All of the discussion of thermal vias vs. direct-connect vias leaves one
important question -- is this a component via? If so, and you decide to use a
direct-connect via to power and ground planes, your assembly processes should
have a top side preheater to assure that the planes are brought up to
temperature or you will not have adequate top side component lead wetting. This
is even more true if the component is a power (large leaded) component. It is
the same principle if you try to solder a heavy leaded device to a large copper
external pad without adequate spokes out to the trace to avoid puddling of the
tin lead away from the component lead via site. If you do not use the top side
preheater, even a 14 pin dip that does not have thermal relief on the power and
ground plane connections will demonstrate a "different" top side wetting
condition.
______________________________ Reply Separator _________________________________
Subject: DES: VIAS - Thermal vs. Direct
Author: [log in to unmask] at SMTPpost
Date: 9/24/96 2:33 PM
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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