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

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 surface mounting, on a printed circuit board the electrically conductive pattern is made of foil using an additive or subtractive method. In the additive method, a conductive pattern is formed on a non-foil material, usually by chemical copper plating through a protective mask previously applied to the material. In the subtractive method, a conductive pattern is formed on the foil material by removing unnecessary sections of the foil, usually using chemical etching.

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

Types of printed circuit boards

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

In terms of flexibility:
-Hard
-Flexible

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

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

Materials

The basis of the printed circuit board is a dielectric; the most commonly used materials are textolite, fiberglass, and getinax.
Also the basis of printed circuit boards can be 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 glass fabric based material with epoxy resin as a binder (fiberglass). Usually a matte yellowish color or a transparent, familiar green color, it is given by a solder mask applied to the surface of the printed circuit board. Flammability class UL94-V0.
Depending on the properties and application of FR-4
-standard, with glass transition temperature Tg ~130°C, 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, with low dielectric constant ε ≤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 somewhat 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 PCB solderability even after long-term storage. HAL coating is technologically advanced and inexpensive. Compatible with all known methods installation and soldering - manual, wave soldering, reflow in an oven, etc. The disadvantages of this type of finishing coating include the presence lead - one of the most toxic metals, prohibited for use in the European Union by the RoHS directive (Restriction of Hazardous Substances Directives), and also the fact that the HAL coating does not meet the conditions of flatness of contact pads for mounting microcircuits with a very high degree of integration. The coating is not suitable for the technology of bonding crystals onto a board (COB - Chip on board) and application to end contacts (lamellas).

HAL lead free - HAL coating option, but using lead-free solders, for example, Sn100, Sn96.5/Ag3/Cu0.5, SnCuNi, SnAgNi. The coating fully complies with RoHS requirements and has very good safety and solderability. This finishing coat applied at more high temperature than PIC-based HAL, which imposes higher temperature requirements on the base material of the printed circuit board and electronic components. The coating is compatible with all mounting and soldering methods, both using lead-free solders (which is most recommended) and using tin-lead solders, but requires careful attention to temperature conditions rations. Compared to Sn/Pb-based HAL, this coating is more expensive due to the higher cost of lead-free solders and also due to its higher energy 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 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 1.55 mm thick.
-Drawing the dimensions (edges) of the board in a CAD program in the BOARD layer.
-Location of large radio components: connectors, etc. This usually occurs in the top layer (TOP):
-It is assumed that the drawings of each component, the location and number of pins, etc. have already been determined (or ready-made libraries of components are used).
“Scattering” the remaining components across the top layer, or, less commonly, across both layers for 2-sided boards.
-Start the tracer. If the result is unsatisfactory, the components are repositioned. These two steps are often performed dozens or hundreds of times in a row.
In some cases, tracing of printed circuit boards (drawing of tracks) is done manually in whole or in part.
-Checking the board for errors (DRC, Design Rules Check): checking for gaps, short circuits, overlapping components, etc.
-Export the file to a format accepted by the PCB manufacturer, such as Gerber.

Manufacturing

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

Obtaining a wire pattern

In the manufacture of circuit boards, chemical, electrolytic or mechanical methods 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 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, e.g. 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 for mechanical removal 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.

Printed circuit board (English: printed circuit board, PCB, or printed wiring board, PWB) - a plate made of dielectric, on the surface and/or in the volume of which electrically conductive circuits are formed electronic circuit. A printed circuit board is designed to electrically and mechanically connect various electronic components. Electronic components on a printed circuit board are connected by their terminals to elements of a conductive pattern, usually by soldering.

Unlike surface mounting, on a printed circuit board the electrically conductive pattern is made of foil, located entirely on a solid insulating base. The printed circuit board contains mounting holes and pads for mounting leaded or planar components. In addition, printed circuit boards have vias for electrically connecting sections of foil located on different layers of the board. WITH external parties The board is usually coated with a protective coating (“solder mask”) and markings (supporting drawing and text according to the design documentation).

Depending on the number of layers with an electrically conductive pattern, printed circuit boards are divided into:

single-sided (OSP): there is only one layer of foil glued to one side of the dielectric sheet.

double-sided (DPP): two layers of foil.

multilayer (MLP): foil not only on two sides of the board, but also in the inner layers of the dielectric. Multilayer printed circuit boards are made by gluing together several single-sided or double-sided boards.

As the complexity of the designed devices and installation density increases, the number of layers on the boards increases.

The basis of the printed circuit board is a dielectric; the most commonly used materials are 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 materials used for printed circuit boards operating in the microwave range and at temperatures up to 260 °C are fluoroplastic reinforced with glass fabric (for example, FAF-4D) and ceramics. Flexible circuit boards are made from polyimide materials such as Kapton.

What material will we use to make the boards?

The most common, affordable materials for making boards are Getinax and Fiberglass. Getinax paper impregnated with bakelite varnish, fiberglass textolite with epoxy. We will definitely use fiberglass!

Foil fiberglass laminate is sheets made from glass fabrics, impregnated with a binder based on epoxy resins and lined on both sides with copper electrolytic galvanic resistant foil 35 microns thick. Maximum permissible temperature from -60ºС to +105ºС. It has very high mechanical and electrical insulating properties and can be easily machined by cutting, drilling, stamping.

Fiberglass is mainly used single or double-sided with a thickness of 1.5 mm and with copper foil with a thickness of 35 microns or 18 microns. We will use one-sided fiberglass laminate with a thickness of 0.8 mm with a foil with a thickness of 35 microns (why will be discussed in detail below).

Methods for making printed circuit boards at home

Boards can be produced chemically and mechanically.

With the chemical method, in those places where there should be tracks (pattern) on the board, a protective composition (varnish, toner, paint, etc.) is applied to the foil. Next, the board is immersed in a special solution (ferric chloride, hydrogen peroxide and others) which “corrodes” copper foil, but does not affect the protective composition. As a result, copper remains under the protective composition. The protective composition is subsequently removed with a solvent and the finished board remains.

The mechanical method uses a scalpel (for manual production) or a milling machine. A special cutter makes grooves on the foil, ultimately leaving islands with foil - the necessary pattern.

Milling machines are quite expensive, and the milling machines themselves are expensive and have a short resource. So we won't use this method.

The simplest chemical method is manual. Using a risograph varnish, we draw tracks on the board and then etch them with a solution. This method does not allow making complex boards with very thin traces - so this is not our case either.

The next method of making circuit boards is using photoresist. This is a very common technology (boards are made using this method at the factory) and is often used at home. There are a lot of articles and methods for making boards using this technology on the Internet. It gives very good and repeatable results. However, this is also not our option. The main reason is rather expensive materials (photoresist, which also deteriorates over time), as well as additional tools (UV illumination lamp, laminator). Of course, if you have a large-scale production of circuit boards at home - then photoresist is unrivaled - we recommend mastering it. It is also worth noting that the equipment and photoresist technology allows us to produce silk-screen printing and protective masks on circuit boards.

With the advent of laser printers, radio amateurs began to actively use them for the manufacture of circuit boards. As you know, a laser printer uses “toner” to print. This is a special powder that sinteres under temperature and sticks to the paper - the result is a drawing. The toner is resistant to various chemicals, which allows it to be used as a protective coating on the surface of copper.

So, our method is to transfer toner from paper to the surface of copper foil and then etch the board with a special solution to create a pattern.

Due to its ease of use, this method has become very widespread in amateur radio. If you type in Yandex or Google how to transfer toner from paper to a board, you will immediately find a term such as “LUT” - laser ironing technology. Boards using this technology are made like this: the pattern of the tracks is printed in a mirror version, the paper is applied to the board with the pattern on the copper, the top of this paper is ironed, the toner softens and sticks to the board. The paper is then soaked in water and the board is ready.

There are “a million” articles on the Internet about how to make a board using this technology. But this technology has many disadvantages that require direct hands and a very long time to adapt yourself to it. That is, you need to feel it. The payments don't come out the first time, they come out every other time. There are many improvements - using a laminator (with modification - the usual one does not have enough temperature), which allows you to achieve very good results. There are even methods for constructing special heat presses, but all this again requires special equipment. The main disadvantages of LUT technology:

overheating - the tracks spread out - become wider

underheating - the tracks remain on the paper

the paper is “fried” to the board - even when wet it is difficult to come off - as a result, the toner may be damaged. There is a lot of information on the Internet about what paper to choose.

Porous toner - after removing the paper, micropores remain in the toner - through them the board is also etched - corroded tracks are obtained

repeatability of the result - excellent today, bad tomorrow, then good - it is very difficult to achieve a stable result - you need a strictly constant temperature for warming up the toner, you need stable contact pressure on the board.

By the way, I was unable to make a board using this method. I tried to do it both on magazines and on coated paper. As a result, I even spoiled the boards - the copper swelled due to overheating.

For some reason, there is unfairly little information on the Internet about another method of toner transfer - the cold chemical transfer method. It is based on the fact that toner is not soluble in alcohol, but is soluble in acetone. As a result, if you choose a mixture of acetone and alcohol that will only soften the toner, then it can be “re-glued” onto the board from paper. I really liked this method and immediately bore fruit - the first board was ready. However, as it turned out later, I could not find detailed information anywhere that would give 100% results. We need a method that even a child could make the board with. But the second time it didn’t work out to make the board, then again it took a long time to select the necessary ingredients.

As a result, after much effort, a sequence of actions was developed, all components were selected that give, if not 100%, then 95% of a good result. And most importantly, the process is so simple that the child can make the board completely independently. This is the method we will use. (of course, you can continue to bring it to the ideal - if you do better, then write). The advantages of this method:

all reagents are inexpensive, accessible and safe

no additional tools needed (irons, lamps, laminators - nothing, although not - you need a saucepan)

there is no way to damage the board - the board does not heat up at all

the paper comes off on its own - you can see the result of the toner transfer - where the transfer did not come out

there are no pores in the toner (they are sealed with paper) - therefore, there are no mordants

we do 1-2-3-4-5 and we always get the same result - almost 100% repeatability

Before we start, let's see what boards we need and what we can do at home using this method.

Basic requirements for manufactured boards

We will make devices on microcontrollers, using modern sensors and microcircuits. Microchips are getting smaller and smaller. Accordingly, the following requirements for boards must be met:

the boards must be double-sided (as a rule, it is very difficult to wire a single-sided board, making four-layer boards at home is quite difficult, microcontrollers need a ground layer to protect against interference)

the tracks should be 0.2mm thick - this size is quite enough - 0.1mm would be even better - but there is a possibility of etching and the tracks coming off during soldering

the gaps between tracks are 0.2mm - this is enough for almost all circuits. Reducing the gap to 0.1mm is fraught with merging of tracks and difficulty in monitoring the board for short circuits.

We will not use protective masks, nor will we do silk-screen printing - this will complicate production, and if you are making the board for yourself, then there is no need for this. Again, there is a lot of information on this topic on the Internet, and if you wish, you can do the “marathon” yourself.

We will not tin the boards, this is also not necessary (unless you are making a device for 100 years). For protection we will use varnish. Our main goal is to quickly, efficiently, and cheaply make a board for the device at home.

This is what the finished board looks like. made by our method - tracks 0.25 and 0.3, distances 0.2

How to make a double-sided board from 2 single-sided ones

One of the challenges of making double-sided boards is aligning the sides so that the vias line up. Usually a “sandwich” is made for this. Two sides are printed on a sheet of paper at once. The sheet is folded in half, and the sides are accurately aligned using special marks. Double-sided textolite is placed inside. With the LUT method, such a sandwich is ironed and a double-sided board is obtained.

However, with the cold toner transfer method, the transfer itself is carried out using a liquid. And therefore it is very difficult to organize the process of wetting one side at the same time as the other side. This, of course, can also be done, but with the help of a special device - a mini press (vice). Thick sheets of paper are taken - which absorb the liquid to transfer toner. The sheets are wetted so that the liquid does not drip and the sheet holds its shape. And then a “sandwich” is made - a moistened sheet, a sheet of toilet paper to absorb excess liquid, a sheet with a picture, a double-sided board, a sheet with a picture, a sheet of toilet paper, a moistened sheet again. All this is clamped vertically in a vice. But we won’t do that, we’ll do it simpler.

A very good idea came up on board manufacturing forums - what a problem it is to make a double-sided board - take a knife and cut the PCB in half. Since fiberglass is a layered material, this is not difficult to do with a certain skill:

As a result, from one double-sided board with a thickness of 1.5 mm we get two single-sided halves.

Next we make two boards, drill them and that’s it - they are perfectly aligned. It was not always possible to cut the PCB evenly, and in the end the idea came to use a thin one-sided PCB with a thickness of 0.8 mm. The two halves then do not need to be glued together; they will be held in place by soldered jumpers in the vias, buttons, and connectors. But if necessary, you can glue it with epoxy glue without any problems.

The main advantages of this hike:

Textolite with a thickness of 0.8 mm is easy to cut with paper scissors! In any shape, that is, it is very easy to cut to fit the body.

Thin PCB - transparent - by shining a flashlight from below you can easily check the correctness of all tracks, short circuits, breaks.

Soldering one side is easier - the components on the other side do not interfere and you can easily control the soldering of the microcircuit pins - you can connect the sides at the very end

You need to drill twice as many holes and the holes may slightly mismatch

The rigidity of the structure is slightly lost if you do not glue the boards together, but gluing is not very convenient

Single-sided fiberglass laminate with a thickness of 0.8mm is difficult to buy; most people sell 1.5mm, but if you can’t get it, you can cut thicker textolite with a knife.

Let's move on to the details.

Necessary tools and chemistry

We will need the following ingredients:

Now that we have all this, let’s take it step by step.

1. Layout of board layers on a sheet of paper for printing using InkScape

Automatic collet set:

We recommend the first option - it is cheaper. Next, you need to solder wires and a switch (preferably a button) to the motor. It is better to place the button on the body to make it more convenient to quickly turn the motor on and off. All that remains is to choose a power supply, you can take any power supply with 7-12V current 1A (less is possible), if there is no such power supply, then USB charging at 1-2A or a Krona battery may be suitable (you just have to try it - not everyone likes charging motors, the motor may not start).

The drill is ready, you can drill. But you just need to drill strictly at an angle of 90 degrees. You can build a mini machine - there are various schemes on the Internet:

But there is a simpler solution.

Drilling jig

To drill exactly 90 degrees, it is enough to make a drilling jig. We will do something like this:

It is very easy to make. Take a square of any plastic. We place our drill on a table or other flat surface. And drill a hole in the plastic using the required drill. It is important to ensure an even horizontal movement of the drill. You can lean the motor against the wall or rail and the plastic too. Next, use a large drill to drill a hole for the collet. From the reverse side, drill out or cut off a piece of plastic so that the drill is visible. You can glue a non-slip surface to the bottom - paper or rubber band. Such a jig must be made for each drill. This will ensure perfectly accurate drilling!

This option is also suitable, cut off part of the plastic on top and cut off a corner from the bottom.

Here's how to drill with it:

We clamp the drill so that it sticks out 2-3mm when the collet is fully immersed. We put the drill in the place where we need to drill (when etching the board, we will have a mark where to drill in the form of a mini hole in the copper - in Kicad we specially put a checkmark for this, so that the drill will stand there on its own), press the jig and turn on the motor - hole ready. For illumination, you can use a flashlight by placing it on the table.

As we wrote earlier, you can only drill holes on one side - where the tracks fit - the second half can be drilled without a jig along the first guide hole. This saves a little effort.

8. Tinning the board

Why tin the boards - mainly to protect copper from corrosion. The main disadvantage of tinning is overheating of the board and possible damage to the tracks. If you don’t have a soldering station, definitely don’t tin the board! If it is, then the risk is minimal.

You can tin a board with ROSE alloy in boiling water, but it is expensive and difficult to obtain. It is better to tin with ordinary solder. To do this efficiently, you need to make a simple device with a very thin layer. We take a piece of braid for desoldering parts and put it on the tip, screw it to the tip with wire so that it does not come off:

We cover the board with flux - for example LTI120 and the braid too. Now we put tin into the braid and move it along the board (paint it) - we get an excellent result. But as you use the braid, it comes apart and copper fluff begins to remain on the board - they must be removed, otherwise there will be a short circuit! You can see this very easily by shining a flashlight on the back of the board. With this method, it is good to use either a powerful soldering iron (60 watt) or ROSE alloy.

As a result, it is better not to tin the boards, but to varnish them at the very end - for example, PLASTIC 70, or simple acrylic varnish purchased from auto parts KU-9004:

Fine tuning of the toner transfer method

There are two points in the method that can be tuned and may not work right away. To configure them, you need to make a test board in Kicad, tracks in a square spiral of different thicknesses, from 0.3 to 0.1 mm and with different intervals, from 0.3 to 0.1 mm. It is better to immediately print several such samples on one sheet and make adjustments.

Possible problems that we will fix:

1) tracks can change geometry - spread out, become wider, usually very little, up to 0.1mm - but this is not good

2) the toner may not stick well to the board, come off when the paper is removed, or stick poorly to the board

The first and second problems are interconnected. I solve the first one, you come to the second one. We need to find a compromise.

The tracks can spread for two reasons - too much pressure, too much acetone in the resulting liquid. First of all, you need to try to reduce the load. The minimum load is about 800g, it is not worth reducing below. Accordingly, we place the load without any pressure - we just put it on top and that’s it. There must be 2-3 layers of toilet paper to ensure good absorption of excess solution. You must ensure that after removing the weight, the paper should be white, without purple smudges. Such smudges indicate severe melting of the toner. If you can’t adjust it with a weight and the tracks still blur, then increase the proportion of nail polish remover in the solution. You can increase to 3 parts liquid and 1 part acetone.

The second problem, if there is no violation of the geometry, indicates insufficient weight of the load or a small amount of acetone. Again, it’s worth starting with the load. More than 3 kg does not make sense. If the toner still does not stick well to the board, then you need to increase the amount of acetone.

This problem mainly occurs when you change your nail polish remover. Unfortunately, this is not a permanent or pure component, but it was not possible to replace it with another. I tried to replace it with alcohol, but apparently the mixture is not homogeneous and the toner sticks in some patches. Also, nail polish remover may contain acetone, then less of it will be needed. In general, you will need to carry out such tuning once until the liquid runs out.

The board is ready

If you do not immediately solder the board, it must be protected. The easiest way to do this is to coat it with alcohol rosin flux. Before soldering, this coating will need to be removed, for example, with isopropyl alcohol.

Alternative options

You can also make a board:

Additionally, custom board manufacturing services are now gaining popularity - for example Easy EDA. If you need a more complex board (for example, a 4-layer board), then this is the only way out.

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. The most important characteristics of the material are 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. Various requirements for manufacturing technology have been developed. 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 mechanical processing. 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 performance characteristics determined the popularity of this material.

FR-4 has a nominal thickness of 1.6 mm, lined with 35 micron copper foil 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, standard green color determined by the color of the solder mask 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 can be stamped well. 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.

The physical and mechanical properties of 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 changes 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 they 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 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. 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 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 using a chemical method in a 5% solution of caustic soda 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 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. The dry resist polymerizes when exposed to ultraviolet radiation, the maximum of its spectral sensitivity is in the region of 350 nm, so 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:

– the 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. Behind optimal time manifestations, the time taken is 1.5 times longer than necessary 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 closed loop using 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.

Printed circuit board(eng. printed circuit board, PCB, or printed wiring board, PWB) is a dielectric plate on the surface and/or volume of which electrically conductive circuits of an electronic circuit are formed. A printed circuit board is designed to electrically and mechanically connect various electronic components. Electronic components on a printed circuit board are connected by their terminals to elements of a conductive pattern, usually by soldering.
Unlike surface mounting, on a printed circuit board the electrically conductive pattern is made of foil, located entirely on a solid insulating base. The printed circuit board contains mounting holes and pads for mounting leaded or planar components. In addition, printed circuit boards have vias for electrically connecting sections of foil located on different layers of the board. On the outside of the board, a protective coating (“solder mask”) and markings (supporting drawing and text according to the design documentation) are usually applied.

Depending on the number of layers with an electrically conductive pattern, printed circuit boards are divided into:

• single-sided (OSP): there is only one layer of foil glued to one side of the dielectric sheet.
• double-sided (DPP): two layers of foil.
• multilayer (MLP): foil not only on two sides of the board, but also in the inner layers of the dielectric. Multilayer printed circuit boards are made by gluing together several single-sided or double-sided boards

As the complexity of the designed devices and mounting density increases, the number of layers on the boards increases]. According to the properties of the base material:

• Hard
• Thermally conductive
• Flexible

Printed circuit boards may have their own characteristics, due to their purpose and requirements for special operating conditions (for example, an extended temperature range) or application features (for example, boards for devices operating at high frequencies).
Materials The basis of the printed circuit board is a dielectric; the most commonly used materials are 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 materials used for printed circuit boards operating in the microwave range and at temperatures up to 260 °C are fluoroplastic reinforced with glass fabric (for example, FAF-4D) and ceramics.
Flexible circuit boards are made from polyimide materials such as Kapton.

Getinax used under average operating conditions.

• Advantages: cheap, less drilling, hot integration.
• Disadvantages: it can delaminate during mechanical processing, can absorb moisture, reduces its dielectric properties and warps.

It is better to use getinax lined with galvanic-resistant foil.

Foil fiberglass- obtained by pressing, impregnation with epoxy resin of layers of fiberglass and glued surface film VF-4R of copper electrical foil with a thickness of 35-50 microns.

• Advantages: good dielectric properties.
• Disadvantages: 1.5-2 times more expensive.

Used for single-sided and double-sided boards. For multilayer PCBs, thin foil dielectrics FDM-1, FDM-2 and semi-flexible RDME-1 are used. The basis of such materials is an impregnating epoxy layer of fiberglass. The thickness of electrical copper galvanized foil is 35.18 microns. For the manufacture of multilayer PP, cushioning fabric is used, for example SPT-2 with a thickness of 0.06-0.08 mm, which is a non-foil material.

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.

Foil material- 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 widespread are foils with a thickness of 18 and 35 microns; 70, 105 and 140 microns are much less common. These values ​​are based on standard imported copper thicknesses, 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. Sometimes, instead of aluminum, copper or steel is used, laminated with a thin insulator and foil. Copper has great thermal conductivity, and the stainless steel of the board provides corrosion resistance.
• 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.

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.

The chemical method for manufacturing printed circuit boards from finished foil material consists of two main stages: applying a protective layer to the foil and etching unprotected areas using chemical methods. In industry, the protective layer is applied by photolithography using an ultraviolet-sensitive photoresist, a photomask, and an ultraviolet light source. The copper foil is completely covered with photoresist, after which the pattern of tracks from the photomask is transferred to the photoresist by illumination. The exposed photoresist is washed off, exposing the copper foil for etching; the unexposed photoresist is fixed on the foil, protecting it from etching.

Photoresist can be liquid or film. Liquid photoresist is applied in industrial conditions, as it is sensitive to non-compliance with the application technology. Film photoresist is popular for hand-made circuit boards, but is more expensive. The photomask is a UV-transparent material with a track pattern printed on it. After exposure, the photoresist is developed and fixed as in a conventional photochemical process. IN amateur conditions a protective layer in the form of varnish or paint can be applied by silk-screening or manually. To form an etching mask on foil, radio amateurs use toner transfer from an image printed on a laser printer (“laser-iron technology”). Foil etching refers to the chemical process of converting copper into soluble compounds. Unprotected foil is etched, most often, in a solution of ferric chloride or in a solution of other chemicals, for example, copper sulfate, ammonium persulfate, ammonium copper chloride, ammonia copper sulfate, chlorite-based, chromic anhydride-based. When using ferric chloride, the board etching process proceeds as follows: FeCl3+Cu → FeCl2+CuCl. Typical solution concentration is 400 g/l, temperature up to 35°C. When using ammonium persulfate, the board etching process proceeds as follows: (NH4)2S2O8+Cu → (NH4)2SO4+CuSO4]. After etching, the protective pattern is washed off from the foil.

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.

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, laser-based industrial prototyping installations have now begun to appear.

Metallization of holes Via and mounting holes can be drilled, punched mechanically (in soft materials such as getinax) or laser (very thin vias). Metallization of holes is usually done chemically or mechanically.
Mechanical metallization of holes is carried out with special rivets, soldered wires or by filling the hole with conductive glue. The mechanical method is expensive to produce and therefore is used extremely rarely, usually in highly reliable one-piece solutions, special high-current equipment or amateur radio conditions.
During chemical metallization, holes are first drilled in a foil blank, then they are metallized, and only then the foil is etched to obtain a print pattern. Chemical metallization of holes - 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 deposition of metal 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 run, 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.

Multilayer boards (with more than 2 layers of metallization) are assembled from a stack of thin two- or single-layer printed circuit boards made traditional way(except for the outer layers of the bag - they are left with the foil untouched for now). 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.

Possible coatings include:

• Protective and decorative varnish coatings (“soldering mask”). Usually has a characteristic green color. When choosing a solder mask, keep in mind that some of them are opaque and the conductors under them are not visible.
• Decorative and information coverings (labeling). Usually applied using silk-screen printing, less often - inkjet or laser.
• Tinning of conductors. 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. The main disadvantage is the significant thickness of the coating, which makes it difficult to install high-density components. To reduce the thickness, excess solder during tinning is blown off with a stream of air.
• Chemical, immersion or galvanic coating of conductor foil with inert metals (gold, silver, palladium, tin, etc.). Some types of such coatings are applied before the copper etching stage.
• Coating with conductive varnishes to improve the contact properties of connectors and membrane keyboards or create an additional layer of conductors.

After mounting printed circuit boards, it is possible to apply additional protective coatings that protect both the board itself and the soldering and components.
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 fall apart. 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.