RE>Tented Vias 6/25/97
Hi Frank,
I'm not sure that there are many problems with tented vias entrapping
contaminants, but I'd believe that *plugged* vias after HASL can cause
trouble. My guess is that the underlying reason for this is that it can be
expensive and time-consuming to perform the failure analysis on single vias
which might have failed. It requires microsectioning and metallographic
inspection equipment, as well as skilled analysts, that most companies don't
have.
In a past life, I was with a large OEM that had these capabilities, and we
actually discovered some vias which had corroded through because of the
presence of entrapped water soluble fluxes. I seem to remember that it only
took a month or so to eat through the 1 mil of copper plating.
At Mercury, we haven't plugged vias for quite some time now because of the
entrapped contaminant issue, but also because we had a problem with the
solder stencil sitting too far from the board. This gave us lousy printing
of our BGA patterns. We don't do any ICT, and we partially cover our via
pads with soldermask. This, in conjunction with a bare copper/OSP surface
finish, tends to prevent bridging through the solder wave.
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
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--------------------------------------
Date: 6/24/97 9:05 PM
To: Greg Bartlett
From: Frank Hinojos
A couple of weeks ago I asked all of you about the dangers of using
tented LPI vias. I received an overwhelming response that while it is
good for assembly, it may be very detrimental to the boards'
reliability due to chemical interaction of contaminants and uncured
LPI in the via. Why then does it seem that everyone is using tented
vias and why haven't we heard of problems arising due to your
concerns?
Frank Hinojos
Watkins-Johnson Co.
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