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Old 14th Jul 2017, 12:25 pm   #11
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 3,280
Default Re: UV Dry Film PCB technique - experiences to date.

Originally Posted by Oldcodger View Post
David -can you advise on how well the attached will come out ?
Firstly, well done on getting to grips with Express PCB and for creating your layout.

As I've said before I'm not a fan of Express PCB because basically, like similar PCB programs, its intention is for the DIY creation of a PCB design but for that design to be commercially produced. I'd reiterate that to my mind, the tracks are usually too thin and the pads far too small for DIY etching and drilling. This risks the tracks being undercut during etching, leaving even less copper on the board, the holes being miss-drilled and the copper lifting due to heat when soldered. No reason at all that you can't create the design in Express PCB and use that as a basis to refine the layout in Photoshop, PAINT or whatever.

In your design, the track widths looks fine, but in my experience, the pads are too small to survive etching, drilling and soldering. Not a big job to 'beef them up' but for DIL I.C. pads, about 1.8mm is about as large as you can go. Whenever there's space, I try to make the pad size no less than 3mm diam. As an example, I've clipped a portion of the bottom RH corner of your design, and using MS PAINT have increased the size of the pads to show what I mean. I also note that there seems to be a grid of feint holes on the tracks, which I've highlighted. If they're intended to be etched, they'll weaken the track so would need to have pads created around the holes. Otherwise, if not to be etched they'd need to be blacked out.

Personally, without wishing to drone on about it, whenever I can, I try to have as thick a track width or as large a pad size as is possible, and as much clearance between tracks. Also, the more copper that can be left on the board, the quicker the etching time. If I see a design which I'd like to use, which has thin tracks and small pads, (often showing the hallmarks of being an Express PCB design), I use that as a basis to retain the same basic layout, but to increase clearance between tracks and a groundplane if there is one, and to increase the track thickness and pad size. As an example, pic 2 shows the track layout of the 'mini-mod' kindly provided by Ian, the designer, and pic 3 the pattern that I created from that, which retains exactly the same holes and component placement, but for me at any rate, maximised the chance of successful DIY etching & drilling. (A PCB production company would of course have no difficulty in making PCBs from the original design, and maybe some home-brewers too).

The fourth pic is of a small PCB for a sawtooth generator which I didn't design, but modified the artwork to leave more copper on the board.
That last pic shows the PCB that I etched and drilled from that artwork.

Not in any way intended to be critical of your efforts - just a kindred spirit who's trodden this well-worn path on a 'voyage of discovery' in the black arts of home PCB production, trying to give constructive tips and advice in helping to avoid pitfalls. Every good wish in your efforts - you've already invested much time to get to this stage and I'm sure it won't be long before your first PCB project will be up and running! The overall aim is to maximise the chance of success at every stage - artwork, exposure, developing, etching, drilling and building, so as to achieve consistent rather than hit & miss results.

Hope that helps.
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