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-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:in-reply-to:references:from:date:message-id:subject:to; bh=HDVtnEomKRw4G1g+R0ybZQwJWcArQxeT1TaeFssR4Kk=; b=KPA5a7OpIkOMoK4PaHbzZ89J4PtPpPz/wwRbN9pPOI1bQqGsEACfaaG/Dwt6Xq3JlU ShWbxwHgRmHm4JkscDu5D7PTzk9dZGaZi5fQsUuyhibfdu1/uZQQLRY/mjyCVvZJw9OR AvjzMoj2PNbg3FluMyMcPLGm1k7r+I0iTVY9nNxuwPMuFzEpnkgbZ1sM5feYJHlgeE5Y 2DdbVjACGLgtwhCQ/5ZQelvrpHnDrJ2HDMKZIf0K7B8jsf5pCWWnF1tjR8RZzqXuIyMf vZ8vK5+qJE5CgCajDk5EyGsnKjuOQ951wsgmxFTDrWINFDHpHHGLlTc8GQAg7dDiTEir bemA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to; bh=HDVtnEomKRw4G1g+R0ybZQwJWcArQxeT1TaeFssR4Kk=; b=D3lNxsiBetLK+ApK/+RSGyJBVvbv2ZL+QDJjUzE1xT7EK1hfwaCOSSaABkesMLthZg D9OXQX+M517a7Bu8A3o1FWMBwfbiyHtiNZ62K2C/dV/Y9E5Zyg/RWbgJz8EqVFZsHgrC OnhnGwWwVoe9RWl+p7hLMuSvsU6RF8U4qP+N88HfRBG2FWnBKLziOW9Lh0+qofxgJTLp K8xU/osmS7FHOoYXKTVRsikzEAoE2QWXoj7F3MUbNPBW5BZsbtyEq2lvakfj43PMxniI 01VaoFdzpFDLJWf7lkgC8Ssa/lkmN5l8JpiAgisEIqX3q1ZxcI9eD+pMMPhZPWsrFOPm 1tUA== X-Gm-Message-State: AIkVDXIwgwmvKoEC4k6yjFzim5HMV1+Xd12Moq9ZeVVeumGkXr2ft7NaILOVzaZDTZKaaPV44tC3snMzk9/SNA== X-Received: by 10.25.216.156 with SMTP id r28mr955744lfi.28.1484741031765; Wed, 18 Jan 2017 04:03:51 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <59149c35-79a3-2bd7-4b04-6d0967fcfe0a@ecosensory.com> References: <2df480cc-5ef2-9ac6-b7ad-d17788a6b8b9 AT ecosensory DOT com> <59149c35-79a3-2bd7-4b04-6d0967fcfe0a AT ecosensory DOT com> From: "John Luciani (jluciani AT gmail DOT com) [via geda-user AT delorie DOT com]" Date: Wed, 18 Jan 2017 07:03:51 -0500 Message-ID: Subject: Re: [geda-user] QFN packages solder mask To: geda-user AT delorie DOT com Content-Type: multipart/alternative; boundary=001a1140d76ac82b2605465d3511 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 --001a1140d76ac82b2605465d3511 Content-Type: text/plain; charset=UTF-8 What type of stencil openings are you using? 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. John L On Tue, Jan 17, 2017 at 11:58 PM, John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com] wrote: > 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. > -- http://www.wiblocks.com --001a1140d76ac82b2605465d3511 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
What type of stencil opening= s are you using?

For the thermal pads I typical reduce the cov= erage 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.
John L





<= /div>

On Tue= , Jan 17, 2017 at 11:58 PM, John Griessen (john AT ecosensory DOT com) [via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wr= ote:
On 01/17/2017 10:00= PM, Dan McMahill (da= n 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.=C2=A0 Did you = look at the QFN footprints in the ~geda library?=C2=A0 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.=C2=A0 They suggest all the gaps betw= een paste areas and metal areas to make channels
that volatile gas can escape from during reflowing.=C2=A0 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 t= hey are
a help against bubbles forming, and they're needed to connect any separ= ate pads
for current flow.=C2=A0 The goal is often heat flow also, and vias help wit= h that a lot.

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



--
http://www.wiblocks.com =C2=A0
--001a1140d76ac82b2605465d3511--