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From: "David Carr (junk AT dcarr DOT org) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date: Tue, 13 Sep 2016 23:42:28 -0500
Message-ID: <CADKOf5Fy+OeaSZ-DVMMhWwyHoW_H6S+RyXWgGm-hEn8LHMC51Q@mail.gmail.com>
Subject: [geda-user] Tenting exposed vias in QFN paddle
To: geda-user AT delorie DOT com
Content-Type: multipart/alternative; boundary=94eb2c124e34ae6bc5053c705d5b
Reply-To: geda-user AT delorie DOT com

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Content-Type: text/plain; charset=UTF-8

More and more often these days manufacturers are calling for via arrays in
ground pads of QFNs.  One of the challenges with this is that these vias
need to be "sealed off" so that all of your solder doesn't just wick down
these vias (starving the exposed QFN pad).

The recommended low cost approach is to place "tented vias" in the middle
of the pad, basically little soldermask dots that cover each via.  Here's a
nice description:
http://blog.screamingcircuits.com/.services/blog/
6a00d8341c008a53ef00d8341c008c53ef/search?filter.q=tenting

How might one accomplish this with PCB?
Placing vias on pads is no issue, but the pads clear the entire mask
(including the area over the via hole that we wan't to keep).  One could
reduce/eliminate the mask clearance over the pad so that the via tent
remains, but then there's the problem of how to clear the mask around each
via.  Basically one needs to have the soldermask gerber (which is inverted)
look like a square with small holes punched in it for each tent.

Thanks for your ideas,
David

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<div dir=3D"ltr"><span style=3D"font-size:12.8px">More and more often these=
 days manufacturers are calling for via arrays in ground pads of QFNs.=C2=
=A0 One of the challenges with this is that these vias need to be &quot;sea=
led off&quot; so that all of your solder doesn&#39;t just wick down these v=
ias (starving the exposed QFN pad).</span><div style=3D"font-size:12.8px"><=
br></div><div style=3D"font-size:12.8px">The recommended low cost approach =
is to place &quot;tented vias&quot; in the middle of the pad, basically lit=
tle soldermask dots that cover each via.=C2=A0 Here&#39;s a nice descriptio=
n:</div><div style=3D"font-size:12.8px"><a href=3D"http://blog.screamingcir=
cuits.com/.services/blog/6a00d8341c008a53ef00d8341c008c53ef/search?filter.q=
=3Dtenting" target=3D"_blank">http://blog.screamingcircuits.<wbr>com/.servi=
ces/blog/<wbr>6a00d8341c008a53ef00d8341c008c<wbr>53ef/search?filter.q=3Dten=
ting</a><br></div><div style=3D"font-size:12.8px"><br></div><div style=3D"f=
ont-size:12.8px">How might one accomplish this with PCB?</div><div style=3D=
"font-size:12.8px">Placing vias on pads is no issue, but the pads clear the=
 entire mask (including the area over the via hole that we wan&#39;t to kee=
p).=C2=A0 One could reduce/eliminate the mask clearance over the pad so tha=
t the via tent remains, but then there&#39;s the problem of how to clear th=
e mask around each via.=C2=A0 Basically one needs to have the soldermask ge=
rber (which is inverted) look like a square with small holes punched in it =
for each tent.</div><div style=3D"font-size:12.8px"><br></div><div style=3D=
"font-size:12.8px">Thanks for your ideas,</div><div style=3D"font-size:12.8=
px">David</div></div>

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