There have been quite a few help requests for info on how to successfully
make DIY PCB's by the U.V. exposure method. Over the years I've experimented a
lot with various techniques and have now evolved a system which gives consistent
results. I thought it might help others if I shared my experiences and gave
advice on equipment, materials and methods. I'll cover this in more than one
bull so the bulls aren't too long.
EQUIPMENT:
You need a U.V. light box consisting of at least two 8 watt U.V. tubes for smaller boards, up to say 6" (15cms) square and ideally four tubes for larger boards. Two tubes on large PCB layouts can cause under exposure at the edges of the board. Though the tubes are not cheap, at about œ7.00 to œ10 each, the simplest of commercial U.V. boxes with only two tubes and no timer costs œ85 from Maplins and is very inferior to what you can make for yourself. NOTE: The "black light" disco type tubes are NOT suitable, you must use the proper U.V. tubes!!
In adition to the tubes you need a starter and a choke for each tube, though since the tubes are only used intermittently for a few minutes at a time they could no doubt share the starters and chokes. (My box has a starter and choke for each tube). These items and such things as tube end caps are available from Farnell and other sources. (I obtained my tube end caps from two new double striplights bought on a rally stand for £3.00 each).
For convenience, it's better to have a built in timer with a range of preset times up to about 8 mins, plus manual start and finish, plus a piezo sounder to indicate end of exposure. It's from an article in Everyday Electronics, Sept 1985, but there have been several similar articles in other mags over the years. If anyone is stuck and really wants to make a box I'd be happy to advise further.
Ordinary 4mm thick glass is used, about 3" above the tubes. The box has a lid lined with dense foam rubber to hold the U.V. lacquered PCB and artwork tight against the glass during exposure.
The box is lined with metal baking sheet which has a semi-shiny silver finish. This was obtained cheaply from the kitchen implement section of a supermarket and cut to size. It ensures maximum light reflection upwards to the developing PCB.
Apart from the light box, the only other "special" equipment I use is an etching "bubble bath" made fom a plastic cereal container, with a 6" long aquarium airstone glued in the bottom and a plastic tube glued up the inside of the container. The air is supplied from an aquarium air pump. The airstones cost 50p and the pump was about a fiver. A plasic mesh "cradle" made from a drainer of the type which you place on a sink draining board is used to immerse and remove the PCB during etching. I find that by using this type of bubble etch tank the etching process is greatly speed up (about 10 mins), though for years I simply used a margarine container which I rocked back and forth to speed up the etching process.
PCB MATERIALS:
There are two options, firstly, to use pre-sensitised boards, which are expensive and have only a thin coating of lacquer. You need to waste quite a lot, to find the correct exposure time and also end up with useless offcuts too small to use but for which you've paid a lot! Secondly, to coat your own boards with U.V. lacquer, which is the method I use. It's best to coat a piece of board slightly larger than the finished size as you can get either a thining or thickening of the lacquer at the edges of self coated boards. Use only fibreglass board, NEVER SRBP and especially not the horrible blue stuff sometimes seen at rallies. Only fibreglass has good adherance of the copper on the board, on other types the etched tracks will lift as soon as a soldering iron is used. Rememeber that in commercial production, soldering is done by the solder bath ripple method not by a ham with a 25 watt iron!! The best type of board, often seen cheaply at rallies, is that which shines like glass on the plain side, (unless of course it's double sided!) You can get several square feet of offcuts at most rallies for about a fiver.
Clean the board with steel wool or an abrasive kitchen pad and scouring powder and wipe it over with meths or Iso-propanol from the chemists. Ensure that the surface is dust free.
Spray in a dust free atmosphere using U.V. lacquer (Maplin's code No: YM62S at£7.99 a can which lasts for ages and covers several square feet). Spray in as dust free an atmosphere as you can. I do mine in the greemhouse. Don't wear a wooly jumper, (Nor an anorak!) Spray one good coat with the board on a dead level surface and immediately cover the pcb with a box to preclude daylight and dust settling on it. After an hour or so remove it to where it can fully dry overnight. I made a small "drying oven" but now I simply put the PCB in a desk drawer overnight. (You don't need to worry about limited exposure to daylight as there's not enough UV even in strong sunlight to cause any bother for a minute or two).
ARTWORK:
I produce PCB's from magazine articles and from my own designs, as follows:
MAGAZINES:
I have found to my pleasure, that two, sometimes three, photocopies onto acetate at the local copy shop, laid one on top of the other, will give sufficient opacity when using self coated boards, to give consistently good results. Hold the copies up to the light to see if you have two identical ones as the copying process, especially on larger boards, can distort the image so that the images don't quite accurately overlay one another. In that case try taking about four copies and select a couple for best fit. (Any feint lines can be touched up with rub down transfers). Sellotape the acetates together and handle them with care so you dont scratch off any of the images. |
