This message was sent by David G4EBT to the UK packet BBSs in August, 1996 and will be of interest to anyone wishing to make their own PCBs using the UV method.
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.
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 7.00 to 10 each, the simplest of commercial U.V. boxes with only two tubes and no timer costs 85 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.
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).
I produce PCB's from magazine articles and from my own designs, as follows:
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.
I have not yet tried the "PCB software/printout to Deskjet onto acetate" technique, nor scanning in from other artwork. I simply use rub-down transfers onto plain acetates, then sellotape a clear acetate on top to prevent damage to the artwork.
EXPOSING THE BOARD: Place the artwork onto the glass plate of the UV exposure box, (making sure it's the right side down!) and place the lacquered board on top. Close the lid to tightly clamp the board down. If it's not clamped tightly, UV will creep behind the thinner tracks and over expose them.
EXPOSURE TIME: Using the Maplin lacquer with one good coat in a four X 8 watt tube box, I find that an exposure time of four to 5 mins gives consistently good results. (The exposure time doesn't seem too critical). Once exposed, do not expect to see anthing but the feintest sign of a pattern on the board. The pattern will not emerge till developed.
DEVELOPING: I have had no success using caustic soda (sodium hydroxide). I've found that either all or none of the lacquer dissolves. I'm told that you only need a weak solution, just enough for it to feel "soapy" between finger and thumb. Well it won't work for me! I use Maplin's Photoresist Developer, Code: YM62S at £3.79 a 250 ml bottle which dilutes four to one with water. I make about two PCB's a week on average and this developer lasts for at least a year.
I pour it into a "Tupperware" food container, about 10" by 6" so that the developer is shallow and you can see the board developing. (The developer soon goes deep blue) As soon as the lacquer has dissolved and your pattern is fully visible, take the board out and rinse it.
If you find for some reason that the result is unsuccessful, all is not lost. You can clean off all the lacquer, go back to square one and recoat the board! In any event carefully inspect the board for any blemishes such as bridges between track, often caused by a build up of lacquer around dust specks, or holes in tracks due to over exposure. These can be repaired with rub down transfers. Meths on a cotton bud will remove any underexposed lacquer/dust specks.
In the final part, I'll cover etching drilling and tinning.
(All information given in good faith, without liability! I have no connection with Maplin's or any other supplier).
PCB TECHNIQUES, FINAL PART.
In the first two parts I covered equipment, materials and techniques for the UV method. Now a few final comments, firstly on double sided board production and generally finishing off your boards:
DOUBLE SIDED PCBS: There are two situations encountered, firstly when you simply need a groundplane on the reverse side as in RF circuits. All you need to do is to protect the reverse side of the board with sticky back plastic such as Fablon during etching of the other side.
Secondly, when you need tracks on both sides. In this case, place the artwork for both sides accurately on top of each other and drill through them and the board at three register points, such as mounting holes. This will help you to position the artwork at the exposure stage and ensure that when the board is completed the tracks on both sides will accurately overlay one another. Spray, develope and etch one side, protecting the second side with Fablon during etching the first side. Then repeat the process for the second side, protecting the first with Fablon during etching. Do not, incidentally, be tempted to drill any other holes until both sides are etched as the ferric chloride will creap through the holes and start to etch away at the edges of any holes!
TINNING AND DRILLING:
Once etched and cleaned, tin my boards as follows: Smear the copper with a thin coating of plumbers flux. The white stuff used for lead free solder works very well, obtained from DIY stores. Using a wide tip to your iron, put a small blob of solder onto the tip and quickly pass it over the fluxed board in a "brushing" motion. This quickly tins the whole board with a thin even coating and avoids overheating any thin tracks and pads.
Rinse the board and for I.C's, resistors, caps and diodes, drill with a 0.8mm HSS drill (Maplin's code: QY64U, 82p ea.). The Tungsten Carbide drill bits are superior but don't last too much longer and cost £3.00 each. I re-sharpen the HSS ones on a small modelling grindstone ( Mapin's code KW16S, £3.99 for three), held in a PCB drill. For Preset pots, trimmer caps and pcb pins I use a 1mm Maplin's code: QY65V or 1.2mm code QY90X, both 75p each.
SAFETY PRECAUTIONS: Commonsense dictates that all chemicals should be kept well away from chidren, pets and the XYL and containers clearly marked so they don't get used for storing the porrage or feeding the cat! Rubber gloves and eye protection are sensible and don't switch on the UV box without the lid closed!
In conclusion, yes, it's messy, and the cost of the UV box is quite high but once you've geared up, the intitial outlay is soon recovered and you can achieve professional results quickly and at negligible cost. I've made boards for such things as the P.W. Robin Frequency Counter (excellent), the G3TSO TX/RX, a masthead preamp, Baycom/JVFAX interfaces, PSU's, etc etc. Presently I'm making the P.W Droitwich locked frequency standard, which has become something of a nightmare because there were errors in the article and despite an apology and assurance from the Editor in March 96 PW, and two letters from me which have so far been ignored, the errors remain uncorrected, except what I've worked out for myself. However, 'twas ever thus with all radio mags and we soldier (and solder!) on! :-)
Disclaimer: As I've said before, all info and advice given in good faith with no acceptance of liability. I have no connection with any company such as Maplin. I'm just a retired old buffer having fun while my wife is at keep fit and can't see what I'm up to! ( She has, however, long since discovered that Ferric chloride makes holes in shirts and sink unit tops and I've discovered that such mishaps induce earache in the culprit!)
I'll be happy to answer any queries but I don't take orders for boards!!
A couple of notes on PCB making:
1. Exposing the board to the UV/Sunlight etc. -rather than use several sheets of acetate to provide a dark enough 'negative'...I have found that a good photocopy on plain white paper will do the job just as well...yeap ! well sure the exposure takes a little longer but the final results are the same. (I dont have a UV light - but there is plenty of sun here in Oz)
2. The etchant which I use is a 50%-50% mixture of HCL Hydrochloric Acid (Swimming Pool Acid in Oz comes in 5 litre containers) and Hydrogen Peroxide (20 Volume - 6% W/V) - The actual mix doesn't appear to be that critical - (though if its too HCL strong you may have to snatch the board before it all dissoves!!). I much prefer it to Ferric Chloride - much cleaner to handle! safer for drains also, and much cheaper, and kinder to the environment.
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