How to make a really good board at home. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies

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This page is a guide to producing high-quality printed circuit boards (PCBs) quickly and efficiently, especially for professional PCB production layouts. Unlike most other guides, the emphasis is on quality, speed and minimal cost of materials.

Using the methods on this page, you can make a single-sided and double-sided board of good enough quality, suitable for surface mounting with a pitch of 40-50 elements per inch and with a hole pitch of 0.5 mm. The methodology described here is the summary of experience collected over 20 years of experimentation in this area. If you strictly follow the methodology described here, you will be able to get excellent quality PP every time. Of course, you can experiment, but remember that careless actions can lead to a significant reduction in quality. Only photolithographic methods of PCB topology formation are presented here - other methods, such as transfer, printing on copper, etc., which are not suitable for fast and efficient use, are not considered.

Drilling

If you are using FR-4 as your base material, then you will need tungsten carbide coated bits - HSS drills wear out very quickly, although steel can be used for large diameter single holes (greater than 2mm) as 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. The top-down movement is the most optimal in terms of the load on the tool. Carbide drills are made with a rigid shank (i.e. the drill exactly matches the diameter of the hole), or with a 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 PP, because. the drill can pull out a fragment of the board when moving up. Small diameter drills are usually inserted into either a collet chuck of various sizes or a 3-jaw chuck - sometimes a XNUMX-jaw chuck is the best option.

For precise fixing, however, this fastening is not suitable, and the small size of the drill (less than 1 mm) quickly makes grooves in the clamps that provide good fixation. 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, i.e., firstly, to provide illumination of the board during drilling. To do this, you can use a 12V halogen lamp (or 9V to reduce brightness) and attach it to a tripod to be able to choose the position (illuminate the right side). Secondly, raise the work surface about 6" above the height of the table, for better visual control of the process. It would be nice to remove dust (you can use a regular vacuum cleaner), but this is not necessary - accidentally shorting the circuit with a dust particle is a myth. It should be noted, that the fiberglass dust generated during drilling is very caustic and irritates the skin if it comes into contact.Finally, it is very convenient to use the foot switch of the drill press during work, especially when changing drills frequently.Typical hole sizes:

• Via holes - 0.8 mm or less
• Integrated circuit, resistors, etc. - 0.8 mm.
• Large diodes (1N4001) - 1.0 mm;
• · Contact blocks, trimmers - from 1.2 to 1.5 mm;

cutting

If you are mass-producing PP, you will need guillotine shears for cutting (they cost about 150 USD).

Conventional saws dull quickly, except for carbide-coated saws, and sawing dust can cause skin irritation. A saw can accidentally damage the protective film and destroy the conductors on the finished board. If you want to use guillotine shears, then be very careful when cutting the board, remember that the blade is very sharp. If you need to cut the board along a complex contour, then this can be done either by drilling many small holes and breaking off the PCB along the perforations obtained, or using a jigsaw or a small hacksaw, but be prepared to change the blade often. It is practically possible to make a corner cut with guillotine shears, but be very careful.

through plating

When you make a double-sided board, there is a problem of combining elements on the top side of the board. Some components (resistor, surface integrated circuits) are much easier to solder than others (eg a pin capacitor), so the thought is to surface connect only "light" components. And for DIP components, use pins, and it is preferable to use a model with a thick pin, rather than a connector.

Slightly lift the DIP component off the surface of the board and solder a couple of pins from the solder side, making a small hat on the end. Then you need to solder the required components to the top side using reheat, and when soldering, wait until the solder fills the space around the pin (see figure). For very densely packed boards, the layout needs to be well thought out to facilitate soldering of DIP components. After you have finished assembling the board, it is necessary to carry out a two-way quality control of the installation. For vias, 0.8 mm quick-mount tie pins are used (see figure).

This is the most affordable way of electrical connection. You just need to accurately insert the end of the device into the hole for the entire length, repeat the same with other holes. If you need to do through metal plating, for example to connect inaccessible elements, or for DIP components (bond pins), you will need the "Copperset" system. This setup is very convenient, but expensive ($350). It uses "plate bars" (see picture), which consist of a solder bar with a copper sleeve plated on the outside.

Notches are cut on the bushing with an interval of 1.6 mm, corresponding to the thickness of the board. The bar is inserted into the hole using a special applicator. Then the hole is punched with a core, which causes the plated bushing to warp and also pushes the bushing out of the hole. Pads are soldered on each side of the board to attach the sleeve to the pads, then the solder is removed along with the braid. Fortunately, this system can be used to plate standard 0.8mm holes without purchasing a complete kit.

The applicator can be any 0.8mm diameter automatic pencil that has a tip similar to the one shown, and works much better than a real applicator. Hole plating must be done before mounting, while the board surface is completely flat. The holes must be drilled with a diameter of 0.85 mm, because after metallization, their diameters decrease. Note that if your program drew the pads the same size as the drill bit, then the holes could extend beyond the pads, causing the board to malfunction. Ideally, the contact pad extends beyond the hole by 0.5 mm.

Hole plating based on graphite

The second option for obtaining conductivity through holes is metallization with graphite, followed by galvanic copper deposition. After drilling, the surface of the board is covered with an aerosol solution containing fine particles of graphite, which is then pressed into the holes with a squeegee (scraper or spatula). You can use the CRAMOLIN "GRAPHITE" aerosol. This aerosol is widely used in electroforming and other electroplating processes, as well as in obtaining conductive coatings in radio electronics.

If the base is a volatile substance, then immediately shake the board in a direction perpendicular to the plane of the board, so that excess paste is removed from the holes before the base evaporates. Excess graphite from the surface is removed with a solvent or mechanically - by grinding. It should be noted that the size of the resulting hole can be 0.2 mm smaller than the original diameter. Clogged holes can be cleaned with a needle or otherwise. In addition to aerosols, colloidal solutions of graphite can be used. Next, copper is deposited onto the conductive cylindrical surfaces of the holes. The galvanic deposition process is well developed and widely described in the literature.

The installation for carrying out this operation is a container filled with an electrolyte solution (saturated solution of Cu₂SO₄+10% H solution₂SO₄), into which the copper electrodes and the workpiece are lowered. A potential difference is created between the electrodes and the workpiece, which should provide a current density of not more than 3 amperes per square decimeter of the workpiece surface. The high current density makes it possible to achieve high copper deposition rates. So, for deposition on a workpiece with a thickness of 1.5 mm, it is necessary to deposit up to 25 microns of copper; at such a density, this process takes a little more than half an hour.

To intensify the process, various additives can be added to the electrolyte solution, and the liquid can be subjected to mechanical mixing, bubbling, etc. If copper is applied unevenly to the surface, the workpiece can be polished. The process of metallization with graphite is usually used in subtractive technology, i.e. before applying photoresist. Any paste left before applying the copper reduces the free volume of the hole and gives the hole an irregular shape, which complicates further mounting of components. A more reliable method of removing conductive paste residue is by vacuuming or purging with overpressure.

Photomask formation

You need to produce a positive (ie black = copper) translucent photomask film. You will never make a really good PCB without a quality photomask, so this operation is of great importance. It is very important to have a clear and extremely opaquePCB topology image. Today and in the future, the photomask will be created using computer programs of the PCAD family or graphics packages suitable for this purpose. In this work we will not discuss the merits of the software, we will only say that you can use any software products, but it is absolutely necessary that the program prints the holes located in the center of the contact pad, which are used as markers during the subsequent drilling operation. It is almost impossible to manually drill holes without these guidelines. If you want to use general-purpose CAD or graphics packages, then in the program settings, define the pads either as an object containing a black filled area with a white concentric circle of smaller diameter on its surface, or as an unfilled circle, having previously set a large line thickness (i.e. .black ring). Once we have determined the location of the pads and line types, we set the recommended minimum dimensions: drill diameter - (1 mil = 1/1000 of an inch) 0.8 mm.

You can make PCB with smaller through holes, but it will be much more difficult. · Pads for normal components and DIL LCS: 65 mil round or square pads with a hole diameter of 0.8 mm. · line width - 12.5 mils, if you need, you can get 10 mils. · 12.5 mil to 25 mil center-to-center spacing (perhaps slightly less if your printer model permits).

It is necessary to take care of the correct diagonal connection of the tracks at the cut corners (grid - 25 mil, track width - 12.5 mil). The photomask must be printed in such a way that when exposed, the side on which the ink is applied is turned towards the surface of the PCB, in order to ensure a minimum gap between the image and the PCB. In practice, this means that the top side of the double-sided PCB must be printed in a mirror image. The quality of a photomask is highly dependent on both the output device and the material of the photomask, as well as factors that we will discuss next.

Photomask material

This is not about using a photomask of medium transparency - since a translucent one will be enough for ultraviolet radiation, this is not essential, because. for less transparent material, the exposure time increases quite a bit. Line legibility, black area opacity and toner/ink drying speed are much more important. Possible alternatives when printing a photomask: Transparent acetate film (OHP) - this may seem like the most obvious alternative, but this replacement can be costly. The material tends to bend or distort when heated by the laser printer, and the toner/ink may crack and flake off easily. NOT RECOMMENDED Polyester drawing film- good but expensive, excellent dimensional stability. Rough surface holds ink or toner well.

When using a laser printer, it is necessary to take thick film, because... When heated, the thin film is susceptible to warping. But even thick film can be deformed under the influence of some printers. Not recommended, but possible. Tracing paper.Take the maximum thickness that you can find - at least 90 grams per square meter. meter (if you take it thinner, it can warp), 120 grams per sq. a meter would be even better, but harder to find. It is inexpensive and easily available in offices. Tracing paper has good permeability to ultraviolet radiation and is close to the drawing film in terms of its ability to hold ink, and even surpasses it in terms of its properties not to be distorted when heated.

Output device

Pen plotters- painstaking and slow. You will need to use expensive polyester drawing film (tracing paper is not good, because the ink is applied in single lines) and special inks. The pen will have to be cleaned periodically, because... it gets clogged easily. NOT RECOMMENDED. Inkjet printers- the main problem when using - to achieve the necessary opacity. These printers are so cheap that they are certainly worth a try, but their print quality is not comparable to that of laser printers. You can also try to print on paper first, and then use a good copier to transfer the image to tracing paper.

Typesetters- for the best quality of the photomask, a Postscript or PDF file is created and sent to a DTP or compositor. A photomask made in this way will have a resolution of at least 2400DPI, absolute opacity of the black areas, and perfect image sharpness. The cost is usually given for one page, excluding the area used, ie. if you can replicate copies of the PCB, or put both sides of the PCB on the same page, you'll save money. On such devices, you can also make a large board, the format of which is not supported by your printer.

Laser printers- Easily provide the best resolution, affordable and fast. The printer used must have a resolution of at least 600dpi for all PCBs. we need to make 40 strips per inch. 300DPI will not be able to divide an inch by 40 unlike 600DPI. It is also important to note that the printer produces good black prints with no toner blotches. If you are planning to buy a PCB printer, then you first need to test this model on a regular sheet of paper. Even the best laser printers may not completely cover large areas, but this is not a problem if thin lines are printed. When using tracing paper or drawing film, you must have a manual for loading paper into the printer and correctly change the film in order to avoid jamming the equipment. Remember that in the production of small PCBs, to save film or tracing paper, you can cut the sheets in half or to the desired size (for example, cut A4 to get A5). Some laser printers print with poor accuracy, but since any error is linear, it can be compensated for by scaling the data when printed.

Photoresist

It is best to use FR4 fiberglass already with film resist applied. Otherwise, you will have to coat the workpiece yourself. You don't need a dark room or dim lighting, just avoid direct sunlight by minimizing excess light and develop immediately after UV exposure. Rarely used are liquid photoresists, which are spray-applied and coat the copper with a thin film. I would not recommend using them unless you have the conditions to produce a very clean surface or want a low resolution PCB.

Exposure

The photoresist-coated board must be exposed to ultraviolet light through a photomask using a UV machine. When exposed, standard fluorescent lamps and UV cameras can be used. For a small PCB, two or four 8W 12" bulbs will suffice, for larger ones (A3) four 15" 15W bulbs are ideal. To determine the distance from the glass to the lamp during exposure, place a sheet of tracing paper on the glass and adjust the distance to obtain the desired level of illumination of the paper surface. The UV lamps you need are sold either as replacement parts for medical installations or "black light" lamps for disco lighting. They are colored white or sometimes black/blue and glow with a violet light that makes the paper fluorescent (it glows brightly). DO NOT use short wavelength UV lamps such as erasable ROM or germicidal lamps that have clear glass. They emit short wavelength UV radiation which can cause skin and eye damage and are not suitable for PP production. The exposure setting can be equipped with a timer that displays the duration of exposure to radiation on the PP, the limit of its measurement should be from 2 to 10 minutes in increments of 30 s.

It would be nice to provide the timer with an audible signal that indicates the end of the exposure time. It would be ideal to use a mechanical or electronic microwave timer. You will have to experiment to find the required exposure time. Try exposing every 30s, starting at 20 seconds and ending at 10 minutes. Show the software and compare the permissions received. Note that overexposure produces a better image than underexposure.

So, to expose one-sided PCB, turn the photomask with the printed side up on the installation glass, remove the protective film and place the PCB with the sensitive side down on top of the photomask. The PCB should be pressed against the glass to get the minimum gap for the best resolution. This can be achieved either by placing a weight on the surface of the PCB or by attaching a hinged cover with a rubber seal to the UV unit, which presses the PCB against the glass. In some installations, for better contact, the PCB is fixed by creating a vacuum under the cap using a small vacuum pump. When exposing a double-sided board, the side of the photomask with toner (rougher) is applied to the solder side of the PP normally, and to the opposite side (where the components will be placed) - mirrored.

After placing the photomasks print side by side and aligning them, check that all areas of the film match. To do this, it is convenient to use a table with backlight, but it can be replaced by ordinary daylight if you combine photomasks on the window surface. If the coordinate accuracy was lost during printing, this may lead to misregistration of the image with holes; try to align the films by the average error value, making sure that the vias do not extend beyond the edges of the pads. After the photomasks are connected and correctly aligned, attach them to the PCB surface with adhesive tape in two places on opposite sides of the sheet (if the board is large, then on 3 sides) at a distance of 10 mm from the edge of the plate.

Leaving a gap between the paper clips and the edge of the PCB is important, because this will prevent damage to the edge of the image. Use the smallest size paper clips you can find so that the thickness of the paper clip is not much thicker than the PP. Expose each side of the PCB in turn. After irradiating the PCB, you will be able to see an image of the topology on the photoresist film. Finally, it can be noted that a short exposure of the radiation to the eyes is not harmful, but a person may feel discomfort, especially when using powerful lamps. For the installation frame it is better to use glass, not plastic, because. it is more rigid and less prone to cracking on contact. It is possible to combine UV lamps and white light tubes. If you have many orders for the production of double-sided boards, then it would be cheaper to purchase a double-sided exposure setup, where the PCBs are placed between two light sources, and both sides of the PCB are exposed to radiation at the same time.

Manifestation

The main thing to say about this operation - DO NOT USE SODIUM HYDROXIDE when developing photoresist. This substance is completely unsuitable for the manifestation of PP - in addition to the causticity of the solution, its disadvantages include strong sensitivity to changes in temperature and concentration, as well as instability. This substance is too weak to develop the entire image and too strong to dissolve the photoresist. Those. it is impossible to get an acceptable result with this solution, especially if you set up your laboratory in a room with frequent temperature changes (garage, shed, etc.). Much better as a developer is a solution made on the basis of silicic acid ester, which is sold as a liquid concentrate. Its chemical composition is Na₂SiO₃* 5H₂O. This substance has a great number of virtues.

The most important thing is that it is very difficult to overexpose PP in it. You can leave the PP for a non-fixed time. This also means that it almost does not change its properties with temperature changes - there is no risk of decomposition with increasing temperature. This solution also has a very long shelf life, and its concentration remains constant for at least a couple of years. The absence of the problem of overexposure in the solution will allow you to increase its concentration to reduce the time of development of PP. It is recommended to mix 1 part of the concentrate with 180 parts of water, i.e. 200 ml of water contains just over 1,7 grams. silicate, but it is possible to make a more concentrated mixture so that the image develops in about 5 seconds without the risk of destruction of the surface during overexposure, if sodium silicate cannot be obtained, sodium carbonate or potassium carbonate (Na₂CO₃).

You can control the development process by immersing the PCB in ferric chloride for a very short time - the copper will immediately fade, and the shape of the lines of the image can be discerned. If there are shiny areas remaining or the spaces between the lines are blurry, rinse the board and soak it in the developing solution for a few more seconds. A thin layer of resist may remain on the surface of underexposed PP that has not been removed by the solvent. To remove any remaining film, gently wipe the PCB with a paper towel that is rough enough to remove the photoresist without damaging the conductors. You can use either a photolithographic developing bath or a vertical developing tank - the bath is convenient because it allows you to control the development process without removing the PP from the solution. You will not need heated baths or tanks if the solution temperature is maintained at least 15 degrees. Another recipe for a developing solution: Take 200 ml of “liquid glass”, add 800 ml of distilled water and stir.

Then add 400 g of sodium hydroxide to this mixture. Precautions: Never handle solid sodium hydroxide with your hands, use gloves. When sodium hydroxide is dissolved in water, a large amount of heat is released, so it must be dissolved in small portions. If the solution has become too hot, then allow it to cool before adding another portion of the powder. The solution is very caustic and therefore protective goggles must be worn when working with it. Liquid glass is also known as "sodium silicate solution" and "egg conservator". It is used to clean drainpipes and is sold at any hardware store. This solution cannot be made by simply dissolving solid sodium silicate. The developing solution described above has the same intensity as the concentrate, and therefore it must be diluted - 1 part concentrate with 4-8 parts water, depending on the resist used and temperature.

Etching

Typically, ferric chloride is used as an etchant. This is a very harmful substance, but it is easy to obtain and much cheaper than most analogues. Ferric chloride will etch any metal, including stainless steels, so when installing etching equipment, use a plastic or ceramic weir, with plastic screws and screws, and when attaching any materials with bolts, their heads should have a silicon rubber seal. If you have metal pipes, then protect them with plastic (when installing a new drain, it would be ideal to use heat-resistant plastic).

Evaporation of the solution usually does not occur very intensely, but when the baths or tank are not in use, it is better to cover them. It is recommended to use ferric chloride hexahydrate, which is yellow in color and is sold in powder or granule form. To obtain a solution, they must be poured with warm water and stirred until completely dissolved. Production can be significantly improved from an environmental point of view by adding a teaspoon of table salt to the solution. Anhydrous iron chloride is sometimes found, which has the appearance of brown-green granules.

Avoid using this substance if possible.It can only be used as a last resort, because. when dissolved in water, it releases a large amount of heat. If you still decide to make an etching solution out of it, then in no case do not fill the powder with water. Granules must be added very carefully and gradually to the water. If the resulting ferric chloride solution does not completely etch the resist, then try adding a small amount of hydrochloric acid and leave it for 1-2 days. All manipulations with solutions must be carried out very carefully.

Do not allow splashing of both types of etchants, because. when mixed, a small explosion may occur, causing the liquid to splash out of the container and may get into the eyes or on clothing, which is dangerous. Therefore, wear gloves and goggles while working and immediately wash off any drops that come into contact with the skin. If you are producing PCB on a professional basis where time is money, you can use heated pickling pots to speed up the process. With fresh hot FeCl, PP will be completely etched in 5 minutes at a solution temperature of 30-50 degrees. This results in better edge quality and a more uniform image line width. Instead of using heated baths, you can place the pickling pan in a larger container filled with hot water. If you are not using a container with air to agitate the solution, you will need to move the board periodically to ensure even etching.

Tinning

The application of tin to the surface of the PP is carried out to facilitate soldering. The metallization operation consists in deposition of a thin layer of tin (no more than 2 microns) on the copper surface. Surface preparation of PCB is a very important step before the start of plating. First of all, you need to remove the remaining photoresist, for which you can use special cleaning solutions. The most common solution for stripping the resist is a 40% solution of KOH or NaOH, heated to 50 - XNUMX degrees. The board is immersed in this solution, and the photoresist peels off the copper surface after a while. After straining, the solution can be reused. Another recipe is with methanol (methyl alcohol).

Cleaning is done as follows: holding the PCB (washed and dried) horizontally, drop a few drops of methanol onto the surface, then, slightly tilting the board, try to spread the drops of alcohol over the entire surface. Wait about 10 seconds and wipe the board with a tissue, if the resist remains, repeat the operation again. Then rub the surface of the PCB with a wire cloth (which gives a much better result than sandpaper or abrasive rollers) until you achieve a shiny surface, wipe with a tissue to remove particles left by the scourer, and immediately place the board in the tinning solution. Do not touch the board surface with your fingers after cleaning.

During the soldering process, the tin can be wetted by the solder melt. It is better to solder with soft solders with acid-free fluxes. It should be noted that if there is a certain period of time between technological operations, then the board must be decapitated in order to remove the resulting copper oxide: 2-3 s in a 5% hydrochloric acid solution, followed by washing in running water. It is enough to simply carry out chemical tinning, for this the board is dipped into 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. The change in the potential in the desired direction is facilitated by the introduction of a complexing additive into the solution of the tin salt - thiocarbamide (thiourea), alkali metal cyanide. Solutions of this type have the following composition (g/l):

Among the above, solutions 1 and 2 are the most common.

Attention!Potassium cyanide solution is extremely poisonous! Sometimes it is suggested to use Progress detergent in an amount of 1 ml/l as a surfactant for 1 solution. Adding 2-2 g/l bismuth nitrate to solution 3 leads to the deposition of an alloy containing up to 1,5% bismuth, which improves the sappability of the coating and preserves it for several months. 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 such substances is "SOLDERLAC" from Cramolin. The subsequent soldering passes directly on the processed surface without additional removal of a varnish.

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 regularly have large orders, then try to immediately prepare a small amount of solution, sufficient for tinning the required amount of PP, store the rest of the solution in a closed container (it is ideal to use one of the bottles used in the photo that does not let air through). It is also necessary to protect the solution from contaminants, which can greatly degrade the quality of the substance.

Thoroughly clean and dry the workpiece before each process step. You should have a special tray and tongs for this purpose. Tools must also be thoroughly cleaned after use. The most popular and simple melt for tinning is the fusible alloy - "Rose" (tin - 25%, lead - 25%, bismuth - 50%), the melting point of which is 130 C^o. The board is placed with tongs under the level of the liquid melt for 5-10 s, and when removed, it is checked whether all copper surfaces are evenly covered. If necessary, the operation is repeated. Immediately after removing the board from the melt, it is removed either with a rubber squeegee or by sharp shaking in the direction perpendicular to the plane of the board, holding it in the clamp. Another way to remove residues of the Rose alloy is to heat it 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, nitroglycerin is added to the solution so that its level covers the melt by 10 mm. After the operation, the board is washed from glycerin in running water.

Attention!These operations involve working with installations and materials exposed to high temperatures, so to prevent burns it is necessary to use protective gloves, goggles and aprons. The operation of tinning with a tin-lead alloy proceeds in a similar way, but the higher temperature of the melt limits the scope of application of this method in handicraft production conditions.

Recommended Equipment

· Three-tank plant: heated pickling bath, bubbling bath and developing tray. As a guaranteed minimum: an pickling bath and a container for rinsing boards. Photo trays can be used for developing and tinning boards.
A set of tinning trays of various sizes
· Guillotine for PP or small guillotine shears.
· The drilling machine, with a foot pedal of inclusion.
If you cannot get a washing bath, you can use a hand-held sprinkler to wash the boards (for example, for watering flowers).

OK it's all over Now. We wish you to successfully master this technique and get excellent results every time.

Publication: cxem.net

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