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Mail Archives: geda-user/2019/03/21/07:04:25

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To: geda-user AT delorie DOT com
From: "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject: [geda-user] Modify SMD stencil aperture size to prevent short circuits?
Message-ID: <999a6a2b-c727-4f22-3aaf-b6c20cf15807@linetec.nl>
Date: Thu, 21 Mar 2019 12:01:44 +0100
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Reply-To: geda-user AT delorie DOT com

With decreasing SMD pin pitch sizes and ever more pinless (QFN) 
housings, I increasingly run into problems with short circuits or open 
pins, at least with manual assembly. With more complex PCB's, I 
sometimes get 20%-30% with solder problems.

The main problem seems to be an excess of solder paste ending up on the 
SMD pads. One cause I found is pre-tinned pads, which tend to slightly 
raise the stencil. This is solved easily by requesting gold plated 
surface finishing, so that the stencil lies as much as possible flush to 
the surface when applying the solder paste.

But even then, and with the thinnest (100 um) stainless steel stencils, 
I occasionally get shorts.

(Open pins result when a largish QFN housing has a central ground pad 
that accumulates more solder paste relative to some of the small pins, 
making the device and thus those smaller pins float above the paste. 
This too could probably be fixed by making the central aperture smaller, 
although this seems mostly a matter of being very careful about pressure 
when applying the paste, and of course keeping the stencil absolutely 
clean.)

After some searching, I found that decreasing the size of the openings 
in the stencil and thus the amount of solder paste ending up on the PCB 
appears to be a common way to prevent shorts of this nature.

Can anyone confirm that this is indeed a good idea? It would probably 
mean that I'd need to make two copies of each layout: one with the 
desired PCB pad sizes, and one with reduced pad (and thus aperture) 
sizes for stencil production. And what reduction percentage would be 
recommended? At several dozen dollars plus at least a week delivery time 
per stencil, I don't want to mess around too much with trial-and-error.

Or is it a matter of (still) better workmanship on my behalf? I'm 
already using a stereoscopic magnifying headset, I have quite a steady 
hand, and the solder paste usually looks quite good right after 
application (and if it doesn't, I simply scrape it off, clean the PCB, 
and do it again).

Any thoughts on this are appreciated!

Thanks in advance, best regards,

Richard

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