Design and manufacturing of the PCB’s has been explained in detail in the second issue of P.E. Here, we’ll consider how to make a PCB whose drawing is already done. As an example, we’ll take a drawing of the board of the receiver from pic.3.19, which measures 45 mm x 30 mm.a. The PCB is being made of pertinax or vitroplast, i.e. a thin plate (about 1.5 mm) made of isolation material, which has a lean layer of copper put on one side. From the plate you buy in some electronic components’ shop, a 45 mm x 30 mm piece should be cut. In amateur conditions, this means refracting. First, points A and B are marked on the non-copper side of the plate, acc. to pic.5.1-b. A ruler is placed over them and a groove is made by pressing with a screwdriver or a bodkin along it. Its depth should be about 0.5 mm (on picture it is shown in dashed line). When this is done, the plate is placed on the edge of the table, with copper facing downwards. With one hand the plate is pressed firmly to the table, and with the other, the piece that has to be refracted. And - it cracks just along the groove.On the refracted piece, a new groove, measuring 30 mm from the edge, is made, and the procedure is repeated. In this way we finally have our 45 mm x 30 mm plate.b. All of the copper has to be clean and shiny, since only in this case the etching and, later, soldering is performed quick and easy and well. If it seems to you that the copper you have just bought is clean enough, you’re probably wrong. The plate must have spent some time in the shop, and the copper surface is certainly more-less corroded. The cleaning is most efficiently done with some abrasive powder (VIM or similar) which is otherwise used for cleaning of the cookers, bathrooms etc, but also the sodium bicarbonate, laundry detergent and even plain salt can well serve the purpose. Take a piece of cloth, wipe it with water, extract the water well and muss it to be ball-shaped. Dip it then in the powder, and scrub the copper until it “shines like the shiny sun”. After that rinse the plate, and pay attention not to touch the copper with your fingers, since that will make it dirty again.c. Put the plate, facing the copper up, beneath the sheet that contains the PCB layout, right under this drawing. In our example, that would be the one on the pic.5.2-a. With the pike of a bodkin the holes are made through the centers of all the contacts, and in the centers of two bigger holes that are placed sidewise, taking care not to move the plate. The bodkin has to be pressed firmly, in order to obtain good prods on the copper. When this is finished, the plate should look as on pic.5.2-b. i.e. it has to contain as much prods as there are contacts, plus two. If the drawing contains many contacts, the plate can be easily dislocated, and the procedure is to be done all over again. It is better practice then to make a copy of the picture, cut it out, and attach it to the plate with two pieces of scotch tape. d. Drawing the contacts and lines on the plate is done with the acid-resistant marker paintstick. It can be recognized by its characteristic “alcohol smell”, and is being sold in bookstores as a marker for “writing on glass”. You can test it: write in the store (it will be later afterwards) something on the glass, piece of plastic and similar, wait for a couple of seconds, then try to wipe it out with your fingertip. If the paint remains - the marker is OK. Nevertheless, this test isn’t 100% certain, it is much better to buy the marker in the electronic components store (you have to accent to the salesman that you need a marker for drawing lines on PCB’s). With the tip of the marker draw a circle around every prod (except those two that are for bigger holes), measuring 2-3 mm in diameter. Move the marker slowly, in order to leave a thick layer of paint on the plate. Take care to leave a small copper isle around every hole. Then,you should, carefully and slowly, draw all the lines, by looking at the pic.5.2-a. They do not need to have the same shape as on the picture, especially they don’t have to be that “chamfered”. Line thickness should be about 1 mm, but that either is not obligatory, they can be somewhat thinner or a lot thicker (where applicable). The important thing is not to connect the nearby lines or contacts during the drawing, i.e. not to make junctions that do not exist on the drawing. If that happens anyhow, remove the paint surplus with a razor or a small, sharp screwdriver. Pic.5.2-c shows the beginning of drawing, several contacts and 3 lines are drawn. The drawing is finished when you have a pic.5.2-a on the copper foil.e. Next step is etching, i.e. removing the copper that is not covered with marker paint. For this purpose, a mixture of hydrochloric acid (HCl), hydrogen peroxide (H2O2) and water (H2O). Pure hydrochloric acid is not used, but its 35% solution, that is being sold as a household cleaning agent. Hydrogen peroxide is being sold in drug stores and cosmetic stores. It is being sold as 30% solution, or even more diluted, 8-12%. Hydrochloric acid and hydrogen peroxide are very aggressive media, especially for the eyes and skin, therefore care should be exercised when working with them. It would be the best for you to work with them in the bathroom, or some other place close to the running water supply. If some of these liquids spills on your skin, metal tool or clothing, wash them down with water immediately.The etching mixture is being made directly before the etching, and is CERTAINLY being disposed of, right after the process. The plate is put at the bottom of a plastic, glass or porcelain dish, with copper facing upwards, and the acid is poured, in quantity enough to fully cover the plate (pic.5.2-d). Hydrogen peroxide is then added, being poured from the container directly over the plate. The amount of peroxide depends on its concentration, as well as on the concentration of the acid. So, put some peroxide, raise a little left end of the dish, then the right one, to allow a liquids to mix, and observe the plate. The mixture is transparent, and if the copper starts changing the colour after a dozen seconds - the etching has begun. During this process, the bubbles are formed in the mixture, in the amount somewhat more than in a glass of mineral water. If too little bubbles are present, add some more peroxide. Be careful, however, not to exaggerate, since if you happen to have too much bubbles, the mixture is going to heat up and the marker paint can be destroyed. From time to time, you should raise one end of a plate with a pointed wooden or plastic stick, in order to remove the old liquid from its surface, and allow for fresh mixture to take its place. Etching is finished when there is no more uncovered copper on the plate. Raise one plate end with the stick, wait for the liquid to decant, take a plate with a laundry clip and wash it thoroughly in a jet of running water. You can then remove the paint by scrubbing, as previously described, with a wet cloth dipped in some powder. The copper contacts and lines will emerge on the plate. f. If you were careful enough to leave a uncovered isle of copper in the centre of every contact, after etching this will be a small cavity, in the centre of the contact. Through these cavities, that will guide your drill, a 1 mm holes should be drilled (it is better if the holes are 0.8 mm in diameter, but such drills are harder to find, and a lot easier to break). Two holes for the fixing screws are usually about 3 mm in diameter. While drilling, a piece of thicker plywood or some flat hard-wood plank (beech, oak) should be put beneath the plate, and not a piece of polystyrene or something similar. Do not press the drill too hard, since the tool will be plucking tiny pieces of plastic on the other side of the plate.
you should, carefully and slowly, draw all the lines, by looking at the pic.5.2-a. They do not need to have the same shape as on the picture, especially they don’t have to be that “chamfered”. Line thickness should be about 1 mm, but that either is not obligatory, they can be somewhat thinner or a lot thicker (where applicable). The important thing is not to connect the nearby lines or contacts during the drawing, i.e. not to make junctions that do not exist on the drawing. If that happens anyhow, remove the paint surplus with a razor or a small, sharp screwdriver. Pic.5.2-c shows the beginning of drawing, several contacts and 3 lines are drawn. The drawing is finished when you have a pic.5.2-a on the copper foil.
e. Next step is etching, i.e. removing the copper that is not covered with marker paint. For this purpose, a mixture of hydrochloric acid (HCl), hydrogen peroxide (H2O2) and water (H2O). Pure hydrochloric acid is not used, but its 35% solution, that is being sold as a household cleaning agent. Hydrogen peroxide is being sold in drug stores and cosmetic stores. It is being sold as 30% solution, or even more diluted, 8-12%.
Hydrochloric acid and hydrogen peroxide are very aggressive media, especially for the eyes and skin, therefore care should be exercised when working with them. It would be the best for you to work with them in the bathroom, or some other place close to the running water supply. If some of these liquids spills on your skin, metal tool or clothing, wash them down with water immediately.
The etching mixture is being made directly before the etching, and is CERTAINLY being disposed of, right after the process. The plate is put at the bottom of a plastic, glass or porcelain dish, with copper facing upwards, and the acid is poured, in quantity enough to fully cover the plate (pic.5.2-d). Hydrogen peroxide is then added, being poured from the container directly over the plate. The amount of peroxide depends on its concentration, as well as on the concentration of the acid. So, put some peroxide, raise a little left end of the dish, then the right one, to allow a liquids to mix, and observe the plate. The mixture is transparent, and if the copper starts changing the colour after a dozen seconds - the etching has begun. During this process, the bubbles are formed in the mixture, in the amount somewhat more than in a glass of mineral water. If too little bubbles are present, add some more peroxide. Be careful, however, not to exaggerate, since if you happen to have too much bubbles, the mixture is going to heat up and the marker paint can be destroyed. From time to time, you should raise one end of a plate with a pointed wooden or plastic stick, in order to remove the old liquid from its surface, and allow for fresh mixture to take its place.
Etching is finished when there is no more uncovered copper on the plate. Raise one plate end with the stick, wait for the liquid to decant, take a plate with a laundry clip and wash it thoroughly in a jet of running water. You can then remove the paint by scrubbing, as previously described, with a wet cloth dipped in some powder. The copper contacts and lines will emerge on the plate.
f. If you were careful enough to leave a uncovered isle of copper in the centre of every contact, after etching this will be a small cavity, in the centre of the contact. Through these cavities, that will guide your drill, a 1 mm holes should be drilled (it is better if the holes are 0.8 mm in diameter, but such drills are harder to find, and a lot easier to break). Two holes for the fixing screws are usually about 3 mm in diameter. While drilling, a piece of thicker plywood or some flat hard-wood plank (beech, oak) should be put beneath the plate, and not a piece of polystyrene or something similar. Do not press the drill too hard, since the tool will be plucking tiny pieces of plastic on the other side of the plate.
