Remember to keep the edge of via to edge of smd land at least 20 mils apart,
otherwise there could be soldering issues. Other than this, there have been
no problems reported either by fabrication or assembly.
Ken Barrett
Cisco Systems
San Jose CA
408.526.5625
[log in to unmask]
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>From [log in to unmask] Tue Sep 24 10:16:41 1996
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Date: Tue, 24 Sep 1996 09:57:03 -0700
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Organization: Circuit Packaging and Layout
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Subject: DES: VIAS - Thermal vs. Direct
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I was thrilled when the subject of thermal vias vs. direct-connect vias came
up, because we were just discussing whether we should "go direct" last week.
After reading all the responses... well, I'm just a little worried about it.
Rather than quote everybody, here is a summary of what my boss will read if
I show him this discussion:
Two people, K.Barret and [log in to unmask], have used direct connections
extensively, in all environments, all sizes, varied layer count, no
problems. (and one ships about a million boards per year to prove it)
B.Luthor concerned about heat transfer to plane, one reply stated trace from
SMT pad to via was a thermal in itself. (I would tend to agree on that one)
One side discussion about a PINK RING problem, without enough explanation to
learn how pink ring would affect a board or how it is related to direct vias.
Another side discussion about clearance vs. fab allowance, and another about
reduced clearance (anti-pad) sizes. Won't affect our decision.
A warning from Norm about CTE mismatch and the danger of board damage if
direct connections are used.
One reminder from G.Ferrari about the value of formal test procedures and a
warning that what works for one design may not be appropriate for others.
-=x=-
Thanks to all who participated, this thread came along at the PERFECT time.
Our board vendors don't care WHAT we do, so it is up to us. But before I go
show this to others and we make the decision, are there any other issues
that got left out? any elaboration necessary? any final comments?
NOW THAT I THINK ABOUT IT, ALL OF THE REAL-WORLD RESPONSES WERE FAIRLY
POSITIVE, AND ALL OF THE NEGATIVE RESPONSES WERE KIND OF THEORETICAL..!
Am I misinterpreting? Are there any REAL horror stories out there?
Also, if we remove the thermal apertures from our pwr/gnd via padstacks,
should we replace them with a small aperture to "show" something there, or
just leave it empty? Seems like it would be hard to track down connections
looking at "direct-connected" film, any problems there?
thanks in advance, (and Sorry to those who were tired of this subject)
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