What material are the boards made from? Materials for printed circuit boards. Problems of industrial production technology

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% within 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 methods are mainly used. copper foil, one side of which must have 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 height of micro-irregularities 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.

As metal base boards use steel and aluminum. 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. For the manufacture of microboards and microassemblies in the microwave range, ceramic materials with stable properties are also used. electrical characteristics and geometric parameters.

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 price.

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. Transferring a drawing printed circuit assembly on foil dielectric is carried out by methods of 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 diagram is the most cost-effective for mass and large-scale production 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 high-density and precision boards. 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 solution next line-up:

– 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, 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.

Issued in the CIS following types dry film photoresists:

– 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 PP base protective film The dry photoresist is removed from polyethylene and 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 to acidic environment, 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;

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

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.

The geometric accuracy and 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.

Printed circuit board

A printed circuit board with electronic components mounted on it.

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

Board drawing in CAD program and finished board

Device

Also the basis of printed circuit boards can be metal base, coated with a dielectric (for example, anodized aluminum), copper foil of the tracks is applied on top of the dielectric. Such printed circuit boards are used in power electronics for efficient heat removal from electronic components. In this case, the metal base of the board is attached to the radiator.

The material used for printed circuit boards operating in the microwave range and at temperatures up to 260 °C is fluoroplastic, fiberglass reinforced (for example, FAF-4D) and ceramics.

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

Other PCB standards:

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

Typical process

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

• Translation of a circuit diagram into a CAD database for PCB layout. The drawings of each component, the location and purpose of the pins, etc. are determined in advance. Ready-made libraries of components supplied by CAD developers are usually used.
• Check with the future manufacturer of the printed circuit board about its technological capabilities (available materials, number of layers, accuracy class, permissible hole diameters, possibility of coatings, etc.).
• Determination of the design of the printed circuit board (dimensions, mounting points, permissible heights of components).
  • Drawing the dimensions (edges) of the board, cutouts and holes, areas where the placement of components is prohibited.
  • Location of structurally related parts: connectors, indicators, buttons, etc.
  • Selecting the board material, number of metallization layers, material thickness and foil thickness (the most commonly used is 1.5 mm thick fiberglass with foil 18 or 35 microns thick).
• Perform automatic or manual component placement. Usually they try to place components on one side of the board since mounting parts on both sides is noticeably more expensive to manufacture.
• Launch the tracer. If the result is unsatisfactory, the components are repositioned. These two steps are often performed dozens or hundreds of times in a row. In some cases, PCB tracing (routing tracks) is produced entirely or partially by hand.
• Checking the board for errors ( DRC, Design Rules Check): checking for gaps, short circuits, overlapping components, etc.
• Export the file to a format accepted by the PCB manufacturer, such as Gerber.
• Preparation of an accompanying note which, as a rule, indicates the type of foil material, drilling diameters of all types of holes, type of vias (closed with varnish or open, tinned), areas of galvanic coatings and their type, solder mask color, the need for marking, method of board separation ( milling or scribing), etc.

Manufacturing

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

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

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

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

Production of foil material

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

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

Aluminum PCBs

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

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

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

Workpiece processing

Obtaining a wire pattern

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

Chemical method

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

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

Photoresist can be liquid or film. Liquid photoresist is applied in industrial conditions as it is sensitive to non-compliance with the application technology. Film photoresist is popular for hand-made circuit boards, but is more expensive. The photomask is a UV-transparent material with a track pattern printed on it. After exposure, the photoresist is developed and fixed as in a conventional photochemical process.

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

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

After etching, the protective pattern is washed off from the foil.

Mechanical method

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

Laser engraving

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

Metallization of holes

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

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

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

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

Pressing of multilayer boards

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

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

Coating

Possible coatings include:

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

Mechanical restoration

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

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

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

Installation of components

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

Wave soldering

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

Soldering in ovens

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

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

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

Installing components

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

Finish coatings

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

Similar technologies

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

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

The quality of supplied materials complies with the IPC4101B standard, and the manufacturers' quality management system is confirmed by international certificates ISO 9001:2000.

FR4 – fiberglass laminate with fire resistance class 94V-0 is the most common material for the production of printed circuit boards. Our company supplies the following types of materials for the production of single- and double-sided printed circuit boards:

• Fiberglass laminate FR4 with a glass transition temperature of 135ºС, 140ºС and 170ºС for the production of single-sided and double-sided printed circuit boards. Thickness 0.5 - 3.0 mm with foil 12, 18, 35, 70, 105 microns.
• Basic FR4 for internal layers of MPP with glass transition temperatures of 135ºС, 140ºС and 170ºС
• FR4 prepregs with glass transition temperatures of 135ºС, 140ºС and 170ºС for pressing MPP
• Materials XPC, FR1, FR2, CEM-1, CEM-3, HA-50
• Materials for boards with controlled heat dissipation:
  • (aluminum, copper, stainless steel) with a dielectric with thermal conductivity from 1 W/m*K to 3 W/m*K produced by Totking and Zhejiang Huazheng New Material Co.
  • Material HA-30 CEM-3 with thermal conductivity 1 W/m*K for the production of single- and double-sided printed circuit boards.

For some purposes, a high-quality non-foil dielectric is required that has all the advantages of FR4 (good dielectric properties, stability of characteristics and dimensions, high resistance to adverse influences). climatic conditions). For these applications we can offer non-foil FR4 fiberglass laminate.

In many cases where fairly simple printed circuit boards are required (in the production of household equipment, various sensors, some components for automobiles, etc.), the excellent properties of fiberglass are redundant, and indicators of manufacturability and cost come to the fore. Here we can offer the following materials:

• XPC, FR1, FR2 - foil getinaks (base made of cellulose paper impregnated with phenolic resin), widely used in the manufacture of printed circuit boards for consumer electronics, audio and video equipment, in the automotive industry (arranged in ascending order of properties, and, accordingly, price ). Excellent stamping.
• CEM-1 - laminate based on a composition of cellulose paper and fiberglass with epoxy resin. Stamps beautifully.

Our assortment also includes electrodeposited copper foil for pressing MPP produced by Kingboard. Foil is supplied in rolls of various widths, foil thicknesses are 12, 18, 35, 70, 105 microns, foil thicknesses of 18 and 35 microns are almost always available from our warehouse in Russia.

All materials are manufactured in accordance with the RoHS directive, contents harmful substances confirmed by relevant certificates and RoHS test reports. Also, all materials, many items have certificates, etc.

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

Types of boards

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

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

Types of printed circuit boards

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

In terms of flexibility:
-Hard
-Flexible

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

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

Materials

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

FR-4

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

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

CEM-3

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

CEM-1

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

FR-1/FR-2

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

PCB Finishes

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

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

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

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

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

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

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

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

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

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

Development

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

Manufacturing

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

Obtaining a wire pattern

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

Chemical method

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

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

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

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

Mechanical method

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

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

Multilayer PCBs

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

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

Multilayer PCB Design

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

Blind and hidden holes

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

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

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

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

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

PCB pad coatings

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Metal boards are used in products with high current loads.

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

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

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

– the fifth class has the highest pattern density and the width of the conductor and spaces within 0.1 mm.

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

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