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December 1998

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Subject:
From:
KELLY M SCHRIVER <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 8 Dec 1998 09:25:00 -0500
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Hi Tom -

I don't have a total set of answers for you, but a few pointers which may fit
with the inputs of others.

1] There are so many different urethane formulations that you need to query
the specific material manufacturer regarding strippers.  Urethanes tend to be
rather tough materials and may be very difficult to strip without harm to
other parts on the board, particularly if you have trimpots, switches, etc.  I
have good success desoldering individual components without removing coating,
cleaning up the discolored coating and soldering in the replacement parts,
followed by spot cleaning and patch coating.  In general, the less you can do,
the better.

2] Depends on the temperature and the material involved.  At a minimum, I
would be leery of reflow temperatures setting up some extreme stresses with
the encapsulant.

3] Most clear urethane coatings turn a distinct amber, commencing at about
130deg.C.  The manufacturer can best advise you on any loss of properties.
Reflow temperatures may be close to the carbonization point for some of these
materials.

4] Your cleaning method needs to be governed by the contaminant (what type of
flux residues, etc.) and the geometry of the assembly.  Dependent on the
assembly and extent of work performed, simple spot cleaning may be adequate.
Passing a fully assembled, partially conformal coated CCA thru an in line or
diswasher cleaner does pose some risks.

5] At best, a conformal coating is a vapor permeable barrier.  I would
recommend that any form of overall fluid cleaning performed on a coated board
be followed by a dehumidification cycle in an air circulating oven at whatever
temperature the components will stand.  If you happen to apply conformal
coating over moisture or contaminants, it will become apparent by the
formation of tiny bubble clusters following a bit of environmental testing.


Good luck - Kelly

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