X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f X-Recipient: geda-user AT delorie DOT com X-Virus-Scanned: Debian amavisd-new at gag.com From: Bdale Garbee To: "John Luciani \(jluciani\@gmail.com\) \[via geda-user\@delorie.com\]" , geda-user AT delorie DOT com Subject: Re: [geda-user] QFN packages solder mask In-Reply-To: References: <2df480cc-5ef2-9ac6-b7ad-d17788a6b8b9 AT ecosensory DOT com> <59149c35-79a3-2bd7-4b04-6d0967fcfe0a AT ecosensory DOT com> Date: Sat, 21 Jan 2017 10:25:44 -0700 Message-ID: <87d1fg1g13.fsf@rover.gag.com> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha512; protocol="application/pgp-signature" Reply-To: geda-user AT delorie DOT com Errors-To: nobody AT delorie DOT com X-Mailing-List: geda-user AT delorie DOT com X-Unsubscribes-To: listserv AT delorie DOT com Precedence: bulk --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable "John Luciani (jluciani AT gmail DOT com) [via geda-user AT delorie DOT com]" writes: > For the thermal pads I typical reduce the coverage to > between 50 - 70% (depending on the aperature dimensions). Yep, good plan. You really want the reflow process to "suck the part down" against the ground pad, or you'll have marginal results on the connectivity of the signal pads. A number of years ago, I built a footprint for a TI/Chipcon QFN-36 part (the CC1111 RF system on chip), in which I meticulously followed their recommendation to go even farther. They called for small spots of paste on the exposed pad area between vias with mask over them. On these parts, that pad is the *only* ground for the part, and since it's an RF part getting the ground paths right is a big deal... so you *must* use vias in the pad and handle them well or the part won't work very well. This was a pain in the ass to figure out how to do in pcb. I wrote a script that did the math and output a footprint with a lot of small elements, some overlapping, to control exactly where copper, mask, and paste get placed. Worked great, and we've shipped a lot of product using this footprint and others like it that I've created in similar fashion in the years since.=20 One of the lessons I learned from this was that I really prefer using scripts to describe footprint geometry. I've never used a GUI to create a footprint for PCB, and frankly just can't imagine wanting to... In any case, if you want to see what I did, have a look at TI-QFN36.py in the 'hw/altusmetrum' repo on git.gag.com, here's a direct link to the file in question: http://git.gag.com/?p=3Dhw/altusmetrum;a=3Dblob;f=3Dpackages/TI-QFN36.py The script isn't "pretty", as it was hacked together over at least one long night, but I suspect a real-life example could help some of you trying to figure out how to do this stuff. This footprint has just about all the craziness I've ever put in a QFN footprint (mask over vias, partial exposure of the pad through the mask, small spots of paste on the exposed pad, etc) all in one place...=20=20 Thanks to DJ, et al, for patiently answering questions for me on IRC way back when as I first figured out how to do things like this in pcb... All of the products shipped by Altus Metrum, LLC, to date have used a makefile-driven gschem -> pcb workflow, and all of our designs are in the hw/ tree on git.gag.com if anyone wants to learn more from studying what we've done.=20 Bdale --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEhHDyCwYlkhh8unuzOpNhlsCV2UEFAliDmZgACgkQOpNhlsCV 2UEBmg//YD8HNQ3sQExXJVlGPycO34H3/j3xx6xr7kT8dSehOWr0T4kfGeQJ3NPj RQt5Ct7oPMKHLIhNz5F3Ac0Hhrz85NaOSasNhBS5iNjchmQh1kDDEvWgDURA7YDF 67Tg284Gh4PPklhKn4Se3bWLEdLwyMbixQVFcY5uINJf/9akaB7o1N3rbsUWZXvr ilMLa6jVeGsU9lxnvRdToiAxRd/AlknoGfBHsQn25xamSlHHtD5S30Rk+ILTzUuL 4p0cCuI4zB9Dz1SYIZ+2ctfoogLOIFBRxZjZ+yxD2gNtOTo9uFpaVXKd/RgBQ9d1 omBmV1b3VhaE6iphHc6wRcpYNERDOqIRKHz4LRApd8fxhQmFRJqwtYxxT5GLl0uj NndhPNCfZ4Dshdw7FpVf7nhwb2tmkuh1SyHXN4brePz0lS/1DAI5HkNUOnElwvd8 +hUInsXhv4sMVhHR6/sGOkq69ebtCarytrQsO+Xoi0lgDksBFABvTXzFbmBoLyOw 8o2nksVwsZkHk0+T1gJgtZ4MuSvqx0PAnKsfbZ6vas2MtFayzZc8F8aGCgV4xQDm bP13mhEEKmNSEsg5uE/H8cx95HaPHr3i+7lEtRjnl6Q9bM1vD7b4GDzvBs8ZARj2 U1KLuO9MO1CdzDRyZnsizG2Kf1kK+c1GttJ+CZiAekXCqn4jZDI= =6Pz4 -----END PGP SIGNATURE----- --=-=-=--