X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f
X-Recipient: geda-user AT delorie DOT com
X-Original-DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=sendgrid.net; 
	h=to:from:subject:mime-version:content-type:content-transfer-encoding; 
	s=smtpapi; bh=NdecKcMzqVP5DB3RkmUkrtSahBg=; b=ONmpA19lvqGkfux+XO
	YY8aaMC+8aI4yv3yIrnGNeWFu57p6hGj8HRzu9JXb/MS0if74067Ae/7SfA+2YmO
	zUD8cxmci2W5N8n9JPihQ31Dv7E3IMME3pehknQuNt1GmzRBiVp59Zvh8BFzS+ip
	GuW/zvsE71JGA6pUvpy1z8W2g=
To: gEDA users mailing list <geda-user AT delorie DOT com>
From: "John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject: [geda-user] QFN packages solder mask
Message-ID: <2df480cc-5ef2-9ac6-b7ad-d17788a6b8b9@ecosensory.com>
Date: Tue, 17 Jan 2017 15:47:09 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Icedove/45.4.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-SG-EID: V53lTA/kUP1+IqXnzXuv0M/cu/N8aMtf7nxyAyKnAkvpA3CJe/tK46fZd1SL2tTYyES+rAaH4/jt4J
 U5UySmYXyaN4Kh1BDLbSshMlPuPGplqNSsJrMHc2XExyypTQJuzSdc/Xhlg5c5w89Vricae25b++IR
 hi1g3SvUg7Fm6WtLoCTLNpnFdEdBwKI/7lgacJOzALXO5yxdu3OJ7q9eT2U5/yIXe98Krz9y23whn6
 I=
Reply-To: geda-user AT delorie DOT com

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.

So I was wondering if there is any way to get zones of solder mask added.  PCB generates solder mask,
it does not define it directly, so I'm looking for a trick.

I experimented with the thickness clearance and mask values in the pad line of an element.
The sharp square corners of the mask layer cause problems at QFN package corners.

The reasons are graphically shown in these files:

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

Is there a gerber editor to use?

In a while, pcb-rnd will be probably able to handle solder mask directly and go beyond
layer growing limitations of solder mask and clearance, but I need something now.

If fabs would let you, a speck of metal that would be gone in the etch could have
solder mask opening and put it where I want it...

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