Re: gEDA-user: DIY brass stencils

[DJ Delorie <dj@xxxxxxxxxxx>]

> The next thing to try would be compensating the shapes for the
> distortion caused by the etching.  You should try bowing in the
> sides of the openings to see if you could square up the holes, like:

The issue is that, if you're etching through 5 mil of brass, you
should expect 5 mil of undercutting.  I don't think there's much of a
way around that, so I'm kinda limited to a 5 mil radius by the time
I'm through the panel.  I thought of masking both sides, which would
cut both etch time and undercutting in half, but lining up the masks
proved to be difficult.

If you look at the lower right on this picture, for example:
http://www.delorie.com/pcb/brass/20070221-60-top.jpg You'll see that a
pinhole in the toner results in an etched cone about 10 mil in
diameter on the toner side.

What I need to do is try to second-guess the over-etching and figure
out the right ps-bloat setting so that the final size is what I want.
Note that it differs depending on pad size; the thin holes for the 60
pin connector (0.5mm pitch) didn't over-etch as much as the bigger
holes.  In fact, they came out pretty much perfect with a ps-bloat of
-200 (2 mils smaller on each edge).  I might go to -250 depending on
the results of the squeegee testing.


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