fixed on the surface, their position is fixed, which makes it possible to calculate their mutual connections. In principle, printed circuit boards are now approaching flat structures.

At the initial stage of application, printed circuit boards had single-sided or double-sided conductive tracks.

Single Sided PCB- this is a plate on one side of which there are printed conductors. In double-sided printed circuit boards, the conductors also occupied the empty reverse side of the plate. And for their connection, various options have been proposed, among which metallized transition holes are the most widespread. Fragments of the design of the simplest single-sided and double-sided printed circuit boards are shown in Fig. 1.

Double sided PCB- their use instead of one-sided ones was the first step towards the transition from plane to volume. If we abstract ourselves (mentally discard the substrate of the double-sided printed circuit board), we get a three-dimensional structure of conductors. By the way, this step was taken quite quickly. Albert Hanson's application already indicated the possibility of placing conductors on both sides of the substrate and connecting them using through holes.

Rice. 1. Fragments of the design of printed circuit boards a) single-sided and 6) double-sided: 1 - mounting hole, 2 - contact pad, 3 - conductor, 4 - dielectric substrate, 5 - transition metallized hole

Further development of electronics - microelectronics led to the use of multi-pin components (chips can have more than 200 pins), and the number of electronic components increased. In turn, the use of digital microcircuits and the increase in their performance have led to increased requirements for their shielding and power distribution to components, for which special shielding conductive layers were included in multilayer boards of digital devices (for example, computers). All this led to an increase in interconnections and their complexity, which resulted in an increase in the number of layers. In modern printed circuit boards it can be much more than ten. In a sense, the multilayer PCB has gained volume.

Multilayer PCB Design

In the first, most common, option, the internal layers of the board are formed from double-sided copper-laminated fiberglass, which is called the “core”. The outer layers are made of copper foil, pressed with the inner layers using a binder - a resinous material called "prepreg". After pressing at high temperatures, a “pie” of a multilayer printed circuit board is formed, in which holes are then drilled and metallized. Less common second option, when the outer layers are formed from “cores” held together by prepreg. This is a simplified description; there are many other designs based on these options. However, the basic principle is that prepreg acts as the bonding material between the layers. Obviously, there cannot be a situation where two double-sided "cores" are adjacent without a prepreg spacer, but a foil-prepreg-foil-prepreg... etc. structure is possible, and is often used in boards with complex combinations of blind and hidden holes.

Multilayer printed circuit boards now account for two-thirds of global printed circuit board production in terms of price, although in quantitative terms they are inferior to single- and double-sided boards.

A schematic (simplified) fragment of the design of a modern multilayer printed circuit board is shown in Fig. 2. Conductors in such printed circuit boards are placed not only on the surface, but also in the volume of the substrate. At the same time, the layer arrangement of the conductors relative to each other was preserved (a consequence of the use of planar printing technologies). Layering is inevitably present in the names of printed circuit boards and their elements - single-sided, double-sided, multilayer, etc. Layering actually reflects the design and the manufacturing technologies of printed circuit boards corresponding to this design.

Rice. 2. Fragment of the design of a multilayer printed circuit board: 1 - through metallized hole, 2 - blind microvia, 3 - hidden microvia, 4 - layers, 5 - hidden interlayer holes, 6 - contact pads

In reality, the design of multilayer printed circuit boards differs from those shown in Fig. 2.

In terms of its structure, MPPs are much more complex than double-sided boards, just as their production technology is much more complex. And their structure itself differs significantly from that shown in Fig. 2. They include additional shield layers (ground and power), as well as several signal layers.

In reality they look like this:

a) Schematically	To ensure switching between MPP layers, interlayer vias and microvias are used (Fig. 3.a. Interlayer transitions can be made in the form through holes, connecting the outer layers to each other and to the inner layers. Blind and hidden passages are also used. A blind via is a metallized connecting channel visible only from the top or bottom side of the board. Hidden vias are used to connect the internal layers of the board to each other. Their use makes it possible to significantly simplify the layout of boards; for example, a 12-layer MPP design can be reduced to an equivalent 8-layer one. switching Microvias have been developed specifically for surface mounting, connecting contact pads and signal layers.
c) for clarity in 3D view	To manufacture multilayer printed circuit boards, several dielectrics laminated with foil are connected to each other using adhesive gaskets - prepregs. In Figure 3.c the prepreg is shown in white. Prepreg glues the layers of a multilayer printed circuit board together during thermal pressing. The overall thickness of multilayer printed circuit boards grows disproportionately quickly with the number of signal layers. In this regard, it is necessary to take into account the large ratio of the thickness of the board to the diameter of the through holes, which is a very strict parameter for the process of through metallization of holes. However, even taking into account the difficulties with metallization of through holes of small diameter, manufacturers of multilayer printed circuit boards prefer to achieve high packaging density through a larger number of relatively cheap layers rather than a smaller number of high-density but, therefore, more expensive layers.
With) Drawing 3	Figure 3.c shows an approximate structure of the layers of a multilayer printed circuit board, indicating their thicknesses.

Vladimir Urazaev [L.12] believes that the development of designs and technologies in microelectronics proceeds in accordance with the objectively existing law of the development of technical systems: problems associated with the placement or movement of objects are solved by moving from a point to a line, from a line to a plane, from a plane to three-dimensional space.

I think that printed circuit boards will have to obey this law. There is a potential possibility of implementing such multi-level (infinitely level) printed circuit boards. This is evidenced by the rich experience of using laser technologies in the production of printed circuit boards, the equally rich experience of using laser stereolithography to form three-dimensional objects from polymers, the tendency to increase the thermal resistance of base materials, etc. Obviously, such products will have to be called something else. Since the term “printed circuit board” will no longer reflect either their internal content or manufacturing technology.

Perhaps this will happen.

But it seems to me that three-dimensional designs in the design of printed circuit boards are already known - these are multilayer printed circuit boards. And the volumetric installation of electronic components with the location of contact pads on all surfaces of radio components reduces the manufacturability of their installation, the quality of interconnections and complicates their testing and maintenance.

Future will tell!

Flexible printed circuit boards

To most people, a printed circuit board is simply a rigid plate with electrically conductive interconnections.

Rigid printed circuit boards are the most popular product used in radio electronics, which almost everyone knows about.

But there are also flexible printed circuit boards, which are increasingly expanding their range of applications. An example is the so-called flexible printed cables (loops). Such printed circuit boards perform a limited range of functions (the function of a substrate for radioelements is excluded). They serve to combine conventional printed circuit boards, replacing harnesses. Flexible printed circuit boards gain elasticity due to the fact that their polymer “substrate” is in a highly elastic state. Flexible printed circuit boards have two degrees of freedom. They can even be folded into a Mobius strip.

Drawing 4

One or even two degrees of freedom, but very limited freedom, can also be given to conventional rigid printed circuit boards, in which the polymer matrix of the substrate is in a rigid, glassy state. This is achieved by reducing the thickness of the substrate. One of the advantages of relief printed circuit boards made from thin dielectrics is the ability to give them “roundness”. Thus, it becomes possible to coordinate their shape and the shape of the objects (rockets, space objects, etc.) in which they can be placed. The result is a significant saving in the internal volume of products.

Their significant drawback is that as the number of layers increases, the flexibility of such printed circuit boards decreases. And the use of conventional inflexible components creates a need to fix their shape. Because the bending of such PCBs with non-flexible components results in high mechanical stress at the points where they connect to the flexible PCB.

An intermediate position between rigid and flexible printed circuit boards is occupied by “ancient” printed circuit boards, consisting of rigid elements folded like an accordion. Such “accordions” probably gave rise to the idea of ​​​​creating multilayer printed circuit boards. Modern rigid-flex printed circuit boards are implemented in a different way. We are talking mainly about multilayer printed circuit boards. They can combine rigid and flexible layers. If the flexible layers are moved beyond the rigid ones, you can get a printed circuit board consisting of a rigid and flexible fragment. Another option is to connect two rigid fragments with a flexible one.

Classification of printed circuit board designs based on the layering of their conductive pattern covers most, but not all, printed circuit board designs. For example, for the production of woven circuit boards or cables, weaving equipment, rather than printing, turned out to be suitable. Such “printed circuit boards” already have three degrees of freedom. Just like ordinary fabric, they can take on the most bizarre shapes and shapes.

Printed circuit boards on a base with high thermal conductivity

Recently, there has been an increase in heat generation electronic devices which is related to:

Increased productivity of computing systems,

High power switching needs,

Increasing use of electronic components with increased heat generation.

The latter is most clearly manifested in LED lighting technology, where interest in creating light sources based on powerful ultra-bright LEDs has sharply increased. The luminous efficiency of semiconductor LEDs has already reached 100lm/W. Such ultra-bright LEDs replace conventional incandescent lamps and find their application in almost all areas of lighting technology: street lighting lamps, automotive lighting, emergency lighting, advertising signs, LED panels, indicators, tickers, traffic lights, etc. These LEDs have become indispensable in decorative lighting and dynamic lighting systems due to their monochrome color and switching speed. It is also beneficial to use them where it is necessary to strictly save energy, where frequent maintenance is expensive and where electrical safety requirements are high.

Studies show that approximately 65-85% of the electricity when operating an LED is converted into heat. However, provided that the thermal conditions recommended by the LED manufacturer are followed, the LED service life can reach 10 years. But, if the thermal conditions are violated (usually this means working with a transition temperature of more than 120...125°C), the service life of the LED can drop by 10 times! And if the recommended thermal conditions are grossly violated, for example, when emitter-type LEDs are turned on without a radiator for more than 5-7 seconds, the LED may fail during the first turn on. An increase in the transition temperature, in addition, leads to a decrease in the brightness of the glow and a shift in the operating wavelength. Therefore, it is very important to correctly calculate the thermal regime and, if possible, dissipate the heat generated by the LED as much as possible.

Large manufacturers powerful LEDs, such as Cree, Osram, Nichia, Luxeon, Seoul Semiconductor, Edison Opto, etc., have long been manufacturing them in the form of LED modules or clusters on printed circuit boards with a metal base (in international classification IMPCB - Insulated Metal Printed Circuit Board, or AL PCB - printed circuit boards on an aluminum base).

Figure 5

These printed circuit boards on an aluminum base have a low and fixed thermal resistance, which makes it possible, when installing them on a radiator, to simply ensure heat removal from the p-n junction of the LED and ensure its operation throughout its entire service life.

Copper, Aluminum, and various types of ceramics are used as materials with high thermal conductivity for the bases of such printed circuit boards.

Problems of industrial production technology

The history of the development of printed circuit board production technology is a history of improving quality and overcoming problems that arise along the way.

Here are some of its details.

Printed circuit boards manufactured by metallization of through holes, despite their widespread use, have a very serious drawback. From a design point of view, the weakest link of such printed circuit boards is the junction of the metallized posts in the vias and the conductive layers (contact pads). The connection between the metallized column and the conductive layer occurs along the end of the contact pad. The length of the connection is determined by the thickness of the copper foil and is usually 35 microns or less. Galvanic metallization The walls of vias are preceded by a stage of chemical metallization. Chemical copper, unlike galvanic copper, is more friable. Therefore, the connection of the metallized column with the end surface of the contact pad occurs through an intermediate sublayer of chemical copper that is weaker in strength characteristics. The coefficient of thermal expansion of fiberglass laminate is much greater than that of copper. When passing through the glass transition temperature of the epoxy resin, the difference increases sharply. During thermal shocks, which a printed circuit board experiences for a variety of reasons, the connection is subjected to very large mechanical loads and... breaks. As a result, the electrical circuit is broken and performance is impaired. electrical diagram.

Rice. 6. Interlayer vials in multilayer printed circuit boards: a) without dielectric undercut, 6) with dielectric undercut 1 - dielectric, 2 - contact pad of the inner layer, 3 - chemical copper, 4 - galvanic copper

Rice. 7. Fragment of the design of a multilayer printed circuit board made by layer-by-layer building: 1 - interlayer junction, 2 - inner layer conductor, 3 - mounting pad, 4 - outer layer conductor, 5 - dielectric layers

In multilayer printed circuit boards, increasing the reliability of internal junctions can be achieved by introducing an additional operation - undercutting ( partial removal) dielectric in vias before metallization. In this case, the connection of metallized posts with contact pads is carried out not only at the end, but also partially along the outer annular zones of these pads (Fig. 6).

Higher reliability of metallized vias of multilayer printed circuit boards was achieved using the technology of manufacturing multilayer printed circuit boards using the layer-by-layer building method (Fig. 7). The connections between the conductive elements of the printed layers in this method are made by galvanic growth of copper into the holes of the insulation layer. Unlike the method of metallization of through holes, in this case the vias are filled entirely with copper. The connection area between the conductive layers becomes much larger, and the geometry is different. Breaking such connections is not so easy. Still, this technology is also far from ideal. The transition “galvanic copper - chemical copper - galvanic copper” still remains.

Printed circuit boards made by metallization of through holes must withstand at least four (multilayer at least three) re-solderings. Embossed printed circuit boards allow a much larger number of re-solderings (up to 50). According to the developers, metallized vias in relief printed circuit boards do not reduce, but increase their reliability. What caused such a sharp qualitative leap? The answer is simple. In the technology of manufacturing relief printed circuit boards, conductive layers and metallized columns connecting them are implemented in a single technological cycle (simultaneously). Therefore, there is no transition “galvanic copper - chemical copper - galvanic copper”. But such a high result was obtained as a result of abandoning the most widespread technology for manufacturing printed circuit boards, as a result of the transition to a different design. It is not advisable to abandon the method of metallization of through holes for many reasons.

How to be?

Responsibility for the formation of a barrier layer at the junction of the ends of the contact pads and metallized pistons mainly falls on the technologists. They were able to solve this problem. Revolutionary changes in the technology of manufacturing printed circuit boards have been made by methods of direct metallization of holes, which eliminates the stage of chemical metallization, limiting itself only to preliminary activation of the surface. Moreover, direct metallization processes are implemented in such a way that a conductive film appears only where it is needed - on the surface of the dielectric. As a consequence, the barrier layer in metallized vias of printed circuit boards manufactured by direct metallization of holes is simply absent. Isn't it a beautiful way to resolve a technical contradiction?

It was also possible to overcome the technical contradiction related to the metallization of vias. Plated holes can become a weak link in printed circuit boards for another reason. The thickness of the coating on the walls of vias should ideally be uniform over their entire height. Otherwise, reliability problems arise again. The physical chemistry of electroplating processes counteracts this. The ideal and actual coating profile in metallized vias are shown in Fig. 5. The thickness of the coating at the depth of the hole is usually less than at the surface. The reasons are very different: uneven current density, cathodic polarization, insufficient electrolyte exchange rate, etc. In modern printed circuit boards, the diameter of transition holes to be metalized has already exceeded 100 microns, and the ratio of height to hole diameter in some cases reaches 20:1. The situation has become extremely complicated. Physical methods (using ultrasound, increasing the intensity of fluid exchange in the holes of printed circuit boards, etc.) have already exhausted their capabilities. Even the viscosity of the electrolyte begins to play a significant role.

Rice. 8. Cross-section of a metallized via hole in a printed circuit board. 1 - dielectric, 2 - ideal metallization profile of the hole walls, 3 - real metallization profile of the hole walls,
4 - resist

Traditionally, this problem has been solved by using electrolytes with leveling additives that are adsorbed in areas where the current density is higher. The sorption of such additives is proportional to the current density. Additives create a barrier layer to counteract excess plating on sharp edges and adjacent areas (closer to the surface of the printed circuit board).

Another solution to this problem has been known theoretically for a long time, but in practice it was possible to implement it quite recently - after the industrial production of high-power switching power supplies was mastered. This method is based on the use of pulsed (reverse) power supply mode for galvanic baths. Most time, direct current is supplied. In this case, coating deposition occurs. Reverse current is supplied a minority of the time. At the same time, the deposited coating dissolves. Uneven current density (more sharp corners) in this case brings only benefits. For this reason, the dissolution of the coating occurs first and to a greater extent at the surface of the printed circuit board. In that technical solution a whole “bouquet” of techniques for resolving technical contradictions is used: use a partially redundant action, turning harm into benefit, apply a transition from a continuous process to a pulsed one, do the opposite, etc. And the result obtained corresponds to this “bouquet”. With a certain combination of the duration of forward and reverse pulses, it is even possible to obtain a coating thickness in the depth of the hole that is greater than at the surface of the printed circuit board. This is why this technology has proven to be indispensable for filling blind vias with metal (a common feature of modern printed circuit boards), due to which the interconnect density in the PCB is approximately doubled.

Problems associated with the reliability of metallized vias in printed circuit boards are local in nature. Consequently, the contradictions that arise in the process of their development in relation to printed circuit boards as a whole are also not universal. Although such printed circuit boards occupy the lion's share of the market for all printed circuit boards.

Also, in the process of development, other problems that technologists face are solved, but consumers do not even think about them. We obtain multilayer printed circuit boards for our needs and use them.

Microminiaturization

At the initial stage, the same components were installed on printed circuit boards that were used for volumetric installation of electronic devices, albeit with some modification of the pins to reduce their size. But the most common components could be installed on printed circuit boards without modification.

With the advent of printed circuit boards, it became possible to reduce the size of components used on printed circuit boards, which in turn led to a reduction in the operating voltages and currents consumed by these elements. Since 1954, the Ministry of Power Plants and Electrical Industry has mass-produced the Dorozhny tube portable radio receiver, which used a printed circuit board.

With the advent of miniature semiconductor amplifier devices - transistors, printed circuit boards began to dominate in household appliances, and a little later in industry, and with the advent of fragments of electronic circuits - functional modules and microcircuits - combined on one chip, their design already provided for the installation of exclusively non-printed circuit boards.

With the continued reduction in the size of active and passive components, a new concept has emerged - “Microminiaturization”.

In electronic components, this resulted in the emergence of LSI and VLSI containing many millions of transistors. Their appearance forced an increase in the number of external connections (see the contact surface of the graphics processor in Figure 9.a), which in turn caused a complication in the layout of conductive lines, which can be seen in Figure 9.b.

Such a GPU panel, and CPU too - nothing more than a small multilayer printed circuit board on which the processor chip itself, the wiring of connections between the chip pins and the contact field, and external elements (usually filter capacitors of the power distribution system) are located.

Figure 9

And don’t let it seem like a joke to you, the 2010 CPU from Intel or AMD is also a printed circuit board, and a multilayer one at that.

Figure 9a

The development of printed circuit boards, as well as electronic equipment in general, is a line of reducing its elements; their compaction on the printed surface, as well as the reduction of electronic elements. In this case, “elements” should be understood as both the own property of printed circuit boards (conductors, vias, etc.), and elements from the supersystem (printed circuit assembly) - radioelements. The latter are ahead of printed circuit boards in terms of speed of microminiaturization.

Microelectronics is involved in the development of VLSI.

Increasing the density of the element base requires the same from the conductors of the printed circuit board - the carrier of this element base. In this regard, many problems arise that require solutions. We will talk in more detail about two such problems and ways to solve them.

The first methods of producing printed circuit boards were based on gluing copper foil conductors to the surface of a dielectric substrate.

It was assumed that the width of the conductors and the gaps between the conductors are measured in millimeters. In this version, such technology was quite workable. The subsequent miniaturization of electronic equipment required the creation of other methods for manufacturing printed circuit boards, the main versions of which (subtractive, additive, semi-additive, combined) are still used today. The use of such technologies has made it possible to implement printed circuit boards with element sizes measured in tenths of a millimeter.

Achieving a resolution level of approximately 0.1 mm (100 µm) in printed circuit boards was a landmark event. On the one hand, there was a transition “down” by another order of magnitude. On the other hand, it is a kind of qualitative leap. Why? The dielectric substrate of most modern printed circuit boards is fiberglass - a layered plastic with a polymer matrix reinforced with fiberglass. Reducing the gaps between the conductors of the printed circuit board has led to the fact that they have become commensurate with the thickness of glass threads or the thickness of the weaves of these threads in fiberglass. And the situation in which conductors are “shorted” by such knots has become quite real. As a result, the formation of peculiar capillaries in fiberglass laminate, “shoring” these conductors, has become real. In humid environments, capillaries eventually lead to deterioration of the insulation levels between PCB conductors. To be more precise, this happens even in normal humidity conditions. Moisture condensation in the capillary structures of fiberglass is also observed under normal conditions. Moisture always reduces the level of insulation resistance.

Since such printed circuit boards have become commonplace in modern electronic equipment, we can conclude that the developers of basic materials for printed circuit boards managed to solve this problem using traditional methods. But will they cope with the next significant event? Another qualitative leap has already occurred.

It is reported that Samsung specialists have mastered the technology of manufacturing printed circuit boards with conductor widths and gaps between them of 8-10 microns. But this is not the thickness of a glass thread, but of fiberglass!

The task of providing insulation in the ultra-small gaps between the conductors of current and especially future printed circuit boards is complex. By what methods it will be solved - traditional or non-traditional - and whether it will be solved - time will tell.

Rice. 10. Etching profiles of copper foil: a - ideal profile, b - real profile; 1 - protective layer, 2 - conductor, 3 - dielectric

There were difficulties in obtaining ultra-small (ultra-narrow) conductors in printed circuit boards. For many reasons, subtractive methods have become widespread in printed circuit board manufacturing technologies. In subtractive methods, an electrical circuit pattern is formed by removing unnecessary pieces of foil. Back during the Second World War, Paul Eisler developed the technology of etching copper foil with ferric chloride. Such an unpretentious technology is still used by radio amateurs today. Industrial technology Not far from this “kitchen” technology. The only difference is that the composition of the etching solutions has changed and elements of process automation have appeared.

The fundamental disadvantage of absolutely all etching technologies is that etching occurs not only in the desired direction (towards the dielectric surface), but also in an undesired transverse direction. The lateral undercut of the conductors is comparable to the thickness of the copper foil (about 70%). Usually, instead of an ideal conductor profile, a mushroom-shaped profile is obtained (Fig. 10). When the width of the conductors is large, and in the simplest printed circuit boards it is measured even in millimeters, people simply turn a blind eye to the lateral undercut of the conductors. If the width of the conductors is commensurate with their height or even less than it (the realities of today), then “lateral aspirations” call into question the feasibility of using such technologies.

In practice, the amount of lateral undercut of printed conductors can be reduced to some extent. This is achieved by increasing the etching speed; using jet pouring (etchant jets coincide with the desired direction - perpendicular to the plane of the sheet), as well as other methods. But when the width of the conductor approaches its height, the effectiveness of such improvements becomes clearly insufficient.

But advances in photolithography, chemistry and technology now make it possible to solve all these problems. These solutions come from microelectronics technologies.

Amateur radio technologies for the production of printed circuit boards

The manufacture of printed circuit boards in amateur radio conditions has its own characteristics, and the development of technology is increasingly increasing these possibilities. But processes continue to be their basis

The question of how to cheaply produce printed circuit boards at home has worried all radio amateurs, probably since the 60s of the last century, when printed circuit boards found widespread use in household appliances. And if back then the choice of technologies was not so great, today, thanks to the development of modern technology, radio amateurs have the opportunity to quickly and efficiently produce printed circuit boards without the use of any expensive equipment. And these possibilities are constantly expanding, allowing the quality of their creations to become closer and closer to industrial designs.

Actually, the entire process of manufacturing a printed circuit board can be divided into five main stages:

• preliminary preparation of the workpiece (surface cleaning, degreasing);
• applying a protective coating in one way or another;
• removing excess copper from the surface of the board (etching);
• cleaning the workpiece from the protective coating;
• drilling holes, coating the board with flux, tinning.

We consider only the most common “classical” technology, in which excess copper is removed from the surface of the board by chemical etching. In addition, it is possible, for example, to remove copper by milling or using an electric spark installation. However, these methods are not widely used either in the amateur radio environment or in industry (although the production of circuit boards by milling is sometimes used in cases where it is necessary to very quickly produce simple printed circuit boards in single quantities).

And here we will talk about the first 4 points of the technological process, since drilling is performed by a radio amateur using the tool that he has.

At home, it is impossible to make a multilayer printed circuit board that can compete with industrial designs, therefore, usually in amateur radio conditions, double-sided printed circuit boards are used, and in microwave device designs only double-sided.

Although one should strive when making printed circuit boards at home, one should strive when developing a circuit to use as many surface-mount components as possible, which in some cases makes it possible to place almost the entire circuit on one side of the board. This is due to the fact that no technology for metallizing vias has yet been invented that is actually feasible at home. Therefore, if the board layout cannot be done on one side, the layout should be done on the second side using the pins of various components installed on the board as interlayer vias, which in this case will have to be soldered on both sides of the board. Of course, there are various ways to replace the metallization of holes (using a thin conductor inserted into the hole and soldered to the tracks on both sides of the board; using special pistons), but they all have significant drawbacks and are inconvenient to use. Ideally, the board should be routed on only one side using a minimum number of jumpers.

Let us now take a closer look at each of the stages of manufacturing a printed circuit board.

Preliminary preparation of the workpiece

This stage is the initial one and consists of preparing the surface of the future printed circuit board for applying a protective coating to it. In general, surface cleaning technology has not undergone any significant changes over a long period of time. The whole process comes down to removing oxides and contaminants from the surface of the board using various abrasives and subsequent degreasing.

To remove heavy dirt, you can use fine-grained sandpaper (“zero”), fine abrasive powder, or any other product that does not leave deep scratches on the surface of the board. Sometimes you can simply wash the surface of the printed circuit board with a hard dishwashing sponge with detergent or powder (for these purposes it is convenient to use an abrasive dishwashing sponge, which looks like felt with small inclusions of some substance; often such a sponge is glued to a piece of foam rubber) . In addition, if the surface of the printed circuit board is sufficiently clean, you can skip the abrasive treatment step altogether and go straight to degreasing.

If there is only a thick oxide film on the printed circuit board, it can be easily removed by treating the printed circuit board for 3-5 seconds with a ferric chloride solution, followed by rinsing in cold running water. It should be noted, however, that it is advisable to either perform this operation immediately before applying the protective coating, or after it, store the workpiece in a dark place, since copper quickly oxidizes in light.

The final stage surface preparation consists of degreasing. To do this, you can use a piece of soft, fiber-free cloth moistened with alcohol, gasoline or acetone. Here you should pay attention to the cleanliness of the board surface after degreasing, since recently acetone and alcohol with a significant amount of impurities have begun to appear, which leave whitish stains on the board after drying. If this is the case, then you should look for another degreaser. After degreasing, the board should be washed in running cold water. The quality of cleaning can be controlled by monitoring the degree of water wetting of the copper surface. A surface completely wetted with water, without the formation of drops or breaks in the water film, is an indicator of a normal level of cleaning. Disturbances in this film of water indicate that the surface has not been sufficiently cleaned.

Application of protective coating

Applying a protective coating is the most important stage in the process of manufacturing printed circuit boards, and it is they who determine 90% of the quality of the manufactured board. Currently, three methods of applying a protective coating are the most popular in the amateur radio community. We will consider them in order of increasing quality of the boards obtained when using them.

First of all, it is necessary to clarify that the protective coating on the surface of the workpiece must form a homogeneous mass, without defects, with smooth, clear boundaries and resistant to the effects of the chemical components of the etching solution.

Manual application of protective coating

With this method, the drawing of the printed circuit board is transferred to fiberglass laminate manually using some kind of writing device. Recently, many markers have appeared on the market, the dye of which is not washed off with water and provides a fairly durable protective layer. In addition, for hand drawing you can use a drawing board or some other device filled with dye. For example, it is convenient to use for drawing a syringe with a thin needle (insulin syringes with a needle diameter of 0.3-0.6 mm) cut to a length of 5-8 mm are best suited for these purposes. In this case, the rod should not be inserted into the syringe - the dye should flow freely under the influence of capillary effect. Also, instead of a syringe, you can use a thin glass or plastic tube extended over the fire to achieve the desired diameter. Particular attention should be paid to the quality of processing of the edge of the tube or needle: when drawing, they should not scratch the board, otherwise the already painted areas may be damaged. When working with such devices, you can use bitumen or some other varnish diluted with a solvent, tsaponlak, or even a solution of rosin in alcohol as a dye. In this case, it is necessary to select the consistency of the dye so that it flows freely when drawing, but at the same time does not flow out and form drops at the end of the needle or tube. It is worth noting that the manual process of applying a protective coating is quite labor-intensive and is only suitable in cases where it is necessary to very quickly produce a small circuit board. The minimum track width that can be achieved when drawing by hand is about 0.5 mm.

Using "laser printer and iron technology"

This technology appeared relatively recently, but immediately became widespread due to its simplicity and high quality of the resulting boards. The basis of the technology is the transfer of toner (powder used when printing in laser printers) from any substrate to a printed circuit board.

In this case, two options are possible: either the substrate used is separated from the board before etching, or, if the substrate is used aluminum foil, it is etched together with copper .

The first stage of using this technology is to print mirror image printed circuit board pattern on the substrate. The printer's print settings should be set to maximum print quality (since in this case the thickest toner layer is applied). As a backing, you can use thin coated paper (covers from various magazines), fax paper, aluminum foil, film for laser printers, backing from Oracal self-adhesive film or some other materials. If you use paper or foil that is too thin, you may need to glue it around the perimeter onto a piece of thick paper. Ideally, the printer should have a paper path without kinks, which prevents such a sandwich from collapsing inside the printer. This is also of great importance when printing on foil or Oracal film base, since the toner adheres to them very weakly, and if the paper inside the printer is bent, there is a high probability that you will have to spend several unpleasant minutes cleaning the printer oven from adhering toner residues. It's best if the printer can pass paper horizontally through itself while printing on the top side (like the HP LJ2100, one of the best printers for PCB manufacturing). I would like to immediately warn owners of printers such as HP LJ 5L, 6L, 1100 so that they do not try to print on foil or base from Oracal - usually such experiments end in failure. Also, in addition to the printer, you can also use a copy machine, the use of which sometimes gives even better results compared to printers due to the application of a thick layer of toner. The main requirement for the substrate is that it can be easily separated from the toner. Also, if you use paper, it should not leave any lint in the toner. In this case, two options are possible: either the substrate is simply removed after transferring the toner to the board (in the case of film for laser printers or the base from Oracal), or it is pre-soaked in water and then gradually separated (coated paper).

Transferring toner to a board involves applying a substrate with toner to a previously cleaned board and then heating it to a temperature slightly above the melting point of the toner. There are a huge number of options for how to do this, but the simplest is to press the substrate to the board with a hot iron. At the same time, to evenly distribute the pressure of the iron on the substrate, it is recommended to lay several layers of thick paper between them. A very important issue is the temperature of the iron and the holding time. These parameters vary in each specific case, so you may have to run more than one experiment before you get good results. There is only one criterion here: the toner must have time to melt enough to stick to the surface of the board, and at the same time it must not have time to reach a semi-liquid state so that the edges of the tracks do not flatten. After “welding” the toner to the board, it is necessary to separate the substrate (except when used as a substrate aluminum foil: it should not be separated, since it dissolves in almost all etching solutions). The laser film and Oracal backing simply peel off carefully while plain paper requires pre-soaking in hot water.

It is worth noting that due to the printing features of laser printers, the toner layer in the middle of large solid polygons is quite small, so you should avoid using such areas on the board whenever possible, or you will have to retouch the board manually after removing the backing. In general, the use of this technology, after some training, allows you to achieve the width of the tracks and the gaps between them down to 0.3 mm.

I have been using this technology for many years (since a laser printer became available to me).

Application of photoresists

A photoresist is a light-sensitive substance (usually in the near ultraviolet region) that changes its properties when exposed to light.

Recently, several types of imported photoresists in aerosol packaging have appeared on the Russian market, which are especially convenient for use at home. The essence of using photoresist is as follows: a photomask () is applied to a board with a layer of photoresist applied to it and it is illuminated, after which the illuminated (or unexposed) areas of the photoresist are washed off with a special solvent, which is usually caustic soda (NaOH). All photoresists are divided into two categories: positive and negative. For positive photoresists, the track on the board corresponds to a black area on the photomask, and for negative ones, accordingly, a transparent area.

Positive photoresists are most widespread as they are the most convenient to use.

Let us dwell in more detail on the use of positive photoresists in aerosol packaging. The first step is preparing a photo template. At home, you can get it by printing a board design on a laser printer on film. In this case, it is necessary to pay special attention to the density of black color on the photomask, for which you need to disable all modes of saving toner and improving print quality in the printer settings. In addition, some companies offer output of a photomask on a photoplotter - and you are guaranteed a high-quality result.

At the second stage, a thin film of photoresist is applied to the previously prepared and cleaned surface of the board. This is done by spraying it from a distance of about 20 cm. In this case, one should strive for maximum uniformity of the resulting coating. In addition, it is very important to ensure that there is no dust during the sputtering process - every speck of dust that gets into the photoresist will inevitably leave its mark on the board.

After applying the photoresist layer, it is necessary to dry the resulting film. It is recommended to do this at a temperature of 70-80 degrees, and first you need to dry the surface at a low temperature and only then gradually increase the temperature to the desired value. Drying time at the specified temperature is about 20-30 minutes. As a last resort, drying the board with room temperature in 24 hours. Boards coated with photoresist should be stored in a cool, dark place.

After applying the photoresist, the next step is exposure. In this case, a photomask is applied to the board (with the printed side facing the board, this helps to increase clarity during exposure), which is pressed against thin glass or. When enough small sizes For pressing the boards, you can use a photographic plate washed from the emulsion. Since the region of maximum spectral sensitivity of most modern photoresists is in the ultraviolet range, for illumination it is advisable to use a lamp with a large proportion of UV radiation in the spectrum (DRSh, DRT, etc.). As a last resort, you can use a powerful xenon lamp. The exposure time depends on many reasons (type and power of the lamp, distance from the lamp to the board, thickness of the photoresist layer, etc.) and is selected experimentally. However, in general, exposure time is usually no more than 10 minutes, even when exposed in direct sunlight.

(I don’t recommend using plastic plates that are transparent in visible light for pressing, as they have a strong absorption of UV radiation)

Most photoresists are developed with a solution of sodium hydroxide (NaOH) - 7 grams per liter of water. It is best to use a freshly prepared solution at a temperature of 20-25 degrees. The development time depends on the thickness of the photoresist film and ranges from 30 seconds to 2 minutes. After development, the board can be etched in ordinary solutions, since the photoresist is resistant to acids. When using high-quality photomasks, the use of photoresist allows you to obtain tracks up to 0.15-0.2 mm wide.

Etching

There are many known compounds for chemical etching of copper. All of them differ in the speed of the reaction, the composition of the substances released as a result of the reaction, as well as the availability of the chemical reagents necessary for preparing the solution. Below is information about the most popular etching solutions.

Ferric chloride (FeCl)

Perhaps the most famous and popular reagent. Dry ferric chloride is dissolved in water until a saturated solution of golden yellow color is obtained (this will require about two tablespoons per glass of water). The etching process in this solution can take from 10 to 60 minutes. The time depends on the concentration of the solution, temperature and stirring. Stirring significantly speeds up the reaction. For these purposes, it is convenient to use an aquarium compressor, which provides mixing of the solution with air bubbles. The reaction also accelerates when the solution is heated. After etching is complete, the board must be washed big amount water, preferably with soap (to neutralize acid residues). The disadvantages of this solution include the formation of waste during the reaction, which settles on the board and interferes with the normal course of the etching process, as well as the relatively low reaction rate.

Ammonium persulfate

A light crystalline substance that dissolves in water based on the ratio of 35 g of substance to 65 g of water. The etching process in this solution takes about 10 minutes and depends on the area of ​​the copper coating being etched. To ensure optimal conditions for the reaction, the solution must have a temperature of about 40 degrees and be constantly stirred. After etching is complete, the board must be washed in running water. The disadvantages of this solution include the need to maintain the required temperature and stirring.

Solution of hydrochloric acid(HCl) and hydrogen peroxide(H2O2)

- To prepare this solution, you need to add 200 ml of 35% hydrochloric acid and 30 ml of 30% hydrogen peroxide to 770 ml of water. The prepared solution should be stored in a dark bottle, not hermetically sealed, since the decomposition of hydrogen peroxide releases gas. Attention: when using this solution, all precautions must be taken when working with caustic chemicals. All work must be carried out only on fresh air or under the hood. If the solution gets on your skin, rinse it immediately with plenty of water. The etching time is highly dependent on stirring and solution temperature and is on the order of 5-10 minutes for a well-mixed fresh solution at room temperature. The solution should not be heated above 50 degrees. After etching, the board must be washed with running water.

This solution after etching can be restored by adding H 2 O 2. Assessment of the required amount of hydrogen peroxide is carried out visually: immersed in a solution copper board should be repainted from red to dark brown. The formation of bubbles in the solution indicates an excess of hydrogen peroxide, which leads to a slowdown in the etching reaction. The disadvantage of this solution is the need to strictly observe all precautions when working with it.

Solution citric acid and hydrogen peroxide from Radiokot

In 100 ml of pharmaceutical 3% hydrogen peroxide, 30 g of citric acid and 5 g of table salt are dissolved.

This solution should be enough to etch 100 cm2 of copper, 35 µm thick.

There is no need to skimp on salt when preparing the solution. Since it plays the role of a catalyst, it is practically not consumed during the etching process. Peroxide 3% should not be diluted further because when other ingredients are added, its concentration decreases.

The more hydrogen peroxide (hydroperite) is added, the faster the process will go, but do not overdo it - the solution is not stored, i.e. is not reused, which means hydroperite will simply be overused. Excess peroxide can be easily determined by the abundant “bubbling” during etching.

However, adding citric acid and peroxide is quite acceptable, but it is more rational to prepare a fresh solution.

Cleaning the workpiece

After etching and washing of the board is completed, it is necessary to clean its surface from the protective coating. This can be done with any organic solvent, for example, acetone.

Next you need to drill all the holes. This must be done with a sharply sharpened drill at maximum engine speed. If, when applying the protective coating, no empty space was left in the centers of the contact pads, it is necessary to first mark the holes (this can be done, for example, with a core). After that, defects (fringe) on the back side of the board are removed by countersinking, and on a double-sided printed circuit board on copper - with a drill with a diameter of about 5 mm in a manual clamp for one turn of the drill without applying force.

The next step is to coat the board with flux, followed by tinning. You can use special industrial fluxes (best washed off with water or do not require rinsing at all) or simply coat the board weak solution rosin in alcohol.

Tinning can be done in two ways:

Immersion in molten solder

Use a soldering iron and a metal braid impregnated with solder.

In the first case, it is necessary to make an iron bath and fill it with a small amount of low-melting solder - Rose or Wood alloy. The melt must be completely covered with a layer of glycerin on top to avoid oxidation of the solder. To heat the bath, you can use an inverted iron or hotplate. The board is dipped into the melt and then removed while removing excess solder with a hard rubber squeegee.

Conclusion

I think this material will help readers get an idea of ​​the design and manufacture of printed circuit boards. And for those who are starting to get involved in electronics, get the basic skills of making them at home. For a more complete acquaintance with printed circuit boards, I recommend reading [L.2]. It can be downloaded on the Internet.

Literature

1. Polytechnic Dictionary. Editorial team: Inglinsky A. Yu. et al. M.: Soviet Encyclopedia. 1989.
2. Medvedev A. M. Printed circuit boards. Designs and materials. M.: Technosphere. 2005.
3. From the history of printed circuit board technologies // Electronics-NTB. 2004. No. 5.
4. New items in electronic technology. Intel is ushering in the era of three-dimensional transistors. Alternative to traditional planar devices // Electronics-NTB. 2002. No. 6.
5. Truly three-dimensional microcircuits - the first approximation // Components and Technologies. 2004. No. 4.
6. Mokeev M. N., Lapin M. S. Technological processes and systems for the production of woven circuit boards and cables. L.: LDNTP 1988.
7. Volodarsky O. Does this computer suit me? Electronics woven into fabric is becoming fashionable // Electronics-NTB. 2003. No. 8.
8. Medvedev A. M. Printed circuit board production technology. M.: Technosphere. 2005.
9. Medvedev A. M. Pulse metallization of printed circuit boards // Technologies in the electronic industry. 2005. No. 4
10. Printed circuit boards - development lines, Vladimir Urazaev,

Our company produces printed circuit boards from high-quality imported materials, ranging from standard FR4 to microwave materials and polyimide. In this section, we define the basic terms and concepts used in the field of printed circuit board design and manufacturing. This section talks about very simple things familiar to every design engineer. However, there are a number of nuances here that many developers do not always take into account.

*** Additional information can be obtained from

Multilayer PCB Design
Let's consider a typical design of a multilayer board (Fig. 1). In the first, most common, option, the internal layers of the board are formed from double-sided copper-laminated fiberglass, which is called the “core”. The outer layers are made of copper foil, pressed with the inner layers using a binder - a resinous material called "prepreg". After pressing at high temperatures, a “pie” of a multilayer printed circuit board is formed, in which holes are then drilled and metallized. The second option is less common, when the outer layers are formed from “cores” held together with prepreg. This is a simplified description; there are many other designs based on these options. However, the basic principle is that prepreg acts as the bonding material between the layers. Obviously, there cannot be a situation where two double-sided "cores" are adjacent without a prepreg spacer, but a foil-prepreg-foil-prepreg...etc structure is possible, and is often used in boards with complex combinations of blind and hidden holes.

Blind and hidden holes
The term "blind holes" refers to vias that connect the outer layer to the nearest inner layers and do not have access to a second outer layer. It comes from the English word blind, and is similar to the term "blind holes". Hidden, or buried (from English buried), holes are made in the inner layers and have no exit to the outside. The simplest options for blind and hidden holes are shown in Fig. 2. Their use is justified in the case of very dense wiring or for boards very saturated with planar components on both sides. The presence of these holes increases the cost of the board from one and a half to several times, but in many cases, especially when routing microcircuits in a BGA package with a small pitch, you cannot do without them. There are various ways to form such vias; they are discussed in more detail in the section, but for now we will consider in more detail the materials from which the multilayer board is constructed.

Tg—glass transition temperature (structure destruction)
Dk - dielectric constant

Basic dielectrics for printed circuit boards
The main types and parameters of materials used for the manufacture of MPPs are given in Table 1. Typical designs of printed circuit boards are based on the use of standard fiberglass laminate type FR4, with an operating temperature, usually from -50 to +110 °C, glass transition (destruction) temperature Tg about 135 °C. Its dielectric constant Dk can be from 3.8 to 4.5, depending on the supplier and type of material. For increased requirements for heat resistance or when mounting boards in an oven using lead-free technology (t up to 260 °C), high-temperature FR4 High Tg or FR5 is used. When requirements for constant operation at high temperatures or sudden temperature changes are required, polyimide is used. In addition, polyimide is used for the manufacture of high-reliability circuit boards, for military applications, and also in cases where increased electrical strength is required. For boards with microwave circuits (more than 2 GHz), separate layers of microwave material are used, or the entire board is made of microwave material (Fig. 3). The most well-known suppliers of special materials are Rogers, Arlon, Taconic, and Dupont. The cost of these materials is higher than FR4 and is roughly shown in the last column of Table 1 relative to the cost of FR4. Examples of boards with different types of dielectric are shown in Fig. 4, 5.

Material thickness
Knowing the available material thicknesses is important for an engineer not only for determining the overall thickness of the board. When designing MPP, developers are faced with the following tasks:
- calculation of the wave resistance of conductors on the board;
- calculation of the value of interlayer high-voltage insulation;
- selection of the structure of blind and hidden holes.
Available options and thicknesses of various materials are shown in tables 2-6. It should be taken into account that the tolerance on the thickness of the material is usually up to ±10%, therefore the tolerance on the thickness of the finished multilayer board cannot be less than ±10%.

Table 2. Double-sided FR4 “cores” for the internal layers of the printed circuit board

Typically in stock;
h - On request (not always available)
m - Can be manufactured;
Note: to ensure the reliability of the finished boards, it is important to know that for foreign internal layers we prefer to use cores with 35 micron foil rather than 18 micron (even with a conductor and gap width of 0.1 mm). This increases the reliability of printed circuit boards.
The dielectric constant of FR4 cores can range from 3.8 to 4.4 depending on the brand.

Table 3. Prepreg (“bonding” layer) for multilayer printed circuit boards

The dielectric constant of FR4 prepreg can range from 3.8 to 4.4 depending on the brand.
Please check this parameter for a specific material with our engineers by email

Table 4. Rogers microwave materials for printed circuit boards

* Dk - dielectric constant

Table 5. Arlon microwave materials for MPP

Note: Microwave materials are not always in stock, and their delivery time can take up to 1 month. When choosing a board design, you need to check the stock status of the MPP manufacturer.

Dk — Dielectric constant
Tg—glass transition temperature

I would like to note the importance of the following points:
1. In principle, all FR4 core values ​​from 0.1 to 1.0mm are available in 0.1mm increments. However, when designing urgent orders, you should check in advance the availability of materials in the warehouse of the PCB manufacturer.
2. When it comes to the thickness of the material - for materials intended for the manufacture of double-sided circuit boards, the thickness of the material is indicated including copper. The “core” thicknesses for the internal layers of the MPP are specified in the documentation without the copper thickness.
Example 1: material FR4, 1.6/35/35 has a dielectric thickness: 1.6-(2x35 µm)=1.53 mm (with a tolerance of ±10%).
Example 2: FR4, 0.2/35/35 core has dielectric thickness: 200 µm (with tolerance ±10%) and total thickness: 200 µm+(2x35 µm)=270 µm.
3. Ensuring reliability. The permissible number of adjacent layers of prepreg in MPP is no less than 2 and no more than 4. The possibility of using a single layer of prepreg between the “cores” depends on the nature of the pattern and the thickness of the adjacent copper layers. The thicker the copper and the richer the pattern of the conductors, the more difficult it is to fill the space between the conductors with resin. And the reliability of the board depends on the quality of the filling.
Example: copper 17 microns - you can use 1 layer 1080, 2116 or 106; copper 35 microns - you can use 1 layer only for 2116.

PCB pad coatings
Let's look at what types of coatings there are for copper pads. Most often, sites are coated with a tin-lead alloy, or PIC. The method of applying and leveling the surface of solder is called HAL or HASL (from English Hot Air Solder Leveling - leveling solder with hot air). This coating provides the best solderability of the pads. However, it is being replaced by more modern coatings, usually compatible with the requirements of the international RoHS directive. This directive requires the prohibition of the presence of harmful substances, including lead, in products. So far, RoHS does not apply to the territory of our country, but it is useful to remember its existence. The problems associated with RoHS will be described in one of the subsequent sections, but for now let's take a look at possible options coverage of MPP sites in Table 7. HASL is applied everywhere, unless there are other requirements. Immersion (chemical) gold plating is used to provide a smoother board surface (this is especially important for BGA pads), but has slightly lower solderability. Oven soldering is performed using approximately the same technology as HASL, but hand soldering requires the use of special fluxes. Organic coating, or OSP, protects the copper surface from oxidation. Its disadvantage is the short shelf life of solderability (less than 6 months). Immersion tin provides a smooth surface and good solderability, although it also has a limited solder life. Lead-free HAL has the same properties as lead-containing HAL, but the composition of the solder is approximately 99.8% tin and 0.2% additives. The contacts of the blade connectors, which are subject to friction during operation of the board, are electroplated with a thicker and more rigid layer of gold. For both types of gilding, a nickel underlayer is used to prevent diffusion of gold.

Table 7. PCB pad coatings

Note: All coatings except HASL are RoHS compliant and suitable for lead-free soldering.

Protective and other types of printed circuit board coatings
To complete the picture, let’s consider the functional purpose and materials of printed circuit board coatings.
- Solder mask - applied to the surface of the board to protect conductors from accidental short circuits and dirt, as well as to protect fiberglass laminate from thermal shock during soldering. The mask does not carry any other functional load and cannot serve as protection against moisture, mold, breakdown, etc. (except when special types of masks are used).
- Marking - applied to the board with paint over the mask to simplify identification of the board itself and the components located on it.
- Peelable mask - applied to specified areas of the board that need to be temporarily protected, for example, from soldering. It is easy to remove in the future, since it is a rubber-like compound and simply peels off.
- Carbon contact coating - applied to certain areas of the board as contact fields for keyboards. The coating has good conductivity, does not oxidize and is wear-resistant.
- Graphite resistive elements - can be applied to the surface of the board to perform the function of resistors. Unfortunately, the accuracy of the denominations is low - no more accurate than ±20% (with laser adjustment - up to 5%).
- Silver contact jumpers - can be applied as additional conductors, creating another conductive layer when there is not enough space for routing. Mainly used for single-layer and double-sided printed circuit boards.

Table 8. PCB Surface Coatings

Conclusion
The choice of materials is large, but, unfortunately, often when producing small and medium-sized series of printed circuit boards, the stumbling block becomes the availability of the necessary materials in the warehouse of the plant that produces the MPP. Therefore, before designing an MPP, especially if we are talking about creating a non-standard design and using non-standard materials, it is necessary to agree with the manufacturer on the materials and layer thicknesses used in the MPP, and perhaps order these materials in advance.

Printed circuit board (in English PCB - printed circuit board)- a plate made of dielectric on which at least one electrically conductive circuit (electronic circuit) is formed (usually by printing). A printed circuit board is designed for electrical and mechanical connection of various electronic components or connection of individual electronic components. Electronic components on a printed circuit board are connected at their pins to elements of a conductive pattern, usually by soldering, or wrapping, or riveting, or pressing, resulting in an electronic module (or assembled printed circuit board) being assembled.

Types of boards

Depending on the number of layers with an electrically conductive pattern, printed circuit boards are divided into single-sided, double-sided and multilayer.
Unlike wall-mounted, on the printed circuit board, the electrically conductive pattern is made of foil using an additive or subtractive method. In the additive method, a conductive pattern is formed on a non-foil material, usually by chemical copper plating through a protective mask previously applied to the material. In the subtractive method, a conductive pattern is formed on the foil material by removing unnecessary sections of the foil, usually using chemical etching.

A printed circuit board usually contains mounting holes and pads, which can be additionally coated with a protective coating: tin-lead alloy, tin, gold, silver, organic protective coating. In addition, printed circuit boards have vias for electrical connection of the board layers and an external insulating coating (“ protective mask") which covers the surface of the board not used for contact with an insulating layer, the marking is usually applied using silk-screen printing, less often - by inkjet or laser.

Types of printed circuit boards

By the number of layers of conductive material:
-Single-sided
-Double-sided
-Multilayer (MPP)

In terms of flexibility:
-Hard
-Flexible

According to installation technology:
-For hole mounting
-Surface Mount

Each type of printed circuit board may have its own characteristics, due to the requirements for special operating conditions (for example, an extended temperature range) or application features (for example, in devices operating at high frequencies).

Materials

The basis of the printed circuit board is a dielectric; the most commonly used materials are textolite, fiberglass, and getinax.
Also, the basis of printed circuit boards can be a metal base coated with a dielectric (for example, anodized aluminum); copper foil of the tracks is applied on top of the dielectric. Such printed circuit boards are used in power electronics for efficient heat removal from electronic components. In this case, the metal base of the board is attached to the radiator.
The material used for printed circuit boards operating in the microwave range and at temperatures up to 260 °C is fluoroplastic reinforced with glass fabric (for example, FAF-4D) and ceramics. Flexible boards made from polyimide materials such as Kapton.

FR-4

A family of materials under the general name FR-4 according to the NEMA classification (National Electrical Manufacturers Association, USA). These materials are the most common for the production of DPP, MPP and OPP with increased requirements for mechanical strength. FR-4 is a material based on fiberglass with epoxy resin as a binder (fiberglass). Usually a matte yellowish color or a transparent, familiar green color, it is given by a solder mask applied to the surface of the printed circuit board. Flammability class UL94-V0.
Depending on the properties and application of FR-4
-standard, with glass transition temperature Tg ~130°C, with or without UV blocking. The most common and widely used type, it is also the least expensive of the FR-4;

With a high glass transition temperature, Tg ~170°C-180°C;
-halogen-free;
-with a standardized tracking index, CTI ≥400, ≥600;
- high-frequency, with low dielectric constant ε ≤3.9 and small tangent of angle dielectric losses df ≤0.02.

CEM-3

CEM-3 material family according to NEMA classification. The fiberglass-epoxy composite material is typically milky white or clear. Consists of two outer layers of fiberglass, between which non-woven glass fiber (fiberglass felt) is placed. Widely used in the production of metallized fibreboard. Its properties are very close to FR-4 and differ in by and large, only less mechanical strength. It is an excellent low cost alternative to FR-4 for the vast majority of applications. Excellent mechanical processing (milling, stamping). Flammability class UL94-V0.
Depending on the properties and scope of application, CEM-3 is divided into the following subclasses:
-standard, with or without UV blocking;

CEM-1

Material class CEM-1 according to NEMA classification. These composite materials are made on a paper base with two layers of fiberglass on the outside. Usually milky white, milky yellow or brownish brown. Incompatible with the process of metallization of holes, therefore they are used only for the production of OPP. Dielectric properties are close to FR-4, mechanical properties are slightly worse. CEM-1 is a good alternative to FR-4 in single-sided PCB production where cost is a determining factor. Excellent mechanical processing (milling, stamping). Flammability class UL94-V0.
Divided into the following subclasses:
-standard;
-high temperature, compatible with lead-free tinning and soldering technologies;
-halogen-free, without phosphorus and antimony;
-with a standardized tracking index, CTI ≥600
-moisture resistant, with increased dimensional stability

FR-1/FR-2

Material class FR-1 and FR-2 according to NEMA classification. These materials are made on a phenolic paper basis and are used only for the production of OPP. FR-1 and FR-2 have similar characteristics, FR-2 differs from FR-1 only in the use of a modified phenolic resin with a higher glass transition temperature as a binder. Due to the similar characteristics and applications of FR-1 and FR-2, most material manufacturers produce only one of these materials, usually FR-2. Excellent mechanical processing (milling, stamping). Cheap. Flammability class UL94-V0 or V1.
Divided into the following subclasses:
-standard;
-halogen-free, without phosphorus and antimony, non-toxic;
-moisture resistant

PCB Finishes

To maintain the solderability of printed circuit boards after storage, ensure reliable installation of electronic components and preserve the properties of soldered or welded connections during operation, it is necessary to protect the copper surface of the contact pads of the printed circuit board with a solderable surface coating, the so-called finishing coating. We offer you a wide range of finishing coatings, which allows you to optimally choose one or even several of them at the same time in the production of your printed circuit boards.

HAL or HASL (from English Hot Air Leveling or Hot Air Solder Leveling - hot air leveling) using solders based on a tin-lead alloy (Sn/Pb), for example, OS61, OS63, and leveling with an air knife. It is applied at the final stage of manufacturing to an already formed printed circuit board with a solder mask applied by dipping it into a melt bath and then leveling and removing excess solder using an air knife. This coating, currently the most common, is classic, the most famous and has been used for a long time. Provides excellent PCB solderability even after long-term storage. HAL coating is technologically advanced and inexpensive. Compatible with all known methods installation and soldering - manual, wave soldering, reflow in an oven, etc. The disadvantages of this type of finishing coating include the presence lead - one of the most toxic metals, prohibited for use in the European Union by the RoHS directive (Restriction of Hazardous Substances Directives), and also the fact that the HAL coating does not meet the conditions of flatness of contact pads for mounting microcircuits with a very high degree of integration. The coating is not suitable for the technology of bonding crystals onto a board (COB - Chip on board) and application to end contacts (lamellas).

HAL lead free - HAL coating option, but using lead-free solders, for example, Sn100, Sn96.5/Ag3/Cu0.5, SnCuNi, SnAgNi. The coating fully complies with RoHS requirements and has very good safety and solderability. This finishing coat is applied at a higher temperature than PIC-based HAL, which imposes increased temperature requirements on the base material of the printed circuit board and electronic components. The coating is compatible with all mounting and soldering methods, both using lead-free solders (which is most recommended) and using tin-lead solders, but requires careful attention to temperature conditions rations. Compared to Sn/Pb-based HAL, this coating is more expensive due to the higher cost of lead-free solders and also due to its higher energy consumption.

The main problem with HAL coating , is a significant unevenness in the thickness of the coating. The problem is especially acute for components with small pin pitches, such as QFPs with a pitch of 0.5 mm or less, BGAs with a pitch of 0.8 mm or less. The thickness of the coating can vary from 0.5 microns to 40 microns, depending on the geometric dimensions of the contact pad and the uneven impact of the air knife. Also, as a result of thermal shock when applying HASL, warping of the printed circuit board in the form of deflection/torsion is possible. This is especially true for boards with thickness<1,0 мм и для плат с несимметричным стеком слоев, несбалансированных по меди, имеющих несимметричные по слоям сплошные медные заливки, ряды металлизированных отверстий, а также для бессвинцового покрытия.

Immersion gold (ENIG - Electroless Nickel/Immersion Gold) - coating of the Ni/Au family. Coating thickness: Ni 3-7 microns, Au 0.05-0.1 microns. Applied chemically through windows in a solder mask. A widely available lead-free coating that provides flat pads, good solderability, high surface conductivity of pads, and long shelf life. Ideal for fine pitch components and in-circuit testing. The coating fully complies with RoHS requirements. Compatible with all mounting and soldering methods. More expensive compared to HASL.

There are many manufacturers of chemicals for applying immersion gold, and the technology for applying it varies from chemical manufacturer to chemical manufacturer. The final result also depends on the choice of chemicals and application process. Some chemicals may not be compatible with a particular type of solder mask. This type of coating is prone to the formation of two types of critical defects - “black pad” (black pad, non-wetting of the surface of the pad with solder) and cracking under mechanical or thermal loads (cracking occurs between the nickel and copper layer, along the intermetallic layer). Also, when applying plating, the amount of gold should be controlled to prevent brittleness of the solder joint. Exact adherence to the technology of applying immersion gold and timely replacement of solutions guarantee the quality of the coating and the absence of black pad defects. To prevent cracking under mechanical loads, it is recommended to increase the thickness of the printed circuit board to 2.0 mm or more when using BGA packages larger than 25x25 mm or when the board size is more than 250 mm. Increasing the thickness of the board reduces the mechanical stress on components when the board bends.

Gold Fingers - coating of the Ni/Au family. Coating thickness: Ni 3-5 microns, Au 0.5-1.5 microns. Applied by electrochemical deposition (electroplating). Used for application to end contacts and lamellas. It has high mechanical strength, resistance to abrasion and adverse environmental influences. Indispensable where it is important to ensure reliable and durable electrical contact.

Immersion tin - chemical coating that meets RoHS requirements and ensures high flatness of the printed circuit boards. Technological coating compatible with all soldering methods. Contrary to popular misconception based on the experience of using outdated types of coating, immersion tin provides good solderability after a sufficiently long storage period - a guaranteed shelf life of 6 months. (coating solderability lasts up to a year or more if stored correctly). Such long periods of maintaining good solderability are ensured by the introduction of an organometal sublayer as a barrier between the copper of the contact pads and the tin itself. The barrier sublayer prevents the mutual diffusion of copper and tin, the formation of intermetallic compounds and the recrystallization of tin. The final coating with immersion tin with an organometal sublayer, with a thickness of about 1 micron, has a smooth, flat surface, retains solderability and the possibility of several re-solderings even after quite a long period of storage.

OSP (from English Organic Solderability Preservatives) - a group of organic finishing coatings applied directly to copper pads and providing protection of the copper surface from oxidation during storage and soldering. As component pitches decrease, interest in coatings that provide the necessary flatness, and in particular OSP, is constantly growing. Recently, OSP coatings have been rapidly progressing; varieties of coatings have appeared that provide multi-pass soldering without copper oxidation, even with fairly long time intervals between passes (days). A distinction is made between a thin coating, about 0.01 microns, and a relatively thick coating, 0.2 - 0.5 microns or more. To ensure two- or multi-pass soldering, choose a thick coating. OSP provides flat surface pads, is lead free and RoHS compliant and, when properly stored and handled, provides a very reliable solder connection. Thin OSP coating is cheaper than HAL. Thick - almost as much as HAL.

However, OSP does not ensure that the ends of the copper pad are covered with solder during the reflow process. Solder flow over the surface is worse than with HASL coating. Therefore, when applying the paste, the holes in the stencil should be made the same size as the contact pad. Otherwise, not the entire surface of the pad will be covered with solder (although this defect is only cosmetic, the reliability of the connection remains very good). A copper surface not covered with solder will oxidize over time, which can negatively affect repairs. There is also the problem of wetting metallized holes when wave soldering. It is necessary to apply a sufficiently large amount of flux before soldering, the flux must get into the holes so that the solder wets the hole from the inside and forms a fillet on the back of the board. The disadvantages of this coating also include: short storage time before use, incompatibility with terpene solvents, limitations on testability for in-circuit and functional tests (which is partially solved by applying solder paste to the test points). If you have chosen OSP, we recommend using ENTEK coatings from Enthone (ENTEK PLUS, ENTEK PLUS HT), as they provide the best combination of wettability, connection reliability and multi-pass.

Development

Let's look at a typical development process for a 1-2 layer board.
-Determination of dimensions (not important for a breadboard).
-Choice of board material thickness from a range of standard ones:
-The most commonly used material is 1.55 mm thick.
-Drawing the dimensions (edges) of the board in a CAD program in the BOARD layer.
-Location of large radio components: connectors, etc. This usually occurs in the top layer (TOP):
-It is assumed that the drawings of each component, the location and number of pins, etc. have already been determined (or ready-made libraries of components are used).
“Scattering” the remaining components across the top layer, or, less commonly, across both layers for 2-sided boards.
-Start the tracer. If the result is unsatisfactory, the components are repositioned. These two steps are often performed dozens or hundreds of times in a row.
In some cases, tracing of printed circuit boards (drawing of tracks) is done manually in whole or in part.
-Checking the board for errors (DRC, Design Rules Check): checking for gaps, short circuits, overlapping components, etc.
-Export the file to a format accepted by the PCB manufacturer, such as Gerber.

Manufacturing

The manufacture of printed circuit boards usually refers to the processing of a workpiece (foil material). A typical process consists of several stages: drilling vias, obtaining a conductor pattern by removing excess copper foil, plating the holes, applying protective coatings and tinning, and applying markings.

Obtaining a wire pattern

In the manufacture of circuit boards, chemical, electrolytic or mechanical methods are used to reproduce the required conductive pattern, as well as their combinations.

Chemical method

The chemical method for manufacturing printed circuit boards from finished foil material consists of two main stages: applying a protective layer to the foil and etching unprotected areas using chemical methods.

In industry, the protective layer is applied photochemically using an ultraviolet-sensitive photoresist, a photomask, and an ultraviolet light source. Photoresist can be liquid or film. Liquid photoresist is applied in industrial conditions as it is sensitive to non-compliance with the application technology. Film photoresist is popular for hand-made circuit boards. The photomask is a UV-transparent material with a track pattern printed on it. After exposure, the photoresist is developed and cured as in a conventional photographic process.

A protective layer in the form of varnish or paint can be applied by silk-screening or manually. To form an etching mask on foil, radio amateurs use toner transfer from an image printed on a laser printer (“laser-iron technology”).

The unprotected foil is then etched in a solution of ferric chloride or (much less commonly) other chemicals such as copper sulfate. After etching, the protective pattern is washed off from the foil.

Mechanical method

The mechanical manufacturing method involves the use of milling and engraving machines or other tools to mechanically remove a layer of foil from specified areas.
-Metalization of holes
-Coating

Possible coatings include:
-Protective varnish coatings (“soldering mask”).
-Tinning.
-Coating of foil with inert metals (gold plating, palladizing) and conductive varnishes to improve contact properties.
-Decorative and information coverings (labeling).

Multilayer PCBs

Multilayer printed circuit boards (abbreviated MPP[source?], English multilayer printed circuit board) are used in cases where the wiring of connections on a double-sided board becomes too complex. As the complexity of the designed devices and mounting density increases, the number of layers on the boards increases.

In multilayer boards, the outer layers (as well as vias) are used to mount components, and the inner layers contain interconnects or solid power plans (polygons). Metallized vias are used to connect conductors between layers. In the manufacture of multilayer printed circuit boards, the inner layers are first manufactured, which are then glued together through special adhesive pads (prepregs). Next, pressing, drilling and metallization of the via holes are performed.

Multilayer PCB Design

Let's consider a typical design of a multilayer board (Fig. 1). In the first, most common, option, the internal layers of the board are formed from double-sided copper-laminated fiberglass, which is called the “core”. The outer layers are made of copper foil, pressed with the inner layers using a binder - a resinous material called "prepreg". After pressing at high temperatures, a “pie” of a multilayer printed circuit board is formed, in which holes are then drilled and metallized. The second option is less common, when the outer layers are formed from “cores” held together with prepreg. This is a simplified description; there are many other designs based on these options. However, the basic principle is that prepreg acts as the bonding material between the layers. Obviously, there cannot be a situation where two double-sided "cores" are adjacent without a prepreg spacer, but a foil-prepreg-foil-prepreg... etc. structure is possible, and is often used in boards with complex combinations of blind and hidden holes.

Blind and hidden holes

The term " blind holes "means transitions that connect the outer layer with the nearest inner layers and do not have access to the second outer layer. It comes from the English word blind, and is similar to the term "blind holes". Hidden, or buried (from English buried), holes are made in the inner layers and have no exit to the outside. The simplest options for blind and hidden holes are shown in Fig. 2. Their use is justified in the case of very dense wiring or for boards very saturated with planar components on both sides. The presence of these holes increases the cost of the board from one and a half to several times, but in many cases, especially when routing microcircuits in a BGA package with a small pitch, you cannot do without them. There are various ways to form such vias, they are discussed in more detail in the section Boards with blind and hidden holes, but for now let’s take a closer look at the materials from which a multilayer board is constructed.

Basic dielectrics for printed circuit boards
The main types and parameters of materials used for the manufacture of MPPs are given in Table 1. Typical designs of printed circuit boards are based on the use of standard fiberglass laminate type FR4, with an operating temperature, usually from –50 to +110 °C, glass transition (destruction) temperature Tg about 135 °C. Its dielectric constant Dk can be from 3.8 to 4.5, depending on the supplier and type of material. For increased requirements for heat resistance or when mounting boards in an oven using lead-free technology (t up to 260 °C), high-temperature FR4 High Tg or FR5 is used. When requirements for constant operation at high temperatures or sudden temperature changes are required, polyimide is used. In addition, polyimide is used for the manufacture of high-reliability circuit boards, for military applications, and also in cases where increased electrical strength is required. For boards with microwave circuits (more than 2 GHz), separate layers of microwave material are used, or the entire board is made of microwave material (Fig. 3). The most well-known suppliers of special materials are Rogers, Arlon, Taconic, and Dupont. The cost of these materials is higher than FR4 and is roughly shown in the last column of Table 1 relative to the cost of FR4. Examples of boards with different types of dielectric are shown in Fig. 4, 5.

Material thickness
Knowing the available material thicknesses is important for an engineer not only for determining the overall thickness of the board. When designing MPP, developers are faced with the following tasks:
- calculation of the wave resistance of conductors on the board;
- calculation of the value of interlayer high-voltage insulation;
- selection of the structure of blind and hidden holes.
Available options and thicknesses of various materials are shown in tables 2–6. It should be taken into account that the tolerance on the thickness of the material is usually up to ±10%, therefore the tolerance on the thickness of the finished multilayer board cannot be less than ±10%.

Table 2. Double-sided FR4 “cores” for the internal layers of the printed circuit board Dielectric thickness and copper thickness 5 µm 17 µm 35 µm 70 µm 105 µm
0.050 mm w/w
0.075 mm m z z
0.100 mm w/w
0.150 mm
0.200 mm m z z
0.250 mm
0.300 mm
0.350 mm m z z
0.400 mm w/w
0.450 mm
0.710 mm m z z
0.930 mm m z
1,000 mm w
More than 1 mm

Typically in stock;
h - On request (not always available)
m - Can be manufactured;
Note: to ensure the reliability of the finished boards, it is important to know that for foreign internal layers we prefer to use cores with 35 micron foil rather than 18 micron (even with a conductor and gap width of 0.1 mm). This increases the reliability of printed circuit boards.
The dielectric constant of FR4 cores can range from 3.8 to 4.4 depending on the brand.

PCB pad coatings

Let's look at what types of coatings there are for copper pads. Most often, sites are coated with a tin-lead alloy, or PIC. The method of applying and leveling the surface of solder is called HAL or HASL (from English Hot Air Solder Leveling - leveling solder with hot air). This coating provides the best solderability of the pads. However, it is being replaced by more modern coatings, usually compatible with the requirements of the international RoHS directive. This directive requires the prohibition of the presence of harmful substances, including lead, in products. So far, RoHS does not apply to the territory of our country, but it is useful to remember its existence. The problems associated with RoHS will be described in one of the subsequent sections, but for now let's take a look at the possible options for covering MPP sites. HASL is used everywhere unless otherwise required. Immersion (chemical) gold plating is used to provide a smoother board surface (this is especially important for BGA pads), but has slightly lower solderability. Oven soldering is performed using approximately the same technology as HASL, but hand soldering requires the use of special fluxes. Organic coating, or OSP, protects the copper surface from oxidation. Its disadvantage is the short shelf life of solderability (less than 6 months). Immersion tin provides a smooth surface and good solderability, although it also has a limited solder life. Lead-free HAL has the same properties as lead-containing HAL, but the composition of the solder is approximately 99.8% tin and 0.2% additives. The contacts of the blade connectors, which are subject to friction during operation of the board, are electroplated with a thicker and more rigid layer of gold. For both types of gilding, a nickel underlayer is used to prevent diffusion of gold.

Protective and other types of printed circuit board coatings
To complete the picture, let’s consider the functional purpose and materials of printed circuit board coatings.
- Solder mask - applied to the surface of the board to protect conductors from accidental short circuits and dirt, as well as to protect fiberglass laminate from thermal shock during soldering. The mask does not carry any other functional load and cannot serve as protection against moisture, mold, breakdown, etc. (except when special types of masks are used).
- Marking - applied to the board with paint over the mask to simplify identification of the board itself and the components located on it.
- Peelable mask - applied to specified areas of the board that need to be temporarily protected, for example, from soldering. It is easy to remove in the future, since it is a rubber-like compound and simply peels off.
- Carbon contact coating - applied to certain areas of the board as contact fields for keyboards. The coating has good conductivity, does not oxidize and is wear-resistant.
- Graphite resistive elements - can be applied to the surface of the board to perform the function of resistors. Unfortunately, the accuracy of the denominations is low - no more accurate than ±20% (with laser adjustment - up to 5%).
- Silver contact jumpers - can be applied as additional conductors, creating another conductive layer when there is not enough space for routing. Mainly used for single-layer and double-sided printed circuit boards.

Conclusion
The choice of materials is large, but, unfortunately, often when producing small and medium-sized series of printed circuit boards, the stumbling block becomes the availability of the necessary materials in the warehouse of the plant that produces the MPP. Therefore, before designing an MPP, especially if we are talking about creating a non-standard design and using non-standard materials, it is necessary to agree with the manufacturer on the materials and layer thicknesses used in the MPP, and perhaps order these materials in advance.

Our company produces printed boards s from high-quality domestic and imported materials, ranging from standard FR4 to Microwave-FAF materials.

Typical designs printed boards based on the use of standard fiberglass and type FR4, with an operating temperature from –50 to +110 °C, and a glass transition temperature Tg (softening) of about 135 °C.

For increased heat resistance requirements or installation e boards in the furnace using lead-free technology (t up to 260 °C), high-temperature FR4 High Tg is used.

Basic materials for printed boards:

"+" - Typically in stock

"+/-" - On request (not always available)

Prepreg (“bonding” layer) for multilayer printed boards

The dielectric constant of FR4 prepreg can range from 3.8 to 4.4 depending on the brand.

FR-4

VT-47 (FR-4 Tg 180°C)

• High glass transition temperature FR-4 Tg 180°C
• Excellent heat resistance
• Resistance of glass fiber and resin to electrochemical corrosion processes (Conductive Anodic Filament (CAF))
• UV blocking
• Low temperature coefficient of expansion along the Z axis

MI 1222

• surface electrical resistance (Ohm): 7 x 1011;
• specific volumetric electrical resistance (Ohm m): 1 x 1012;
• dielectric constant: 4.8;
• foil peel strength (N): 1.8.

FAF-4D

• Adhesion strength of foil to base per 10 mm strip, N (kgf), not less than 17.6(1.8)
• Dielectric loss tangent at a frequency of 106 Hz, no more than 7 x 10-4
• Dielectric constant at frequency 1 MHz 2.5 ± 0.1

F4BM350

• Dielectric loss tangent at a frequency of 10 GHz, no more than 7x10-4
• Dielectric constant at 10 GHz 3.5 ± 2%
• Operating temperature -60 +260° C
• Available sheet sizes, mm (maximum deviation in sheet width and length 10 mm) 500x500

HA50

Attention: Type 1 and Type 3 are available, please indicate type when order e.

T111

• Aluminum base thickness – 1.5 mm
• Dielectric thickness - 100 microns
• Copper foil thickness – 35 microns
• Thermal conductivity of the dielectric - 2.2 W/mK
• Dielectric thermal resistance - 0.7°C/W
• Thermal conductivity of aluminum substrate (5052 - analogue of AMg2.5) - 138 W/mK
• Breakdown voltage – 3 KV
• Glass transition temperature (Tg) – 130
• Volume resistance – 108 MΩ×cm
• Surface resistance - 106 MΩ
• Highest operating voltage (CTI) – 600V

Protective solder masks used in production printed boards

Soldering mask(aka “green stuff”) is a layer of durable material designed to protect conductors from the ingress of solder and flux during soldering, as well as from overheating. Mask covers conductors and leaves pads and blade connectors exposed. The method of applying a solder mask is similar to applying photoresist - using a photomask with a pattern of pads, the mask material applied to the PCB is illuminated and polymerized, the areas with soldering pads are unexposed and mask is washed off from them after development. More often soldering mask applied to a layer of copper. Therefore, before its formation, the protective layer of tin is removed - otherwise the tin under the mask will swell from heating boards s when soldering.

PSR-4000 H85

Green color, liquid photosensitive heat-hardening, 15-30 microns thick, TAIYO INK (Japan).

Has approval for use by the following organizations and end product manufacturers: NASA, IBM, Compaq, Lucent, Apple, AT&T, General Electric, Honeywell, General Motors, Ford, Daimler-Chrysler, Motorola, Intel, Micron, Ericsson, Thomson, Visteon, Alcatel , Sony, ABB, Nokia, Bosch, Epson, Airbus, Philips, Siemens, HP, Samsung, LG, NEC, Matsushita(Panasonic), Toshiba, Fujitsu, Mitsubishi, Hitachi, Toyota, Honda, Nissan and many, many others;

IMAGECURE XV-501

– colored (red, black, blue), two-component liquid soldering mask, Coates Electrografics Ltd (England), thickness 15-30 microns;

PSR-4000 LEW3

– white, liquid two-component soldering mask, TAIYO INK (Japan), thickness 15-30 microns;

Laminar D5030

– dry, filmy mask from DUNACHEM (Germany), thickness 75 microns, provides tenting of vias, has high adhesion.

Marking

SunChemical XZ81(white)

SunChemical XZ85(black)

Thermosetting marking paints applied using the grid-graphic method SunChemical (UK).

Marking ink AGFA DiPaMat Legend Ink Wh04 (white)

Acrylic UV + thermosetting ink, for inkjet printing of markings on an industrial printer.

The base used is foil and non-foil dielectrics (getinax, textolite, fiberglass, fiberglass, lavsan, polyamide, fluoroplastic, etc.), ceramic materials, metal plates, insulating cushioning material (prepreg).

Foil dielectrics are electrical insulating bases, usually clad with electrolytic copper foil with an oxidized galvanic-resistant layer adjacent to the electrical insulating base. Depending on the purpose, foil dielectrics can be single-sided or double-sided and have a thickness from 0.06 to 3.0 mm.

Non-foil dielectrics, intended for semi-additive and additive manufacturing methods of boards, have a specially applied adhesive layer on the surface, which serves for better adhesion of chemically deposited copper to the dielectric.

PCB bases are made of a material that can adhere well to the metal of the conductors; have a dielectric constant of no more than 7 and a small dielectric loss tangent; have sufficiently high mechanical and electrical strength; allow the possibility of processing by cutting, stamping and drilling without the formation of chips, cracks and delamination of the dielectric; maintain their properties when exposed to climatic factors, be non-flammable and fire resistant; have low water absorption, low thermal coefficient of linear expansion, flatness, and resistance to aggressive environments during circuit design and soldering.

The base materials are layered pressed plates impregnated with artificial resin and possibly lined on one or both sides with copper electrolytic foil. Foil dielectrics are used in subtractive methods of manufacturing PCBs, non-foil dielectrics are used in additive and semi-additive ones. The thickness of the conductive layer can be 5, 9, 12, 18, 35, 50, 70 and 100 microns.

In production, materials are used, for example, for OPP and DPP - foil fiberglass laminate grades SF-1-50 and SF-2-50 with a copper foil thickness of 50 microns and an intrinsic thickness of 0.5 to 3.0 mm; for MPP - foil-etched fiberglass laminate FTS-1-18A and FTS-2-18A with a copper foil thickness of 18 microns and its own thickness from 0.1 to 0.5 mm; for GPP and GPK - foil-coated lavsan LF-1 with a copper foil thickness of 35 or 50 microns and its own thickness from 0.05 to 0.1 mm.

Compared to getinaks, fiberglass laminates have better mechanical and electrical characteristics, higher heat resistance, and lower moisture absorption. However, they have a number of disadvantages, for example, low heat resistance compared to polyamides, which contributes to contamination of the ends of the inner layers with resin when drilling holes.

To manufacture PCBs that provide reliable transmission of nanosecond pulses, it is necessary to use materials with improved dielectric properties, these include PCBs made from organic materials with a relative dielectric constant below 3.5.

For the manufacture of PCBs used in conditions of increased fire hazard, fire-resistant materials are used, for example, fiberglass laminates of the SONF, STNF, SFVN, STF brands.

For the manufacture of GPCs that can withstand repeated bends of 90 degrees in both directions from the initial position with a radius of 3 mm, foil-coated lavsan and fluoroplastic are used. Materials with a foil thickness of 5 microns make it possible to produce PCBs of the 4th and 5th accuracy classes.

Insulating cushioning material is used for gluing PP layers. They are made of fiberglass impregnated with under-polymerized thermosetting epoxy resin with an adhesive coating applied on both sides.

To protect the surface of PP and GPC from external influences, polymer protective varnishes and protective coating films are used.

Ceramic materials are characterized by stability of electrical and geometric parameters; stable high mechanical strength over a wide temperature range; high thermal conductivity; low moisture absorption. The disadvantages are a long manufacturing cycle, large shrinkage of the material, fragility, high cost, etc.

Metal bases are used in heat-loaded PCBs to improve heat removal from the IC and ERE in EAs with high current loads operating at high temperatures, as well as to increase the rigidity of PCBs made on thin bases; they are made from aluminum, titanium, steel and copper.

For high-density printed circuit boards with microvias, materials suitable for laser processing are used. These materials can be divided into two groups:

1. Strengthened non-woven glass materials and preprigs (a composite material based on fabrics, paper, continuous fibers, impregnated with resin in an uncured state) with a given geometry and thread distribution; organic materials with a non-oriented arrangement of fibers Preprig for laser technology has a smaller thickness of fiberglass along the Z axis compared to standard fiberglass.

2. Unreinforced materials (resin coated copper foil, polymerized resin), liquid dielectrics and dry film dielectrics.

Of the other materials used in the manufacture of printed circuit boards, the most widely used are nickel and silver as a metal resist for soldering and welding. In addition, a number of other metals and alloys are used (for example, tin - bismuth, tin - indium, tin - nickel, etc.), the purpose of which is to provide selective protection or low contact resistance, improve soldering conditions. Additional coatings that increase the electrical conductivity of printed conductors are in most cases performed by galvanic deposition, less often by vacuum metallization and hot tinning.

Until recently, foil dielectrics based on epoxy-phenolic resins, as well as dielectrics based on polyimide resins used in some cases, satisfied the basic requirements of printed circuit board manufacturers. The need to improve heat dissipation from ICs and LSIs, the requirements for a low dielectric constant of the board material for high-speed circuits, the importance of matching the coefficients of thermal expansion of the board material, IC packages and crystal carriers, and the widespread introduction of modern mounting methods have led to the need to develop new materials. Ceramic-based MPPs are widely used in modern designs of computer hardware. The use of ceramic substrates for the manufacture of printed circuit boards is primarily due to the use of high-temperature methods for creating a conductive pattern with a minimum line width, but other advantages of ceramics are also used (good thermal conductivity, matching the coefficient of thermal expansion with IC packages and media, etc.). In the manufacture of ceramic MPPs, thick film technology is most widely used.

In ceramic bases, aluminum and beryllium oxides, as well as aluminum nitride and silicon carbide are widely used as starting materials.

The main disadvantage of ceramic boards is their limited size (usually no more than 150x150 mm), which is mainly due to the fragility of ceramics, as well as the difficulty of achieving the required quality.

The formation of a conductive pattern (conductors) is carried out by screen printing. Pastes consisting of metal powders, an organic binder and glass are used as conductor materials in ceramic substrate boards. For conductor pastes, which must have good adhesion, the ability to withstand repeated heat treatment, and low electrical resistivity, powders of noble metals are used: platinum, gold, silver. Economic factors also force the use of pastes based on compositions: palladium - gold, platinum - silver, palladium - silver, etc.

Insulating pastes are made on the basis of crystallizing glasses, glass-crystalline cements, and glass ceramics. Pastes made from powders of refractory metals: tungsten, molybdenum, etc. are used as conductor materials in batch-type ceramic boards. Tapes made from ceramic cheeses based on aluminum and beryllium oxides, silicon carbide, and aluminum nitride are used as the base of the workpiece and insulators.

Rigid metal bases coated with a dielectric are characterized (like ceramic ones) by high-temperature burning of thick-film pastes based on glasses and enamels into the substrate. Features of boards on a metal base are increased thermal conductivity, structural strength and speed limitations due to the strong connection of conductors with the metal base.

Plates made of steel, copper, titanium, coated with resin or fusible glass are widely used. However, the most advanced in terms of a range of indications is anodized aluminum and its alloys with a fairly thick oxide layer. Anodized aluminum is also used for thin-film multilayer PCB layout.

The use of bases with a complex composite structure, including metal spacers, as well as bases made of thermoplastics, in printed circuit boards is promising.

PTFE bases with fiberglass are used in high-speed circuits. Various composite bases from "Kevlar and quartz" as well as copper - Invar - copper are used in cases where it is necessary to have a thermal expansion coefficient close to the expansion coefficient of aluminum oxide, for example, in the case of mounting various ceramic crystal carriers (microcases) on a board. Complex polyimide-based substrates are used primarily in high-power circuits or high-temperature PCB applications.