I'd agree with Greg Bartlett's memo (below). I've actually performed 
the cross sections and have seen the damage. Also see the memo from 
Gerry Gagnon on this topic. We have photos showing the plugged vias 
bulge up and interfere with SMT components.

Paul Terranova
[log in to unmask]
Phone: 508-467-3109
FAX: 508-467-6796

DIGITAL's Analytical and Environmental Test Services Lab
Digital Equipment Corporation
M/S: MRO1-3/D2
200 Forest Street
Marlboro, MA  01752

From:	NAME: Greg Bartlett <[log in to unmask]@PMDF@INTERNET>
To:	NAME: Frank Hinojos <[log in to unmask]@PMDF@INTERNET>,
	NAME: technet <[log in to unmask]@PMDF@INTERNET>

                      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
[log in to unmask]
--------------------------------------
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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