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Old 11th Jan 2017, 7:38 pm   #5
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: UV Photo-Resist Dry Film PCB Process

Thanks for reading the thread Joe, and for your helpful comments. I've been making PCBs using positive UV spray since the late 1980s, but only learned recently of the 'dry film' negative process which I thought might be easier and less time consuming than spray and once the exposure time and developer concentration has been optimised, would give consistent results. It certainly looks that way in the video. For me, it's early days yet. One reason for the wrinkles and bubbles might be that the laminator is too high a temperature.

As to artwork, I've had a go with various programs and haven't got very far with them. Some of the freeware ones have shortcomings for DIY PCBs as the tracks are needlessly thin and the pads needlessly small. In part, that's because the software is intended for the user to use a PCB Pool to get the boards professionally made. If - as occasionally happens - I see a PCB designs created by such programs, ('Easy PCB'?), I set about editing the design by increasing the size of the pads and thickening the tracks to prevent 'undercutting' when the board is etched. I use MS 'PAINT' for that, and for creating artwork to my own requirements. For simple one-off boards, I occasionally use 'rub-down' PCB transfers.

I print the artwork onto 'micro porous film' from a UK firm called 'Crafty Computer Paper' which creates excellent opaque masks, and print them so that the ink side is to the PCB to prevent 'undercutting' as the broad is exposed to UV.

As to etchant, I supposed much depends on what chemicals are availed which might differ from county to country, but I abandoned Ferric Chloride years ago - it's really messy stuff and I'm surprised that it's still quite widely used. Sodium persulphate (outside the UK often spelt 'persulphate') is a far better option. It's fine white crystals and clear when mixed with water, so when the board is etching, you can see the progress. Over time, it turns pale blue, and can be sorted for re-use. No nasty brown sludge. It works quicker if heated to 50C, for which I use an acquarium heater with the thermostat shorted out, and I've made a little tank which has an acquarium pump and 'airstone' to bubble air through the etchant to speed up the process.

I always clean the board scrupulously, rinse it in water then wipe it with IPA. When using the positive resist spray, I do it in the garage as that's as dust free as I can get, and having placed the board on a level surface, I shield in with a large plastic browl hovering above it, and spray the board beneath the bowl, then lower the bowl to cover the board till dry. That way, very few specks of dust fall onto the lacquer.

I generally get good results, but the dry film process seems much less of a faff than spraying the board with lacquer and leaving it overnight to harden off. There are quite a lot of videos on YouTube which indicate very successful results with the negative film, so when I get back home from London next week (counting the days!), I'll see if I can find out which the film getting little bubbles and wrinkles When passed through the laminator. The film is cheap enough to experiment with!

Thanks once more for your input Joe.
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