My last desulfator design was Rev B. I decided it should be Rev A (even though Rev B is etched on the board), as it was the first desulfator board I actually built. For future reference I'm doing major revisions A, B, C, but minor (and or pre release) tweaks 001, 002, etc. So this should be Rev A 003, I think.
For more information on the desulfator see my previous post here.
Specs / Limitations
- When I do the layout I've found I can etch down to about 0.010"(50% success), but 0.020" is much more forgiving (80+% success). So I do most of my traces as 0.020". but they won't fit between some pins, so I run a 0.020" trace as close as I can, end it, put a 0.010-0.012 trace in the tight spot, end it, and continue with a 0.020 trace.
- I try to maintain 0.025" clearance around traces, since I don't use a solder mask it helps me avoid bridging traces.
- I flood-fill everything I can, the more copper I leave on the board the quicker it etches.
- So far I only do single-sided boards. Hasn't been a huge issue yet. I'd be tempted to just order 2 sided boards when I need them.
I use the toner transfer method for both the etch resist and the silkscreen. As far as prep it goes like this:
- Sand board with 400grit (wet sandpaper) wet with just a drop of dish soap
- Dry board with a paper towel
- Apply rubbing alcohol and dry with a paper towel.
- Print resist on a piece of shiny paper, I have varing results with this, seems to depend on the paper I find. I read that the (glossy) whites of newspaper / magazine ads worked the best, but I find that some heavily colored ads have more of the wax / clay stuff on them and work better for me, with the exception of the silk screen side (more on that later)
- Align board and paper, either fold the ends over to keep it in place or use a tiny piece of tape. Then run through laminator 5+ times.
- Soak in cold water until paper starts to dissolve. Some times it peels off, other times I have to rub it until just the toner remains. sometimes I loose traces and stuff, if it's not too tight I continue, otherwise back to step 1.
- Gently dry and examine carefully, touch up little holes in the flood fills and broken traces with a permanent sharpe(3 coats).
For etching I use cupric chloride, it works really good above 60F, somewhere around 40-50F and below it's really slow. It needs to work fairly fast, the majority of the problems I've had with it were when it's cold out and instead of 20-40 minutes it took 2-3 hours.
I keep it out in the garage, as I prefer not to risk spilling stuff like that in the house. (I don't want to spill it out there either, but I have better ventilation and access to a garden hose for emergency clean-up)
8. Etch until traces are isolated.
like this: (note the 2 is missing from the 2013? yep, didn't transfer perfect)
10. Dry then drill two holes (for through-hole parts) as far apart as possible
(either corner would be good) and use them to align your silk screen (do
steps 3-6). I poke a wire right through the board and paper for alignment.
(remove wire before running through laminator) Note: I try to use mostly
white paper for this as the colors tend to stick.