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MAKE YOUR OWN PROTOTYPE PRINTED CIRCUIT BOARDS
Looking back over some recent copies of CQTV I was struck by the number of constructional items which were built with strip-board pcb. This is an obvious way to quickly prototype designs, but can be rather cumbersome and obviously will be tricky as more and more ICs go over to surface mount. One of the worst jobs in my view for the home constructor is the drilling of boards. Even quite simple boards can require dozens of holes, so the attraction of smd components all soldered onto the one side of the pcb requiring no drilling at all is clear. PCB manufacturers are not interested in small runs and charge nearly as much for a couple of boards as for ten. So what other options are there? Photo-resist requires a negative sheet with a photographic surface painted onto a copper pcb. The process then requires a light-box and careful timing of exposure followed by a developing and then removal of unwanted material leaves a pcb ready for etching. Quite a few tricky processes and opportunities to fail. I discovered a couple of years ago another option using an iron-on system. Discussing the product with some local operators I realised that many had never even heard of this way of producing boards. At first I was rather sceptical, particularly when it came to fine tracking, but I decided to give it a try and have found that Press N Peel is ideal for anyone with access to a laser printer or photocopier. Armed with a vector based computer drawing programme such as CorelDraw a simple circuit can be quickly prototyped. Of course a CAD program will do the work for you. A 1:1 scale picture in a magazine can be scanned into your pc.Settings for the printout can be checked on paper first.The settings for the printer should be 'print colors as black' and 'mirror'. (selected on the printing options via the drawing program). Sheets available at several suppliers such as Maplin, CPC and on Ebay. By way of a demonstration I will briefly take you through the process of the design and making of a couple of useful circuit boards for the ubiquitous Comtech 23cms receivers and transmitters.
The blue A4 sized PressNPeel sheets have a shiny side and a dull, coated side. It is this coated side that is printed on with a mirror image of the track layout.Use the printer options from CorelDraw etc to set all colours as black and mirrored. Dont use scale to fit etc as you need the sizes to remain unchanged.
This cannot be done with an inkjet printer, but a laser printer or photocopier work equally well. The toner has to be fused with heat onto the sheet to mix with the impregnated chemicals. It is not necessary to etch from the whole sheet at once, a sharp knife or scissors can be used to cut out a single image.Store the remnants in a air-tight plastic bag.
Using light pressure, clean the copper surface with some fine wet/dry paper.Acetone (nail varnish remover) wiped over the surface should clear any grease/fingerprints etc.Don't use any cleaning liquids.The very clean pcb copper board is then laid flat with the dull (printed) side of the sheet touching the copper and use a smoothing iron (set to temperature around ‘silk’) is gently wiped across the area of the design until the copper is heated to a point where the chemicals on the sheet deposit the toner onto the board rather like using an iron-on transfer.It takes quite a while if using a lot of copper as it acts as a heat sink.
Allow about 5-10 minutes ironing for a board of about 100mm x 100mm, less if smaller. If the temperature is too high the blue plastic sheet will start to crinkle. Ensure that there is a fairly even heat across the board but there is no need to press hard as that will tend to make the tracks splay wider.
After ironing for five minutes, allow to cool a few minutes and gently peel back the transfer sheet from a corner, looking for any tracks that may not have transferred fully. If there are any, simply let the sheet back down and iron for a few more minutes then repeat. Cool the board gently with cold water, or by waiting, down to room temperature then gently peel off the sheet. Any spots missing can be repaired by a Etch Resist Pen before etching.
An etch resist pen can be used for odd spots that need attention in large ground areas or to increase the saved area of copper for example.Nail varnish is ideal for large areas. Etch as usual with ferric chloride crystals (about a heaped tablespoonfull to 1/2pint [300ml] depending on area of exposed copper to deal with) and hot, but not boiling, water (dangerous chlorine fumes), sufficient to submerge the board.
Wear old clothing as F.C. stains permanently.Use latex gloves and a plastic tray (such as those use in supermarkets for whole chickens) cover it with cling film to protect clothes from staining splashes and agitate the mixture vigorously to get air bubbles into the ferric chloride and to prevent any etched copper settling back onto the board.. After 10-15 minutes or so the board will be etched.If it is going a bit slowly because the liquid has cooled, you can remove the board, rinse under cold water tap and place the etchant tray in the microwave for 10 seconds or so to re-warm the liquid.
	Rinse under cold water. The etch resist can be removed with Jif cream and wire wool or wet/dry paper. If not being assembled that day the resist should be left on to protect the copper tracks and removed when needed and the board placed into a sealable plastic bag. To tin the board you will need a small artwork brush (~10mm) and some solder paste which should be stored in a cool place. When buying ensure that it is solder paste not soldering flux which some sellers seem to confuse . Brush it over the tracks and use a hot air blower to flow the solder. Utube has some examples of this to watch. When disposing of used FC remember it will stain clothes and metals incl.cutlery if it splashes, so take care.
	Do let me know how you get on with the above method or if you think there is more detail needed anywhere.