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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>
From: "John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Message-ID: <59149c35-79a3-2bd7-4b04-6d0967fcfe0a@ecosensory.com>
Date: Tue, 17 Jan 2017 22:58:56 -0600
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On 01/17/2017 10:00 PM, Dan McMahill (dan AT mcmahill DOT net) [via geda-user AT delorie DOT com] wrote:
>> I've been making footprints for QFN packages and came across guidelines
>> by the chip makers that say
>> it is beneficial to have some solder mask under the edge of the QFN
>> package, but not if it gets too thin
>> anywhere, as it can come off and cause trouble if it is.
>
> I don't have great suggestions about the soldermask bit.  Did you look at the QFN footprints in the ~geda library?  I thought we
> had a decent coverage of packages but it has been a while since I looked.

When you dig into the docs about what I saw at first, it seems like solder mask under the edge of the QFN
package could be a negative or a plus.  They suggest all the gaps between paste areas and metal areas to make channels
that volatile gas can escape from during reflowing.  So there are not bubbles left in the solder.

So, the basic shapes we have from footprint generators are OK really.

Vias in a center pad can be a minus if they pull out too much solder, but they are
a help against bubbles forming, and they're needed to connect any separate pads
for current flow.  The goal is often heat flow also, and vias help with that a lot.

Now I'm more confused than when I started reading about it.