gEDA-dev: code sprint ideas

[Peter Clifton]

On Mon, 2007-10-15 at 12:14 -0400, DJ Delorie wrote:
> > You want rounded clearances, but square soldermask apertures.
> > 
> > In the example above, all but the pad would be covered with soldermask.
> 
> Except that fabs won't let you do that; they want the mask to be
> bigger than the pad by some amount, in case of registration errors.
> If you're using a 3 mil gap and 6/6 rules, an offset mask could
> uncover some adjacent copper near the corner.  With a 3 mil gap
> (i.e. +- 3 mil mask accuracy), you could end up with 6 mil on two
> adjacent sides, or 8.4 mils from the pad's corner to the mask's
> corner.  If you have a 6 mil copper clearance, you've just uncovered
> 2.4 mils of the wrong copper.

With a rounded mask, you take the risk that the registration error is
within the fab's spec, and you get some mask on the pad. (Is that ok, or
would it cause production issues?)

http://www2.eng.cam.ac.uk/~pcjc2/geda/notches2.png

At the risk of being provocative.. what does other board layout software
do? (Incl. the high end ones which might get used for this kind of
fiddly work). This isn't completely rhetorical.. I "think" I've only
ever seen boards with square mask cutouts, but haven't used high end SW
to know for myself.

Peter