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

*** Additional information can be obtained from

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

Blind and hidden holes
The term "blind holes" refers to vias that connect the outer layer to the nearest inner layers and do not have access to a second outer layer. It comes from 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 leads to an increase in the cost of the board from one and a half to several times, but in many cases, especially when routing microcircuits in BGA package with small steps, you can’t do without them. There are various ways to form such vias; they are discussed in more detail in the section, but for now we will consider in more detail the materials from which the multilayer board is constructed.

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

Basic dielectrics for printed circuit boards
The main types and parameters of materials used for the manufacture of MPPs are given in Table 1. Typical designs of printed circuit boards are based on the use of standard fiberglass laminate type FR4, with an operating temperature, usually from -50 to +110 °C, glass transition (destruction) temperature Tg about 135 °C. Its dielectric constant Dk can be from 3.8 to 4.5, depending on the supplier and type of material. For increased requirements for heat resistance or when mounting boards in an oven using lead-free technology (t up to 260 °C), high-temperature FR4 High Tg or FR5 is used. When required to work permanently high temperatures or when sharp changes temperatures 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 dielectrics are shown in Fig. 4, 5.

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

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

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

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

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

Table 4. Rogers microwave materials for printed circuit boards

* Dk - dielectric constant

Table 5. Arlon microwave materials for MPP

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

Dk — Dielectric constant
Tg—glass transition temperature

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

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

Table 7. PCB pad coatings

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

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

Table 8. PCB Surface Coatings

Conclusion
The choice of materials is great, but, unfortunately, often when producing small and medium-sized series of printed circuit boards, the stumbling block becomes the availability necessary materials in the warehouse of the plant - manufacturer of 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, the printed circuit boards have vias for electrical connection 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 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 lends itself well to machining 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 (usually separated single sided board very difficult, 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 for absorption excess liquid, sheet with a picture, double-sided board, sheet with a picture, sheet of toilet paper, again a dampened sheet. 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.

Required 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 at complete immersion collets. 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 with high quality, very thin layer you need to make a simple device. We take a piece of braid for soldering 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 different thicknesses, from 0.3 to 0.1 mm and at 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.

What does it represent printed boards A?

Printed boards A or boards A, is a plate or panel consisting of one or two conductive patterns located on the surface of a dielectric base, or a system of conductive patterns located in the volume and on the surface of a dielectric base, interconnected in accordance with a circuit diagram, intended for electrical connection and mechanical fastening products installed on it electronic technology, quantum electronics and electrical products - passive and active electronic components.

Simplest printed boards oh is boards A, which contains copper conductors on one side printed boards s and connects the elements of the conductive pattern on only one of its surfaces. Such boards s known as single layer printed boards s or unilateral printed boards s(abbreviated as AKI).

Today, the most popular in production and the most widespread printed boards s, which contain two layers, that is, containing a conductive pattern on both sides boards s– double-sided (double-layer) printed boards s(abbreviated DPP). Through connections are used to connect conductors between layers. installation metalized and transitional holes. However, depending on the physical complexity of the design printed boards s, when the wiring is on both sides boards does not become too complex in production order available multilayer printed boards s(abbreviated MPP), where the conductive pattern is formed not only on two external sides boards s, but also in the inner layers of the dielectric. Depending on complexity, multi-layer printed boards s can be made of 4,6,...24 or more layers.

For installation A electronic components on printed boards s, a technological operation is required - soldering, used to obtain a permanent connection of parts made of different metals by introducing molten metal - solder, which has more low temperature melting than the materials of the parts being joined. The soldered contacts of the parts, as well as the solder and flux, are brought into contact and subjected to heating at a temperature above the melting point of the solder, but below the melting temperature of the parts being soldered. As a result, the solder goes into liquid state and wets the surfaces of parts. After this, the heating stops and the solder goes into the solid phase, forming a connection. This process can be done manually or using specialized equipment.

Before soldering, components are placed on printed boards e leads of components into through holes boards s and are soldered to the contact pads and/or the metallized inner surface of the hole - the so-called. technology installation A into holes (THT Through Hole Technology - technology installation A into holes or other words - pin installation or DIP installation). Also, the increasing spread, especially in the mass and large-scale production, received more advanced surface technology installation A- also called TMP (technology installation A to the surface) or SMT(surface mount technology) or SMD technology (from surface mount device - a device mounted on a surface). Its main difference from “traditional” technology installation A into holes is that the components are mounted and soldered onto land pads, which are part of the conductive pattern on the surface printed boards s. In surface technology installation A Typically, two soldering methods are used: solder paste reflow soldering and wave soldering. The main advantage of the wave soldering method is the ability to simultaneously solder both surface-mounted components boards s, and into the holes. At the same time, wave soldering is the most productive soldering method when installation e into the holes. Reflow soldering is based on the use of a special technological material - solder paste. It contains three main components: solder, flux (activators) and organic fillers. Soldering paste applied to the contact pads either using a dispenser or through stencil, then the electronic components are installed with the leads on the solder paste and then, the process of reflowing the solder contained in the solder paste is carried out in special ovens by heating printed boards s with components.

To avoid and/or prevent accidental short circuit conductors from different circuits during the soldering process, manufacturers printed boards a protective solder mask is used (English solder mask; also known as “brilliant”) - a layer of durable polymer material designed to protect conductors from the ingress of solder and flux during soldering, as well as from overheating. Soldering mask covers conductors and leaves pads and blade connectors exposed. The most common solder mask colors used in printed boards A x - green, then red and blue. It should be kept in mind that soldering mask doesn't protect boards from moisture during operation boards s and special organic coatings are used for moisture protection.

In the most popular systems programs computer-aided design printed boards and electronic devices (abbreviated CAD - CAM350, P-CAD, Protel DXP, SPECCTRA, OrCAD, Allegro, Expedition PCB, Genesis), as a rule, there are rules associated with the solder mask. These rules define the distance/setback that must be maintained between the edge of the solder pad and the edge of the solder mask. This concept is illustrated in Figure 2(a).

Silk-screen printing or marking.

Marking (eng. Silkscreen, legend) is a process in which the manufacturer applies information about electronic components and which helps to facilitate the process of assembly, inspection and repair. Typically, markings are applied to indicate reference points and the position, orientation and rating of electronic components. It can also be used for any design purpose printed boards, for example, indicate the company name, setup instructions (this is widely used in old motherboards boards A X personal computers) etc. Marking can be applied to both sides boards s and it is usually applied using screen printing (silk-screen printing) with a special paint (with thermal or UV curing) of white, yellow or black color. Figure 2 (b) shows the designation and area of ​​the components, made with white markings.

Structure of layers in CAD

As noted at the beginning of this article, printed boards s can be made of several layers. When printed boards A designed using CAD, can often be seen in the structure printed boards s several layers that do not correspond to the required layers with wiring of conductive material (copper). For example, the marking and solder mask layers are non-conductive layers. The presence of conductive and non-conductive layers can lead to confusion, as manufacturers use the term layer when they only mean conductive layers. From now on, we will use the term "layers" without "CAD" only when referring to conductive layers. If we use the term "CAD layers" we mean all types of layers, that is, conductive and non-conductive layers.

Structure of layers in CAD:

Figure 3 shows three various structures layers. Orange color highlights the conductive layers in each structure. Structure height or thickness printed boards s may vary depending on the purpose, but the most commonly used thickness is 1.5mm.

Types of Electronic Component Housings

There are a wide variety of electronic component housing types on the market today. Typically, there are several types of housings for one passive or active element. For example, you can find the same microcircuit in both a QFP package (from the English Quad Flat Package - a family of microcircuit packages with planar pins located on all four sides) and in an LCC package (from the English Leadless Chip Carrier - is a low-profile square ceramic housing with contacts located on its bottom).

There are basically 3 large families of electronic enclosures:

Contact pad printed boards s(English land)

Contact pad printed boards s- part of the conductive pattern printed boards s, used for electrical connection of installed electronic products. Contact pad printed boards s It represents parts of the copper conductor exposed from the solder mask, where the component leads are soldered. There are two types of pads - contact pads installation holes for installation A into holes and planar pads for surface installation A- SMD pads. Sometimes, SMD via pads are very similar to via pads. installation A into the holes.

Figure 4 shows the pads for 4 different electronic components. Eight for IC1 and two for R1 SMD pads, respectively, as well as three pads with holes for Q1 and PW electronic components.

Copper conductors

Copper conductors are used to connect two points on printed boards e - for example, for connecting between two SMD pads (Figure 5.), or for connecting an SMD pad to a pad installation hole or to connect two vias.

Conductors can have different calculated widths depending on the currents flowing through them. Also, at high frequencies, it is necessary to calculate the width of the conductors and the gaps between them, since the resistance, capacitance and inductance of the conductor system depends on their length, width and their relative position.

Through plated vias printed boards s

When you need to connect a component that is on the top layer printed boards s with a component located on the bottom layer, through-plated vias are used that connect the elements of the conductive pattern on different layers printed boards s. These holes allow current to pass through printed boards u. Figure 6 shows two wires that start on the pads of a component on the top layer and end on the pads of another component on the bottom layer. Each conductor has its own via hole, which conducts current from the upper layer to the lower layer.

Figure 6. Connection of two microcircuits through conductors and metallized vias on different sides printed boards s

Figure 7 gives a more detailed view of the cross section of 4-layer printed boards. Here the colors indicate the following layers:

On the model printed boards s, Figure 7 shows a conductor (red) that belongs to the upper conductive layer, and which passes through boards y using a through-via, and then continues its path along the bottom layer (blue).

Figure 7. Conductor from the top layer passing through printed boards y and continuing its path on the lower layer.

"Blind" metallized hole printed boards s

In HDI (High Density Interconnect - high density connections) printed boards A x, it is necessary to use more than two layers, as shown in Figure 7. Typically, in multi-layer structures printed boards s On which many ICs are installed, separate layers are used for power and ground (Vcc or GND), and thus the outer signal layers are freed from power rails, which makes it easier to route signal wires. There are also cases where signal conductors must pass from the outer layer (top or bottom) along the shortest path in order to provide the necessary characteristic impedance, galvanic isolation requirements and ending with the requirements for resistance to electrostatic discharge. For these types of connections, blind metallized hole(Blind via - “deaf” or “blind”). This refers to holes connecting the outer layer with one or more inner layers, which allows the connection to be kept to a minimum height. A blind hole starts on the outer layer and ends on the inner layer, which is why it is prefixed with "blind".

To find out which hole is present on boards e, you can put printed boards above the light source and look - if you see light coming from the source through the hole, then this is a transition hole, otherwise it is blind.

Blind vias are useful to use in design boards s, when you are limited in size and have too little space for placing components and routing signal wires. You can place electronic components on both sides and maximize space for wiring and other components. If the transitions are made through through holes rather than blind ones, you will need extra space for holes because the hole takes up space on both sides. At the same time, blind holes can be located under the chip body - for example, for wiring large and complex BGA components.

Figure 8 shows three holes that are part of a four-layer printed boards s. If we look from left to right, the first thing we will see is a through hole through all the layers. The second hole starts at the top layer and ends at the second inner layer - the L1-L2 blind via. Finally, the third hole starts in the bottom layer and ends in the third layer, so we say it is a blind via L3-L4.

The main disadvantage of this type of hole is the higher manufacturing cost printed boards s with blind holes, compared to alternative through holes.

Hidden vias

English Buried via - “hidden”, “buried”, “built-in”. These vias are similar to blind vias, except that they start and end on the inner layers. If we look at Figure 9 from left to right, we can see that the first hole goes through all the layers. The second is a blind via L1-L2, and the last is a hidden via L2-L3, which starts on the second layer and ends on the third layer.

Figure 9. Comparison of via via, blind hole, and buried hole.

Manufacturing technology for blind and hidden vias

The technology for manufacturing such holes can be different, depending on the design that the developer has laid down, and depending on the capabilities factory a-manufacturer. We will distinguish two main types:

The hole is drilled in a double-sided workpiece DPP, metallized, etched and then this workpiece, essentially a finished two-layer printed boards A, pressed through prepreg as part of a multilayer preform printed boards s. If this blank is on top of the “pie” MPP, then we get blind holes, if in the middle, then we get hidden vias.

1. A hole is drilled in a compressed workpiece MPP, the drilling depth is controlled to accurately hit the pads of the inner layers, and then metallization of the hole occurs. This way we only get blind holes.

In complex structures MPP Combinations of the above types of holes can be used - Figure 10.

Figure 10. Example of a typical combination of via types.

Note that the use of blind holes can sometimes lead to a reduction in the cost of the project as a whole, due to savings on the total number of layers, better traceability, and reduction in size printed boards s, as well as the ability to apply components with finer pitches. However, in every specific case the decision to use them should be made individually and reasonably. However, one should not overuse the complexity and variety of types of blind and hidden holes. Experience shows that when choosing between adding another type of blind hole to a design and adding another pair of layers, it is better to add a couple of layers. In any case, the design MPP must be designed taking into account exactly how it will be implemented in production.

Finish metal protective coatings

Getting the correct and reliable solder connections in electronic equipment depends on many design and technological factors, including the proper level of solderability of the connected elements, such as components and printed conductors. To maintain solderability printed boards before installation A electronic components, ensuring the flatness of the coating and for reliable installation A solder joints, the copper surface of the pads must be protected printed boards s from oxidation, the so-called finishing metal protective coating.

When looking at different printed boards s, you can notice that the contact pads almost never have a copper color, often and mostly they are silver, shiny gold or matte gray. These colors determine the types of finishing metal protective coatings.

The most common method of protecting soldered surfaces printed boards is the coating of copper contact pads with a layer of silver tin-lead alloy (POS-63) - HASL. Most manufactured printed boards protected by the HASL method. Hot tinning HASL - hot tinning process boards s, by immersion for a limited time in a bath of molten solder and with rapid removal by blowing a stream of hot air, removing excess solder and leveling the coating. This coating dominates for several recent years, despite its severe technical limitations. Plat s, produced in this way, although they retain solderability well throughout the entire storage period, are unsuitable for some applications. Highly integrated elements used in SMT technologies installation A, require ideal planarity (flatness) of the contact pads printed boards. Traditional HASL coatings do not meet planarity requirements.

Coating technologies that meet planarity requirements are applied chemical methods coatings:

Immersion gold plating (Electroless Nickel / Immersion Gold - ENIG), which is a thin gold film applied over a nickel sublayer. The function of gold is to provide good solderability and protect nickel from oxidation, and nickel itself serves as a barrier preventing the mutual diffusion of gold and copper. This coating ensures excellent planarity of the contact pads without damage printed boards, ensures sufficient strength of solder joints made with tin-based solders. Their main disadvantage is the high cost of production.

Immersion Tin (ISn) – gray matte chemical coating that provides high flatness printed sites boards s and compatible with all soldering methods than ENIG. The process of applying immersion tin is similar to the process of applying immersion gold. Immersion tin provides good solderability after long-term storage, which is ensured by the introduction of an organometal sublayer as a barrier between the copper of the contact pads and the tin itself. However, boards s, coated with immersion tin, require careful handling and should be stored vacuum-packed in dry storage cabinets and boards s with this coating are not suitable for the production of keyboards/touch panels.

When operating computers and devices with blade connectors, the contacts of the blade connectors are subject to friction during operation. boards s Therefore, the end contacts are electroplated with a thicker and more rigid layer of gold. Galvanic gilding of knife connectors (Gold Fingers) - coating of the Ni/Au family, coating thickness: 5 -6 Ni; 1.5 – 3 µm Au. The coating is applied by electrochemical deposition (electroplating) and is used primarily on end contacts and lamellas. Thick, gold coating has high mechanical strength, resistance to abrasion and adverse environmental influences. Indispensable where it is important to ensure reliable and durable electrical contact.

Printed circuit board

A printed circuit board with electronic components mounted on it.

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

Board drawing in CAD program and finished board

Device

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

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

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

Other PCB standards:

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

Typical process

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

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

Manufacturing

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

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

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

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

Production of foil material

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

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

Aluminum PCBs

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

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

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

Workpiece processing

Obtaining a wire pattern

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

Chemical method

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

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

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

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

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

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

Mechanical method

The mechanical manufacturing method involves the use of milling and engraving machines or other tools for mechanical removal layer of foil from specified areas.

Laser engraving

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

Metallization of holes

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

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

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

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

Pressing of multilayer boards

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

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

Coating

Possible coatings include:

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

Mechanical restoration

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

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

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

Installation of components

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

Wave soldering

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

Soldering in ovens

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

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

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

Installing components

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

Finish coatings

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

Similar technologies

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

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

Duration: 2 hours (90 min.)

25.1 Basic questions

PP base materials;

Materials for creating printed design elements;

Technological materials for the manufacture of PP.

25.2 Lecture text

25.2.1 Basic mPP base materials – up to 40 min

Basic materials of printed circuit boards include:

foil-coated (on one or both sides) and non-foil-coated dielectrics (getinax, textolite, fiberglass, fiberglass, lavsan, polyimide, fluoroplastic, etc.), ceramic materials and metal (with a surface dielectric layer) plates from which printed circuit board bases are made;

insulating spacer material (adhesive gaskets - prepregs) used for gluing MPP layers.

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

When choosing a PP base material, you need to pay attention to the following: expected mechanical effects (vibrations, shocks, linear acceleration, etc.); accuracy class PP (distance between conductors); implemented electrical functions; performance; terms of Use; price.

The base material must adhere well to the metal of the conductors, have high mechanical strength, retain its properties when exposed to climatic factors, and have a similar coefficient of thermal expansion compared to the metal of the conductors.

The choice of material is determined by:

electrical insulating properties;

mechanical strength;

stability of parameters when exposed to aggressive environments and changing conditions;

machinability;

cost.

Foil dielectrics are produced with a conductive coating of copper (less commonly nickel or aluminum) electrolytic foil with a thickness of 5 to 105 microns. To improve adhesion strength, the foil is coated on one side with a layer of chromium 1…3 microns thick. The foil is characterized by purity of composition (impurities no more than 0.05%), ductility. Foiling is carried out by pressing at a temperature of 160...180 0 C and a pressure of 5...15 MPa.

Non-foil dielectrics are produced in two types:

with an adhesive (adhesive) layer with a thickness of 50...100 microns (for example, an epoxy rubber composition), which is applied to increase the adhesion strength of chemical copper deposited during the manufacturing process of PP;

with a catalyst introduced into the volume of the dielectric, which promotes the deposition of chemical copper.

Laminated plastics consisting of a filler (electrical insulating paper, fabric, fiberglass) and a binder (phenolic or phenolic epoxy resin) are used as the dielectric base of rigid PP. Laminated plastics include getinax, textolite and fiberglass.

Getinax is made from paper and is used under normal climatic operating conditions for household equipment. It has low cost, good workability, and high water absorption.

Textolite is made from cotton fabric.

Fiberglass laminates are made from fiberglass. Compared to getinaks, fiberglass laminates have better mechanical and electrical characteristics, higher heat resistance, and lower moisture absorption. However, they have a number of disadvantages: worse machinability; higher cost; a significant difference (about 30 times) in the coefficient of thermal expansion of copper and fiberglass in the direction of the material thickness, which can lead to rupture of the metallization in the holes during soldering or during operation.

For the manufacture of PCBs used in conditions of increased fire hazard, fire-resistant getinaks and fiberglass laminates are used. Increasing the fire resistance of dielectrics is achieved by introducing fire retardants into their composition.

The introduction of 0.1...0.2% palladium or cuprous oxide into the varnish that impregnates fiberglass improves the quality of metallization, but slightly reduces the insulation resistance.

To manufacture PCBs that provide reliable transmission of nanosecond pulses, it is necessary to use materials with improved dielectric properties (reduced dielectric constant and dielectric loss tangent). Therefore, the use of bases made of organic materials with a relative dielectric constant below 3.5 is considered promising. Non-polar polymers (fluoroplastic, polyethylene, polypropylene) are used as the basis for PP in the microwave range.

To produce GPP and GPC that can withstand repeated bending, dielectrics based on polyester film (lavsan or polyethylene terephthalate), fluoroplastic, polyimide, etc. are used.

Insulating cushioning material (prepregs) is made from fiberglass impregnated with under-polymerized thermosetting epoxy resin (or other resins); made of polyimide with an adhesive coating applied on both sides and other materials.

Ceramics can be used as the base material for the PP.

The advantage of ceramic PP is better heat removal from active elements, high mechanical strength, stability of electrical and geometric parameters, reduced noise levels, low water absorption and gas emission.

The disadvantage of ceramic boards is fragility, large mass and small dimensions (up to 150x150 mm), long manufacturing cycle and large shrinkage of the material, high cost.

PP on metal base used in products with high current loads and at elevated temperatures. Aluminum, titanium, steel, copper, and an alloy of iron and nickel are used as base materials. To obtain an insulating layer on a metal base, special enamels, ceramics, epoxy resins, polymer films, etc. are used; an insulating layer on an aluminum base can be obtained by anodic oxidation.

The disadvantage of metal enameled boards is the high dielectric constant of the enamel, which precludes their use in high-frequency equipment.

The metal base of the PCB is often used as power and ground buses, as a shield.

25.2.2 Materials of printed design elements – up to 35 min

Metal coatings are used as the material for printed pattern elements (conductors, contact pads, end contacts, etc.). Copper is most often used to create the main current-carrying layer. Ceramic PCBs use graphite.

The materials used to create metal coatings are presented in Table 25.1.

Table 25.1 – Metal coatings used to create printed design elements

25.2.3 Technological (consumables) mmaterials for the manufacture of PP – up to 15 min

Technological materials for the manufacture of PCB include photoresists, special screen paints, protective masks, copper plating electrolytes, etching, etc.

Requirements for consumables are determined by the design of the PCB and the manufacturing process.

Photoresists must provide the necessary resolution when obtaining a circuit pattern and appropriate chemical resistance. Photoresists can be liquid or dry film (SPF).

Negative and positive photoresists are used. When using negative photoresists, the exposed areas of the PCB blank remain on the board, and the unexposed areas are washed out during development. When using positive photoresistors, the exposed areas are washed out during development.

Etching solutions must be compatible with the resist used for etching, be neutral to insulating materials, and have a high etching rate. Acid and alkaline solutions of copper chloride, solutions based on ferric chloride, solutions based on ammonium persulfate, and iron-copper chloride solutions are widely used as etching electrolytes.

All materials must be economical and environmentally friendly.