To make the base of a printed circuit board, foil and non-foil dielectrics are used - getinax, fiberglass, fluoroplastic, polystyrene, ceramic and metal (with a surface insulating layer) materials.

Foil materials- These are multilayer pressed plastics made of electrically insulating paper or fiberglass impregnated with artificial resin. They are covered on one or both sides with electrolytic foil with a thickness of 18; 35 and 50 microns.

Foil-coated fiberglass laminate of the SF grade is produced in sheets with dimensions of 400×600 mm and a sheet thickness of up to 1 mm and 600×700 mm with a larger sheet thickness; it is recommended for boards that are operated at temperatures up to 120°C.

Glass fiber laminates of SFPN grades have higher physical and mechanical properties and heat resistance.

The dielectric slofodite has a copper foil 5 microns thick, which is obtained by evaporating copper in a vacuum.

For multilayer and flexible boards, heat-resistant fiberglass laminates of the STF and FTS brands are used; they are operated in the temperature range from minus 60 to plus 150°C.

Non-foil STEF dielectric is metallized with a layer of copper during the manufacturing process of a printed circuit board.

The foil is made from high-purity copper, the impurity content does not exceed 0.05%. Copper has high electrical conductivity and is relatively resistant to corrosion, although it requires a protective coating.

For printed circuit assembly the permissible current value is selected: for foil 100–250 A/mm2, for galvanic copper 60–100 A/mm2.

For the production of printed cables, reinforced fluoroplastic foil films are used.

Ceramic boards can operate in the temperature range of 20...700ºС. They are made from mineral raw materials (for example, quartz sand) by pressing, injection molding or film casting.

Metal boards used in products with high current load.

Aluminum or alloys of iron and nickel are used as a base. An insulating layer on the surface of aluminum is obtained by anodic oxidation with a thickness of tens to hundreds of micrometers and an insulation resistance of 109–1010 Ohms.

The thickness of the conductor is 18; 35 and 50 microns. Based on the density of the conductive pattern, printed circuit boards are divided into five classes:

– the first class is characterized by the lowest density of the conductive pattern and the width of the conductor and spaces more than 0.75 mm;

- fifth grade has highest density pattern and the width of the conductor and spaces within 0.1 mm.

Since the printed conductor has a low mass, the force of its adhesion to the base is sufficient to withstand alternating mechanical overloads acting on the conductor up to 40 q in the frequency range 4–200Hz.

Standards for printed circuit board materials are presented below in the corresponding section “Standardization of Printed Circuit Board Manufacturing”.

To manufacture a printed circuit board, we need to select the following materials: material for the dielectric base of the printed circuit board, material for the printed conductors, and material for the protective coating against moisture. First we will determine the material for the dielectric base of the PCB.

There is a wide variety of copper foil laminates. They can be divided into two groups:

– on paper;

– based on fiberglass.

These materials, in the form of rigid sheets, are formed from several layers of paper or fiberglass, bonded together with a binder by hot pressing. The binder is usually phenolic resin for paper or epoxy for fiberglass. In some cases, polyester, silicone resins or fluoroplastic may also be used. Laminates are covered on one or both sides with copper foil of standard thickness.

The characteristics of the finished printed circuit board depend on the specific combination of raw materials, as well as on the technology including machining plat.

Depending on the base and impregnation material, there are several types of materials for the dielectric base of a printed circuit board.

Phenolic getinax is a paper base impregnated with phenolic resin. Getinaks boards are intended for use in household equipment because they are very cheap.

Epoxy getinax is a material on the same paper base, but impregnated with epoxy resin.

Epoxy fiberglass is a fiberglass-based material impregnated with epoxy resin. This material combines high mechanical strength and good electrical properties.

Flexural strength and impact strength The printed circuit board must be high enough so that the board can be loaded without damage by elements with a large mass installed on it.

As a rule, phenolic and epoxy laminates are not used in boards with metallized holes. In such boards, it is applied to the walls of the holes. thin layer copper Since the temperature coefficient of expansion of copper is 6-12 times less than that of phenolic getinax, there is a certain risk of cracks in the metallized layer on the walls of the holes during thermal shock to which the printed circuit board is exposed in a group soldering machine.

A crack in the metallized layer on the walls of the holes sharply reduces the reliability of the connection. In the case of using epoxy fiberglass laminate, the ratio of temperature coefficients of expansion is approximately equal to three, and the risk of cracks in the holes is quite small.

From a comparison of the characteristics of the bases it follows that in all respects (except for cost) bases made of epoxy fiberglass laminate are superior to bases made of getinax. Printed circuit boards made of epoxy fiberglass laminate are characterized by less deformation than printed circuit boards made of phenolic and epoxy getinax; the latter have a degree of deformation ten times greater than fiberglass.

Some characteristics various types laminates are presented in Table 4.

Table 4 - Characteristics of various types of laminates

Comparing these characteristics, we conclude that only epoxy fiberglass should be used for the manufacture of double-sided printed circuit boards. In this course project, fiberglass laminate grade SF-2-35-1.5 was selected.

The foil used to foil the dielectric base can be copper, aluminum or nickel foil. However, aluminum foil is inferior to copper, since it is difficult to solder, and nickel foil has a high cost. Therefore, we choose copper as the foil.

Copper foil is available in various thicknesses. Standard foil thicknesses for the most widespread use are 17.5; 35; 50; 70; 105 microns. During etching of copper along the thickness, the etchant also acts on the copper foil from the side edges under the photoresist, causing the so-called “etching”. To reduce it, thinner copper foil with a thickness of 35 and 17.5 microns is usually used. Therefore, we choose copper foil with a thickness of 35 microns.

1.7 Selecting a PCB manufacturing method

All printed circuit board manufacturing processes can be divided into subtractive and semi-additive.

Subtractive process ( subtraction-subtract) obtaining a conductive pattern involves selectively removing sections of conductive foil by etching.

Additive process (additio-add) - in the selective deposition of conductive material onto a non-foil base material.

The semi-additive process involves the preliminary application of a thin (auxiliary) conductive coating, which is subsequently removed from the gap areas.

In accordance with GOST 23751 - 86, the design of printed circuit boards should be carried out taking into account the following manufacturing methods:

– chemical for GPC

– combined positive for DPP

Metallization of through holes for MPP

Thus, this printed circuit board, developed in the course project, will be manufactured on the basis of a double-sided foil dielectric using a combined positive method. This method makes it possible to obtain conductors up to 0.25 mm wide. The conductive pattern is obtained using the subtractive method.

2 CALCULATION OF CONDUCTING PATTERN ELEMENTS

2.1 Calculation of mounting hole diameters

Structural and technological calculation of printed circuit boards is carried out taking into account production errors in the design of conductive elements, photomask, basing, drilling, etc. The limit values ​​of the main parameters of printed wiring, which can be ensured during design and production for five classes of mounting density, are given in Table 4.

Table 4 – Limit values ​​of the main parameters of printed wiring

The table shows:

t – conductor width;

S – distance between conductors, contact pads, conductor and contact pad or conductor and metallized hole;

b – distance from the edge of the drilled hole to the edge of the contact pad of this hole (guarantee belt);

g – the ratio of the minimum diameter of the metallized hole to the thickness of the board.

The dimensions selected in accordance with Table 1 must be coordinated with the technological capabilities of a particular production.

The limiting values ​​of the technological parameters of the structural elements of the printed circuit board (Table 5) were obtained as a result of the analysis of production data and experimental studies of the accuracy of individual operations.

Table 5 – Limit values ​​of process parameters

Continuation of table 5

Minimum diameter of metallized (via) hole:

d min V H calculated ´ g = 1.5 ´ 0.33 = 0.495 mm;

where g = 0.33 is the printed circuit density for the third accuracy class.

H calculated – thickness of the foil dielectric of the board.

An electronic printed circuit board (Russian abbreviation - PP, English - PCB) is a sheet panel that houses interconnected microelectronic components. Printed circuit boards are used as part of various electronic equipment, ranging from simple doorbells, household radios, studio radios and ending with complex radar and computer systems. Technologically, the manufacture of electronics printed circuit boards involves the creation of connections with conductive “film” material. Such material is applied (“printed”) on an insulating plate, which is called a substrate.

Electronic printed circuit boards marked the beginning of the formation and development of electrical interconnection systems developed in the mid-19th century.

Metal strips (rods) were originally used for bulky electrical components mounted on a wood base.

Gradually, metal strips replaced conductors with screw terminal blocks. The wooden base was also modernized, giving preference to metal.

This is what the prototype of modern PP production looked like. Similar design solutions were used in the mid-19th century

The practice of using compact, small-sized electronic parts required unique solution on a basic basis. And so, in 1925, a certain Charles Ducasse (USA) found such a solution.

An American engineer proposed a unique way of organizing electrical connections on an insulated plate. He used electrically conductive ink and a stencil to transfer the circuit diagram onto a plate.

A little later, in 1943, the Englishman Paul Eisler also patented the invention of etching conductive circuits on copper foil. The engineer used an insulator plate laminated with foil material.

However, the active use of Eisler technology was noted only in the period 1950-60, when they invented and mastered the production of microelectronic components - transistors.

The technology for manufacturing through holes on multilayer printed circuit boards was patented by Hazeltyne (USA) in 1961.

Thus, thanks to the increase in the density of electronic parts and the close arrangement of connecting lines, the new era PCB design.

Electronic printed circuit board - manufacturing

A generalized vision of the process: individual electronic parts are distributed over the entire area of ​​the insulating substrate. The installed components are then connected by soldering to the circuit circuits.

The so-called contact “fingers” (pins) are located along the extreme areas of the substrate and act as system connectors.

Through contact “fingers”, communication with peripheral printed circuit boards or connection of external control circuits is organized. The electronic printed circuit board is designed for wiring a circuit that supports one function or several functions simultaneously.

Three types of electronic printed circuit boards are manufactured:

1. One-sided.
2. Double-sided.
3. Multilayer.

Single-sided printed circuit boards are characterized by the placement of parts exclusively on one side. If the complete circuit parts do not fit on a single-sided board, a double-sided option is used.

Substrate material

The substrate traditionally used in printed circuit boards is typically made from fiberglass combined with epoxy resin. The substrate is covered with copper foil on one or two sides.

Electronics printed circuit boards made from phenolic resin paper, also coated with copper film, are considered cost-effective for production. Therefore, more often than other variations, they are used to equip household electronic equipment.

The connections are made by coating or by etching the copper surface of the substrate. Copper tracks are coated with a tin-lead composition to protect against corrosion. Contact pins on printed circuit boards are coated with a layer of tin, then nickel, and finally gold.

Performing strapping operations

Surface mounting technology involves the use of straight (J-shaped) or angled (L-shaped) branches. Due to such branches, each electronic part is directly connected to a printed circuit.

By using a complex paste (glue + flux + solder), electronic parts are temporarily held at the point of contact. The hold continues until the printed circuit board is inserted into the oven. There the solder melts and connects the circuit parts.

Despite the challenges of component placement, surface mount technology has another important advantage.

This technique eliminates the lengthy drilling process and insertion of bonding gaskets, as is practiced with the outdated through-hole method. However, both technologies continue to be actively used.

Electronic PCB Design

Each individual electronics printed circuit board (batch of boards) is designed for unique functionality. Electronic printed circuit board designers turn to design systems and specialized “software” to layout the circuit on a printed circuit board.

The gap between conductive tracks is usually measured in values ​​of no more than 1 mm. Hole locations for component conductors or contact points are calculated.

All this information is translated into the software format of the computer that controls the drilling machine. An automatic machine for the production of electronic printed circuit boards is programmed in the same way.

Once the circuit diagram is laid out, the negative image of the circuit (mask) is transferred to transparent sheet plastic. Areas of the negative image that are not included in the circuit image are marked in black, and the circuit itself remains transparent.

Industrial manufacturing of electronics printed circuit boards

Electronics printed circuit board manufacturing technologies provide for production conditions in a clean environment. The atmosphere and objects of production premises are controlled automatically for the presence of contaminants.

Many electronic printed circuit board manufacturing companies practice unique manufacturing. And in standard form, the production of double-sided printing electronic board traditionally involves the following steps:

Making the base

1. The fiberglass is taken and passed through the process module.
2. Impregnated with epoxy resin (immersion, spraying).
3. The glass fiber is rolled on a machine to the desired thickness of the substrate.
4. Dry the substrate in an oven and place it on large panels.
5. The panels are arranged in stacks, alternating with copper foil and a backing coated with glue.

Finally, the stacks are placed under a press, where at a temperature of 170°C and a pressure of 700 kg/mm ​​2, they are pressed for 1-2 hours. The epoxy resin hardens and the copper foil is bonded under pressure to the backing material.

Drilling and tinning holes

1. Several backing panels are taken, laid one on top of the other, and firmly fixed.
2. The folded stack is placed in a CNC machine, where holes are drilled according to the schematic pattern.
3. The holes made are cleared of excess material.
4. The internal surfaces of the conductive holes are coated with copper.
5. Non-conductive holes are left uncoated.

Producing a drawing of a printed circuit board

A sample PCB circuit is created using an additive or subtractive principle. In the case of the additive option, the substrate is coated with copper according to the desired pattern. In this case, the part outside the scheme remains unprocessed.

The subtractive process primarily covers the overall surface of the substrate. Then individual areas that are not included in the diagram are etched or cut out.

How does the additive process work?

The foil surface of the substrate is pre-degreased. The panels go through a vacuum chamber. Due to the vacuum, the layer of positive photoresist material is tightly compressed over the entire foil area.

The positive material for photoresist is a polymer that has the ability to solubilize under ultraviolet radiation. Vacuum conditions eliminate any possible remaining air between the foil and the photoresist.

The circuit template is laid on top of the photoresist, after which the panels are exposed to intense ultraviolet light. Since the mask leaves areas of the circuit transparent, the photoresist at these points is exposed to UV radiation and dissolves.

Then the mask is removed and the panels are pollinated with an alkaline solution. This, a kind of developer, helps to dissolve the irradiated photoresist along the boundaries of the areas of the circuit design. Thus, the copper foil remains exposed on the surface of the substrate.

Next, the panels are galvanized with copper. Copper foil acts as the cathode during the galvanization process. Exposed areas are galvanized to a thickness of 0.02-0.05 mm. The areas remaining under the photoresist are not galvanized.

Copper traces are additionally coated with a tin-lead composition or other protective coating. These actions prevent oxidation of copper and create a resist for the next stage of production.

Unneeded photoresist is removed from the substrate using an acid solvent. The copper foil between the circuit design and the coating is exposed. Since the copper of the PCB circuit is protected by a tin-lead compound, the conductor here is not affected by acid.

Techniques for industrial manufacturing of electronic circuit boards

Printed circuit board

A printed circuit board with electronic components mounted on it.

Flexible printed circuit board with installed volumetric and surface mounting parts.

Board drawing in CAD program and finished board

Device

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, fiberglass reinforced (for example, FAF-4D) and ceramics.

• GOST 2.123-93 Unified system of design documentation. Completeness of design documentation for printed circuit boards in computer-aided design.
• GOST 2.417-91 Unified system of design documentation. Printed circuit boards. Rules for the execution of drawings.

Other PCB standards:

• GOST R 53386-2009 Printed circuit boards. Terms and Definitions.
• GOST R 53429-2009 Printed circuit boards. Basic design parameters. This GOST specifies the accuracy classes of printed circuit boards and the corresponding geometric parameters.

Typical process

Let's look at a typical process for developing a board from a ready-made circuit diagram:

• Translation of a circuit diagram into a CAD database for PCB layout. The drawings of each component, the location and purpose of the pins, etc. are determined in advance. Ready-made libraries of components supplied by CAD developers are usually used.
• Check with the future manufacturer of the printed circuit board about its technological capabilities (available materials, number of layers, accuracy class, permissible hole diameters, possibility of coatings, etc.).
• Determination of the design of the printed circuit board (dimensions, mounting points, permissible heights of components).
  • Drawing the dimensions (edges) of the board, cutouts and holes, areas where the placement of components is prohibited.
  • Location of structurally related parts: connectors, indicators, buttons, etc.
  • Selecting the board material, number of metallization layers, material thickness and foil thickness (the most commonly used is 1.5 mm thick fiberglass with foil 18 or 35 microns thick).
• Perform automatic or manual component placement. Usually they try to place components on one side of the board since mounting parts on both sides is noticeably more expensive to manufacture.
• Launch 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, PCB tracing (routing tracks) is produced entirely or partially by hand.
• 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.
• Preparation of an accompanying note which, as a rule, indicates the type of foil material, drilling diameters of all types of holes, type vias(varnished or open, tinned), areas of galvanic coatings and their type, solder mask color, the need for marking, method of separating the boards (milling or scribing), etc.

Manufacturing

PP can be manufactured using additive or subtractive methods. In the additive method, a conductive pattern is formed on a non-foil material by chemical copper plating through a previously applied coating on the material. protective mask. In the subtractive method, a conductive pattern is formed on the foil material by removing unnecessary sections of the foil. In modern industry, exclusively the subtractive method is used.

The entire process of manufacturing printed circuit boards can be divided into four stages:

• Manufacturing of blanks (foil material).
• Processing the workpiece to obtain the desired electrical and mechanical appearance.
• Installation of components.
• Testing.

Often, the manufacture of printed circuit boards refers only to the processing of the workpiece (foil material). Typical process processing of foil material consists of several stages: drilling vias, obtaining a conductor pattern by removing excess copper foil, metallization of holes, applying protective coatings and tinning, and marking. For multilayer printed circuit boards, pressing of the final board from several blanks is added.

Production of foil material

Foil material is a flat sheet of dielectric with copper foil glued to it. As a rule, fiberglass is used as a dielectric. In old or very cheap equipment, textolite is used on a fabric or paper basis, sometimes called getinax. Microwave devices use fluorine-containing polymers (fluoroplastics). The thickness of the dielectric is determined by the required mechanical and electrical strength; the most common thickness is 1.5 mm.

A continuous sheet of copper foil is glued onto the dielectric on one or both sides. The thickness of the foil is determined by the currents for which the board is designed. The most widely used foils are 18 and 35 microns thick. These values ​​are based on standard copper thicknesses in imported materials, in which the thickness of the copper foil layer is calculated in ounces (oz) per square foot. 18 microns corresponds to ½ oz and 35 microns corresponds to 1 oz.

Aluminum PCBs

A separate group of materials consists of aluminum metal printed circuit boards. They can be divided into two groups.

The first group is solutions in the form of an aluminum sheet with a high-quality oxidized surface, onto which copper foil is glued. Such boards cannot be drilled, so they are usually made only one-sided. The processing of such foil materials is carried out using traditional chemical printing technologies.

The second group involves creating a conductive pattern directly in the aluminum base. For this purpose, the aluminum sheet is oxidized not only on the surface but also throughout the entire depth of the base according to the pattern of conductive areas specified by the photomask.

Workpiece processing

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 by photolithography using an ultraviolet-sensitive photoresist, a photomask, and an ultraviolet light source. The copper foil is completely covered with photoresist, after which the pattern of tracks from the photomask is transferred to the photoresist by illumination. The exposed photoresist is washed off, exposing the copper foil for etching; the unexposed photoresist is fixed on the foil, protecting it from etching.

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, but is more expensive. The photomask is a UV-transparent material with a track pattern printed on it. After exposure, the photoresist is developed and fixed as in a conventional photochemical process.

IN amateur conditions 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”).

Foil etching refers to the chemical process of converting copper into soluble compounds. Unprotected foil is etched, most often, in a solution of ferric chloride or in a solution of other chemicals, such as copper sulfate, ammonium persulfate, ammonia copper chloride, ammonia copper sulfate, chlorite-based, chromic anhydride-based. When using ferric chloride, the board etching process proceeds as follows: FeCl 3 +Cu → FeCl 2 +CuCl. Typical solution concentration is 400 g/l, temperature up to 35°C. When using ammonium persulfate, the board etching process proceeds as follows: (NH 4) 2 S 2 O 8 +Cu → (NH 4) 2 SO 4 +CuSO 4.

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.

Laser engraving

Until recently, laser engraving of printed circuit boards was not widespread due to the good reflective properties of copper at the wavelength of the most common high-power gas CO lasers. Due to progress in the field of laser technology, industrial installations laser-based prototyping.

Metallization of holes

Via and mounting holes can be drilled, punched mechanically (in soft materials such as getinax) or laser (very thin vias). Metallization of holes is usually done chemically or mechanically.

Mechanical metallization of holes is carried out with special rivets, soldered wires or by filling the hole with conductive glue. The mechanical method is expensive to produce and therefore is used extremely rarely, usually in highly reliable one-piece solutions, special high-current equipment or amateur radio conditions.

During chemical metallization, holes are first drilled in a foil blank, then they are metallized, and only then the foil is etched to obtain a print pattern. Chemical metallization of holes is a multi-stage complex process that is sensitive to the quality of reagents and adherence to technology. Therefore, it is practically not used in amateur radio conditions. Simplified, it consists of the following steps:

• Applying the walls of the hole of a conductive substrate to the dielectric. This substrate is very thin and fragile. Applied by chemical metal deposition from unstable compounds such as palladium chloride.
• Electrolytic or chemical deposition of copper is performed on the resulting base.
• At the end of the production cycle, either hot tinning is used to protect the rather loose deposited copper or the hole is protected with varnish (solder mask). Poor quality, untinned vias are one of the most common causes of electronic failure.

Pressing of multilayer boards

Multilayer boards (with more than 2 layers of metallization) are assembled from a stack of thin two- or single-layer printed circuit boards manufactured in the traditional way (except for the outer layers of the package - they are still left with the foil intact). They are assembled in a “sandwich” with special gaskets (prepregs). Next, pressing is carried out in an oven, drilling and metallization of vias. Lastly, the foil of the outer layers is etched.

Via holes in such boards can also be made before pressing. If the holes are made before pressing, then it is possible to obtain boards with so-called blind holes (when there is a hole in only one layer of the sandwich), which allows the layout to be compacted.

Coating

Possible coatings include:

• Protective and decorative varnish coatings (“soldering mask”). Usually has a characteristic green color.
• Tinning. Protects the copper surface, increases the thickness of the conductor, and facilitates the installation of components. Typically performed by immersion in a solder bath or wave of solder.
• Electroplating of foil with inert metals (gold plating, palladizing) and conductive varnishes to improve the contact properties of connectors and membrane keyboards.
• Decorative and information coverings (labeling). Usually applied using silk-screen printing, less often - inkjet or laser.

Mechanical restoration

Many individual boards are often placed on one sheet of workpiece. They go through the entire process of processing the foil blank as one board and only at the end are they prepared for separation. If the boards are rectangular, then non-through grooves are milled, which facilitate the subsequent breaking of the boards (scribing, from the English. scribe to scratch). If the boards have a complex shape, then through milling is done, leaving narrow bridges so that the boards do not crumble. For boards without metallization, instead of milling, a series of holes with small pitches are sometimes drilled. Drilling of mounting (non-metalized) holes also occurs at this stage.

See also: GOST 23665-79 Printed circuit boards. Contour processing. Requirements for standard technological processes.

According to the standard technical process, the separation of boards from the workpiece occurs after the components are installed.

Installation of components

Soldering is the primary method of assembling components onto printed circuit boards. Soldering can be done either manually with a soldering iron or using specially developed specific technologies.

Wave soldering

The main method of automated group soldering for lead components. Using mechanical activators, a long wave of molten solder is created. The board is passed over the wave so that the wave barely touches the bottom surface of the board. In this case, the leads of pre-installed lead components are wetted by a wave and soldered to the board. Flux is applied to the board using a sponge stamp.

Soldering in ovens

The main method of group soldering of planar components. A special solder paste (solder powder in a paste-like flux) is applied to the contact pads of the printed circuit board through a stencil. Then the planar components are installed. The board with the components installed is then fed into a special oven where the solder paste flux is activated and the solder powder melts, soldering the component.

If such installation of components is performed on both sides, then the board undergoes this procedure twice - separately for each side of the installation. Heavy planar components are mounted on beads of adhesive that prevent them from falling off the inverted board during the second soldering. Lightweight components are held on the board by the surface tension of the solder.

After soldering, the board is treated with solvents to remove flux residues and other contaminants, or, when using no-clean solder paste, the board is immediately ready for certain operating conditions.

Installing components

Installation of components can be performed either manually or using special automatic installers. Automatic installation reduces the likelihood of errors and significantly speeds up the process (the best automatic installations install several components per second).

Finish coatings

After soldering, the printed circuit board with components is coated with protective compounds: water repellents, varnishes, means of protecting open contacts.

Similar technologies

Hybrid chip substrates are something similar to a ceramic printed circuit board, but usually use different technical processes:

• silk-screen printing of conductors with metallized paste followed by sintering of the paste in an oven. The technology allows multilayer wiring of conductors due to the possibility of applying an insulator layer to a layer of conductors using the same silk-screen printing methods.
• Metal deposition through a stencil.

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, transitional plated 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 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%.

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- the 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 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.

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. Soldering in a furnace 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 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 composition of the solder 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 to connect them it was proposed various options, among which metallized transition holes are 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 of 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 considering the difficulties in plating small through holes, multilayer PCB manufacturers prefer to achieve high density installation due to a larger number of relatively cheap layers rather than a smaller number of high-density but, accordingly, 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 equipment, 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

IN Lately, there is an increase in heat generation from electronic devices, which is associated with:

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 of high-power LEDs, such as Cree, Osram, Nichia, Luxeon, Seoul Semiconductor, Edison Opto, etc., have long manufactured them in the form of LED modules or clusters on printed circuit boards to simplify the inclusion and expand the applications of LEDs. metal base (in the 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.

As materials with high thermal conductivity for the bases of such printed circuit boards, Copper, Aluminum, different kinds ceramics.

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, it breaks electrical circuit and the functionality of the electrical circuit is disrupted.

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). Connections between conductive elements printed layers In this method, they are carried out 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. This technical solution uses a whole “bouquet” of techniques for resolving technical contradictions: use a partially redundant action, turning the harm into a 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 conditions high humidity capillaries ultimately 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 in normal conditions Moisture always reduces the insulation resistance level.

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 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 of surface preparation is 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

The application of a protective coating is the most important stage in the process of manufacturing printed circuit boards, and it is this that determines 90% of the quality of the manufactured board. Currently, three methods of applying 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 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 for the case of using aluminum foil as a substrate: it should not be separated, since it dissolves in almost all etching solutions). Oracal's laser film and base simply peel off carefully, while regular 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 Special attention pay 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 at room temperature for 24 hours is allowed. 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. If the size of the boards is sufficiently small, you can use a photographic plate washed from the emulsion for clamping. 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 process, which settles on the board and interferes with the normal course of the etching process, as well as relatively low speed reactions.

Ammonium persulfate

A light crystalline substance that dissolves in water based on the ratio of 35 g of substance per 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 regime 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. The required amount of hydrogen peroxide is assessed visually: a copper board immersed in the solution 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.

A solution of 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. Special fluxes can be used industrial production(best washed off with water or not requiring rinsing at all) or simply cover the board with a weak solution of 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, you need to make an iron bath and fill it with a small amount fusible 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

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6. Mokeev M. N., Lapin M. S. Technological processes and systems for the production of woven circuit boards and cables. L.: LDNTP 1988.
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