Produce high-quality printed circuit boards at home

How did you make printed circuit boards  in home and laboratory conditions before? There are several methods-for example:

Paint the guide of robbers in the future;
Carve and cut with a knife;
Stick tape or tape, and then use a scalpel to cut off the pattern.
Make a simple mold and then paint with a spray gun.
The missing elements are completed with a drawing feeder and modified with a scalpel.

This is a long and time-consuming process that requires the outstanding artistic ability and accuracy of the “painter”. The thickness of the line is almost unsuitable for 0.8 mm, there is no repeat accuracy, and each board must be drawn separately, which greatly hinders the production of even small batches of printed circuit boards (hereinafter referred to as PCs).

What do we have today?

Progress will not stand still. The time when radio amateurs painted PP with stone axes on the skin of mammoths has been forgotten. The advent of chemicals commonly used in lithography on the market has opened up completely different prospects for the production of PP without metallization of household holes.

Let us briefly consider the chemicals used to produce PP today.

Photoresist
You can use liquid or film. Due to the scarcity of film, the difficulty of rolling into PP, and the low quality of the printed circuit boards obtained on output, we will not consider film in this article.

After analyzing the market quotation, I chose POSITIV 20 as the best photoresist for household PP.

POSITIV 20-Positive Light Emulsion Appointment:
POSITIV 20-Photosensitive varnish. It is used for small-scale production of printed circuit boards and copper prints in work related to the transfer of images to various materials.
characteristic:
High exposure characteristics provide good contrast for the transferred image.
Application range:
It is used to transfer images to glass, plastic, metal and other related fields in small-scale production. The method of use is marked on the cylinder.
characteristic:
color: blue
Density: 0.87 g/cm 3 at 20°C
Drying time: 15 minutes at 70°C.
Consumption: 15 l / m 2
Maximum light sensitivity: 310-440 nm

Read more about POSITIV 20 here.

The description of the photoresist says that it can be stored at room temperature and is not easily aged. Strongly opposed! It should be stored in a cool place, for example, on the bottom shelf of the refrigerator, the temperature should usually be maintained at + 2 … + 6 °C. However, the freezing temperature is not allowed under any circumstances!

If you use photoresist sold in “bottled” packaging without light-proof packaging, you need to protect it from light. Store in a completely dark environment at a temperature of + 2 … + 6°C.

Educator
Similarly, I think TRANSPARENT 21 is the most suitable illuminator.

Translucent formulation in transparent 21-aerosol Purpose:
Allows you to transfer the image directly to the surface coated with POSITIV 20 photosensitive emulsion or other photoresist.
Attributes:
Make the paper transparent. Provides UV transmission.
application:
Quickly transfer the outlines of drawings and charts to the substrate. It can greatly simplify the reproduction process and reduce the time and cost.
characteristic:
Color: transparent
Density: 0.79 g/cm 3 at 20°C
Drying time: 30 minutes at 20°C.
note:
The transparent film can be used in inkjet printers or laser printers instead of using plain paper with illuminators-depending on how we will print the photomask on it.

Photoresist developer
There are many different solutions for the performance of photoresists.

It is recommended to use “liquid glass” solution. Its chemical composition: Na 2 SiO 3 * 5H 2O. This substance has many advantages. The most important thing is that it is very difficult to overexpose the PP-you can leave the PP in an exact time. The solution hardly changes its properties during temperature changes (there is no risk of decomposition with increasing temperature), and it has a very long shelf life-its concentration remains constant for at least a few years. The absence of excessive contact in the solution will increase its concentration to reduce the time when PP appears. It is recommended to mix 1 part of the concentrate with 180 parts of water (only more than 1.7 grams of silicate in 200 ml of water), but a more concentrated mixture can be prepared so that the image appears in about 5 seconds without overexposure Risk of surface damage. If sodium silicate is not available, use sodium carbonate (Na2 CO 3) or potassium carbonate (K 2 CO 3).

It is also recommended to use household tools to clean the pipes-“moles”.

I haven’t tried the first or the second, so I’ll tell you what I’ve been showing for years without any problems. I use caustic soda in water. For 1 liter of cold water-7 grams of caustic soda. If there is no NaOH, use KOH solution to double the alkali concentration in the solution. The correct exposure time is 30-60 seconds. If the pattern does not appear (or weakly appears) after 2 minutes, and the photoresist starts to wash off the workpiece, the exposure time is not selected correctly: the exposure time needs to be increased. Conversely, if the exposed and unexposed areas are displayed quickly, but the solution concentration is too high or the quality of the photomask is too low (UV light will freely pass through the black): you need to increase the print density of the template.

Copper etching solutions
Etching excess copper in PCB with various etchant. Ammonium persulfate, hydrogen peroxide + hydrochloric acid, copper sulfate solution + sodium chloride are usually very common in the family.

I always etch bleach on the glass plate. When using this solution, you need to be cautious: when it comes in contact with clothing and objects, rust will remain, and it is difficult to remove it with a weak solution of citric acid (lemon juice) or oxalic acid.

We preheated the ferric chloride concentrate to 50-60°C, immersed the workpiece in it, and then gently driven the glass rod with a cotton swab at a location where the copper corrosion was less-this ensures that the entire area of ​​the PP Perform more uniform corrosion. If the speeds are not forced to be equal, the required etching time will increase, which ultimately leads to the fact that in areas where copper has been etched, the traces are etched. As a result, we have nothing we want at all. It is highly desirable to provide continuous mixing of the etching solution.

Chemical cleaning photoresist
What is the easiest way to wash away unnecessary photoresist after etching? After repeated trial and error, I chose ordinary acetone. If not, please clean the nitro paint with any solvent.

Therefore, we make circuit boards
Where do high-quality printed circuit boards start? Correct:

Create high-quality reticle
For its manufacture, almost any modern laser or inkjet printer can be used. Since we are using positive photoresist in this article, the copper should remain on the PCB, so the printer should be painted black. Where there is no copper, the printer should not draw anything. When printing a photomask, it is very important that you need to set the maximum amount of dye water (in the printer driver settings). The darker the shadow area, the greater the possibility of obtaining better results. No color is needed, just a black ink cartridge. From this program (we will not consider these programs: everyone is free to choose-from PCAD to Paintbrush), in the case where a photo template is drawn, we print it on plain paper. The higher the resolution when printing, the better the paper quality and the higher the quality of the photomask. I recommend at least 600 dpi, the paper should not be too thick. When printing, we consider that the side coated with ink on the paper, the template will be placed on the PP blank. If you make a difference, the edges of the PP conductor will become blurred and blurry. If it is an inkjet printer, let the paint dry. Then we impregnated TRANSPARENT 21 paper, let it dry, and…ready to mask.

Instead of even using paper and illuminators, it may even be very desirable to use transparent films for laser printers (when printing on laser printers) or inkjet printers (for inkjet printers). Please note that these movies have a different aspect: only one is valid. If you want to use laser printing, I strongly recommend to “dry” the film before printing-just pass the film through the printer and simulate printing, but nothing is printed. Why do you need this? When printing, the fuser (furnace) will heat the paper, which will inevitably cause deformation. As a result, there is an error in the geometry of PP at the output. In the production of double-sided PP, this causes a mismatch between the layer and all subsequent layers. With the help of the “dry” operation, we heated the sheet to deform it and prepared the printing template. When printing, the paper will pass through the stove again,

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

In the preparation stage, in the sPlan 4.0 program of Russified (~450 KB), it is very convenient to draw a circuit that is not too bulky.

This is what the finished mask printed on the Epson Stylus Color 740 printer looks like:

Preparation of PP surface for photoresist
In order to produce PP, a sheet coated with copper foil was used. The most common options are copper thicknesses of 18 and 35 microns. The most common is that when PP is produced at home, tissues made of tissue paper (a few layers of fabric pressed with glue), fiberglass (same, but using epoxy compounds as glue) and getinax (laminated paper) are used ). Less common are-placenta and polycor (high frequency ceramics-rarely used at home), fluoroplastics (organic plastics). The latter is also used to manufacture high-frequency equipment, and has very good electrical characteristics, can be used anywhere, but its use is limited by the high price​​​​

First, you need to ensure that the workpiece does not have deep scratches, scratches, or areas affected by corrosion. In addition, it is desirable to polish copper to a mirror surface. We are not very enthusiastic about polishing, otherwise we will erase the already thin copper layer (35 microns), or in any case, we will get different thickness of copper on the surface of the workpiece. In turn, this will result in different etch rates: thinner places are more prone to wear. Yes, and the thinner conductors on the board are not always very good. Especially if it is very long and a decent current will flow. If the copper on the workpiece is of high quality and free of burrs, it is enough to degrease the surface.

Application of photoresist on workpiece surface
We placed the circuit board on a horizontal or slightly inclined surface, and applied the ingredients of the aerosol packaging within a distance of about 20 cm. Remember, the most important enemy is dust. Every dust particle on the surface of the workpiece is the source of the problem. To form a uniform coating, spray aerosol in a continuous zigzag movement starting from the upper left corner. Do not use aerosols in excess, as this can cause undesirable smudges and uneven coating thickness, requiring longer exposure times. In summer, at higher ambient temperatures, reprocessing may be required, or aerosols can be sprayed from a shorter distance to reduce evaporation losses. When spraying, do not incline the bottle strongly-this will result in increased consumption of propellant gas. As a result, although there is still photoresist in it, the aerosol may stop working. If aerosol photoresist does not give satisfactory results, use a centrifuge coating. In this case, the photoresist is applied to the board mounted on the rotary table with a driving force of 300-1000 rpm. After coating, the circuit board should not be exposed to strong light. The color of the coating can roughly determine the thickness of the applied layer: in this case, the photoresist is applied to the board mounted on the rotary table with a driving force of 300-1000 rpm. After coating, the circuit board should not be exposed to strong light. The color of the coating can roughly determine the thickness of the applied layer: in this case, the photoresist is applied to the board mounted on the rotary table with a driving force of 300-1000 rpm. After coating, the circuit board should not be exposed to strong light. The color of the coating can roughly 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-greater than 8 microns.
On copper, the color of the coating may be greenish.

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

I always apply photoresist on the centrifuge. In my centrifuge, the rotation speed is 500-600 rpm. Fixing should be simple, clamping only at the end of the workpiece. We fix the workpiece, start the centrifuge, spray it to the center of the workpiece, and observe how the photoresist spreads on the surface of the thin layer. Under the effect of centrifugal force, excess photoresist will be discarded from future software, so I strongly recommend providing a protective wall to prevent the workplace from becoming a pigsty. I use a regular pan with a hole in the center of the bottom. The shaft of the motor passes through the hole, and the mounting platform is mounted on the mounting hole in the form of a cross of two aluminum rails, and the ear of the workpiece “slides” along the hole. The ears are made of aluminum corners and fixed on the guide rail with a butterfly nut. Why is aluminum? When the center of rotation deviates from the center of rotation of the centrifuge shaft, the specific gravity is smaller, so the runout is smaller. The higher the accuracy of the workpiece alignment, the less the jitter caused by the eccentricity of the mass, and the less work is required to firmly fix the centrifuge to the base.

Applied photoresist. Let it dry for 15-20 minutes, turn the workpiece over and apply a layer on the second side. We dry for another 15-20 minutes. Don’t forget to avoid direct sunlight and fingers on the working side of the workpiece.

Tanning photoresist on the surface of the workpiece
We put the workpiece in the oven and steadily raise the temperature to 60-70°C. At this temperature, we can withstand 20-40 minutes. It is important not to let anything touch the surface of the workpiece-only the tip is allowed.

Alignment of upper and lower masks on the workpiece surface
On each photomask (upper and lower), there should be marks, and two holes need to be made in the workpiece-in order to combine the layers. The farther the distance between the marks, the higher the alignment accuracy. Usually I place them on the diagonal of the pattern. Use these marks on the workpiece on the drilling machine, drill two holes strictly at 90° (the thinner the hole, the more precise the alignment-I use a 0.3 mm drill) and combine the pattern on it, don’t forget to apply the template Print on the photoresist side. We use thin glass to press the template onto the workpiece. It is best to use glass quartz-they preferably pass ultraviolet light. Plexiglass (plexiglass) can provide better results, but has an unpleasant scraping ability, which inevitably affects the quality of PP. For small PP sizes, you can use the transparent cover in the CD packaging. In the absence of such glasses, you can use the usual window to increase the exposure time. It is important that the glass is flat to ensure a smooth fit between the photomask and the workpiece, otherwise high-quality track edges will not be obtained on the finished PP.

Blank with photo template under perspex. We use the box below the CD.
Exposure (flares)
The time required for exposure depends on the thickness of the photoresist layer and the intensity of the light source. POSITIV 20 photoresist varnish is sensitive to ultraviolet rays, the maximum sensitivity is in the wavelength range of 360-410 nm.

It is best to expose under a lamp whose emission range is in the ultraviolet range of the spectrum, but if there is no such lamp, a normal powerful incandescent lamp can be used, thereby increasing the exposure time. Do not start flashing until the light source is stable-the lamp must be warmed up for 2-3 minutes. The exposure time depends on the thickness of the coating, which is usually 60-120 seconds when the light source is located at a distance of 25-30 cm. The used glass plate can absorb up to 65% of ultraviolet rays, so in this case it is necessary to increase the exposure time. Use transparent Plexiglas plates for best results. When using a photoresist with a long shelf life, the exposure time may need to be doubled-please remember: photoresist aging!

Examples of using various light sources:

Light source Time Distance Note
Philips mercury lamp HPR125 3 minutes 30 cm 5 mm thick quartz glass coating
1000W mercury lamp 1.5 minutes 50 cm 5 mm thick quartz glass coating
500W mercury lamp 2.5 minutes 50 cm 5 mm thick quartz glass coating
300W quartz lamp 3-4 minutes 30 cm 5 mm thick quartz glass coating
Sunshine 5-10 minutes Summer, noon, cloudless 5mm thick quartz glass coating
Osram-Vitalux lamp 300W 4-8 minutes 40 cm 8 mm thick quartz glass coating

ultra violet light
We expose each side in turn. After the exposure, we let the workpiece sit in a dark place for 20-30 minutes.

Performance of bare artifacts
We appear in NaOH (caustic soda) solution at a solution temperature of 20-25°C-see the beginning of this article for more details. If the performance is up to 2 minutes-a small exposure time. If it performs well, but the useful area is washed away-the solution is too complicated (too high in concentration), or the exposure time is too long under a given radiation source or poor quality photomask-the printed black is not fully saturated, so Ultraviolet light can illuminate the workpiece.

When developing, I always used a cotton swab to “roll” on the glass rod very carefully and effortlessly where the photoresist should be rinsed off-this would speed up the entire process.

Cleaning alkali and residual photoresist on the workpiece
I do this under the tap-normal tap water.

Retanning photoresist
We put the billet in the oven and gradually increase the temperature. At a temperature of 60-100°C, we can withstand 60-120 minutes-the pattern becomes firm and firm.

Quality Control
Briefly (for 5-15 seconds), immerse the workpiece in a ferric chloride solution heated to 50-60°C. Rinse quickly with running water. Where there is no photoresist, copper etching begins strongly. If the photoresist is accidentally left somewhere, carefully remove it mechanically. It is convenient to use optics armed with conventional or ophthalmic scalpels (for welding, magnifying glass and watchmaker’s magnifying glass as well as tripod, microscope glasses).

Etch
We were poisoned in concentrated ferric chloride solution at 50-60°C. It is desirable to provide continuous circulation of the pickling solution. We used a cotton swab on the glass rod to carefully “massage” the etched areas. If it is freshly prepared ferric chloride, the etching time usually does not exceed 5-6 minutes. We rinse the workpiece with running water.

The plank is broken
How to prepare ferric chloride concentrate? Before that, dissolve it in a small amount (up to 40°C) of hot water FeCl 3 until it stops dissolving. Our filtering solution. You need to store it in a cool place in a sealed non-metallic packaging, such as a glass bottle.

Remove unnecessary photoresist
We use acetone or nitro lacquer and nitro enamel to rinse off the photoresist from the track.

drilling
It is desirable to select the diameter of the point of the future hole on the photomask in order to facilitate drilling later. For example, in the case where the required aperture is 0.6-0.8 mm, the diameter of the dot on the photomask should be about 0.4-0.5 mm-in this case, the drill bit will be centered.

It is recommended to use tungsten carbide coated drill bits: drill bits made of high-speed steel wear very quickly, although single holes with large diameters (greater than 2 mm) can be used because steel tungsten carbide drill bits of this diameter are too expensive. When drilling holes with a diameter of less than 1 mm, it is best to use a vertical machine, otherwise the drill bit will break quickly. If a hand drill is used for drilling, deformation will inevitably occur, resulting in incorrect hole connection between layers. In terms of tool load, the top-down movement of the vertical drilling machine is optimal. Carbide drill bits are made with rigid (ie, the drill bit exactly matches the diameter of the hole) or thick (sometimes called a “turbine”) shank with a standard size (usually 3.5 mm).

Usually small-diameter drills are inserted into chucks (various sizes) or three-jaw chucks. For precise fixing, it is not the best choice to use a three-jaw chuck for fixing, and the small size of the drill bit (less than 1 mm) will quickly form a groove in the fixture, thereby losing good fixing effect. Therefore, for drills with a diameter of less than 1 mm, it is best to use a collet chuck. Just in case, please purchase an additional kit that contains spare collets for each size. Some cheap drill bits are made with plastic chucks-throw them away and buy metal.

In order to obtain acceptable accuracy, the workplace must be properly organized, that is, first of all, to ensure good lighting of the board during drilling. To do this, you can use a halogen lamp and mount it on a tripod to choose the location (illuminate the right side). Secondly, raise the work surface to about 15 cm above the work surface to better visually control the process. It is best to remove dust and debris during drilling (a conventional vacuum cleaner can be used), but this is not required. It should be noted that the dust generated by the glass fiber during the drilling process is very prickly, and if it contacts the skin, it will cause irritation. Finally, when working, it is very convenient to use the foot switch of the drilling machine.

Typical hole size:

Through hole-0.8 mm or less;
Integrated circuits, resistors, etc. -0.7-0.8 mm;
Large diode (1N4001)-1.0 mm;
Contact pad, trimmer-up to 1.5 mm.
Try to avoid holes with a diameter less than 0.7 mm. Always keep at least two spare drills of 0.8mm or less, because they will always break when you place an emergency order. Bits of 1 mm or larger are more reliable, although it is better to have a spare bit. When two identical boards need to be made, they can be drilled at the same time to save time. In this case, it is necessary to carefully drill holes in the center of the contact area near each corner of the PCB, and for large boards-holes near the center. Place the circuit boards on top of each other and use 0.3 mm centering holes as pins on the two opposite corners and pins to fix the circuit boards relative to each other.

If necessary, a larger counter hole can be used.

PP tin plating
If you need to irradiate the traces on the PP, you can use a soldering iron, low-melting point soft solder, alcohol rosin flux and a braid of coaxial cable. When used in large quantities, they are tinned in bathrooms with low-temperature flux and flux.

The most popular and simplest melt for tin plating is the low-melting alloy “Rose” (tin-25%, lead-25%, bismuth-50%), which has a melting point of 93-96°C. Place the board under the surface of the liquid melt with pliers for 5-10 seconds. After removing it, check whether the entire copper surface is evenly coated. If necessary, repeat the operation. Immediately after removing the circuit board from the melt, remove its residue with a rubber scraper or shake it vigorously in a direction perpendicular to the plane of the circuit board, and fix it in the fixture. Another way to remove residual rose alloy is to heat the plate in a heating cabinet and shake it. This operation can be repeated to obtain an overall coating. To prevent the hot melt adhesive from oxidizing, add glycerin to the tin-plated can so that its height covers the melt by 10 mm. After the process is over, the plate is rinsed from the glycerin in running water. note! These operations involve equipment and materials installed at high temperatures, so to prevent burns, protective gloves, glasses, and apron must be used.

The process of tin plating with tin-lead alloy is similar, but the higher melt temperature limits the applicability of the method under normal conditions.

I want to share another method of tin plating using rose alloy, which has also been tested in practice. Pour ordinary tap water into a tin can or small bowl, add a little citric acid or vinegar, and place on the stove. Place the circuit board in boiling water, pour out a few drops of frozen rose alloy, and then immediately melt in boiling water. The cotton thread wrapped around the long tweezers or wand (so as not to be burned by steam) gently spreads along the track. At the end of the process, the water is drained and the remaining frozen alloy is stored in the container until the next use.

After tin plating, be sure to remove the flux board and completely degrease.

If the output is large-you can use electroless tin plating.

Protective mask
The operation of applying the protective film can completely repeat all the above operations: apply photoresist, dry, tan, center the mask, then expose again, develop, clean and tan. Of course, we skipped the steps of checking development quality, etching, removing photoresist, tin plating and drilling. Finally, we tan the mask at a temperature of about 90-100°C