John,

Continuing the saga of meniscus in the PTH:

Entrapment of flux and other process chemicals but I would anticipate it to
be rather remote.  The major process variable would be the effectiveness of
the cleaning process itself.  

Entrapment was, and still is a problem with tenting of larger(>20 mils) PTHs
with solder mask, since most mask materials cracked during reflow and at
least partially sealed prior to cleaning.  I do not see the equivalent
possibility here since the meniscus doesn't form a seal to the PTH.  Again,
there are many process variables that should be considered before making a
"never happen" statement.  

TCE of entrapped process chemicals should not present a problem either since
they are not constrained or rigid.  

In any case we have major variables to deal with that are case by case and
process peculiar, but I have no experience with product failure due to
degradation of the PWB PTHs for process chemical entrapment related to
meniscus intrusion.  

As we discussed previously, the two schools of thought were so called "Hi
Rel" which regulated against the anomaly and commercial (not necessarily Lo
Rel) which really didn't see a problem.  The difference being, for the most
part that the Hi Rel parts had to survive environments which were commonly
hazardous to the performance of the product.  In those cases it was easier
to address the potential through process prevention requirements than to
deal with product failure that threatened National Security or loss of life.  

Best regards
Mel Parrish
MTTC Inc.  

At 12:57 PM 11/14/96 +0000, you wrote:
snip
>Yes, I can certainly see that the meniscus would prohibit wetting.  And
especially, in the worst of all situations - meniscus in the solder joint
when in a single sided board application - the premature failure over time
and temperature extremes seems quite likely. What I'm also concerned about,
though, - and this is the predominate debate point, here - involves
potentially compromised PTH's, when the meniscus in the holes is allowed.
Won't the partial blockage of the hole create a potential entrapment of flux
gases or residues?  And, if that is so, it seems like at least two problems
may result: 1 ) a diminished area of wetting inside the PTH (and therefore a
weaker mechanical junction of less current carrying capability)  and 2) the
potential for a breakdown of that hole plating due to either corrosion or
expansion and contraction of the trapped materials (or would it be due to
the relative differences in TCEs of the plated hole and the trapped
materials?). A third possiblity might include the breakdown due to increased
heat generated at these highly resistive pockets.
>
>snip
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