Jean-François
Most "no-clean" flux residues consist of ionic species such as
dicarboxylic acids, their polymers and their metallic salts. The exact
composition is impossible to predict as it depends on many factors, many
of them unique to your conditions of working. For example, if a
particular component you use has an unusual metal or heavy oxidation,
then you may find that the resultant residues are more dangerous than
others. In particular, the residues from manual rework are usually of
greater volume than from other soldering methods and, frequently, rework
is required because a connection was badly soldered, e.g. due to
abnormal oxidation.
If you apply a coating over such residues then any one of a number of
things can happen, but the potential problems may not appear in
practice, at least for a long time. But they are always there. If there
is no intermixing of the flux residues with the coating, then the
conditions are most dangerous as vesication or some other form of
non-adhesion under humid conditions is quasi inevitable, given enough
time. If the residues are dissolved in the coating solvent sufficiently
rapidly that **there is a fairly homogeneous mixture** before solvent
evaporation starts, then the ionic species are held in the resin matrix
and it is possible that little harm will result. This is very unlikely
with solventless formulations. On the other hand, the very presence of
these species may upset the polymerisation chemistry of the coating. The
third and most unlikely scenario is where the dicarboxylic acid residues
actually aid the cross-linking of the coating. Unfortunately, although
this sounds a great idea, the practice is very difficult to achieve
because stoichiometry is extremely difficult to ensure.
Flux and coating manufacturers generally are useless to advise you,
because they do not know your particular conditions of operation. I
suppose that if both products come from the same source and the
manufacturer says there is a compatibility **with guaranteed results**,
then there may be a fairish chance, but tests will still be mandatory to
ensure that the reliability is established.
Finally, some "no-clean" fluxes depend on the sublimation of their
residues over weeks or months to ensure their long term residue
acceptability. If you coat these residues, they can no longer sublimate
and this mechanism is no longer valid.
The answer then can **only** be the same as determining the taste of a
lemon: suck it and see. If this does not appeal to you, then the only
way of playing safe is not to coat over flux residues.
Brian
"Bissonnette, Jean-Francois" wrote:
>
> Did anyone have any kind of problem with coating adhesion when using
> "no clean" flux (manual repair and reworks)? I've been told that by
> some
> tech here and I have no time to experiment with it.
>
> And is "no clean" flux really "NO CLEAN" anyway???
>
> JF
>
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