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Date: Fri, 11 Sep 2015 21:31:48 -0400 (EDT)
From: GENE GLICK <geneglick AT optonline DOT net>
Subject: Re: [geda-user] zero soldermask clearance not caught by DRC
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> Usually solder mask is a little bit larger to not cover any of the 
> pad. I guess you will be able to solder the part even if "visible" pad 
> is a little bit smaller because of tolerance problem with solder mask.
>

Yes, I agree with you.

Presumably, you've designed your pads to meet some guidelines (maybe 
manufacturers recommendation). That gets you a nice heel and toe solder, 
and maybe some solder on the sides of the pad. As you say, you don't 
want to cover the pad with soldermask, and I'd say because you don't 
want to shrink the exposed pad geometry and compromise your solder 
joint. In a perfect world, 0.0 gap from pad to mask is okay. But, 
because the board manufacturer cannot guarantee this precision, they 
like to add a gap such that under worst case the mask doesn't cover the 
pad.

A good fab house will adjust a lot of things on your design in order to 
maintain it's internal design rules. I suppose not all places would do 
this. I also suspect some folks want ultimate control of the board so 
that what they get exactly matches the gerber. In those cases, maybe the 
fab process is iterative, so the fab house suggests changes in the 
layout until it meets their design rules.

As I said before, the place that my company uses will increase or 
decrease mask clearance in order to make their design rules work. That's 
why setting 0.0 mask clearance is okay - in this case. Like you, 
however, I prefer to set the clearance to around 10 mil in my designs. 
If your design house has no checks, and no feedback to you, then maybe 
they do something to your design, maybe they don't. It would be nice to 
know for sure.

gene