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Date: Sat, 24 Mar 2012 15:18:41 -0700
From: Colin D Bennett <colin AT gibibit DOT com>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Solder paste/nopaste flag overlapping, and custom
 paste apertures
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On Sat, 24 Mar 2012 06:50:19 -0400
Bob Paddock <bob DOT paddock AT gmail DOT com> wrote:

> > I read an article somewhere that described a modification of
> > this where the QFN exposed pad is actually segmented by solder
> > mask as well as the paste stencil -- this keeps the solder
> > evenly distributed even if the chip tries to tilt a bit.
> 
> It is only the stencil for the paste that should be segmented, to
> control the amount of the paste.
> The mask and pad should not be segmented.

I took that idea from:

“Novel QFN Land Pattern.”
<http://blog.screamingcircuits.com/2009/01/novel-qfn-land-pattern.html>

Quote, speaking of the idea of segmenting the solder mask in
addition to the paste stencil:

> If only the paste layer was like this, the solder would spread
> out and there would likely be too much voiding. It might not
> connect in the center at all.

The article suggests that segmenting the solder mask to create
separate, smaller openings is the right way to go.  Perhaps that's
not the standard technique, but I have heard it suggested by
another source before.

If the article included comparison test results between various QFN
techniques, it would obviously be more useful.

The fact that the Screaming Circuits Novel QFN Land Pattern
results in SMD (solder-mask-defined) pads might be an issue.
SMD pads are sometimes not recommended since the solder mask on the
copper will potentially prevent the IC from being as close to the
board (and copper pad) as it would otherwise be.

I've used this Novel QFN Land Pattern on a couple of prototype
boards with a two out of three success rate, but that was using
hand-tinned pads instead of solder paste.  Next time I'll try the
standard method with a segmented solder paste stencil and see how
it works.

Regards,
Colin