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Subject: Re: [geda-user] QFN packages solder mask
To: geda-user AT delorie DOT com
References: <2df480cc-5ef2-9ac6-b7ad-d17788a6b8b9 AT ecosensory DOT com>
 <aec326a8-34dd-b47e-837a-b249748918b0 AT mcmahill DOT net>
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From: "John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Message-ID: <d7a0f2d8-697d-9c75-96c2-5945911d3359@ecosensory.com>
Date: Wed, 18 Jan 2017 13:40:39 -0600
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On 01/18/2017 06:03 AM, John Luciani (jluciani AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> What type of stencil openings are you using?

I was thinking about .02mm more than the pad for the 0.5mm spacing pads 0.3mm wide with a 0.2mm gap.

but then I read where all the mfrs want those pad rows to be one open rectangle.

I have about 0.7mm openings for each pad that overlap, so a better way to say it
is the no mask area extends 0.2mm beyong the pads.

>
> For the thermal pads I typical reduce the coverage to
> between 50 - 70% (depending on the aperature dimensions).
> The reduction is done using a layout similar to your footprint.
> The spacing between the paste areas provides the channels.
>
> I also reduce the coverage on the electrical pads.

The small ones around the edge?


I am going with a 9 pad grid in the center like the upper right of

http://ecosensory.com/geda/qfn_lands_problem.png

but with the mask opening like in the lower right.  (No mask under edge, no separation of center and edge row pads
but for lack of metal.)

I'm planning on 4 center zone vias for heat and electrical contact.  5 will only help stick it to the board.

Now that I think of it, I should probably drastically reduce the paste and metal of the non-via connected ones
to let bubbles out even better.  It's not a high power app, just a STM32F401CE.  The go ahead and put the vias
into the footprint also.  For now, I'll just place vias along with parts and use the k key action command to resize pads.

Any hints on success appreciated.

-- 
John Griessen -- building field gear for biologists
Ecosensory  Austin TX  ecosensory.com