Physical and mechanical properties materials must satisfy the established specifications and ensure high-quality production of PCBs in accordance with standard technical specifications. For the manufacture of boards, layered plastics are used - foil dielectrics clad with electrolytic copper foil with a thickness of 5, 20, 35, 50, 70 and 105 microns with a copper purity of at least 99.5%, surface roughness of at least 0.4–0.5 microns, which are supplied in the form of sheets with dimensions of 500×700 mm and a thickness of 0.06–3 mm. Laminated plastics must have high chemical and thermal resistance, moisture absorption of no more than 0.2–0.8%, and withstand thermal shock (260°C) for 5–20 s. Surface resistance of dielectrics at 40°C and relative humidity 93% for 4 days. must be at least 10 4 MOhm. The specific volume resistance of the dielectric is not less than 5·10 11 Ohm·cm. The adhesion strength of the foil to the base (3mm wide strip) is from 12 to 15 MPa. Used as a base in laminated plastics getinaks , which is compressed layers of electrical insulating paper impregnated with phenolic resin; fiberglass laminates are compressed layers of fiberglass impregnated with epoxyphenolic resin, and other materials (Table 2.1).

Table 2.1. Basic materials for making circuit boards.

Getinax, having satisfactory electrical insulating properties in normal climatic conditions, good processability and low cost, has found application in the production of household electronic equipment. For PCBs operated in difficult climatic conditions with a wide range of operating temperatures (–60...+180°C) as part of electronic computing equipment, communications equipment, and measuring equipment, more expensive glass textolites are used. They are distinguished by a wide range of operating temperatures, low (0.2 - 0.8 %) water absorption, high values ​​of volumetric and surface resistance, resistance to warping. Disadvantages - the possibility of peeling off the foil due to thermal shocks, enveloping the resin when drilling holes. Increasing the fire resistance of dielectrics (GPF, GPFV, SPNF, STNF) used in power supplies is achieved by introducing fire retardants into their composition (for example, tetrabromodiphenylpropane).

For the manufacture of foil dielectrics, electrolytic copper foil is mainly used, one side of which must have a smooth surface (not lower than the eighth class of cleanliness) to ensure accurate reproduction of the printed circuit, and the other must be rough with a microroughness height of at least 3 microns for good adhesion to the dielectric. To do this, the foil is subjected to oxidation electrochemically in a solution of sodium hydroxide. The foiling of dielectrics is carried out by pressing at a temperature of 160–180°C and a pressure of 5–15 MPa.

Ceramic materials are characterized by high mechanical strength, which varies slightly in the temperature range of 20–700°C, stability of electrical and geometric parameters, low (up to 0.2%) water absorption and gas release when heated in a vacuum, but are fragile and have a high cost.

Steel and aluminum are used as the metal base of the boards. On steel bases, insulation of current-carrying areas is carried out using special enamels, which include oxides of magnesium, calcium, silicon, boron, aluminum or mixtures thereof, a binder (polyvinyl chloride, polyvinyl acetate or methyl methacrylate) and a plasticizer. The film is applied to the base by rolling between rollers followed by burning. An insulating layer with a thickness of several tens to hundreds of micrometers with an insulation resistance of 10 2 – 10 3 MOhm on the aluminum surface is obtained by anodic oxidation. The thermal conductivity of anodized aluminum is 200 W/(m K), and that of steel is 40 W/(m K). Non-polar (fluoroplastic, polyethylene, polypropylene) and polar (polystyrene, polyphenylene oxide) polymers are used as the basis for microwave PP. Ceramic materials that have stable electrical characteristics and geometric parameters are also used for the manufacture of microboards and microassemblies in the microwave range.

Polyamide film is used for the manufacture of flexible circuit boards with high tensile strength, chemical resistance, and fire resistance. It has the highest temperature stability among polymers, since it does not lose flexibility from the temperatures of liquid nitrogen to the temperatures of eutectic soldering of silicon with gold (400°C). In addition, it is characterized by low gas evolution in a vacuum, radiation resistance, and no envelopment during drilling. Disadvantages: increased water absorption and high cost.

Formation of a diagram drawing.

Drawing a pattern or protective relief of the required configuration is necessary when carrying out metallization and etching processes. The drawing must have clear boundaries with accurate reproduction of fine lines, be resistant to etching solutions, not contaminate circuit boards and electrolytes, and be easy to remove after performing its functions. The transfer of a printed circuit design onto a foil dielectric is carried out using gridography, offset printing and photo printing. The choice of method depends on the design of the board, the required accuracy and density of installation, and the serial production.

Gridographic method drawing a circuit diagram is the most cost-effective for mass and large-scale production of circuit boards with a minimum width of conductors and a distance between them > 0.5 mm, image reproduction accuracy ± 0.1 mm. The idea is to apply special acid-resistant paint to the board by pressing it with a rubber spatula (squeegee) through a mesh stencil, in which the required pattern is formed by open mesh cells (Fig. 2.4).

To make a stencil use metal mesh from stainless steel with a wire thickness of 30–50 microns and a weaving frequency of 60–160 threads per 1 cm, metallized nylon fiber, which has better elasticity, with a thread thickness of 40 microns and a weaving frequency of up to 200 threads per 1 cm, as well as from polyester fibers and nylon

One of the disadvantages of mesh is that it stretches with repeated use. The most durable are meshes made of stainless steel (up to 20 thousand prints), metallized plastics (12 thousand), polyester fibers (up to 10 thousand), nylon (5 thousand).

Rice. 2.4. The principle of screen printing.

1 – squeegee; 2 – stencil; 3 – paint; 4 – base.

The image on the grid is obtained by exposing liquid or dry (film) photoresist, after development of which open (pattern-free) grid cells are formed. The stencil in the mesh frame is installed with a gap of 0.5–2 mm from the surface of the board so that the contact of the mesh with the surface of the board is only in the area where the mesh is pressed with a squeegee. The squeegee is a rectangular sharpened strip of rubber installed in relation to the substrate at an angle of 60–70°.

To obtain a PP pattern, thermosetting paints ST 3.5 are used;

ST 3.12, which are dried either in a heating cabinet at a temperature of 60°C for 40 minutes, or in air for 6 hours, which lengthens the screenography process. More technologically advanced are the photopolymer compositions EP-918 and FKP-TZ with ultraviolet curing for 10–15 s, which is a decisive factor in automating the process. When applied once, the green coating has a thickness of 15–25 microns, reproduces a pattern with a line width and gaps of up to 0.25 mm, withstands immersion in molten POS-61 solder at a temperature of 260°C for up to 10 s, exposure to an alcohol-gasoline mixture for up to 5 min and thermal cycling in the temperature range from – 60 to +120 °C. After applying the design, the board is dried at a temperature of 60 ° C for 5–8 minutes, the quality is controlled and, if necessary, retouched. Removal of the protective mask after etching or metallization is carried out chemical method in a 5% sodium hydroxide solution for 10–20 s.

Table 2.2. Equipment for screen printing.

For screen printing, semi-automatic and automatic equipment is used, differing in print format and productivity (Table 2.2). Automatic screen printing lines from Chemcut (USA), Resco (Italy) have automatic systems for feeding and installing boards, squeegee movement and resist supply. To dry the resist, an IR-tunnel type oven is used.

Offset printing used for large-scale production of PCBs with a small range of circuits. Resolution is 0.5–1 mm, the accuracy of the resulting image is ±0.2 mm. The essence of the method is that paint is rolled into the cliche that carries the image of the circuit (printed conductors, contact pads). Then it is removed with a rubber-coated offset roller, transferred to an insulating base and dried. The cliche and the board base are located one behind the other on the base of the offset printing machine (Fig. 2.5)

Fig.2.5. Offset printing scheme.

1 – offset roller; 2 – cliche; 3 – board;

4 – roller for applying paint; 5 – pressure roller.

The accuracy of printing and the sharpness of the contours are determined by the parallelism of the roller and the base, the type and consistency of the paint. With one cliche you can make an unlimited number of prints. The productivity of the method is limited by the duration of the oscillatory cycle (paint application - transfer) and does not exceed 200–300 impressions per hour. Disadvantages of the method: the duration of the cliche manufacturing process, the difficulty of changing the pattern of the circuit, the difficulty of obtaining non-porous layers, the high cost of the equipment.

Photographic method drawing a pattern allows you to obtain a minimum width of conductors and distances between them of 0.1–0.15 mm with a reproduction accuracy of up to 0.01 mm. From an economic point of view, this method is less cost-effective, but allows for maximum pattern resolution and is therefore used in small-scale and mass production in the manufacture of circuit boards high density and accuracy. The method is based on the use of photosensitive compositions called photoresists , which must have: high sensitivity; high resolution; a homogeneous, non-porous layer over the entire surface with high adhesion to the board material; resistance to chemical influences; ease of preparation, reliability and safety of use.

Photoresists are divided into negative and positive. Negative photoresists under the influence of radiation they form protective relief areas as a result of photopolymerization and hardening. The illuminated areas stop dissolving and remain on the surface of the substrate. Positive photoresists transmit the photomask image without changes. During light processing, the exposed areas are destroyed and washed out.

To obtain a pattern of a circuit when using a negative photoresist, exposure is made through a negative, and a positive photoresist is exposed through a positive. Positive photoresists have a higher resolution, which is explained by differences in the absorption of radiation by the photosensitive layer. The resolution of the layer is affected by the diffraction bending of light at the edge of the opaque element of the template and the reflection of light from the substrate (Fig. 2.6, A).

Fig.2.6. Exposure of the photosensitive layer:

a – exposure; b – negative photoresist; c – positive photoresist;

1 – diffraction; 2 – scattering; 3 – reflection; 4 – template; 5 – resist; 6 – substrate.

In negative photoresist, diffraction does not play a noticeable role, since the template is tightly pressed to the resist, but as a result of reflection, a halo appears around the protective areas, which reduces the resolution (Fig. 2.6, b). In the positive resist layer, under the influence of diffraction, only the upper area of ​​the resist under the opaque areas of the photomask will be destroyed and washed out during development, which will have little effect on protective properties layer. Light reflected from the substrate may cause some destruction of the area adjacent to it, but the developer does not wash out this area, since under the influence of adhesive forces the layer will move down, again forming a clear edge of the image without a halo (Fig. 2.6, V).

Currently, liquid and dry (film) photoresists are used in industry. Liquid photoresists– colloidal solutions of synthetic polymers, in particular polyvinyl alcohol (PVA). The presence of the hydroxyl group OH in each chain link determines the high hygroscopicity and polarity of polyvinyl alcohol. When ammonium dichromate is added to an aqueous solution of PVA, the latter is “sensitized.” A PVA-based photoresist is applied to the pre-prepared surface of the board by dipping the workpiece, pouring, and then centrifuging. Then the photoresist layers are dried in a heating cabinet with air circulation at a temperature of 40°C for 30–40 minutes. After exposure, the photoresist is developed in warm water. To increase the chemical resistance of PVA-based photoresist, chemical tanning of the PP pattern in a solution of chromic anhydride is used, and then thermal tanning at a temperature of 120°C for 45–50 minutes. Tanning (removal) of the photoresist is carried out for 3–6 s in a solution of the following composition:

– 200–250 g/l oxalic acid,

– 50–80 g/l sodium chloride,

– up to 1000 ml of water at a temperature of 20 °C.

The advantages of PVA-based photoresist are low toxicity and fire hazard, development using water. Its disadvantages include the effect of dark tanning (therefore, the shelf life of blanks with applied photoresist should not exceed 3–6 hours), low acid and alkali resistance, the difficulty of automating the process of obtaining a pattern, the complexity of preparing photoresist, and low sensitivity.

Improved properties of liquid photoresists (elimination of tanning, increased acid resistance) are achieved in photoresist based on cinnamate. The photosensitive component of this type of photoresist is polyvinyl cinnamate (PVC), a product of the reaction of polyvinyl alcohol and cinnamic acid chloride. Its resolution is approximately 500 lines/mm, development is carried out in organic solvents - trichloroethane, toluene, chlorobenzene. To intensify the process of developing and removing PVC photoresist, ultrasonic vibrations are used. Diffusion in an ultrasonic field is greatly accelerated due to acoustic microflows, and the resulting cavitation bubbles, when collapsed, tear off sections of the photoresist from the board. The development time is reduced to 10 s, i.e. 5–8 times compared to conventional technology. The disadvantages of PVC photoresist include its high cost and the use of toxic organic solvents. Therefore, PVC resists have not found wide application in the manufacture of PCBs, but are used mainly in the manufacture of ICs.

Photoresists based on diazo compounds are used mainly as positive ones. The photosensitivity of diazo compounds is due to the presence in them of groups consisting of two nitrogen atoms N2 (Fig. 2.7).

Fig.2.7. Molecular bonds in the structure of diazo compounds.

Drying of the photoresist layer is carried out in two stages:

– at a temperature of 20°C for 15–20 minutes to evaporate volatile components;

– in a thermostat with air circulation at a temperature of 80 ° C for 30–40 minutes.

Developers are solutions of trisodium phosphate, soda, and weak alkalis. Photoresists FP-383, FN-11 based on diazo compounds have a resolution of 350–400 lines/mm, high chemical resistance, but their cost is high.

Dry film photoresists Riston brands were first developed in 1968 by Du Pont (USA) and have a thickness of 18 microns (red), 45 microns (blue) and 72 microns (ruby). Dry film photoresist SPF-2 has been produced since 1975 in thicknesses of 20, 40 and 60 microns and is a polymer based on polymethyl methacrylate 2 (Fig. 2.8), located between the polyethylene 3 and lavsan / films with a thickness of 25 microns each.

Fig.2.8. Structure of dry photoresist.

The following types of dry film photoresists are produced in the CIS:

– manifested in organic substances – SPF-2, SPF-AS-1, SRF-P;

– water-alkaline – SPF-VShch2, TFPC;

– increased reliability – SPF-PNShch;

– protective – SPF-Z-VShch.

Before rolling onto the surface of the PCB base, the protective film of polyethylene is removed and dry photoresist is applied to the board using the roller method (cladding, lamination) when heated to 100°C at a speed of up to 1 m/min using a special device called a laminator. Dry resist polymerizes under the influence of ultraviolet radiation, the maximum of its spectral sensitivity is in the region of 350 nm, therefore mercury lamps are used for exposure. Development is carried out in jet-type machines in solutions of methyl chloride and dimethylformamide.

SPF-2 is a dry film photoresist, similar in properties to Riston photoresist, can be processed in both acidic and alkaline environments and is used in all methods of manufacturing DPP. When using it, it is necessary to seal the developing equipment. SPF-VShch has a higher resolution (100–150 lines/mm), is resistant in an acidic environment, and can be processed in alkaline solutions. The composition of the TFPC photoresist (in the polymerizing composition) includes methacrylic acid, which improves performance characteristics. It does not require heat treatment of the protective relief before electroplating. SPF-AS-1 allows you to obtain a PP pattern using both subtractive and additive technologies, since it is resistant in both acidic and alkaline environments. To improve the adhesion of the photosensitive layer to the copper substrate, benzotriazole was introduced into the composition.

The use of dry photoresist significantly simplifies the PCB manufacturing process and increases the yield of suitable products from 60 to 90%. Wherein:

– operations of drying, tanning and retouching, as well as contamination and instability of layers are excluded;

– protection of metallized holes from photoresist leakage is provided;

– high automation and mechanization of the PCB manufacturing process and image control is achieved.

Installation for applying dry film photoresist - laminator (Fig. 2.9) consists of rollers 2, submitting fees 6 and pressing the photoresist to the surface of the workpieces, rollers 3 And 4 for removing the protective polyethylene film, reel with photoresist 5, heater 1 with thermostat.

Fig.2.9. Laminator diagram.

The speed of movement of the board blank reaches 0.1 m/s, the heater temperature is (105 ±5) °C. The design of the ARSM 3.289.006 NPO Raton (Belarus) installation provides a constant pressing force regardless of the gap installed between the heater rollers. The maximum width of the PP workpiece is 560 mm. A feature of rolling is the danger of dust getting under the photoresist layer, so the installation must operate in a hermetic zone. The rolled photoresist film is kept for at least 30 minutes before exposure to complete shrinkage processes, which can cause distortion of the pattern and reduce adhesion.

The development of the pattern is carried out as a result of the chemical and mechanical action of methyl chloroform. The optimal development time is taken to be 1.5 times longer than that required for complete removal of untanned SPF. The quality of the development operation depends on five factors: development time, development temperature, developer pressure in the chamber, contamination of the developing gel, and the degree of final rinsing. As dissolved photoresist accumulates in the developer, the development speed slows down. After development, the board must be washed with water until all solvent residues are completely removed. The duration of the SPF-2 development operation at a developer temperature of 14–18°C, a solution pressure in the chambers of 0.15 MPa and a conveyor speed of 2.2 m/min is 40–42 s.

Removal and development of photoresist is carried out in inkjet machines (GGMZ.254.001, ARSMZ.249.000) in methylene chloride. This is a strong solvent, so the photoresist removal operation must be performed quickly (within 20–30 s). The installations provide a closed cycle for the use of solvents; after irrigating the boards, the solvents enter the distiller, and then the pure solvents are switched to reuse.

Exposure of a photoresist is intended to initiate photochemical reactions in it and is carried out in installations that have light sources (scanning or stationary) and operate in the ultraviolet region. To ensure a tight fit of the photomasks to the board blanks, frames are used where a vacuum is created. The exposure installation SKTSI.442152.0001 NPO "Raton" with a working field of loading frames of 600×600 mm provides a productivity of 15 boards/hour. Exposure time to mercury lamp DRSh-1000 1–5 min. After exposure, to complete the dark photochemical reaction, exposure at room temperature for 30 minutes is required before removing the Mylar protective film.

The disadvantages of dry photoresist are the need to apply mechanical force during rolling, which is unacceptable for glass-ceramic substrates, and the problem of recycling solid and liquid waste. For every 1000 m 2 of material, up to 40 kg of solid and 21 kg of liquid waste are generated, the disposal of which is an environmental problem.

To obtain a conductive pattern on an insulating base, both by gridographic and photochemical methods, it is necessary to use photomasks, which are a graphic image of the pattern on a 1:1 scale on photographic plates or film. Photomasks are made in a positive image when building up conductive areas on the tapes and in a negative image when conductive areas are obtained by etching copper from gap areas.

Geometric accuracy and the quality of the PP pattern are ensured primarily by the accuracy and quality of the photomask, which must have:

– a contrasting black and white image of elements with clear and even boundaries with an optical density of black fields of at least 2.5 units, transparent areas of no more than 0.2 units, measured on a DFE-10 type densitometer;

– minimal image defects (dark dots in white spaces, transparent dots in black fields), which do not exceed 10–30 µm;

– accuracy of the design elements ±0.025 mm.

To a greater extent, the listed requirements are met by high-contrast photographic plates and films “Mikrat-N” (USSR), photographic plates such as FT-41P (USSR), RT-100 (Japan) and Agfalit (Germany).

Currently, two main methods of obtaining photomasks are used: photographing them from photographic originals and drawing them with a light beam on photographic film using program-controlled coordinateographs or a laser beam. When making photo originals, the PP design is made on an enlarged scale (10:1, 4:1, 2:1) on low-shrink material by drawing, making appliqués or cutting into enamel. The application method involves gluing pre-prepared standard elements onto a transparent base (lavsan, glass, etc.). The first method is characterized by low accuracy and high labor intensity, therefore it is used mainly for prototype boards.

Enamel cutting is used for PP with high installation density. To do this, polished sheet glass is covered with an opaque layer of enamel, and the cutting of the circuit design is carried out using a manually controlled coordinateograph. The accuracy of the pattern is 0.03–0.05 mm.

The produced photographic original is photographed with the necessary reduction on a high-contrast photographic plate using photoreproduction printing cameras such as PP-12, EM-513, Klimsch (Germany) and photomasks are obtained, which can be control and working. For replication and production of working, single, and group photo masks, the contact printing method is used from a negative copy of the control photo mask. The operation is performed on a multiplier model ARSM 3.843.000 with an accuracy of ±0.02 mm.

The disadvantages of this method are the high labor intensity of obtaining a photographic original, which requires highly skilled labor, and the difficulty uniform lighting photo originals of a large area, which reduces the quality of photo masks.

The increasing complexity and density of PP patterns and the need to increase labor productivity led to the development of a method for producing photomasks using a scanning beam directly on photographic film. Coordinate machines with program control have been developed to produce a photomask using a light beam. With the transition to machine design of boards, the need to draw a drawing disappears, since the punched paper tape with the coordinates of the conductors obtained from the computer is entered into the reading device of the coordinateograph, on which the photomask is automatically created.

The coordinateograph (Fig. 2.10) consists of a vacuum table 8, on which the film, photo heads and control unit are mounted /. The table moves with high precision in two mutually perpendicular directions using precision lead screws 9 and 3, which are driven by stepper motors 2 And 10. The photo head turns on the illuminator 4, focusing system 5, circular diaphragm 6 and photo shutter 7. The diaphragm has a set of holes (25–70), forming a certain element of the PP pattern, and is fixed on the shaft of the stepper motor. In accordance with the operating program, signals from the control unit are supplied to the stepper motors of the table drive, diaphragm and to the illuminator. Modern coordinateographs (Table 5.4) are equipped with systems for automatically maintaining a constant light mode, outputting information about photomasks from the computer onto film at a scale of 1:2; 1:1; 2:1; 4:1.

Rice. 5.10. Coordinateograph diagram.

Laminate FR4

You can find a detailed description of the laminate.

Laminates in TePro warehouse

Microwave material ROGERS

NOTE: To use ROGERS material in the production of circuit boards, please indicate this in the order form

Since Rogers material is much more expensive than standard FR4, we are forced to introduce an additional markup for boards made on Rogers material. Working fields of used workpieces: 170 × 130; 270 × 180; 370 × 280; 570 × 380.

Metal based laminates

Visual representation of the material

Aluminum laminate ACCL 1060-1 with dielectric thermal conductivity 1 W/(m K)

Description

Aluminum laminate CS-AL88-AD2(AD5) with dielectric thermal conductivity 2(5) W/(m K)

Description

Prepreg

In production we use prepregs 2116, 7628 and 1080 grade A (highest) from ILM.

You can find a detailed description of prepregs.

Solder mask

Properties

You can find a detailed description of the solder mask.

Frequently asked questions and answers on laminates and prepregs

What is XPC?

What's the difference between FR1 and FR2?

What is FR2?

What is FR3?

What is FR4?

What is FR5?

What is CEM-1?

What is CEM-3?

What is G10?

How can laminates be replaced?

What are "prepregs"?

What does FR or CEM stand for?

Is FR4 really green?

Does the color of the logo mean anything?

Does logo orientation mean anything?

What is UV blocking laminate?

What laminates are suitable for plating through holes?

Is there an alternative for plating through holes?

What is "material thickness"?

What is: PF-CP-Cu? IEC-249? GFN?

What is CTI?

What does #7628 mean? What other numbers are there?

What is 94V-0, 94V-1, 94-HB?

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

Printed circuit board

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

There is a more universal formulation:

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

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

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

Historical reference

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

PCB materials

Basic dielectrics for printed circuit boards

Knowledge of the parameters of materials for printed circuit boards, both single-layer and multilayer, is important for everyone involved in their use, especially for printed circuit boards for devices with increased speed and microwaves. When designing MPP, developers are faced with the following tasks:
- calculation of the wave resistance of conductors on the board;
- calculation of the value of interlayer high-voltage insulation;
- selection of the structure of blind and hidden holes.
Available options and thicknesses 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 - 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, as a rule, 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 for their connection, various options have been proposed, among which metallized transition holes are the most widespread. Fragments of the design of the simplest single-sided and double-sided printed circuit boards are shown in Fig. 1.

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

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

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

Multilayer PCB Design

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

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

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

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

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

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

In reality they look like this:

a) Schematically	To ensure switching between MPP layers, interlayer vias and microvias are used (Fig. 3.a. Interlayer transitions can be made in the form 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 taking into account the difficulties with metallization of through holes small diameter, manufacturers of multilayer printed circuit boards prefer to achieve high density by using a larger number of relatively cheap layers rather than using 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 is in accordance with the objectively existing law of development 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

Recently, there has been an increase in the heat generation of 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.

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

Problems of industrial production technology

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

Here are some of its details.

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

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

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

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

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

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

How to be?

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

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

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

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

Another solution to this problem has been known theoretically for a long time, but in practice it was possible to implement it quite recently - after the industrial production of high-power switching power supplies was mastered. This method is based on the use of pulsed (reverse) power supply mode for galvanic baths. Most of the 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 at sharp corners) in this case only brings 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 amplification devices - transistors, printed circuit boards began to dominate household appliances, 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 printed circuit boards.

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

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

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

Figure 9

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

Figure 9a

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

Microelectronics is involved in the development of VLSI.

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

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

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

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

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

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

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

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

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

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

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

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

Amateur radio technologies for the production of printed circuit boards

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

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

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

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

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

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

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

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

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

Preliminary preparation of the workpiece

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

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

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

The final stage surface preparation consists of degreasing. To do this, you can use a piece of soft, fiber-free cloth moistened with alcohol, gasoline or acetone. Here you should pay attention to the cleanliness of the board surface after degreasing, since recently acetone and alcohol with a significant amount of impurities have begun to appear, which leave whitish stains on the board after drying. If this is the case, then you should look for another degreaser. After degreasing, the board should be washed in a running water 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 of protective coating

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

Using "laser printer and iron technology"

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

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

The first stage of using this technology is to print a mirror image of the printed circuit board pattern on a 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). The laser film and Oracal backing simply peel off carefully while plain paper requires pre-soaking in hot water.

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

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

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

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

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

After applying the photoresist layer, it is necessary to dry the resulting film. It is recommended to do this at a temperature of 70-80 degrees, and first you need to dry the surface at a low temperature and only then gradually increase the temperature to the desired value. Drying time at the specified temperature is about 20-30 minutes. As a last resort, drying the board 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, which settles on the board and interferes with the normal course of the etching process, as well as the relatively low reaction rate.

Ammonium persulfate

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

Solution of hydrochloric acid(HCl) and hydrogen peroxide (H 2 O 2)

- 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 somehow organic solvent, for example, acetone.

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

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

Tinning can be done in two ways:

Immersion in molten solder

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

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

Conclusion

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

Literature

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

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 systems electrical connections, developed in the mid-19th century.

Metal strips (rods) were initially used as bulky electrical components, mounted on a wooden 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 a unique solution on the 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 interconnecting lines, a new era of printed circuit board design has opened.

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 single-sided board, a two-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 computer software format that controls 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, a negative image of the circuit (mask) is transferred to a transparent sheet of 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

Fiberglass laminate is used more often than other materials to make the base of a rigid board. Fiberglass laminate has good dielectric properties, mechanical strength and chemical resistance, durability and safety; fiberglass laminate can be used in conditions of high humidity. Most important characteristics material - electrical insulating properties and the second most important characteristic is the glass transition temperature Tg, which limits the scope of application. Temperature of transition of a material from a solid state to a plastic state – glass transition temperature. The higher the glass transition temperature of the resin, the lower the coefficient of linear expansion of the dielectric, which leads to the destruction of the board conductors. The glass transition temperature depends on the molecular weight of the resin molecules used in the manufacture of the material. The appearance and increase in elasticity occurs in a certain temperature range. The central value within this range is called the glass transition temperature. An increase in the glass transition temperature is possible with the improvement of fiberglass production technology.

Fiberglass is a material made by hot pressing of several layers of fiberglass impregnated with a binder - epoxy or phenol-formaldehyde resin. There are many brands available for various operating conditions. Developed different requirements to manufacturing technology. The ignition temperature of various grades of fiberglass is from 300 to 500 °C. STEF A common domestic brand of fiberglass laminate stands for epoxy-phenolic fiberglass laminate. STEF-1 differs from STEF only in its manufacturing technology, which makes it more suitable for machining. STEF-U has improved mechanical and electrical insulating properties compared to the STEF-1 brand.

A variety of this material is foil-coated fiberglass, used in the production of circuit boards.

foil material is the base material of the board, which has a conductive foil on one or both sides - a sheet of conductive material intended to form a conductive pattern on the board. The success of board production and the reliability of the manufactured device depend on the quality and parameters of the material used.

Foil fiberglass laminate has many brands. For the production of boards, domestic brands are used in accordance with GOST, produced by our manufacturers: SF, SONF-U, STF, STNF, SNF, DFM-59, SFVN and brands of imported fiberglass laminates FR-4, FR-5, CEM-3 having many modifications. For the manufacture of boards intended for operation in conditions of normal and high humidity at temperatures from -60 to +85 ° C, the SF brand is used, which has many types, one of them SF-1-35G.

Designations in the name SF-1-35G:

• SF - foil fiberglass laminate
• 1 - one-sided
• 35 - Foil thickness 35 microns
• G - galvanic resistant foil

For the production of most electronic devices, the brand can be used SONF-U, its operating temperature is from -60 to +155 °C. Designations in the name: S and F - foil fiberglass, OH - general purpose, U - contains bromine-containing additive and belongs to the class of non-flammable plastics. The thickness of the foil placed on the base ranges from 18, 35, 50, 70, 105 microns. The thickness of foil fiberglass laminate is in the range from 0.5 to 3 mm.

FR-4 fire-resistant (Fire Retardent) imported foil fiberglass. FR-4 is by far the most common grade of material for the production of printed circuit boards. High technological and operational characteristics determined the popularity of this material.

FR-4 has nominal thickness 1.6 mm, lined with copper foil 35 microns thick on one or both sides. Standard FR-4 is 1.6 mm thick and consists of eight layers (“prepregs”) of fiberglass. The central layer usually contains the manufacturer's logo; its color reflects the flammability class of this material (red - UL94-VO, blue - UL94-HB). Typically, FR-4 is transparent, the standard green color being determined by the solder mask color applied to the finished PCB.

• volumetric electrical resistance after conditioning and restoration (Ohm x m): 9.2 x 1013;
• surface electrical resistance (Ohm): 1.4 x1012;
• peeling strength of foil after exposure to galvanic solution (N/mm): 2.2;
• flammability (vertical test method): class Vо.

Single-sided foil fiberglass CEM-3. CEM-3 is an imported material (Composite Epoxy Material), most similar to foil-clad fiberglass laminate of the FR-4 brand, at a price 10-15% lower. It is a fiberglass base between two outer layers of fiberglass. Suitable for metallization of holes. CEM-3 is milky white or transparent material, very smooth. The material is easy to drill and stamp. In addition to foil PCB, many different materials are used to make boards.

Getinax

Single-sided foil getinaks.

Foil getinax is intended for the manufacture of boards intended to operate at normal air humidity with one- or two-sided installation of parts without metallization of holes. The technological difference between getinax and fiberglass laminate is the use of paper rather than fiberglass in its production. The material is cheap and easy to stamp. Has good electrical characteristics under normal conditions. The material has disadvantages: poor chemical resistance and poor heat resistance, hygroscopicity.

Domestic foil getinaks brands GF-1-35, GF-2-35, GF-1-50 and GF-2-50 designed to operate at a relative humidity of 45 - 76% and a temperature of 15 - 35 C°, the base material is brown. XPC, FR-1, FR-2 – imported foil getinaks. These materials have a base made of paper with a phenolic filler; the materials are easily stamped.

- FR-3– modification of FR-2, but epoxy resin is used as a filler instead of phenolic resin. The material is intended for the production of boards without metallization of holes.

- CEM-1– a material consisting of epoxy resin (Composite Epoxy Material) on a paper base with one layer of fiberglass. Designed for the production of circuit boards without metallization of holes; the material is easily stamped. Usually milky white or milky yellow in color.

Other foil materials are used for more severe operating conditions, but have a higher price. Their base is made on the basis of chemical compounds that improve the properties of boards: ceramics, aramid, polyester, polyimide resin, bismaleinimide-triazine, cyanate ester, fluoroplastic.

PCB pad coatings

Let's look at what types of coatings there are for copper pads. Most often sites are covered tin-lead alloy, or POS. The method of applying and leveling the solder surface 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 directive RoHS. 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. HASL is used everywhere unless otherwise required. Immersion (chemical) gold plating is used to provide a smoother board surface (this is especially important for BGA pads), but has slightly lower solderability. Oven soldering is performed using approximately the same technology as HASL, but hand soldering requires the use of special fluxes. Organic coating, or OSP, protects the copper surface from oxidation. Its disadvantage is the short shelf life of solderability (less than 6 months). Immersion tin provides a smooth surface and good solderability, although it also has a limited solder life. Lead-free HAL has the same properties as lead-containing HAL, but the composition of the solder is approximately 99.8% tin and 0.2% additives. The contacts of the blade connectors, which are subject to friction during operation of the board, are electroplated with a thicker and more rigid layer of gold. For both types of gilding, a nickel underlayer is used to prevent diffusion of gold.

Protective and other types of printed circuit board coatings

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

Solder mask - applied to the surface of the board to protect conductors from accidental short circuits and dirt, as well as to protect fiberglass from thermal shock during soldering. The mask does not carry any other functional load and cannot serve as protection against moisture, mold, breakdown, etc. (except when special types of masks are used).

Marking - applied to the board with paint over a mask to simplify identification of the board itself and the components located on it.

Peelable mask - applied to specified areas of the board that need to be temporarily protected, for example, from soldering. It is easy to remove in the future, since it is a rubber-like compound and simply peels off.

Carbon contact coating - applied to certain areas of the board as contact fields for keyboards. The coating has good conductivity, does not oxidize and is wear-resistant.

Graphite resistive elements - can be applied to the surface of the board to perform the function of resistors. Unfortunately, the accuracy of the denominations is low - no more accurate than ±20% (with laser adjustment - up to 5%).

Silver contact jumpers - can be applied as additional conductors, creating another conductive layer when there is not enough space for routing. Mainly used for single-layer and double-sided printed circuit boards.