Mr. Stadem's comment about conformal coating wicking is true. I have seen
instances where it got inside connectors making them impossible to connect
or gluing them shut.

**Yup, we agree there.

  The IPC should make recommendations regarding conformal coating masking
in their hand book. Should it be left on after coating? Subject to
qualification?

**If there was any recommendation, it should be to never leave tape on.
Too many tapes have too much crap in the adhesive to make it a long term
reliability material.  Especially after you throw varying amounts of
solvent at it from the coating.  What kind of tape you use for masking
depends on what materials you are using, whether you clean afterwards, the
end use environment, etc.  Talk with your coating supplier to get the right
kind of masking tape for the coatings used and their recommendations on how
it should be used.

  Mr. Stadem is mistaken regarding conformal coating and "moisture
condensation". Moisture yes, condensation no. A lot of other engineers make
this incorrect assumption. Conformal coating applied as liquid doesn't
provide water proofing on 3 dimensional objects. If it's expected to then
the qualifying tests should be changed.

**OK, now we have an argument, and it depends on how far you want to go
with the term "water proofing".  In general, conformal coatings are present
to resist the flow of moisture and to prevent shorts from liquid water
contacting the energized assembly.  No coating makes an assembly truly
water proof as all coatings have varying levels of water permeability.  We
have applications here where assemblies are quite likely to have liquid
water running across them due to condensation in the aircraft avionics
bays.  In that case, we might pick a silicone coating and make sure that we
have good coverage.  In these applications, conformal coat does provide
protection on 3 dimensional objects from liquid water.  It all requires
selecting the right material and process.

Doug Pauls

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