How to make a printed circuit board with your own hands: laser ironing technology (LUT) at home. Paper for making printed circuit boards using LUT technology or how to make a printed circuit board at home Make a printed circuit board at home

Tahiti!.. Tahiti!..
We have not been to any Tahiti!
They feed us well here too!
© Cartoon cat

Introduction with digression

How were boards made in the past in domestic and laboratory conditions? There were several ways, for example:

1. future conductors drew drawings;
2. engraved and cut with cutters;
3. they glued it with adhesive tape or tape, then cut out the design with a scalpel;
4. They made simple stencils and then applied the design using an airbrush.

The missing elements were completed with drawing pens and retouched with a scalpel.

It was a long and laborious process, requiring the “drawer” to have remarkable artistic abilities and accuracy. 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 limited the production of even a very small batch printed circuit boards(further 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 PCB without metallization of holes at home.

Let's take a quick look at the chemistry used today to produce PP.

Photoresist

You can use liquid or film. We will not consider film in this article due to its scarcity, difficulties in rolling onto PCBs and the lower quality of the resulting printed circuit boards.

After analyzing market offers, I settled on POSITIV 20 as the optimal photoresist for home production PP.

Purpose:
POSITIV 20 photosensitive varnish. Used in small-scale production of printed circuit boards, copper engravings, and when carrying out work related to transferring images to various materials.
Properties:
High exposure characteristics provide good contrast of transferred images.
Application:
It is used in areas related to the transfer of images onto glass, plastics, metals, etc. in small-scale production. Directions for use are indicated on the bottle.
Characteristics:
Color: blue
Density: at 20°C 0.87 g/cm 3
Drying time: at 70°C 15 min.
Consumption: 15 l/m2
Maximum photosensitivity: 310-440 nm

The instructions for the photoresist say that it can be stored at room temperature and is not subject to aging. I strongly disagree! It should be stored in a cool place, for example, on the bottom shelf of the refrigerator, where the temperature is usually maintained at +2+6°C. But under no circumstances allow negative temperatures!

If you use photoresists that are sold by the glass and do not have lightproof packaging, you need to take care of protection from light. It should be stored in complete darkness and at a temperature of +2+6°C.

Enlightener

Likewise, I consider TRANSPARENT 21, which I constantly use, to be the most suitable educational tool.

Purpose:
Allows direct transfer of images onto surfaces coated with photosensitive emulsion POSITIV 20 or other photoresist.
Properties:
Gives transparency to paper. Provides transmission of ultraviolet rays.
Application:
For quickly transferring the outlines of drawings and diagrams onto a substrate. Allows you to significantly simplify the reproduction process and reduce time s e costs.
Characteristics:
Color: transparent
Density: at 20°C 0.79 g/cm 3
Drying time: at 20°C 30 min.
Note:
Instead of regular paper with transparency, you can use 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 using a solution " liquid glass" His 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 the PP in it; you can leave the PP for a non-fixed exact time. The solution almost does not change its properties with temperature changes (there is no risk of disintegration with increasing temperature), it also has a very long term storage its concentration remains constant for at least a couple of years. The absence of the problem of overexposure in the solution will allow increasing its concentration to reduce the time of development of PP. It is recommended to mix 1 part concentrate with 180 parts water (just over 1.7 g of silicate in 200 ml of water), but it is possible to make a more concentrated mixture so that the image develops in about 5 seconds without the risk of surface damage due to overexposure. If it is impossible to purchase sodium silicate, use sodium carbonate (Na 2 CO 3) or potassium carbonate (K 2 CO 3).

I haven’t tried either the first or the second, so I’ll tell you what I’ve been using without any problems for several years now. I use a water solution of caustic soda. For 1 liter 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. Development time 30-60 seconds with correct exposure. If after 2 minutes the pattern does not appear (or appears weakly), and the photoresist begins to wash off from the workpiece, this means that the exposure time was chosen incorrectly: you need to increase it. If, on the contrary, it quickly appears, but both exposed and unexposed areas are washed away; either the concentration of the solution is too high, or the quality of the photomask is low (ultraviolet light passes freely through the “black”): you need to increase the print density of the template.

Copper etching solutions

Excess copper is removed from printed circuit boards 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 glassware. When working with the solution, you need to be careful and attentive: if it gets on clothes and objects, it leaves rusty stains that are difficult to remove with a weak solution of citric (lemon juice) or oxalic acid.

We heat a concentrated solution of ferric chloride to 50-60°C, immerse the workpiece in it, and carefully and effortlessly move a glass rod with a cotton swab at the end over areas where copper is etched less easily, this achieves a more even etching over the entire area of ​​the PP. If you do not force the speed to equalize, the required etching duration 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 don’t get what we wanted at all. It is highly desirable to ensure continuous mixing pickling solution.

Chemicals for removing photoresist

What is the easiest way to wash off unnecessary photoresist after etching? After repeated trial and error, I settled on ordinary acetone. When it’s not there, I wash it off with any solvent for nitro paints.

So, let's make a printed circuit board

Where does a high quality PCB start? Right:

Create a high-quality photo template

To make it, you can use almost any modern laser or inkjet printer. Considering that we are using positive photoresist in this article, the printer should draw black where copper should remain on the PCB. Where there should be no copper the printer should not draw anything. Very important point when printing a photomask: you need to set the maximum dye flow (in the printer driver settings). The blacker the painted areas, the greater the chances of getting a great result. No color is needed, a black cartridge is enough. From the program (we will not consider programs: everyone is free to choose for themselves - from PCAD to Paintbrush) in which the photo template was drawn, we print it on a regular sheet of paper. The higher the printing resolution and the higher quality the paper, the higher the quality of the photomask. I recommend no lower than 600 dpi; the paper should not be very thick. When printing, we take into account that with the side of the sheet on which the paint is applied, the template will be placed on the PP blank. If done differently, the edges of the PP conductors will be blurred and indistinct. Let the paint dry if it was an inkjet printer. Next, we impregnate the paper with TRANSPARENT 21, let it dry and the photo template is ready.

Instead of paper and enlightenment, it is possible and even very desirable to use 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 side. If you use laser printing, I highly recommend doing a “dry” run of the film sheet before printing simply run the sheet through the printer, simulating printing, but not printing anything. Why is this necessary? When printing, the fuser (oven) will heat the sheet, which will inevitably lead to its deformation. As a consequence, there is an error in the geometry of the output PCB. When producing double-sided PCBs, 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 be deformed and will be ready for printing the template. When printing, the sheet will pass through the oven a second time, but the deformation will be much less significant checked several times.

If the PP is simple, you can draw it manually in a very convenient program with a Russified interface Sprint Layout 3.0R (~650 KB).

On preparatory stage draw not too bulky electrical circuits very convenient in the also Russified program sPlan 4.0 (~450 KB).

This is what the finished photo templates look like, printed on an Epson Stylus Color 740 printer:

We print only in black, with maximum dye addition. Material transparent film for inkjet printers.

Preparing the PP surface for applying photoresist

For the production of PP they are used sheet materials with applied copper foil. The most common options are with copper thickness of 18 and 35 microns. Most often, for the production of PP at home, sheet textolite (fabric pressed with glue in several layers), fiberglass (the same, but epoxy compounds are used as glue) and getinax (pressed paper with glue) are used. Less commonly, sittal and polycor (high-frequency ceramics are 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 its high price.

First of all, you need to make sure that the workpiece does not have deep scratches, scuffs and corroded areas. Next, it is advisable 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 different speeds etching: it will etch 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.

Applying photoresist to 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. We 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 aerosol in a continuous zigzag motion, starting from the upper left corner. Do not use the aerosol in excess quantities, as this will cause unwanted smudges and lead to the formation of a non-uniform coating thickness, requiring a longer exposure time. In summer at high temperatures environment Re-treatment may be necessary, or the aerosol may need to be sprayed from a shorter distance to reduce evaporation losses. When spraying, do not tilt the container too much this leads to increased consumption of propellant gas and, as a result, aerosol can stops working, although there is still photoresist in it. If you are getting unsatisfactory results when spray coating photoresist, use spin coating. In this case, photoresist is applied to a board mounted on a rotating table with a 300-1000 rpm drive. After finishing coating, the board should not be exposed to strong light. Based on 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 coating color may have a greenish tint.

The thinner the coating on the workpiece, the better the result.

I always spin coat the photoresist. My centrifuge has a rotation speed of 500-600 rpm. Fastening should be simple, clamping is carried out only at the ends of the workpiece. We fix the workpiece, start the centrifuge, spray it on the center of the workpiece and watch how the photoresist spreads over the surface in a thin layer. Centrifugal forces will throw off excess photoresist from the future PCB, so I highly recommend providing a protective wall so as not to turn workplace to the pigsty. I use an ordinary saucepan with a hole in the bottom in the center. The axis of the electric motor passes through this hole, on which a mounting platform is installed in the form of a cross of two aluminum slats, along which the workpiece clamping ears “run.” Ears are made of aluminum corners, clamped onto the rail with a wing nut. Why aluminum? Low 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 accurately the workpiece is centered, the less beating will occur due to the eccentricity of the mass and the less effort will be required to rigidly attach the centrifuge to the base.

Photoresist is applied. Let it dry for 15-20 minutes, turn the workpiece over, apply a layer on the other side. 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 photoresist on the surface of the workpiece

Place the workpiece in the oven, gradually bring the temperature to 60-70°C. Maintain at this temperature for 20-40 minutes. It is important that nothing touches the surfaces of the workpiece; only touching the ends is permissible.

Aligning the top and bottom photomasks on the workpiece surfaces

Each of the photo masks (top and bottom) should have marks along which 2 holes need to be made on the workpiece to align the layers. The farther the marks are from each other, the higher the alignment accuracy. I usually place them diagonally on the templates. Using these marks on the workpiece, using a drilling machine, we drill two holes strictly at 90° (than thinner than the hole, the more accurate the alignment (I use a 0.3 mm drill) and we align the templates using them, not forgetting that the template must be applied to the photoresist with the side on which the print was made. We press the templates to the workpiece with thin glasses. It is preferable to use quartz glass as it transmits ultraviolet radiation better. Plexiglas (plexiglass) gives even better results, but it has the unpleasant property of scratching, which will inevitably affect the quality of the PP. At small sizes PP, you can use the transparent cover from the CD packaging. In the absence of such glass, you can use ordinary window glass, increasing the exposure time. It is important that the glass is smooth, ensuring an even fit of the photomasks to the workpiece, otherwise it will be impossible to obtain high-quality edges of the tracks on the finished PCB.

A blank with a photomask under plexiglass. We use a CD box.

Exposure (light exposure)

The time required for exposure depends on the thickness of the photoresist layer and the intensity of the light source. Photoresist varnish POSITIV 20 is sensitive to ultraviolet rays, the maximum sensitivity occurs in 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, increasing the exposure time. Do not start illumination until the lighting from the source has stabilized; it is necessary for the lamp to warm 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 radiation, so in such cases it is necessary to increase the exposure time. top scores achieved using transparent plexiglass plates. When using photoresist with long term During storage, 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 exposure we let the workpiece stand for 20-30 minutes in a dark place.

Development of the exposed workpiece

Develop in NaOH solution ( caustic soda) see the beginning of the article for more details at a solution temperature of 20-25°C. If there is no manifestation within 2 minutes small O exposure time. If it appears well, but useful areas are also washed away, you are too clever with the solution (the concentration is too high) or the exposure time with a given radiation source is too long or the photomask is of poor quality; the printed black color is not saturated enough to allow ultraviolet light to illuminate the workpiece.

When developing, I always very carefully, effortlessly “roll” a cotton swab on a glass rod over the 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 it under water tap ordinary tap water.

Re-tanning photoresist

We place the workpiece in the oven, gradually raise the temperature and hold it at a temperature of 60-100°C for 60-120 minutes; the pattern becomes strong and hard.

Checking the development quality

Briefly (for 5-15 seconds) immerse the workpiece in a ferric chloride solution heated to a temperature of 50-60°C. Rinse quickly with running water. In places where there is no photoresist, intensive etching of the copper begins. If photoresist accidentally remains somewhere, carefully remove it mechanically. It is convenient to do this with a regular or ophthalmic scalpel, armed with optics (soldering glasses, magnifying glass A watchmaker, loupe A on a tripod, microscope).

Etching

We poison in a concentrated solution of ferric chloride at a temperature of 50-60°C. It is advisable to ensure continuous circulation of the etching solution. We carefully “massage” poorly bleeding areas with a cotton swab on a glass rod. If ferric chloride is freshly prepared, the etching time usually does not exceed 5-6 minutes. We rinse the workpiece with running water.

Board etched

How to prepare a concentrated solution of ferric chloride? Dissolve FeCl 3 in slightly (up to 40°C) heated water until it stops dissolving. Filter the solution. Should be stored in a cool, dark place in sealed non-metallic packaging in glass bottles, For example.

Removing unnecessary photoresist

We wash off the photoresist from the tracks with acetone or a solvent for nitro paints and nitro enamels.

Drilling holes

It is advisable to select the diameter of the point of the future hole on the photomask such that it will be convenient to drill later. For example, with a required hole diameter of 0.6-0.8 mm, the diameter of the point 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 drills coated with tungsten carbide: drills made from high-speed steels wear out very quickly, although steel can be used for drilling single holes large diameter(more than 2 mm), since drills coated with tungsten carbide of this diameter are too expensive. When drilling holes with a diameter of less than 1 mm, it is better to use vertical machine, otherwise your drill bits will break quickly. If you drill hand drill distortions are inevitable, leading to inaccurate joining of holes between layers. Movement from top to bottom on vertical drilling machine the most optimal from the point of view of the load on the tool. Carbide drills are made with a rigid (i.e. the drill fits exactly to the hole diameter) or a thick (sometimes called "turbo") shank that has a standard size (usually 3.5 mm). When drilling with carbide-coated drills, it is important to firmly secure the PCB, since such a drill, when moving upward, can lift the PCB, skew the perpendicularity and tear out a fragment of the board.

Drills of small diameters are usually inserted either into collet(various sizes), or in a three-jaw chuck. For precise clamping, clamping in a three-jaw chuck is not the best option, and the small drill size (less than 1 mm) quickly makes grooves in the clamps, losing good clamping. Therefore for drill diameter less than 1 mm, it is better to use a collet chuck. To be on the safe side, purchase an extra set containing spare collets for each size. Some inexpensive drills come 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 boards 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 work surface approximately 15 cm above the tabletop for better visual control over the process. It would be a good idea to remove dust and chips while drilling (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 blocks, 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 of 0.8 mm or smaller, as they always break just at the moment when you urgently need to order. Drills 1 mm 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 simultaneously to save time. In this case, it is necessary to very carefully drill holes in the center of the contact 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 0.3mm centering holes in two opposite corners and pins as pegs, secure the boards to each other.

If necessary, you can countersink the holes with larger diameter drills.

Copper tinning on PP

If you need to tin the tracks on the PCB, you can use a soldering iron, soft low-melting solder, alcohol-rosin flux and coaxial cable braid. For large volumes, they tin in baths 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. Using tongs, place the board under the level of the liquid melt for 5-10 seconds and, after removing it, check whether the entire copper surface is evenly covered. If necessary, the operation is repeated. Immediately after removing the board from the melt, its remains are removed either using a rubber squeegee or by sharp shaking in a direction perpendicular to the plane of the board, holding it in the clamp. Another way to remove residual Rose alloy is to heat the board in a heating cabinet and shake it. The operation can be repeated to achieve a mono-thickness coating. To prevent oxidation of the hot melt, glycerin is added to the tinning container so that its level covers the melt by 10 mm. After the process is completed, the board is washed from glycerin in running water. Attention! These operations involve working with installations and materials exposed to high temperatures, therefore, to prevent burns, it is necessary to use protective gloves, goggles and aprons.

The operation of tinning with a tin-lead alloy proceeds similarly, but more heat melt limits the scope of application of this method in artisanal production conditions.

After tinning, do not forget to clean the board from flux and thoroughly degrease it.

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 photoresist, dry it, tan it, center the mask photomasks, expose it, develop it, wash it and tan it again. Of course, we skip the steps of checking the quality of development, etching, removing photoresist, tinning and drilling. At the very end, 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 PP from external influences and protects against theoretically possible short circuits during operation. It also plays an important role in automatic soldering: it prevents the solder from “sitting” on adjacent areas, short-circuiting them.

That's it, the double-sided printed circuit board with mask is ready

I had to make a PP in this way with the width of the tracks and the step between them up to 0.05 mm (!). But this is already jewelry work. 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 installing components on it

And here is the device itself for which the PP was made:

This is a cellular telephone bridge that allows you to reduce the cost of mobile communication services by 2-10 times for this it was worth bothering with the PP;). The PCB with soldered components is located in the stand. Previously, there was an ordinary charger for mobile phone batteries.

Additional Information

Metallization of holes

You can even metallize holes at home. For this inner surface holes are 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 influence of light, silver nitrate decomposes, and silver inclusions remain on the board. Next, chemical precipitation of copper from solution is carried out: copper sulfate ( copper sulfate) 2 g, caustic soda 4 g, ammonia 25% 1 ml, glycerin 3.5 ml, formaldehyde 10% 8-15 ml, water 100 ml. The shelf life of the prepared solution is very short; it must be prepared immediately before use. After the copper is deposited, the board is washed and dried. The layer turns out to be very thin; its thickness must be increased to 50 microns by galvanic means.

Solution for applying copper plating by electroplating:
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 being 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.

A fragment of a printed circuit board showing metallization 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 the gelatin swells, place the container in a water bath at a temperature of 30-40°C until the gelatin is completely dissolved.
Second solution: dissolve 5 g of potassium dichromate (chrompic, bright orange powder) in 40 ml of boiled water. Dissolve in low, diffused light.
Pour the second into the first solution with vigorous stirring. Add a few drops to the resulting mixture using a pipette. ammonia until straw color is achieved. The emulsion is applied to the prepared board under very low light. The board is dried until it is tack-free at room temperature in complete darkness. After exposure, rinse the board under low ambient light in warm running water until the untanned gelatin is removed. To better evaluate the result, you can paint areas with unremoved gelatin with a solution of potassium permanganate.

Improved homemade photoresist:
First solution: 17 g of wood glue, 3 ml of ammonia aqueous solution, 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 unacceptable!). The emulsion is applied at a temperature of 30-40°C. Continue as in the first recipe.

Photoresist based on ammonium dichromate and polyvinyl alcohol:
Prepare a solution: polyvinyl alcohol 70-120 g/l, ammonium bichromate 8-10 g/l, ethyl alcohol 100-120 g/l. Avoid bright light! Apply in 2 layers: first layer drying 20-30 minutes at 30-45°C second layer drying 60 minutes at 35-45°C. Developer 40% ethyl alcohol solution.

Chemical tinning

First of all, the board must be picked out to remove the formed copper oxide: 2-3 seconds in a 5% solution of hydrochloric acid, 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 a copper coating occurs when immersed in a tin salt solution in which the potential of the copper is more electronegative than the coating material. The change in potential in the desired direction is facilitated by the introduction of a complexing additive, thiocarbamide (thiourea), into the tin salt solution. This type of solution has the following composition (g/l):

Among those listed, the most common are solutions 1 and 2. Sometimes it is proposed to use as a surfactant for the 1st solution detergent"Progress" in the amount of 1 ml/l. Adding 2-3 g/l 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 of the finished PCB before soldering components.

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 particularly 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 have large orders infrequently, then try to prepare a small amount of solution at once, sufficient to tinning the required amount of PP, and store the remaining solution in a closed container (bottles of the type used in photography that do not allow air to pass 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 metalizing holes at home; you still won’t get great results.

Many thanks to the candidate of chemical sciences Filatov Igor Evgenievich for consultations on issues related to chemistry.
I also want to express my gratitude Igor Chudakov."