Re: gEDA-user: soldermask clearance

[Ben Jackson <ben@xxxxxxx>]

On Sat, Jan 16, 2010 at 03:46:33PM -0500, gene glick wrote:
> What's the right amount of clearance?  Seems like .003" is good, but I 
> don't know.

That has worked for me.  Far better than leaving the random component
defaults (often 10mil) which are far too large.  What it really boils
down to is how accurately your fab can register the mask.  Keep in
mind that if you go even to 4mil clearance on the mask because they
can only hit +/-4, your mask could extend clear across an 8mil gap
with the worst case misregistration.  So to put it another way:  Any
decent fab can hit 8/8 rules and your mask clearance better be less
than half the minimum gap, or <4mil, so 3mil seems good.

> A google search found one article that said for fine 
> pitched parts, it's better to have the entire row blocked.  That is, 
> instead of individual clearances around each pad, the soldermask exposes 
> the entire row at once.  They claim the very thin webs in between pads 
> are too thin to cure correctly, and will smudge later onto the pads.

A good fab will modify the mask in that case even if you specify it.

> If there's no soldermask in between pads, isn't is more likely to get a 
> solder bridge?

I've done .050mm QFP (about 0.020") with and without mask between the
pads and I didn't find a big difference.

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


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