Saturday, September 14, 2013

My PCB Fabrication Process


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:
Copper Side First
  1. Sand board with 400grit (wet sandpaper) wet with just a drop of dish soap
  2. Dry board with a paper towel
  3. Apply rubbing alcohol and dry with a paper towel.
  4. 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)
  5. 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.
  6. 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.
  7. 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.
    9. Rinse thoroughly, then Sand with 400 grit to remove toner, should look
        like this: (note the 2 is missing from the 2013? yep, didn't transfer perfect)
Then the Front (silkscreen)
    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.
When it's all assembled and tested I spray the copper side with several thick coats of Rust-Oleum clear coat.

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