gEDA-user: Rounded copper clearances vs square soldermask apertures

[Ben Jackson <ben@xxxxxxx>]

So I've been thinking about how to do the soldermask DRC.  To recap,
I want to include a soldermask registration tolerance in the DRC parameters,
and warn if the mask is anywhere too close (risking an overlap with the pad)
or too far (risking exposing a nearby trace or adjacent plane).

One particular problem is that a copper pad's clearance (from other
copper) is a uniform distance all the way around the pad, producing a
rounded rectangle.  On the other hand, the hole in the soldermask for
the pad is simply a rectangle that's bigger in X and Y by the mask amount.
So if a pad has a 10mil clearance and a 10mil mask, there will be an arc
of exposed copper even with ideal mask placement.  This is most obvious
when there is a flood of copper (with a polygon) around the pad.  You
can turn on the soldermask and zoom in to see.  The maximum allowable
mask would be the clearance/sqrt(2) (via a bit of trig).  So a 10mil
clearance would allow a max ~7mil mask.  If you had a 3mil registration
tolerance for the mask, it would only allow a mask of either 3 or 4mil.
If it had a 4mil max error there would be no possible mask (given a 10mil
clearance) that would be guaranteed not to possibly hit the pad or expose
the adjacent copper.

Do people think the mask *should* have square corners, or should they
be rounded to conform to the clerance, and leave more leeway?

-- 
Ben Jackson AD7GD
<ben@xxxxxxx>
http://www.ben.com/


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