Source of UV Photoresist lacquer?

[Yorkie]

I use a UV light box, and print a mask onto clear OHP film, or rather I did until recently, when I found a better alternative which will work reasonably well with my Epson printer. I think I mentioned elsewhere that 'Big Clive' (who you know), explained how to alter the printer settings in Epson printers to improve their ability to print onto OHPs, for which Epsons have no settings in the print options. The type of film I've started to use  is called  'micro-porous fim for inkjets', and is described thus:

"A slightly milky, high quality acetate film for inkjet printers which is specifically designed for making stencils for screen printing. The milky micro-porous surface absorbs the ink from the printer to create a dense result, essential when it's being used to block out UV light with screen printing emulsions. The best quality photo-stencils can be made from this film. Can also be used permanent markers".

http://www.craftycomputerpaper.co.uk/.-M..._S048A.htm

The PCB layout is one that I've tweaked for the mini mod. I wasn't keen on the original as it had small pads, narrow tracks and a large area of ground-plane.  I've retained the same basic layout, but for DIY PCBs, I prefer as much space as possible between tracks, and pads for components such as inductors and ICs as large as they can be. You'll see that some of the lettering on the etched PCB (+ - A, L, R, and 'Mini Mod' hasn't come out well, but not worth scrapping the boards just because of that. They're fine in other respects. I tweaked the mask to make the lettering more prominent. I'm not putting forward the artwork as an exemplar - just did it to suit my own purposes.

I print a full sheet of images (six fit on an A4), then tape three images together to make sure that UV doesn't find its way through the black areas.

It could of course be reversed for the laser  iron-on technique, and I guess that if it wasn't for the fact that I've been using a UV box for  more than twenty years for occasional PCBs, I'd perhaps get a cheap mono laser printer and try that technique. I suppose we all do what seems to work for us.

Hope that's of interest

[Nick]

Interesting - the reason I ask is that the images you posted of the completed boards appear to be "undershot" - that is UV has crept in under the edge of the mask creating a jagged finish...

What a lot of folk tend to do is to print a mirror image of the mask, so that you apply it to the coated PCB INK SIDE DOWN, so the mask is in direct contact with the photo-resist, not via one or more thicknesses of acetate.

The mask is then pressed to the board with a sheet of glass (not plastic).

Doing it this way results in far finer edges to the finished board, with consequential greater overall accuracy due to lack of any parallax error. This is what I always did (never used multiple layers) - I used to be able to do 10mil feature boards with ease.

Eagle and I presume other PCB packages have this feature as an output option for exactly the reasons mentioned - otherwise you can take the image and flip it in MS Paint or similar...

HTH

[Yorkie]

Thanks for reading Nick, and for your helpful comments.

Good point re reversing the image. I think the main  reason that the edges look jagged - particularly the sloping ones - is that I used MS Paint to create it, rather than a PCB program, so any circular or sloping areas are 'stepped'. But that said, really, I should reverse the artwork as a matter of course to ensure that the ink side is in contact with the board - not the glass. Using 'Paint' is rather 'hammy' and not the best approach to creating PCB artwork of course, but I just don't have the time to become proficient in any of the PCB progs for the few PCBs that I make - maybe half a dozen a year.

[Alan Douglas]

Years ago when I was making boards for the company I worked for, the UV source was a mercury-vapor suntan lamp, a foot or so above the sensitized board and negative. This might possibly give a sharper "shadow" than a light box.

[ed731pdh]

For years now I've been laser printing direct onto 63gsm tracing paper, the best stuff is from WHSmith as it has a really nice fine grain. My donkeys years old HP5P has a single sheet loader into which the tracing paper goes and generally does not crease during printing.

[Alan Douglas]

You can buy sheets of clear film (Mylar I presume) for laser printers. I got some where I worked; otherwise I'd have had to buy a box of 100. I never tried it for printed circuits.

[Yorkie]

Just a little update.

The UV lacquer I bought has worked really well. Sprayed the PCBs on a dead level surface - five or six passes to give a good layer of lacquer, sprayed in the garage where the dust is minimal, then covered the boards with a plastic container to exclude dust and light. Exposure time not critical - I used 8 minutes to make sure they were fully exposed. A pic of an etched board is attached, alongside a built board. I prefer to spray my own boards rather than to use pre-sensitised board, because apart from the cost advantage, if there are any hitches in exposure or development, the lacquer can be cleaned off with meths and re-sprayed. Too often in the past, when using pre-sensitised boards, I've ended up with just a piece of PCB, and even when successful, lots of useless sized offcuts.

Just need to box it up and finish it off now!

Hope that's of interest to someone.

[Alan Douglas]

Etched looks a whole lot better than a routed board.