Steve adroitly gets my attention with:
This topic has come up before, and there has been inputs from Brian Ellis,
and Doug Pauls on this issue...if I remember correctly, it's not a good
idea. If
I'm wrong, please correct me.
Now wait just a minute here. I can get in enough trouble by myself. What
should be said is that conformal coating over any electronic assembly
without having a very good idea of the overall cleanliness and/or the
residues involved, or using some pre-set recipe, is not a good idea.
Professor Ellis gives a very good response, though I don't know about the
"Lord" part. The only thing around here that is noble is some trace gasses
in the air and the gold on the circuit boards.
We conformally coat over low solids flux residues, but we do a great deal
of testing in each and every case to make sure that we have investigated
the overall cleanliness, the impact of the residues on performance and
reliability, and the consistency of the process. When you build avionics
to last 30 years, you don't take shortcuts. We look at the cleanliness of
assemblies prior to fluxing, compatibilities between cored wire flux and
additional fluxes, performance in harsh environments in stress screens,
etc. This also varies somewhat depending on what product we look at, what
customer requirements may be, etc.
As has been pointed out, the only person who can answer the question of
whether coating over low solids residues is YOU. And yes, it is a very
hard question to answer. But, if you get involved with the IPC committees,
as will happen at the upcoming APEX show, you would have the opportunity to
talk to many individuals who do such evaluations for a living and readily
share their knowledge.
One additional point that I will bring up is that if you are selecting a
conformal coating, you might want to consider a solvent based coating over
a 100% solids or non-VOC solvent coating. In many cases, the solvent can
help penetrate the flux residue, creating a better bond to the residue. Of
course, you must do testing to make sure that the residue/solvent mix is
reliable.
Doug Pauls
Rockwell Collins, where it is -5F, 15 mph winds, and snowing.
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