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Let's say you're dreaming of some fun little home brew project. The problem is you don't want to build a kluge on perf board and Far Circuits doesn't make a PC board for it. In the past, making a single PC board was often difficult, expensive and the results not very good. Thanks to the work of SEITS member Dave Helton, KD0YU, all of that has changed! We now have a simple practical method of making printed circuit boards for our projects. The system he discovered to make PC boards is based on a inexpensive product called Techniks PNP BLUE laser print hot transfer etch resist film. OK, it sounds complicated, I know. Its really not. Here's how it works. You draw your printed circuit layout using your favorite CAD soft- ware on your computer. Then you print the layout on a piece of Techniks PNP BLUE transfer film using a laser printer. The resist pattern is transferred from the film to the raw PC board using an electric iron. The film is peeled off leaving the resist behind. The board is then etched in ferric chloride. The etchant dissolves all the copper not covered by the resist. What remains is the circuit pattern. The etched board is than cleaned, tin plated and the component lead holes are drilled. Once you have your artwork (board layout), it takes about an hour to complete a new PC board. Yes you do need a laser printer. That is the key item other than the PNP Blue film. Although the toner is similar, a Xerox machine will not work for applying the toner. Xerox machines lack the resolution and toner density you need to make PC boards. If you don't have a laser printer, don't despair! Several SEITS members have them and can assist you. Many of you have them at work. Be creative! PNP Blue film consists of 8.5" X 11" high temperature polyester plastic film. On one side it is coated with a high temperature wax that acts as a release agent. On top of the wax is a thin layer of blue micro fine plastic dust. There are other "laser printer" etch resist products on the market. Most simply use the printer toner as the resist. I've tried them and they do not work very well. The problem is that toner alone is not dense enough to fully resist the etchant. The boards these products make are useless. The problem of etch resist density is solved by the blue layer on the PNP film. When you transfer the etch resist pattern to the PC board, the blue film is fused to the board and melts together. The resulting layer of toner and plastic makes a hard durable etch resist that is capable of reproducing traces as narrow .01" wide. A ordinary electric iron supplies the heat and pressure to transfer the find the correct heat setting by experiment. On my iron a setting somewhere between "cotton" and "steam" seems to work best. Too cold a setting and the toner will not melt and the resist won't adhere to the board. Too hot and the PNP film will wrinkle or melt. All through this process the most important thing is to have the PC board as clean as possible. Start by cleaning the board with a Scotch Brite pad till it shines. No trace of oxide can remain on the board. A rough surface also grips the toner layer better giving better adhesion of the resist to the board. After removing the oxide (and it is there, even on the newest PC board stock), wipe the board with household ammonia or rubbing alcohol to remove the last trace of oil. As you do this be very careful not to touch the surface of the board with your fingers. If you do, reclean the board. You could simply run a full sheet of PNP film through your laser printer. This would be a great waste of the film. Here's how to eliminate this waste. First print the PC artwork out on paper. Than cut a piece of film about 1/2" larger than the finished boards area. Tape the PNP film to the paper with the coating side up (smooth side towards the paper). Tape the film only on the top edge that feeds first into the printer. Place the film/paper into the printer tray and print the layout a second time. This time the layout will be printed on the resist side of the PNP film. Let the film cool for a minute and than carefully peel or cut the tape off the film removing it from the paper carrier. Lay the PNP film toner side down on your PC board. Carefully hold the film in position and apply the hot iron. As soon as the film heats, the film will stick to the board and you can let go of it. Move the iron over the entire surface of the board in smooth circular motions. Do not use any pressure, just use the weight of the iron. Iron the film for 60 to 120 seconds. Pay close attention to the edges of the board. The edges won't get as hot as the center of the board and the resist may not adhere. Apply most of the heat and pressure here. Try not to put too much pressure on the IC pads resist areas or you may smear the holes in the pads shut. You should notice the film becoming embossed with the layout pat- tern. When this occurs, the films blue resist material has been adhered to the board by the toner. Remove the iron at this point and allow the board to cool to room temperature. When the board is cool, carefully and slowly peel the PNP film off the board. Start from the edge opposite of the tape that held the film to the carrier paper. As you peel it off, you will see that the blue resist film is transferring from the film to the PC board. If all went well, you should have a perfect resist pattern on the board. If you notice spots where the resist did not adhere (especially on the edges), stop peeling off the film. Lay the film back down and iron that area again. If your careful (and have not completely removed the film) you can occur on the board edges, you can repair problems there using ordinary tape as a resist. Once you have the film peeped off the board, inspect the resist care- fully. You can also touch up any bad spots with etch resist lacquer. If you use ink for touch up you will have to let it dry for several hours before etching. I've had just as good of luck with using vinyl electrical tape as a resist and it is ready to etch immediately. One more note on tape. Many times for RF projects you will want to have a unetched ground plane on the back side of the board. The simplest way to apply etch resist to large areas of copper is to apply vinyl tape. As long as the tape is well burnished on the edges and the joints overlapped it will make a perfect resist. Large boards with simple layouts such as solid state amplifier boards are often made using tape as the resist. The entire board is covered with tape, than the areas to be etched are cut away using an Exacto knife. This method works surprisingly well if you need just a few large pads on a board. One problem with using toner for a resist is that it does melt. The edges of the traces may not be sharp and often the drill holes in the pads will be blocked with resist. Brushing the board with a toothbrush will sharpen the edges of the traces and clear many of the holes. Just be sure not to use to stiff a brush so to damage the traces. While your board is cooling, you can prepare your etchant bath. Ferric Chloride is the most common etchant. You could buy it as a prepared solution from Radio Shack. Their product is pretty thin stuff and does not work all that well. Your better off to buy your Ferric Chloride pure as dry power and mix your own. It lasts a lot longer and etches much faster. A few words of warning on Ferric Chloride. This stuff is nasty! It will not burn holes in your skin or poison you unless you drink it. What it will do is stain or corrode anything it touches. I would not use it in my kitchen or ham shack, ever! Use it somewhere like your garbage work bench or back porch where it can't do any harm. Be sure you have plenty of running water near by and paper towels. Wear old clothing! The chemical reaction that etches the copper from the PC board forms a dark yellow precipitate in the etchant bath. This material can coat the board as it forms, slowing the etching or making it uneven. This is why you need to keep the bath agitated. The agitation washes the precipitate from the board and keeps it exposed to fresh solution. This precipitate is messy to deal with. Professional PC board manufacturers add Hydrochloric Acid to their Ferric Chloride solution to eliminate it. This changes the chemistry of the etchant bath and the deposit does not form. Dilute Hydrochloric Acid is sold in hardware and paint stores under the name Muratic Acid. I used about two ounces of Acid to a pint of Ferric Chloride solution. I have tried this chemistry and it does work very well. The drawback is that the resulting etchant bath is even more corrosive than Ferric Chloride that will corrode any metal in the area. Don't use this mixture unless the etch bath is used outdoors. Wear eye and hand protection and NEVER add water to the acid. Slowly add the acid to the etching solution. Pour your etching solution into a Pyrex dish and place it on a hot plate. Heat the etchant to 140 degrees F. Use a glass thermometer to make sure it does not get hotter then 160 degrees F. I use a small "buffet" hot plate with a thermostat (cost about $17). When the etching solution is warm, place the board in it and check the time. Every few minutes gently agitate the etchant. This speeds things up and ensures that the board will etch evenly. I use a small wooden stick to shove the board up and out of the etchant to check its progress. Check at least once every five minutes at first. When you think the board is done, lift it from the etchant and rinse it off. The easiest way to inspect the board is to hold it in front of a light bulb. The back lighting will clearly show any areas where copper still remains. Do not over etch the board. When you decide the board is finished, rinse it with plenty of water. Clean the etch resist off the board with lacquer cleaner. Just a small amount on a clean rag will do the job. Once the bulk of the resist is off, wipe it off with lacquer cleaner a second time to remove the last trace of resist. Hold your board up to the light again and inspect the traces for any breaks or other flaws. If all is OK clean the board with Scotch Brite and Ammonia again. Remember, you can NEVER get a PC board too clean! I wear gloves at this step to prevent even the possibility of any oil from my hands getting on the board. Clean copper oxidizes very quickly and can be surprisingly hard to solder. I strongly recommend tin plating the board. Commercial boards are plated by hot dipping them in molten solder. A difficult process to do in ones home! Tinit Solution is a simple solution to the problem. Tinit is a electroless chemical metal plating process that deposits a thin layer of pure metallic tin on the clean copper of the board. It is very simple to use. Tinit comes as two small bags of white powder that are mixed with water. The mixed solution is heated to 160 degrees (your Pyrex bowl and hot plate again) and the etched board immersed in it. Leave the board in bath till the Tinit deposits a thick coating of tin on the board. This may take 10 to 15 minutes. Remove the board from the bath and rinse it with running water. If the Tinit solution is not hot enough or if there is any residue from the etch resist stripping on the board, you will have a tinning failure. No mater how clean your hands are, Tinit will not deposit tin anywhere you touch the copper surface of the board. Getting the solution to 160 degrees and keeping the board clean is the secret to success. Immersing the board in household Ammonia for several minutes after plating will neutralize the chemicals remaining in the pores of the metal and prevent the board from oxidizing the board later. This is a very important step! Rinse the finished board with water one last time and polish it with a rag. After trimming the board edges to size, your ready to drill. Fiber glass PC board stock is pretty tough stuff. It can dull regular steel drill bits quickly. For this reason use carbide drill bits if you can find them. On the other hand.... Carbide bits are very easy to break and steel bits are pretty tough. All in all, I've found the steel bits the best to use simply because I don't break them. The common sizes for PC bits are #50 (.070"), #60 (.040") and #70 (.028"). The leads of common 1/4 watt resistors, transistors and IC sockets fit the #70 holes. The larger sizes drill too large a hole that destroy most of the area of the solder pads used for small components. Stock up on the #70 bits when you find them, this is the bit you use the most and the one easiest to break. You cannot hand drill with these tiny bits. They are very brittle and will snap right off if slightly bent. I have used a Dremel Moto Tool for a drill. Dremel makes a small "drill press" that the Moto Tool clamps into. This arrangement is simple and its easy to control the drill. I currently use a large floor mount drill press for drilling my boards. When choosing a drill press be sure that the chuck is the type that closes completely. If not, it won't hold the tiny #70 bits you need to use. Hold the board securely. When the bit penetrates the board, often it will grab. If it does, it will pull the board upward snapping off the bit! This will happen to you. To prevent broken bits, hold the board firmly. Back the board with a piece of hard wood. Drill slowly and don't use too much pressure. Experiment with the drill speed. I've found that I have better luck at lower speeds. How well does this really work? I've made at least thirty or more boards in the last year by this method. Some are as large as 3.5" X 4.5". KD0YU has made dozens of micro-controller boards. Most have traces as narrow as .01"! Those are traces so narrow that they can pass between the solder pads of a standard IC pattern. In other words, you can make serious projects with this system. Several times I've had to apply the resist a second time in order to get the toner/resist to transfer perfectly to the board. If you see errors in the resist pattern (breaks, cracks, etc.) your better off to clean the board off and try again. Don't try to salvage the resist pattern you laid down if it has defects. Its not worth the bother. Do experiment till you get it right. A good board is something to be proud of. What can't you do with this process? Double sided boards would be very difficult. The problem is getting perfect alignment between the pads on both sides of the board. We have not come up with a easy way to do that yet. However, if you can think of a easy way to do it, than that would allow us to design and make boards for some of the more complex microprocessor projects like repeater controllers. Think of the possibilities! What does all this cost? PC board stock scraps sells for about a penny a square inch at hamfests (be sure to buy the single sided board). PNP Blue film sells for $30.00 for 20 8"X 11" sheets. Ferric Chloride costs about $2.50 per 1/4 pound bag. A pint of Tinit solution costs $4.95. Drill bits are $1.50 each new. A good Teflon coated electric iron cost me $19.00 at the local Walmart. Considering the number of small PC boards you can make with the above supplies, the price is cheap! If you do decide to make your own PC boards, please let me know. KD0YU and myself are constantly experimenting with methods to improve our technique. If you have any ideas or questions on the PNP method, BILL OF MATERIALS
SUPPLIERS
Circuit Specialists, Inc.
DC Electronics
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