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 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 available materials for the manufacture of circuit boards - these 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. Extremely 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” the 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.

At mechanical method a scalpel is used (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.

Simplest chemical method- 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 quite 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 the production of silk-screen printing and protective masks on 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, this allows it to be used as a protective coating on the surface of copper.

So, our method is to transfer the toner from the paper to the surface of the copper foil and then etch the board special solution to get the drawing.

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 anywhere detailed information, which 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, it is necessary to perform following requirements to boards:

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 special device- 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 full 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 soldering station- definitely - don’t tinker with 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) - it turns out 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 lacquer 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.

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

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

– on paper;

– based on fiberglass.

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

The characteristics of the finished printed circuit board depend on the specific combination starting materials, as well as from technology, including mechanical processing of boards.

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

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

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

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

The bending strength and impact strength of the printed circuit board must be high enough so that the board can be loaded with heavy components installed on it without damage.

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

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

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

Some characteristics of various types of laminates are presented in Table 4.

Table 4 - Characteristics of various types of laminates

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

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

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

1.7 Selecting a PCB manufacturing method

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

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

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

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

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

– chemical for GPC

– combined positive for DPP

Metallization through holes for MPP

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

2 CALCULATION OF CONDUCTING PATTERN ELEMENTS

2.1 Calculation of mounting hole diameters

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

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

The table shows:

t – conductor width;

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

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

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

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

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

Table 5 – Limit values ​​of process parameters

Continuation of table 5

Minimum diameter of metallized (via) hole:

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

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

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

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. The section tells about completely 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 available,

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. Eat various ways formation of 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. For requirements such as continuous operation at high temperatures or 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). Most famous suppliers special materials- Rogers, Arlon, Taconic, Dupont companies. 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 various materials are given 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 the manufacture of double-sided circuit boards, the thickness of the material 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 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 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 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.

Our company produces printed circuit boards from high-quality domestic and imported materials, ranging from standard FR4 to FAF microwave materials.

Typical designs printed circuit boards are based on the use of standard fiberglass laminate type FR4, with an operating temperature from -50 to +110 °C, and a glass transition temperature Tg (softening) of about 135 °C.

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.

Basic materials for printed circuit boards:

"+" - Typically in stock

"+/-" - On request (not always available)

Prepreg ("tie" layer) for multilayer printed circuit boards

FR-4

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

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

MI 1222

is a layered pressed material based on fiberglass impregnated with an epoxy binder, lined on one or both sides with copper electrolytic foil.

• surface electrical resistance (Ohm): 7 x 1011;
• specific volumetric electrical resistance (Ohm): 1 x 1012;
• dielectric constant (Ohm x m): 4.8;
• foil peel strength (N/mm): 1.8.

FAF-4D

They are glass fiber reinforced fluoroplastic, lined on both sides with copper foil. Application: - as bases printed circuit boards operating in the microwave range; - electrical insulation for printed elements of receiving and transmitting equipment; - capable of long-term operation in the temperature range from +60 to +250° C.

• Adhesion strength of foil to base per 10 mm strip, N (kgf), not less than 17.6(1.8)
• Dielectric loss tangent at a frequency of 106 Hz, no more than 7 x 10-4
• Dielectric constant at frequency 1 MHz 2.5 ± 0.1
• Available sheet sizes, mm (maximum deviation in sheet width and length 10 mm) 500x500

T111

material made from a thermally conductive polymer based on ceramics with an aluminum base, are used in cases where it is intended to use components that emit significant thermal power(for example, ultra-bright LEDs, laser emitters, etc.). The main properties of the material are excellent heat dissipation and increased dielectric strength when exposed to high voltages:

• Aluminum base thickness - 1.5 mm
• Dielectric thickness - 100 microns
• Copper foil thickness - 35 microns
• Thermal conductivity of the dielectric - 2.2 W/mK
• Dielectric thermal resistance - 0.7°C/W
• Thermal conductivity of aluminum substrate (5052 - analogue of AMg2.5) - 138 W/mK
• Breakdown voltage - 3 KV
• Glass transition temperature (Tg) - 130
• Volume resistance - 108 MΩ×cm
• Surface resistance - 106 MΩ
• Highest operating voltage (CTI) - 600V

Protective solder masks used in the production of printed circuit boards

Solder mask (also known as brilliant green) - layer durable material, designed to protect conductors from ingress of solder and flux during soldering, as well as from overheating. The mask covers the conductors and leaves the pads and blade connectors exposed. The method of applying a solder mask is similar to applying photoresist - using a photomask with a pattern of pads, the mask material applied to the PCB is illuminated and polymerized, the areas with soldering pads are unexposed and the mask is washed off from them after development. Most often, the solder mask is applied to the copper layer. Therefore, before its formation, the protective layer of tin is removed - otherwise the tin under the mask will swell from the heating of the board during soldering.

PSR-4000 H85

Green color, liquid photosensitive heat-hardening, 15-30 microns thick, TAIYO INK (Japan).

Has approval for use by the following organizations and end product manufacturers: NASA, IBM, Compaq, Lucent, Apple, AT&T, General Electric, Honeywell, General Motors, Ford, Daimler-Chrysler, Motorola, Intel, Micron, Ericsson, Thomson, Visteon, Alcatel , Sony, ABB, Nokia, Bosch, Epson, Airbus, Philips, Siemens, HP, Samsung, LG, NEC, Matsushita(Panasonic), Toshiba, Fujitsu, Mitsubishi, Hitachi, Toyota, Honda, Nissan and many, many others;

IMAGECURE XV-501

Colored (red, black, blue, white), liquid two-component solder mask, Coates Electrografics Ltd (England), thickness 15-30 microns;

DUNAMASK KM

Dry film mask from DUNACHEM (Germany), thickness 75 microns, provides tenting of vias and has high adhesion.

Laminate FR4

You can find a detailed description of the laminate.

Laminates in TePro warehouse

Microwave material ROGERS

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

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

Metal based laminates

Visual representation of the material

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

Description

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

Description

Prepreg

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

You can find a detailed description of prepregs.

Solder mask

Properties

You can find a detailed description of the solder mask.

Frequently asked questions and answers on laminates and prepregs

What is XPC?

What's the difference between FR1 and FR2?

What is FR2?

What is FR3?

What is FR4?

What is FR5?

What is CEM-1?

What is CEM-3?

What is G10?

How can laminates be replaced?

What are "prepregs"?

What does FR or CEM stand for?

Is FR4 really green?

Does the color of the logo mean anything?

Does logo orientation mean anything?

What is UV blocking laminate?

What laminates are suitable for plating through holes?

Is there an alternative for plating through holes?

What is "material thickness"?

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

What is CTI?

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

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