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Old 30th Jan 2017, 6:27 pm   #10
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

I've just etched and drilled the two boards using sodium persulphate (AKA sodium 'persulfate' in the US and elsewhere) etchant, which have come out OK.

As I've said before, this is an economical & much cleaner & etchant than ferric chloride.

It's crystalline, like salt, comes in an HDPE bottle, makes a clear etchant solution which just turns light blue is use, with no horrid sludge. 100g to 0.5L of warm water (40ºC) will give an etching time of 8 - 10 mins. (A quick etching time avoids 'undercutting', which is a risk with thin tracks and small pads). It can be stored for re-use, but if stored, needs to be heated to 40ºC. Presently, 1KG from a reputable UK supplier costs £13.99 post free and will make up to 5 Litres of etchant. (500G costs £10.49 from the same supplier, so it's more economical to buy 1 KG).

I've attached a couple of pics of the two boards I've made.

I'd reiterate that once you've worked out the optimum exposure time for your light box, and the developer concentration, the main stumbling block is to be able to lay the UV film onto the board with no bubbles - even tiny ones, under the film. The best way I've found to do that is to hold the board vertically between the fingers and thumbs of both hands, with the film drooping down but held with the fingers at the bottom, then to gently smooth the film to the board working upwards from the bottom. If you don't press on too hard, you may get a second chance to ease the film off the board if there are any bubbles, but if you press it down firmly, it will stick fast, and any attempt to lift the film will tear it from the board, leaving some of the UV film stuck to the board.

If there is just the odd bubble here and there after the board has been passed through the laminator, after the board has been developed, any blemishes can be touched up with either nail varnish or modelling paint. If there are lots of bubbles, just remove the protective film and immerse the board in acetone for a minute or so to lift off the UV film so you can attach a new piece and start again.

Having used positive aerosol UV lacquer for the last 25 years or so, using this film has been a voyage of discovery for me, and as yet, the largest board I've made is only 7.5 cms square. Quite how it would work out with a board twice that size I've yet to discover!

One last point, unlike aerosol lacquer, which will fill any rough scratch marks on the PCB sheet, this film will not, so the sheet must be polished smooth and not have any deep scratches or there may be breaks in the tracks. Inspect the board are it's been developed and if any scratches are spotted, make sure that any breaks in tracks are bridged with nail varnish or modelling paint.

It may sound like a lot of faff and I guess it is, but like any new skill, there's a learning curve to climb after which, the pitfalls can be avoided.

Hope that's of interest.
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