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How to plot patterns directly to PC boards Making prototype PC boards is usually time consuming & is sometimes a hit-or-miss procedure. But by using an X-Y flat bed plotter & a special type of pen & ink, good quality single or doublesided PC boards can be made quite quickly. By JOHN CLARKE A necessary part of the work we do at SILICON CHIP involves making PC boards for our prototypes. Every design that we pub­lish, apart from the circuits in Circuit Notebook, must be built and tested before publication. It’s our way of checking that the design works as it should. Making one-off PC boards is an integral part of any elec­tronic design and development process. It enables a prototype to be built and tested before a large number of final production boards are made. Prototype PC boards can be sent out to be made by printed circuit board manufacturers or they 80  Silicon Chip can be made on-site where the design and development takes place. The advantage of making boards on-site is that they can be finished in a relatively short time. Sending out a PC board pattern to be made by a manufacturer may take several days. Of course, for complex multi-layered boards, a professional board manufacturer is the only place where a prototype board can be successfully made. Of the boards produced to date by SILICON CHIP, most have been single sided and, in rare cases, double sided. These can be made with relatively simple equipment. An exception was the main PC board used in the Remote Control Preamplifier pub­ lished in September and October 1993. This board was too large (350 x 230mm) to be made by us at the time and so it was made for us by RCS Radio Pty Ltd. This company can produce prototypes as well as quantity boards and they can supply the majority of PC boards featured in SILICON CHIP. While in most instances we produce our own boards, the process used has not been without its problems. Let’s describe how we have produced prototype boards for the last seven years and why we have now adopted a new process. The old process Our printed circuit boards are designed using Protel Auto­trax, from Protel Technology Pty Ltd, a Tasmanian based company. Prior to 1992 our PC board artworks were produced using Bishop Graphics tapes and pads laid onto clear film. This latter method gave us a ready made artwork but the CAD-produced artwork needs to be printed out before the PC board can be made. We use a laser printer for this task and print directly to 3M transparency film specially made for use with laser printers. Alternatively, we print to paper and then photocopy onto transparency film. In either case, we have a positive artwork. Rather than go through the process of making a negative, as required for Riston-coated boards, we use a positive photo resist material which is brushed onto the blank PC board after it has been thoroughly cleaned. The board is then baked in an oven (or electric frypan) to harden the coating. (Some other types of photo resist available in a spray can do not require baking but they are quite a bit more expensive). The positive artwork is placed over the coated PC board and exposed using UV fluorescent tubes in a light box. Once exposed, the PC board is developed in caustic soda solution to dissolve away the UV-exposed portions. It is then etched using ammonium persulphate or ferric chloride in solution. After etching, the resist is removed using methylated spirits. Finally, the board is drilled, trimmed to size and coated with a clear protective lacquer to prevent the copper from oxidising. Alternatives There are a number of variations to this basic board making process, the most popular being with “Riston” pre­coated board. The disadvantage of precoated boards is that they require “safe” light for handling and cutting and they cannot be stored indefinitely. Safe light handling means that whenever the light proof container holding the precoated board is opened, it must be done in a room where the lighting is of a type that does not affect the UV- sensitive coating. Apart from the high cost and the need to handle it in safe light, Riston precoated board is very good for prototype use. We have usually coated our own boards with positive photo resist because it is quick, cheap and you only need to coat the board needed at the time of processing. However, the particular posi­tive photo resist we used is not supplied any more due to its carcinogenic nature. Spray cans of photo resist which are not carcino- An ultrasonic bath is necessary when it comes to cleaning the pen. The main bath is filled with warm water, while the pen itself is immersed in a small quantity of methylated spirits in a small plastic container. genic are available, although these are very costly. Problems One of the main problems we experience with our PC board production involves the transparency films produced by the photo­copier or laser printer. The black areas tend to be not dense enough to block all the light during UV exposure. This causes pin spots over the copper areas of the board and in severe cases can cause open circuits in the tracks. The problem is more prevalent on larger boards. Another problem occurs when exposing the coated board in the UVlight box. Any warping in the board can mean that the artwork does not sit in close contact with the copper. This causes faulty exposure and the job has to be done again. These problems could be solved but not without considerable time and expense to improve a system which is essentially messy anyway. New method Since all our PC board patterns are produced with a CAD system, on Protel Autotrax, the computerised output could be printed out in many different formats including a dot matrix printer, laser printer, inkjet plotter or X-Y pen plotter. It is this last format that interested us. If the computer could plot out the printed circuit pattern onto paper using an X-Y plotter, it could also print the pattern directly onto the PC board. Then the board would be directly ready for etching. A printed circuit board could then be made without the need for photo resist coat­ings and UV exposure and development. While the concept sounds simple enough it needs the right pen and ink. The ink needs to be resistant to being washed off in the ammonium per­sul­ph­ate solution which is used at about 60°C. And as this story demonstrates, the right pen and ink are now available. Our first tests were done using a Roland 980A A3 flat-bed plotter. This particular plotter did not have sufficient pen height adjustment to cope with the thickness of the PC board material which is typically about 1.5mm thick. We solved the problem by placing a small washer in the pen adaptor to raise the pen tip by 0.5mm. Once we set the pen speed correctly, the plot­ting results onto the copper were excellent. There was no sign of un-inked sections nor was there any tendency to form globs of ink onto the copper surface. We tested the ink in hot (60°C) ammonium persulphate solution and etched the copper from the board. At no time was the ink removed from the copper during this etching process. The ink coating on the etched copper November 1994  81 This photograph shows a fully plotted PC board which is now ready for etching. Note that the discoloration visible in the centre of the board is due to a lighting effect when the photograph was taken – the board itself had been thoroughly cleaned with Ajax® powder & steel wool just prior to plotting. board was then easily removed with methylated spirits. The accompanying photographs show the results which are consistently good. With these initial tests proving successful, we subsequently purchased a Roland DXY-1150 A3 flat-bed plotter and have found the technique to be reliable, producing cleanly etched boards every time. Procedure These are the steps we now use to produce prototype PC boards with a flat-bed plotter. First, the copper surface is thoroughly cleaned with Ajax® powder and soapless steel wool or a Scotchbrite® scourer, to remove any traces of oxide and oil. When rinsing the surface with water, the water should flow across the copper without the tendency to “bead”. Beading means that there is still oil remaining on the copper. The copper surface is then dried with a blow dryer or a hot air gun. Avoid wiping the surface with a towel since it will leave lint on the surface and thus lead to poor plotting results. Once the board is dry, do not touch the copper surface with your fingers. If you do so, you will inevitably leave fingerprints which must be cleaned off again before plotting can proceed. Next, you need to know where to place the PC board onto the bed of the plotter. The way to check this is to make an initial plot onto paper. That done, place the PC board onto the plotter so that it is centred over the plot area and secure it at each corner with adhesive tape. The plotting speed should be set to a slow rate so that the ink has time to flow as the pen traverses the copper surface. We 82  Silicon Chip found 100mm per second (4-inches per second) suitable for our pen size. The pen was a Staedtler Mars Plot tungsten carbide 0.35mm-diameter cross cut type and the ink is a special formulation, also made by Staedtler, for this application. The ink dries quickly, so the pen must be capped immediately after plotting has been completed. The copper is etched in hot (60°C) ammonium persulphate solution. (We have not tried ferric chloride solution, although it should work just as well.) Once etched, the ink is easily washed from the PC board with methy­lated spirits. After drying, the copper is coated with a protective PC board lacquer. Cleaning the pen Since the ink dries so fast, there is a danger that it will clog the pen if it is not cleaned thoroughly, using methy­ lated spirits. Since the emphasis is on thorough cleaning, the only practical way to is use an ultrasonic cleaning • System Requirements CAD software with HPGL print format. • Flat-bed X-Y plotter, Roland DXY-1150 or equivalent. • Staedtler 757PL3CS Mars Plot Tungsten Carbide 0.35mm dia­ meter cross groove pen. • Staedtler 75PL07H2PC plotter adaptor. • Staedtler 48523SAR-9 solventbased ink. • Ultrasonic cleaning bath, Altron­ics A-0100. bath. We used an ultrasonic bath from Altronics (Cat. A-0100) which is currently priced at $219.00. It comes with a small plastic tub so that only a small quantity of methylated spirits is required to clean the disassembled pen. The procedure is to use warm water in the main bath and the tub of methy­ lated spirits is placed into this. The water couples the ultrasonic energy into the small tub and the ink just streams out of the pen. Several bursts of cleaning may be necessary to remove all the ink from the dis­assembled pen, using clean methy­ lated spirits each time. You may wish to use rubber gloves too, to stop the ink from staining your fingers. Some points should also be mentioned. The plotter must be a flat-bed type, preferably A3 size. The type of paper hold system, whether magnetic or electrostatic, is not important since the PC board will need to be held down with tape at each corner. The pen tip height must be sufficient to clear the top of the board and the pen tip should be a cross groove type to allow sufficient ink flow. A tungsten carbide tip is recommended to reduce pen tip wear. Finally, there are other possibilities which are now possi­ble with the plotter. Artwork can be drawn directly onto front panels and overlay diagrams could be plotted on the top side of PC boards. Acknowledgement Our thanks to Mike Matthews of CAD Consumables & Con­ sultancy, Suite 3/83 Hartnett Drive, Seaford, Vic 3198 (PO Box 1049, Frankston, Vic 3199). Phone (03) 782 4000 or fax (03) 782 4011. Mike kindly supplied us with a sample Staedtler tungsten carbide cross groove pen, plotter adaptor and the Staedtler 48523SAR-9 solvent-based ink. CAD Consumables also sells Roland X-Y plotters and the SC full range of Staedtler pens.