A printed circuit board (English printed circuit board, PCB, or printed wiring board, PWB) is a dielectric plate, on the surface and / or in the volume of which electrically conductive circuits of an electronic circuit are formed. The printed circuit board is designed for electrical and mechanical connection of various electronic components. Electronic components on a printed circuit board are connected with their leads to the elements of the conductive pattern, usually by soldering.
In contrast to surface mounting, on a printed circuit board, the electrically conductive pattern is made of foil, entirely located on a solid insulating base. The printed circuit board contains mounting holes and pads for mounting pin or planar components. In addition, printed circuit boards have vias for electrical connection of foil sections located on different layers of the board. From the outside, the board is usually coated with a protective coating (“solder mask”) and markings (an auxiliary figure 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 (SPP): there is only one layer of foil glued to one side of the dielectric sheet.
double-sided (DPP): two layers of foil.
multilayer (MPP): foil not only on two sides of the board, but also in the inner layers of the dielectric. Multilayer printed circuit boards are obtained by gluing several single or double sided boards together.
As the complexity of the designed devices and the density of mounting increase, 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, getinaks. Also, a metal base coated with a dielectric (for example, anodized aluminum) can serve as the basis for printed circuit boards; copper foil tracks are applied over 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. As a material for printed circuit boards operating in the microwave range and at temperatures up to 260 ° C, fluoroplastic reinforced with glass fabric (for example, FAF-4D) and ceramics are used. Flexible boards are made from polyimide materials such as Kapton.
What material will we use for the manufacture of boards
The most common, affordable materials for the manufacture of circuit boards are Getinaks and Steklotekstolit. Getinax paper impregnated with bakelite varnish, fiberglass textolite with epoxy. We will definitely use fiberglass!
Foiled fiberglass is sheets made on the basis of glass fabrics impregnated with a binder based on epoxy resins and lined on both sides with copper electrolytic galvanic-resistant foil 35 microns thick. The maximum allowable temperature is from -60ºС to +105ºС. It has very high mechanical and electrical insulating properties, lends itself well to machining by cutting, drilling, stamping.
Fiberglass is mainly used one or two-sided with a thickness of 1.5mm and with copper foil with a thickness of 35μm or 18μm. We will use a 0.8mm thick single-sided fiberglass with a 35µm thick foil (why will be discussed in detail later).
Methods for making printed circuit boards at home
Boards can be manufactured chemically and mechanically.
With the chemical method, in those places where there should be tracks (drawing) on the board, a protective composition (lacquer, 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.
The mechanical method uses a scalpel (for manual production) or a milling machine. A special cutter makes grooves on the foil, eventually leaving islands with foil - the necessary pattern.
Milling machines are quite expensive, as well as the cutters themselves are expensive and have a small resource. So, we will not use this method.
The simplest chemical method is manual. With a risograph varnish, tracks are drawn on the board and then we etch 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 for making boards is with a photoresist. This is a very common technology (boards are made by this method at the factory) and it is often used at home. There are a lot of articles and methods for manufacturing 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 lamp, laminator). Of course, if you have a bulk production of boards at home - then the photoresist is out of competition - we recommend mastering it. It is also worth noting that the equipment and technology of photoresist allows the production of 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 sinters under temperature and sticks to paper - as a result, a pattern is obtained. The toner is resistant to various chemicals, which allows it to be used as a protective coating on the copper surface.
So, our method is to transfer the toner from the paper to the surface of the copper foil and then etch the board with a special solution to obtain a pattern.
Due to its ease of use, this method has earned a very wide distribution in amateur radio. If you type in Yandex or Google how to transfer the toner from paper to the board, you will immediately find such a term as "LUT" - laser ironing technology. Boards using this technology are made as follows: a pattern of tracks is printed in a mirror version, paper is applied to the board with a pattern to copper, we iron this paper on top, the toner softens and sticks to the board. The paper is further 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 attachment to it. That is, you have to feel it. Payments do not come out the first time, they are obtained every other time. There are many improvements - to use a laminator (with alteration - in the usual one there is not enough temperature), which allow to achieve very good results. There are even methods for building special heat presses, but all this again requires special equipment. The main disadvantages of LUT technology:
overheating - the tracks spread out - become wider
underheating - tracks remain on paper
the paper is “cooked” to the board - even when it is soaked, it is difficult to leave - as a result, the toner may be damaged. There is a lot of information on the Internet about which paper to choose.
Porous toner - after removing the paper, micropores remain in the toner - the board is also etched through them - 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 toner warm-up temperature, you need a stable board pressure.
By the way, this method did not work for me to make a board. Tried to do both on magazines and on coated paper. As a result, he even spoiled the boards - copper swelled from overheating.
For some reason, there is undeservedly little information on the Internet about another method of toner transfer - the method of cold chemical transfer. It is based on the fact that toner does not dissolve with alcohol, but with acetone. As a result, if you choose such a mixture of acetone and alcohol, which will only soften the toner, then it can be “re-pasted” onto the board from paper. I really liked this method and immediately paid off - the first board was ready. However, as it turned out later, I could not find detailed information anywhere that would give a 100% result. We need a method by which even a child could make a payment. But for the second time, the payment did not work out, then again it took a long time to select the necessary ingredients.
As a result, after a long time, 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 payment completely on his own. This is the method we will use. (Of course, it can be further improved to the ideal - if it works out better for you, then write). The advantages of this method:
all reagents are inexpensive, available and safe
no additional tools are needed (irons, lamps, laminators - nothing, although not - you need a pan)
there is no way to spoil the board - the board does not heat up at all
paper moves away by itself - you can see the result of the transfer of toner - where the transfer did not come out
there are no pores in the toner (they are sealed with paper) - accordingly, there are no mordants
do 1-2-3-4-5 and 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 with this method.
Basic requirements for manufactured boards
We will make devices on microcontrollers, using modern sensors and microcircuits. Microcircuits are getting smaller and smaller. Accordingly, the following requirements must be met:
boards must be two-sided (as a rule, it is very difficult to separate a single-sided board, it is rather difficult to make four-layer boards at home, 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 pickling, track departure during soldering
the gaps between the tracks - 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, and also do silk-screening - this will complicate the production, and if you are making the board for yourself, then this is not necessary. Again, there is a lot of information on the Internet on this topic, and if you wish, you can make a “marafet” yourself.
We will not tinker with 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, 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
One of the problems with making double-sided boards is aligning the sides so that the vias line up. Usually a "sandwich" is made for this. 2 sides are printed on a sheet of paper at once. The sheet is bent in half, the sides are precisely aligned with the help of special marks. Double-sided textolite is inserted inside. With the LUT method, such a sandwich is ironed and a double-sided board is obtained.
However, in the cold transfer toner method, the transfer itself is carried out with the help of a liquid. And therefore it is very difficult to organize the process of wetting one side simultaneously with the other side. Of course, this 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 toner transfer liquid. The sheets are wetted so that the liquid does not drip and the sheet holds its shape. And then a “sandwich” is made - a wetted sheet, a sheet of toilet paper to absorb excess liquid, a sheet with a pattern, a double-sided board, a sheet with a pattern, a sheet of toilet paper, again a wetted sheet. All this is clamped vertically in a vise. But we will not do this, we will do it easier.
A very good idea slipped through the board manufacturing forums - what a problem it is to make a double-sided board - we take a knife and cut the textolite in half. Since fiberglass is a puff material, it is not difficult to do this with a certain skill:
As a result, from one double-sided board with a thickness of 1.5 mm, we get two one-sided halves.
Next, we make two boards, drill and that's it - they are perfectly aligned. It was not always possible to cut the textolite evenly, and as a result, the idea came up to immediately use a thin one-sided textolite with a thickness of 0.8 mm. Then you can not glue the two halves, they will be held by soldered jumpers in vias, buttons, connectors. But if necessary, you can glue it with epoxy glue without any problems.
The main advantages of this trip:
Textolite with a thickness of 0.8 mm is easily cut with scissors on paper! In any shape, that is, it is very easy to cut to fit the body.
Thin textolite - transparent - by shining a lantern 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 microcircuit pins - you can connect the sides at the very end
You need to drill twice as many holes and the holes may slightly misalign.
The rigidity of the structure is slightly lost if you do not glue the boards, and gluing is not very convenient
One-sided fiberglass 0.8mm thick is difficult to buy, mostly 1.5mm is sold, but if you couldn’t get it, you can cut a thicker textolite with a knife.
Let's move on to the details.
Necessary tools and chemistry
We will need the following ingredients:
Now that all this is there, let's do 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 to the motor (preferably a button). It is better to place the button on the body, so that it is more convenient to quickly turn the motor on and off. It remains to choose a power supply, you can take any power supply for 7-12V with a current of 1A (or less), if there is no such power supply, then charging via USB at 1-2A or a Kron battery may be suitable (you just need to try - not all chargers like motors, the motor may not start).
The drill is ready, you can drill. But it is only necessary 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 an easier solution.
Drill jig
To drill exactly at 90 degrees, it is enough to make a drilling jig. We'll do something like this:
It is very easy to make it. We take a square of any plastic. We put our drill on a table or other flat surface. And we drill a hole in the plastic with the right drill. It is important to ensure a smooth horizontal displacement of the drill. You can lean the motor against a wall or rail and plastic too. Next, use a large drill to drill a hole for the collet. On the reverse side, drill or cut off a piece of plastic so that the drill can be seen. A non-slip surface can be glued to the bottom - paper or an elastic band. Such a conductor must be made for each drill. This will ensure perfectly accurate drilling!
This option is also suitable, cut off the top part of the plastic and cut off the corner from the bottom.
Here is how drilling is done with it:
We clamp the drill so that it sticks out 2-3 mm when the collet is fully immersed. We put the drill in the place where it is necessary to drill (when etching the board, we will have a mark where to drill in the form of a mini hole in copper - in Kicad we specially set a checkbox for this, so that the drill will get up there by itself), press the conductor and turn on the motor - the 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 some power.
8. Tinning board
Why tin boards - mainly to protect copper from corrosion. The main disadvantage of tinning is overheating of the board, possible damage to the tracks. If you do not have a soldering station - definitely - do not tin the board! If it is, then the risk is minimal.
It is possible to tin the 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 qualitatively, a very thin layer must be made a simple device. We take a piece of braid for soldering parts and put it on the sting, fasten it with a wire to the sting so that it does not come off:
We cover the board with a flux - for example, LTI120 and a braid too. Now we collect tin into the braid and we drive it along the board (we paint it) - we get an excellent result. But with use, the braid falls apart and copper fibers begin to remain on the board - they must be removed, otherwise there will be a short circuit! It is very easy to see this by shining a flashlight on the back of the board. With this method, it is good to use either a powerful soldering iron (60 watts) or ROSE alloy.
As a result, it is better not to tin the boards, but to varnish at the very end - for example, PLASTIC 70, or a simple acrylic varnish bought in auto parts KU-9004:
Fine tuning of the toner transfer method
There are two points in the method that are amenable to tuning, and may not work right away. To set them up, 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 at different intervals, from 0.3 to 0.1 mm. It is better to immediately print several of these samples on one sheet and adjust.
Possible issues we will be fixing:
1) tracks can change geometry - spread, become wider, usually not very much, up to 0.1mm - but this is not good
2) the toner may not adhere well to the board, move away when removing the paper, it may not adhere well to the board
The first and second problems are interrelated. I solve the first, you come to the second. We must find a compromise.
The tracks can spread for two reasons - too much clamping weight, too much acetone in the composition of the resulting liquid. First of all, you need to try to reduce the load. The minimum load is about 800g, you should not reduce it below. Accordingly, we put the load without any pressure - we just put it on top and that's it. Be sure to have 2-3 layers of toilet paper for good absorption of excess solution. You must ensure that after removing the load, the paper should be white, without purple smudges. Such smudges indicate a strong melting of the toner. If it was not possible to adjust the load with the load, the tracks still blur, then we increase the proportion of nail polish remover in the solution. Can be increased to 3 parts liquid and 1 part acetone.
The second problem, if there is no geometry violation, indicates an insufficient weight of the cargo 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 adhere well to the board, then you need to increase the amount of acetone.
This problem mostly occurs when you change your nail polish remover. Unfortunately, this is not a permanent and not a pure component, but it was not possible to replace it with another one. I tried to replace it with alcohol, but apparently the mixture is not homogeneous and the toner sticks with some inclusions. Also, nail polish remover may contain acetone, then it will need less. In general, you will need to carry out such tuning once until the liquid runs out.
Board ready
If you do not immediately solder the board, then it must be protected. The easiest way to do this is to coat with alcohol rosin flux. Before soldering, this coating will need to be removed, for example, with isopropyl alcohol.
Alternatives
You can also make a payment:
Additionally, a custom board manufacturing service is now gaining popularity - for example, Easy EDA. If a more complex board is needed (for example, a 4-layer board), then this is the only way out.
Laser ironing technology (LUT for short) is a simple and common method for drawing and manufacturing printed circuit boards at home. This method is available and beneficial for both beginner radio amateurs and experienced craftsmen. The advantages of this method are the low cost of materials, the availability and ease of doing it yourself.
PCB stencil production
First you need to separate the tracks in special programs for tracing and drawing boards. There are many programs for this purpose, such as Sprint Layout, Pcad, Eagle and Deep Trace. After dividing the tracks on the board, you should print the circuit, be sure to turn off toner saving.
In some cases, it is necessary to print the pattern in a mirror image so that the pins on the board match the pinout of the details, for example, microcircuits in the smd version. For convenience, it is necessary to create a board outline so that after etching it is easier to process the edges of the board, giving them an aesthetic appearance. Then you should remove unnecessary layers for etching or set two layers for tracks and silkscreen in the settings. For reliability, you can print several samples, for possible unsuccessful attempts. You can use any paper with a glossy finish for printing.
Watch a detailed video on making a printed circuit board with your own hands (LUT Technology)
Transferring a drawing to a board
Then you will need an iron, sandpaper, a wooden cutting board and a bath of soapy water. It is necessary to prepare a piece of textolite or getinax suitable for the board, as well as sandpaper of medium grit. Next, you should carefully remove dust and dirt, attach a piece with the print of the board so that the pattern is in the middle of the workpiece. Then wrap tightly, put on a wooden board, put a hot iron on top. The baking temperature of the toner is about 100-180 degrees. Therefore, the temperature of the iron should first be set experimentally, as well as the duration of exposure to the workpiece.
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After this process, the board must be lowered into a bath of water, with the addition of some kind of soapy solution or dishwashing detergent. You should wait until the paper rots off, 10 minutes is enough. After that, carefully it is necessary to tear it off. If there are poorly printed places, you can correct it with a water-resistant marker.
Board etching
There are many solutions for etching circuit boards, but in this article a solution of hydrogen peroxide with citric acid was used for etching. You should lower the board into the solution and watch the etching reaction, sometimes the reaction is so fast and violent that you can feel the heat from the board at the end of the process. After etching the board, you can see the result - the places that were not covered with toner were deprived of the copper layer, only the tracks and characters that were under the toner layer remained. Next, you will need Solvent 646 and a cloth, such as a disposable cloth or dusting cloth. It is necessary to slightly moisten a rag in the solvent, and wipe the toner from the surface of the workpiece.
Workpiece tinning
The next step in the process is tinning the tracks. Rose alloy was used for this workpiece, unlike Wood's alloy, it does not have cadmium and therefore is not so toxic. The advantage of this tinning method over others is the accuracy and aesthetic appearance of the product. Since the Rose alloy melts at a temperature of +94 degrees, liquid glycerin is used to increase the boiling point, which can be purchased at any pharmacy for a penny. You also need to add a teaspoon of citric acid - it serves as a kind of flux. You will also need two wooden sticks, those that are served with Chinese food will do. A special tampon made of tissue material is put on the end of one stick. It is also advisable to purchase a small rubber spatula, for example, from an auto store.
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So, you need to pour a little water into a metal cup, so that it is enough to cover the entire board, plus 3-4 centimeters of water on top, add about a teaspoon of glycerin, sometimes it may take more - you need to install empirically. Then add citric acid, then send the board. Then you have to wait until the solution boils, then, holding the workpiece with a stick with a hard edge, add one granule of the Rosé alloy there.
After the alloy has become liquid, in the form of a light drop similar to mercury, you should drive this drop with a swab with a soft tip over the surface of the board, without sudden movements. It is important to ensure that the alloy covers all areas of the workpiece prepared for tinning. You can take it out and check it visually, for looseness of individual sections. If necessary, repeat the procedure by throwing another alloy granule. Upon completion of the tinning of the board, you should take out a rubber spatula and, holding the board with a stick, remove excess metal on the surface of the workpiece directly in boiling water, running the spatula over it. The rest of the Rose alloy can be collected in the same boiling water in one large drop and used next time. The workpiece should be washed with running water and dried.
Seems like a tough time for overclockers. Manufacturers, as agreed, began to limit the possibility of overclocking their products. I don't know if this is good or bad. I am not a fundamental opponent of overclocking, but I have a pragmatic attitude towards it. If there is any benefit from it - for God's sake. But in my experience, I was convinced that overclocking in itself does little. Well, I overclocked my processor by 40%, slightly overclocked the video card and ... I didn’t see practically any differences in real work, except for the processor temperature. It was 38, now it's 52, I don't know what, but not degrees. He shrugged and put everything back in place. True, I have a fairly powerful computer even without overclocking. So, overclocking seems to give only moral satisfaction. Yes, and it's debatable. Actually, what is the merit of an overclocker? Is it that he got a well-driven processor or was lucky with a specific instance of a video card?
But there have always been, are and will be people for whom it is not enough to buy a good thing and just use it. So the anti-overclocking measures of Intel, AMD, ATI and Nvidia can help direct the energy of people who feel itchy hands in a more promising direction.
advertising
In my opinion, modding is much more useful both from a practical point of view and for obtaining moral satisfaction. But not a simple decoration, but changes and additions that increase functionality and ease of use. So, offhand, we can offer, for example, a multi-channel electronic thermometer for prompt and independent of the arbitrariness of biocoders for temperature control at all critical points, a built-in 6-8 channel amplifier for passive speakers (wow, the wheezing of Chinese cheap stuff got me!), devices for hardware switching of hard drives (useful for placing several conflicting operating systems on one computer and protecting the archive from viruses), an electronic water cooling control system, etc.Here I would like to note the articles "Everything you wanted to do with your hands, but were afraid to ask ..." and "HDD loading indicator". They can be seen as the first signs of this, in my opinion, extremely promising approach.
Even more of those who could repeat the finished development. The problem is technology. Making high-quality printed circuit boards at home is quite problematic, and ordering them from specialized companies is expensive and time-consuming. Yes, and part of the buzz is lost.
Media selection
advertising
As it turned out, only a special film for laser printers can be used as an image carrier. Any type of paper is unsuitable. The film should be thin and with a paper backing. Expensive types of films have a special sublayer for firmly fixing the image and are also unsuitable. Lately I've been using EMTEK film because Xerox film has disappeared from our market, but Xerox is better. It warps less when heated. It is better to use fusible toner. At first, I used the native Samsung ML-1250 toner cartridge. It provides a very good solid image. After refilling the cartridge with Xerox 8T toner, as I was advised in the service center, the image became worse and the boards stopped working at all, which prompted me to research. But by improving the technology, I have achieved excellent results with this toner.
Workpiece preparation
To obtain a good result, the surface preparation of the workpiece is critical. The surface must be perfectly clean and even. Wiping with alcohol, acetone or any cleaning agents is not enough. The surface preparation procedure is as follows. First, we clean the surface from coarse dirt with Pemolux powder. We wash the workpiece with a cotton swab, without touching the surface with our fingers. We place it in a solution of ferric chloride for 10-15 seconds. In this case, a thin top layer is etched along with all contaminants. We wash the workpiece under running water with a cotton swab. Shake off the water and dry without touching the surface with anything. If everything is done correctly, you should get a dark pink matte surface, possibly with a few streaks. The main thing is that there should be no shiny areas. If they are, repeat the procedure.
Rolling a pattern
It is usually recommended to put the workpiece, the media on it and iron it with an iron. Under ideal conditions, this is possible and will pass, but in reality, both the surface of the workpiece and the sole of the iron are not completely smooth and it will not be possible to obtain a uniform pressing of the hot carrier against the surface of the workpiece. In addition, the process cannot be controlled and you have to rely on luck. Therefore, I fix the iron with the sole up, put a clean sheet of paper on it so as not to accidentally damage the sole, but the workpiece on it. The iron must be heated to a temperature at which the paper does not yet turn yellow, but no less. I put a film with a printed pattern on top and roll it with a special device made from a pressure roller of a tape recorder. Rolling should be started from the center, squeezing the air out from under the film to the sides. After the film adheres tightly to the surface of the workpiece, we increase the rolling force and carefully go through the entire board. We remove the workpiece from the iron and cool it. It is possible to remove the film from the workpiece only after complete cooling. If done correctly, all of the toner will transfer to the board, leaving faint pinkish marks on the film. The film cannot be reused.
Pinning a picture
Despite the fact that outwardly the drawing looks almost perfect, you cannot immediately poison the board. The toner layer is porous. If you immediately etch the board, and then look at the resulting conductors under a microscope or a strong magnifying glass, the etched points are clearly visible, and the edges of the conductor are uneven. To avoid this, we cover the pattern on the board with a 10% solution of rosin in alcohol and lay it on the iron again. The temperature should be set to the maximum so that the paper turns yellow and smokes. We stand 10 minutes. In this case, the toner fuses with the rosin, forming a very durable, uniform shiny layer. We cool the board and develop the pattern with a swab with alcohol. Rosin, fused with toner, does not dissolve in alcohol, and the remains of unevaporated rosin from gaps are removed without much difficulty. When wiping, you can apply considerable effort. The alloy of toner and rosin holds very firmly, even with a sandpaper it is difficult to remove it. If somewhere the drawing is damaged, then such is his fate. It is better to detect a badly rolled conductor during the wiping step than after pickling. If unsuccessful, wash off the drawing with acetone and repeat everything from the very beginning. This happens rarely.
Board etching
Etching is carried out in a solution of ferric chloride. The solution can be heated to a temperature of 50-60 degrees. There are no features. After etching, we wash the board with water and wash off the protective coating with acetone.
Results achieved
Using the technology described above, single-sided printed circuit boards up to 100x150 mm in size were produced. The technology allows you to run one conductor between the legs of microcircuits in DIP packages, so I have not yet had a need for double-sided boards. I have an idea to modify the technology for double-sided boards, but I haven't tried it yet. The entire board manufacturing cycle takes about two hours, excluding the time spent on wiring. The payment is received from the first time in 9 cases out of 10.
P.S. This is my first article for you. If this topic is of interest to you, I will send more. I have many materials.
Sincerely, S. Veremeenko.
A printed circuit board is a dielectric plate, on the surface of which conductive tracks are applied and places for mounting electronic components are prepared. Electroradio components are usually installed on the board by soldering.
PCB device
The electrically conductive tracks of the board are made of foil. The thickness of the conductors is, as a rule, 18 or 35 microns, less often 70, 105, 140 microns. The board has holes and pads for mounting radio elements.
Separate holes are used to connect conductors located on different sides of the board. A special protective coating and markings are applied to the outer sides of the board.
Stages of creating a printed circuit board
In amateur radio practice, one often has to deal with the development, creation and manufacture of various electronic devices. Moreover, any device can be built on a printed circuit or conventional board with surface mounting. The printed circuit board works much better, is more reliable and looks more attractive. Creating it involves performing a number of operations:
Layout preparation;
Drawing a picture on textolite;
Etching;
Tinning;
Installation of radio elements.
The manufacture of printed circuit boards is a complex, time-consuming, interesting process.
Development and production of a layout
The board drawing can be done manually or on a computer using one of the special programs.
Manually, it is best to draw the board on paper from recorders at a scale of 1: 1. Graph paper is also suitable. Installed electronic components must be depicted in a mirror image. Tracks on one side of the board are shown as solid lines, and on the other side as dotted lines. Dots mark the places of attachment of radioelements. Ration areas are drawn around these places. All drawings are usually performed by a drawer. Manually, as a rule, simple drawings are made, more complex circuit board designs are developed on a computer in special applications.
Most often, a simple Sprint Layout program is used. Only a laser printer is suitable for printing. The paper must be glossy. The main thing is that the toner does not eat into, but remains on top. The printer must be set up so that the drawing toner thickness is at its maximum.
The industrial production of printed circuit boards begins with the input of the circuit diagram of the device into a computer-aided design system, which creates a drawing of the future board.
Preparing the workpiece and drilling holes
First of all, you need to cut a piece of textolite with the specified dimensions. Finish the edges with a file. Attach the drawing to the board. Prepare the drilling tool. Drill directly according to the drawing. The drill must be of good quality and match the diameter of the smallest hole. If possible, a drilling machine should be used.
Having made all the necessary holes, take a drawing and drill each hole to the specified diameter. Clean the surface of the board with fine sandpaper. This is necessary to eliminate burrs and to improve the adhesion of the paint to the board. To remove traces of fat, treat the board with alcohol.
Drawing a picture on fiberglass
The drawing of the board on the textolite can be applied manually or using one of many technologies. The most popular laser-ironing technology.
Drawing by hand begins with the designation of mounting sites around the holes. They are applied with a drawer or a match. The holes are connected by tracks in accordance with the drawing. It is better to draw with nitro paint, in which rosin is dissolved. This solution provides strong adhesion to the board and good resistance to high temperature etching. As a paint, you can use asphalt-bitumen varnish.
The production of printed circuit boards using laser ironing technology gives good results. It is important to perform all operations correctly and accurately. The degreased board must be placed on a flat surface with the copper up. From above, carefully place the drawing with the toner down. In addition, put a few more sheets of paper. Iron the resulting design with a hot iron for about 30-40 seconds. Under the influence of temperature, the toner should change from a solid to a viscous state, but not to a liquid. Let the board cool down and place it for a few minutes in warm water.
The paper will sag and peel off easily. You should carefully examine the resulting drawing. The absence of individual tracks indicates an insufficient temperature of the iron, wide tracks are obtained when the iron is too hot or the board is heated for an excessively long time.
Small defects can be corrected with a marker, paint or nail polish. If you don’t like the workpiece, then you need to wash everything off with a solvent, clean it with sandpaper and repeat the process again.
Etching
A degreased printed circuit board is placed in a plastic container with a solution. At home, ferric chloride is usually used as a solution. The bath with it needs to be shaken periodically. After 25-30 minutes, the copper will completely dissolve. Etching can be accelerated by using a heated ferric chloride solution. At the end of the process, the printed circuit board is removed from the bath, thoroughly washed with water. Then the paint is removed from the conductive tracks.
Tinning
There are many ways to tin. We have a printed circuit board ready. At home, as a rule, there are no special devices and alloys. Therefore, they use a simple reliable method. The board is covered with flux and tinned with a soldering iron with ordinary solder using a copper braid.
Installation of radioelements
At the final stage, the radio components are alternately inserted into the places intended for them and soldered. The legs of the parts must be fluxed before soldering and, if necessary, shortened.
The soldering iron should be used carefully: with excess heat, the copper foil may begin to peel off, the printed circuit board will be damaged. Remove rosin residues with alcohol or acetone. The finished board can be varnished.
Industrial development
At home, it is impossible to design and manufacture a printed circuit board for high-end equipment. For example, the printed circuit board of an amplifier for high-end equipment is multilayer, copper conductors are coated with gold and palladium, conductive tracks have different thicknesses, etc. It is not easy to achieve this level of technology even in an industrial enterprise. Therefore, in some cases it is advisable to purchase a ready-made high-quality board or place an order for work to be performed according to your own scheme. At present, the production of printed circuit boards has been established at many domestic enterprises and abroad.
Tahiti! .. Tahiti! ..
We have not been to any Tahiti!
We are well fed here!
© Cartoon cat
Introduction with digression
How were boards made before in domestic and laboratory conditions? There were several ways - for example:
- drew future conductors with penguins;
- engraved and cut with cutters;
- they glued adhesive tape or electrical tape, then the drawing was cut out with a scalpel;
- the simplest stencils were made, followed by drawing with an airbrush.
The missing elements were drawn with drawing pen and retouched with a scalpel.
It was a long and laborious process, requiring remarkable artistic abilities and accuracy from the “drawer”. The thickness of the lines hardly fit into 0.8 mm, there was no repetition accuracy, each board had to be drawn separately, which greatly hindered the release of even a very small batch printed circuit boards(hereinafter - PP).
What do we have today?
Progress does not stand still. The times when radio amateurs painted PP with stone axes on mammoth skins have sunk into oblivion. The appearance on the market of publicly available chemistry for photolithography opens up completely different prospects for the production of PP without metallizing holes at home.
Let's take a quick look at the chemistry used to make PP today.
Photoresist
You can use liquid or film. Film in this article will not be considered due to its scarcity, the difficulties of rolling to the PCB and the lower quality of the printed circuit boards obtained at the output.
After analyzing market offers, I settled on POSITIV 20 as the optimal photoresist for home PCB production.
Purpose:
POSITIV 20 is a photosensitive varnish. It is used in small-scale production of printed circuit boards, engravings on copper, when carrying out work related to the transfer of images to various materials.
Properties:
High exposure characteristics ensure good contrast of transferred images.
Application:
It is used in areas related to the transfer of images to glass, plastics, metals, etc. in small-scale production. The method of application is indicated on the bottle.
Characteristics:
Color: blue
Density: at 20°C 0.87 g/cm3
Drying time: at 70°C 15 min.
Consumption: 15 l/m2
Maximum photosensitivity: 310-440nm
The instructions for the photoresist say that it can be stored at room temperature and it is not subject to aging. Strongly disagree! You need to store it in a cool place, for example, on the bottom shelf of the refrigerator, where the temperature is usually maintained at + 2 ... + 6 ° C. But in no case do not allow negative temperatures!
If you use photoresists that are sold "in bulk" and do not have opaque packaging, care must be taken to protect from light. It is necessary to store in complete darkness and at a temperature of +2 ... + 6 ° C.
Enlightener
Similarly, I find TRANSPARENT 21, which I use all the time, to be the most suitable illuminator.
Purpose:
Allows direct transfer of images onto surfaces coated with POSITIV 20 photosensitive emulsion or other photoresist.
Properties:
Gives transparency to paper. Provides UV light transmission.
Application:
For quick transfer of contours of drawings and diagrams to the substrate. Allows you to significantly simplify the process of reproduction and reduce time s e costs.
Characteristics:
Color: transparent
Density: at 20°C 0.79 g/cm3
Drying time: at 20°C 30 min.
Note:
Instead of plain paper with an illuminator, you can use a transparent film for inkjet or laser printers, depending on what we will print the photomask on.
Photoresist Developer
There are many different solutions for developing photoresist.
It is advised to develop with a solution of "liquid glass". Its chemical composition: Na 2 SiO 3 * 5H 2 O. This substance has a huge number of advantages. The most important thing is that it is very difficult to overexpose PP in it - you can leave PP for a non-fixed time. The solution almost does not change its properties with temperature changes (there is no risk of decomposition with increasing temperature), it also has a very long shelf life - its concentration remains constant for at least a couple of years. The absence of the problem of overexposure in the solution will make it possible to increase its concentration in order to reduce the time of manifestation of PP. It is recommended to mix 1 part concentrate with 180 parts water (a little over 1.7 g of silicate in 200 ml of water), but it is possible to make the mixture more concentrated so that the image develops in about 5 seconds without the risk of surface damage due to overexposure. If it is not possible to purchase sodium silicate, use sodium carbonate (Na 2 CO 3) or potassium carbonate (K 2 CO 3).
I have not tried either the first or the second, so I will tell you what I have been showing without any problems for several years now. I use an aqueous solution of caustic soda. For 1 liter of cold water - 7 grams of caustic soda. If there is no NaOH, I use a KOH solution, doubling the concentration of alkali in the solution. The development time is 30-60 seconds with the correct exposure. If, after 2 minutes, the pattern does not appear (or appears weakly), and the photoresist begins to wash off from the workpiece, it means that the exposure time has been chosen incorrectly: you need to increase it. If, on the contrary, it quickly appears, but both the illuminated and unexposed areas are washed off, either the concentration of the solution is too high or the quality of the photomask is low (ultraviolet passes freely through the “black”): you need to increase the print density of the template.
Copper pickling solutions
Excess copper from printed circuit boards is etched using various etchants. Among people doing this at home, ammonium persulfate, hydrogen peroxide + hydrochloric acid, copper sulfate solution + table salt are often common.
I always poison with ferric chloride in glassware. When working with the solution, you need to be careful and attentive: if it gets on clothes and objects, rusty spots remain, which are difficult to remove with a weak solution of citric (lemon juice) or oxalic acid.
We heat the concentrated solution of ferric chloride to 50-60 ° C, immerse the workpiece in it, gently and effortlessly drive the glass rod with a cotton swab at the end over areas where copper is worse etched - this achieves more even etching over the entire area of the PCB. If the speed is not forced to equalize, the required duration of etching increases, and this eventually leads to the fact that in areas where copper has already been etched, etching of the tracks begins. As a result, we do not have what we wanted to get. It is highly desirable to provide continuous mixing of the pickling solution.
Chemistry for removing photoresist
What is the easiest way to wash off the already unnecessary photoresist after etching? After repeated trial and error, I settled on ordinary acetone. When it is not there, I wash it off with any solvent for nitro paints.
So, we make a printed circuit board
Where does a high quality PCB start? Correctly:
Creating a high quality photomask
For its manufacture, you can use almost any modern laser or inkjet printer. Given that we are using a positive photoresist in this article, where copper should remain on the PCB, the printer should draw black. Where there should be no copper, the printer should not draw anything. A very important point when printing a photomask: you need to set the maximum dye watering (in the printer driver settings). The more black the shaded areas are, the more likely you are to get a great result. Color is not needed, a black cartridge is enough. From that program (we will not consider programs: everyone is free to choose for himself - from PCAD to Paintbrush), in which the photomask was drawn, we print on a regular sheet of paper. The higher the resolution when printing and the better the paper, the higher the quality of the photomask will be. I recommend at least 600 dpi, the paper should not be very thick. When printing, we take into account that the side of the sheet on which the paint is applied, the template will be placed on the PP blank. If done otherwise, the edges of the PCB conductors will be blurry, fuzzy. Let the paint dry if it was an inkjet printer. Next, we impregnate TRANSPARENT 21 paper, let it dry and ... the photomask is ready.
Instead of paper and an illuminator, it is possible and even very desirable to use a transparent film for laser (when printing on a laser printer) or inkjet (for inkjet printing) printers. Please note that these films have unequal sides: only one working. If you're using laser printing, I highly recommend doing a "dry run" of a sheet of film before printing - just run the sheet through the printer, simulating printing, but not printing anything. Why is this needed? When printing, the fuser (oven) will heat up the sheet, which will inevitably lead to its deformation. As a result - an error in the geometry of the PP at the output. In the manufacture of double-sided PP, this is fraught with a mismatch of layers with all the consequences ... And with the help of a “dry” run, we will warm up the sheet, it will deform and be ready for printing a template. When printing, the sheet will pass through the oven for the second time, but the deformation will be much less significant - it has been tested repeatedly.
If the PCB is simple, you can draw it manually in a very convenient program with a Russified interface - Sprint Layout 3.0R (~650 KB).
At the preparatory stage, it is very convenient to draw not too bulky electrical circuits in the also Russified sPlan 4.0 program (~ 450 KB).
This is how ready-made photomasks printed on an Epson Stylus Color 740 printer look like:
We print only in black, with the maximum watering of the dye. Material - transparent film for inkjet printers.
Preparing the PCB surface for photoresist application
For the production of PP, sheet materials with applied copper foil are used. The most common options are with a copper thickness of 18 and 35 microns. Most often, for the production of PP at home, sheet textolite (a fabric pressed with glue in several layers), fiberglass (the same thing, but epoxy compounds are used as glue) and getinax (pressed paper with glue) are used. Less often - sittal and polycor (high-frequency ceramics - used extremely rarely at home), fluoroplastic (organic plastic). The latter is also used for the manufacture of high-frequency devices and, having very good electrical characteristics, can be used anywhere and everywhere, but its use is limited by a high price.
First of all, you need to make sure that the workpiece does not have deep scratches, burrs and areas affected by corrosion. Next, it is desirable to polish the copper to a mirror. We polish without being particularly zealous, otherwise we will erase the already thin layer of copper (35 microns) or, in any case, we will achieve different thicknesses of copper on the surface of the workpiece. And this, in turn, will lead to a different etching speed: it is etched faster where it is thinner. And a thinner conductor on the board is not always good. Especially if it is long and a decent current will flow through it. If the copper on the workpiece is of high quality, without sins, then it is enough to degrease the surface.
Deposition of photoresist on the surface of the workpiece
We place the board on a horizontal or slightly inclined surface and apply the composition from an aerosol package from a distance of about 20 cm. Remember that the most important enemy in this case is dust. Every particle of dust on the surface of the workpiece is a source of problems. To create a uniform coating, spray the spray in a continuous zigzag motion, starting from the top left corner. Do not overspray as this causes unwanted streaks and results in uneven coating thickness requiring longer exposure times. In summer, high ambient temperatures may require re-treatment, or spraying from a shorter distance may be necessary to reduce evaporative losses. When spraying, do not tilt the can strongly - this leads to an increased consumption of propellant gas and, as a result, the aerosol can stops working, although there is still photoresist in it. If you get unsatisfactory results with spray coating of photoresist, use spin coating. In this case, the photoresist is applied to a board mounted on a rotating table with a drive of 300-1000 rpm. After finishing the coating, the board should not be exposed to strong light. By the color of the coating, you can approximately determine the thickness of the applied layer:
- light gray blue - 1-3 microns;
- dark gray blue - 3-6 microns;
- blue - 6-8 microns;
- dark blue - more than 8 microns.
On copper, the color of the coating may have a greenish tinge.
The thinner the coating on the workpiece, the better the result.
I always apply photoresist on a centrifuge. In my centrifuge, the rotation speed is 500-600 rpm. Fastening should be simple, clamping is done only at the ends of the workpiece. We fix the workpiece, start the centrifuge, spray on the center of the workpiece and observe how the photoresist spreads over the surface in a thin layer. By centrifugal forces, excess photoresist will be thrown off the future PP, so I highly recommend providing a protective wall so as not to turn the workplace into a pigsty. I use an ordinary pan, in the bottom of which a hole is made in the center. The axis of the electric motor passes through this hole, on which a mounting platform in the form of a cross of two aluminum rails is installed, along which the ears of the workpiece clamp “run”. The ears are made of aluminum corners clamped on the rail with a wing nut. Why aluminum? Small specific gravity and, as a result, less runout when the center of mass of rotation deviates from the center of rotation of the centrifuge axis. The more precisely the workpiece is centered, the less beating will be due to the eccentricity of the mass and the less effort will be required to rigidly fasten the centrifuge to the base.
Photoresist applied. Let it dry for 15-20 minutes, turn the workpiece over, apply a layer on the second side. We give another 15-20 minutes to dry. Do not forget that direct sunlight and fingers on the working sides of the workpiece are unacceptable.
Tanning of photoresist on the workpiece surface
We place the workpiece in the oven, gradually bring the temperature to 60-70 ° C. At this temperature we maintain 20-40 minutes. It is important that nothing touches the surfaces of the workpiece - only touches of the ends are allowed.
Alignment of the upper and lower photomasks on the surfaces of the workpiece
On each of the photomasks (upper and lower) there should be marks, according to which 2 holes must be made on the workpiece - to match the layers. The farther apart the marks, the higher the alignment accuracy. I usually place them diagonally across the templates. Using these marks on the workpiece, using a drilling machine, we drill two holes strictly at 90 ° (the thinner the holes, the more accurate the alignment - I use a 0.3 mm drill) and combine the templates along them, not forgetting that the template must be applied to the photoresist the side that was printed on. We press the templates to the workpiece with thin glasses. It is preferable to use quartz glasses - they transmit ultraviolet better. Plexiglas (plexiglass) gives even better results, but it has an unpleasant scratching property, which will inevitably affect the quality of the PP. For small PCB sizes, you can use a transparent cover from the CD packaging. In the absence of such glasses, ordinary window glass can also be used, increasing the exposure time. It is important that the glass is even, ensuring that the photomasks fit evenly on the workpiece, otherwise it will not be possible to obtain high-quality track edges on the finished PCB.
A blank with a photomask under plexiglass. We use the box from under the CD.
Exposure (flare)
The time required for exposure depends on the thickness of the photoresist layer and the intensity of the light source. POSITIV 20 photoresist lacquer is sensitive to ultraviolet rays, the maximum sensitivity falls on the area with a wavelength of 360-410 nm.
It is best to expose under lamps whose radiation range is in the ultraviolet region of the spectrum, but if you do not have such a lamp, you can also use ordinary powerful incandescent lamps by increasing the exposure time. Do not start illumination until the illumination from the source stabilizes - it is necessary that the lamp warms up for 2-3 minutes. The exposure time depends on the thickness of the coating and is usually 60-120 seconds when the light source is located at a distance of 25-30 cm. The glass plates used can absorb up to 65% of ultraviolet, so in such cases it is necessary to increase the exposure time. Best results are achieved with transparent plexiglass plates. When using photoresist with a long shelf life, the exposure time may need to be doubled - remember: photoresists are subject to aging!
Examples of using different light sources:
UV lamps
We expose each side in turn, after the exposure we let the blank stand for 20-30 minutes in a dark place.
Development of the exposed workpiece
We develop in a solution of NaOH (caustic soda) - see the beginning of the article for details - at a solution temperature of 20-25 ° C. If there is no manifestation up to 2 minutes - small about exposure time. If it appears well, but useful areas are also washed off - you are too smart with the solution (the concentration is too high) or the exposure time is too long with this radiation source or the photomask is of poor quality - insufficiently saturated printed black color allows ultraviolet light to illuminate the workpiece.
When developing, I always very carefully, without effort, “roll” a cotton swab on a glass rod in those places where the exposed photoresist should be washed off - this speeds up the process.
Washing the workpiece from alkali and residues of exfoliated exposed photoresist
I do this under a faucet—ordinary tap water.
Retanning photoresist
We place the workpiece in the oven, gradually raise the temperature and hold at a temperature of 60-100 ° C for 60-120 minutes - the pattern becomes strong and solid.
Checking the development quality
For a short time (for 5-15 seconds) we immerse the workpiece in a solution of ferric chloride heated to a temperature of 50-60 ° C. Rinse quickly with running water. In places where there is no photoresist, intensive etching of copper begins. If a photoresist is accidentally left somewhere, carefully mechanically remove it. It is convenient to do this with a conventional or ophthalmic scalpel, armed with optics (soldering glasses, loupes a watchmaker, loop a on a tripod, microscope).
Etching
We pickle in a concentrated solution of ferric chloride with a temperature of 50-60°C. It is desirable to ensure continuous circulation of the pickling solution. We gently “massage” badly etched places with a cotton swab on a glass rod. If the ferric chloride is freshly prepared, the pickling time usually does not exceed 5-6 minutes. We wash the workpiece with running water.
Board etched
How to prepare a concentrated solution of ferric chloride? We dissolve FeCl 3 in slightly (up to 40 ° C) heated water until it ceases to dissolve. Filter the solution. You need to store in a dark, cool place in a sealed non-metallic package - in glass bottles, for example.
Removing unwanted photoresist
We wash off the photoresist from the tracks with acetone or a solvent for nitro-paints and nitro-enamels.
Hole drilling
It is advisable to select the diameter of the point of the future hole on the photomask in such a way that it would be convenient to drill later. For example, with the required hole diameter of 0.6-0.8 mm, the dot diameter on the photomask should be about 0.4-0.5 mm - in this case, the drill will be well centered.
It is advisable to use tungsten carbide-coated drills: HSS drills wear out very quickly, although steel can be used to drill large diameter single holes (more than 2 mm), since tungsten carbide-coated drills of this diameter are too expensive. When drilling holes with a diameter of less than 1 mm, it is better to use a vertical machine, otherwise your drills will break quickly. If you drill with a hand drill, distortions are inevitable, leading to inaccurate joining of holes between layers. The downward movement on a vertical drilling machine is the most optimal in terms of tool loading. Carbide drills are made with a rigid (i.e., the drill exactly fits the diameter of the hole) or thick (sometimes called "turbo") shank, having a standard size (usually 3.5 mm). When drilling with carbide-coated drills, it is important to firmly fix the PCB, since such a drill, when moving up, can lift the PCB, skew the perpendicularity and tear out a piece of the board.
Small diameter drills are usually inserted into either a collet chuck (various sizes) or a three-jaw chuck. For precise fixing, a three-jaw chuck is not the best option, and a small drill size (less than 1 mm) quickly grooves in the clamps, losing a good hold. Therefore, for drills with a diameter of less than 1 mm, it is better to use a collet chuck. Just in case, get an extra set containing spare collets for each size. Some inexpensive drills are made with plastic collets - throw them away and buy metal ones.
To obtain acceptable accuracy, it is necessary to properly organize the workplace, that is, firstly, to ensure good lighting of the board when drilling. To do this, you can use a halogen lamp, attaching it to a tripod to be able to choose a position (illuminate the right side). Secondly, raise the work surface about 15 cm above the countertop for better visual control over the process. It would be nice to remove dust and chips during the drilling process (you can use a regular vacuum cleaner), but this is not necessary. It should be noted that the dust from fiberglass generated during drilling is very caustic and, if it comes into contact with the skin, causes skin irritation. And finally, when working, it is very convenient to use the foot switch of the drilling machine.
Typical hole sizes:
- vias - 0.8 mm or less;
- integrated circuits, resistors, etc. - 0.7-0.8 mm;
- large diodes (1N4001) - 1.0 mm;
- contact pads, trimmers - up to 1.5 mm.
Try to avoid holes with a diameter of less than 0.7 mm. Always keep at least two spare drills 0.8 mm or less, as they always break just at the moment when you urgently need to order. Drills 1mm and larger are much more reliable, although it would be nice to have spare ones for them. When you need to make two identical boards, you can drill them at the same time to save time. In this case, it is necessary to very carefully drill holes in the center of the pad near each corner of the PCB, and for large boards, holes located close to the center. Lay the boards on top of each other and, using the 0.3mm centering holes in two opposite corners and the pins as pegs, secure the boards against each other.
If necessary, you can countersink holes with drills of a larger diameter.
Copper tinning on PP
If you need to irradiate the tracks on the PCB, you can use a soldering iron, soft low-melting solder, alcohol-rosin flux and coaxial cable braid. With large volumes, they are tinned in bathtubs filled with low-temperature solders with the addition of fluxes.
The most popular and simple melt for tinning is the low-melting alloy "Rose" (tin - 25%, lead - 25%, bismuth - 50%), the melting point of which is 93-96 ° C. The board is placed with tongs under the level of the liquid melt for 5-10 seconds and, having taken it out, it is checked whether the entire copper surface is covered evenly. If necessary, the operation is repeated. Immediately after removing the board from the melt, its remains are removed either with a rubber squeegee or by sharp shaking in a direction perpendicular to the plane of the board, while holding it in the clamp. Another way to remove residues of the Rose alloy is to heat the board in an oven and shake it. The operation can be repeated to achieve a mono-thick coating. To prevent oxidation of the hot melt, glycerin is added to the tinning tank so that its level covers the melt by 10 mm. After the end of the process, the board is washed from glycerin in running water. Attention! These operations involve working with installations and materials that are under the influence of high temperature, therefore, to prevent burns, it is necessary to use protective gloves, goggles and aprons.
The tin-lead tinning operation proceeds similarly, but the higher melt temperature limits the scope of this method in handicraft production.
Do not forget to clean the board from flux after tinning and degrease thoroughly.
If you have a large production, you can use chemical tinning.
Applying a protective mask
The operations with applying a protective mask exactly repeat everything that was written above: we apply a photoresist, dry, tan, center the photomasks of the masks, expose, develop, wash and tan again. Of course, we skip the steps with checking the quality of development, etching, removing photoresist, tinning and drilling. At the very end, we tan the mask for 2 hours at a temperature of about 90-100 ° C - it will become strong and hard, like glass. The formed mask protects the surface of the PCB from external influences and protects against theoretically possible short circuits during operation. It also plays an important role in automatic soldering - it does not allow the solder to “sit down” on neighboring sections, closing them.
That's it, the double-sided printed circuit board with the mask is ready.
I had to make PP in this way with the width of the tracks and the step between them up to 0.05 mm (!). But this is a piece of jewelry. And without much effort, you can make PP with a track width and a step between them of 0.15-0.2 mm.
I did not apply a mask to the board shown in the photographs - there was no such need.
Printed circuit board in the process of mounting components on it
And here is the device itself, for which the software was made:
This is a cellular telephone bridge that allows you to reduce the cost of mobile services by 2-10 times - for this it was worth fiddling with PP;). The PCB with soldered components is in the stand. Previously, there was an ordinary charger for mobile phone batteries.
Additional Information
Hole plating
At home, you can even metallize holes. To do this, the inner surface of the holes is treated with a 20-30% solution of silver nitrate (lapis). Then the surface is cleaned with a squeegee and the board is dried in the light (you can use a UV lamp). The essence of this operation is that under the action of light, silver nitrate decomposes, and inclusions of silver remain on the board. Next, copper is chemically precipitated from the solution: copper sulfate (copper sulfate) - 2 g, sodium hydroxide - 4 g, ammonia 25% - 1 ml, glycerin - 3.5 ml, formalin 10% - 8-15 ml, water - 100 ml. The shelf life of the prepared solution is very short - you need to prepare immediately before use. After the copper is deposited, the board is washed and dried. The layer is obtained very thin, its thickness must be increased to 50 microns by galvanizing.
Electroplating solution for copper plating:
For 1 liter of water, 250 g of copper sulfate (copper sulfate) and 50-80 g of concentrated sulfuric acid. The anode is a copper plate suspended parallel to the part to be coated. The voltage should be 3-4 V, current density - 0.02-0.3 A / cm 2, temperature - 18-30 ° C. The lower the current, the slower the metallization process, but the better the resulting coating.
Fragment of the printed circuit board, where the metallization is visible in the hole
Homemade photoresists
Photoresist based on gelatin and potassium bichromate:
First solution: pour 15 g of gelatin into 60 ml of boiled water and leave to swell for 2-3 hours. After swelling of the gelatin, place the container in a water bath at a temperature of 30-40 ° C until the gelatin is completely dissolved.
The second solution: in 40 ml of boiled water, dissolve 5 g of potassium dichromate (chromic peak, bright orange powder). Dissolve in low ambient light.
Pour the second into the first solution with vigorous stirring. Add a few drops of ammonia to the resulting mixture with a pipette until a straw color is obtained. The photographic emulsion is applied to the prepared board in very low light. The board dries to "tack" at room temperature in complete darkness. After exposure, wash the board under low ambient light in warm running water until untanned gelatin is removed. To better evaluate the result, you can stain areas with unremoved gelatin with a solution of potassium permanganate.
Advanced Homemade Photoresist:
First solution: 17 g of wood glue, 3 ml of an aqueous solution of ammonia, 100 ml of water, leave to swell for a day, then heat in a water bath at 80 ° C until completely dissolved.
Second solution: 2.5 g potassium dichromate, 2.5 g ammonium dichromate, 3 ml aqueous ammonia solution, 30 ml water, 6 ml alcohol.
When the first solution has cooled to 50°C, pour the second solution into it with vigorous stirring and filter the resulting mixture ( this and subsequent operations must be carried out in a darkened room, sunlight is unacceptable!). The emulsion is applied at a temperature of 30-40°C. Further - as in the first recipe.
Photoresist based on ammonium dichromate and polyvinyl alcohol:
We prepare the solution: polyvinyl alcohol - 70-120 g / l, ammonium bichromate - 8-10 g / l, ethyl alcohol - 100-120 g / l. Avoid bright light! It is applied in 2 layers: the first layer - drying for 20-30 minutes at 30-45°C - the second layer - drying for 60 minutes at 35-45°C. The developer is a 40% solution of ethyl alcohol.
Chemical tinning
First of all, the board must be decapitated in order to remove the formed copper oxide: 2-3 seconds in a 5% hydrochloric acid solution, followed by rinsing in running water.
It is enough to simply carry out chemical tinning by immersing the board in an aqueous solution containing tin chloride. The release of tin on the surface of the copper coating occurs when immersed in a solution of tin salt, in which the copper potential is more electronegative than the coating material. A change in the potential in the desired direction is facilitated by the introduction of a complexing additive, thiocarbamide (thiourea), into the tin salt solution. Solutions of this type have the following composition (g/l):
Among the listed solutions, solutions 1 and 2 are the most common. Sometimes, as a surfactant for the 1st solution, it is proposed to use Progress detergent in an amount of 1 ml / l. The addition of 2-3 g/l of bismuth nitrate to the 2nd solution leads to the precipitation of an alloy containing up to 1.5% bismuth, which improves the solderability of the coating (prevents aging) and greatly increases the shelf life before soldering the components of the finished PP.
To preserve the surface, aerosol sprays based on fluxing compositions are used. After drying, the varnish applied to the surface of the workpiece forms a strong, smooth film that prevents oxidation. One of the popular substances is "SOLDERLAC" from Cramolin. Subsequent soldering is carried out directly on the treated surface without additional varnish removal. In especially critical cases of soldering, the varnish can be removed with an alcohol solution.
Artificial tinning solutions deteriorate over time, especially when exposed to air. Therefore, if you do not often have large orders, then try to immediately prepare a small amount of mortar, sufficient to tin the required amount of PP, and store the rest of the mortar in a closed container (bottles like those used in photographs that do not let air through are ideal). It is also necessary to protect the solution from contamination, which can greatly degrade the quality of the substance.
In conclusion, I want to say that it is still better to use ready-made photoresists and not bother with metallizing holes at home - you still won’t get great results.
Many thanks to the candidate of chemical sciences Filatov Igor Evgenievich for advice on chemistry-related matters.
I also want to express my gratitude Igor Chudakov.