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Date: Sun, 13 Sep 2015 00:21:03 +0200
From: "Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To: "Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject: Re: [geda-user] zero soldermask clearance not caught by DRC
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On Fri, Sep 11, 2015 at 01:29:06PM +0200, Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> On Fri, 11 Sep 2015 06:40:53 -0400 (EDT)
> GENE GLICK <geneglick AT optonline DOT net> wrote:
> 
> > I don't think you are going to have any problems. I've worked with 
> > layout guys who routinely have 0 clearance on the soldermask. The board 
> > fab houses will adjust their process, sometimes without telling you. At 
> > work, we use a pretty good place, and they will adjust the clearance up 
> > or down as needed. I think the fab houses have to make adjustments to 
> > your design in order to work with the tolerances of their own processes.
> > 
> > Anyway, you can ask the fab house if they fix that sort of stuff up or 
> > not.
> 
> 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.


While this is the general rule, guidelines for small pitch BGA 
(0.5 mm and lower) often recommend using SMD (solder mask defined) pads:


http://latticesemi.com/view_document?document_id=671

    Gabriel