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From [log in to unmask] Thu Feb 20 14: |
35:11 1997 |
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Jack Crawford posted a concern about the effect of unintentional
migration of silicone from a conformal coating operation to where
soldering is being done. He asked whether the concern is valid and
whether anyone had a history of a similar situation.
I hadn't been in the soldering business very long before I heard that
silicone can be death to solderability. Since I had a wetting bal-
ance, I decided to investigate. I was amazed to discover that I could
smear silicone grease on a component lead and not have any measurable
effect on the wetting time or final wetting force. After the fact, I
came to see that if the component lead's basis metal is covered with a
layer of solder, it doesn't matter what is on the solder, because the
solder melts and breaks up the contamination layer (which just floats
away). The only exception is if the contaminating layer is solid and
so thick that it persists as a sleeve even after the underlying solder
melts. (A thin solid layer breaks up because the solder expands as it
melts.)
As for a contaminant in a plated-through hole, unless it is massive
enough to stay stuck, it will simply be pushed along to the component
(destination) side as the hole fills with solder, where it may be
visible but will not reduce the reliability of the connection.
I have not investigated the effect of silicone contamination on
surfaces whose solderability is protected by means other than solder,
such as gold or an OSP. My guess is that here, trace amounts do get in
the way, and that removing them would be very difficult (plasma
cleaning?).
As for solder-coated surfaces, my take on the matter is that if
silicone is able to interfere with soldering, then the solderability
was marginal to begin with. In any case, it is a very simple matter
to investigate - even without a wetting balance. Just spray conformal
coating in varying amounts on boards (or dip in solutions of varying
concentrations) and see what happens when you try to solder. I predict
that anything short of deliberate coverage will not interfere, except
for possible changes in appearance (but not fillet shape). (If I'm
wrong, then we have an opportunity to learn what is going on, and put
the matter beyond where I see it today, which is at the anecdotal
level.)
Gordon Davy
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