OK, there are three issues at stake here.

1) The residues from "no-clean" products (and yes, there are residues -
always) may impede adhesion of the coating layer to the substrate

2) Many of the "no-clean" products rely on a slow sublimation of the
residues from the surface, requiring a few weeks to achieve optimal
performance. Sealing these in will prevent this from happening.

3) By definition, all soldering fluxes are ionic. Some may be slightly
hygroscopic, as may also the metal salts formed by the reaction between
the oxides and the flux acids. Some of the rheological additives to
pastes may leave slightly hygroscopic residues. This is an open
invitation to promote vesication, with a potentially very serious loss
of reliability. At the very least, a good "pressure cooker" test is a
must both in qualifying the process and at frequent intervals to check
that it stays on the rails.

I am not saying that coating should never be applied over flux residues.
What I am saying is that anyone who does it without a really thorough
qualification and process control is being professionally irresponsible,
if the products must be reliable. After all, conformal coating is really
a high-rel process and "no-clean" chemistry started as a low-rel
process, creeping up to its present status of medium-rel. It is
therefore really a paradoxal conflict to try and combine the two, likely
to present the disadvantages of both processes with none, or few, of the
advantages.

Brian

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