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Message-ID: <4F6F7D80.8050308@ecosensory.com>
Date: Sun, 25 Mar 2012 15:18:08 -0500
From: John Griessen <john AT ecosensory DOT com>
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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 03/24/2012 05:18 PM, Colin D Bennett wrote:
> 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:
>
>> By creating a bunch of openings that are the same size as the side contacts,
>> this QFN will get good grounding and there won't be any issues with too much or
>> too little solder paste getting in the way. It pretty much
>> bypasses the standard QFN problem.
 >>
>> If only the paste layer was like this, the solder would spread
>> out and there would likely be too much voiding.

I don't see how he thinks a sparse solder paste pattern would cause voiding
if you planned how thick it was put on and for "floating the QFN" to a
prescribed height above the PCB surface.

The sparse pattern of paste and mask seems to guarantee a tiny amount of paste
under the QFN and very likely it will suck down to too close to the board.

Like Bob P. has mentioned before, if you solder QFNs  so they are too close to the board,
you can never wash the flux out and they are no good for high Ohms and maybe no good
for longevity even in medium Ohms circuits.

John
avoiding QFNs at the moment...