Kathy Kuhlow asks:

Not to use was explained to me as such: No clean leaves a residue that with
heat can reliquify and then start to move which will cause an air bubble
where the flux was originally at and now isn't.

So the question comes to: does and at what temperature would it take for
the no clean residue you are using to reliquify if it does at all?  Then if
it does liquify is there any activation that is occurring?  Does the
coating withstand the proper adhesion for the application if the air bubble
does occur?


Doug Pauls, now fully tanked up with Dew, responds.

Kathy, you are treating the low solids flux as if it were a single animal
with tightly identified characteristics.  The residues that you have on the
board can be generically called weak organic acids, or dicarboxylic acids.
A paper by Al Schneider (who has forgotten more about flux than I will ever
know) a few years back listed the menu from which a flux manufacturer has
to choose.  If I recall, that information found its way into J-HDBK-001,
section 4.  The list is over 36 materials, each of which would give you a
different blend of properties in the final residue.  A residue that is 50%
adipic and 50% maleic acid would give you different characteristics than
one that is 33% succinnic, 33% glutaric, and 34% fufu dust / gnats noses.
So the question of "what point does the residue soften and creep" depends
entirely on what flux you are using and a generic classification cannot be
made..  Your flux manufacturer would have to answer the question.  They
would also have to address the issue of whether or not the residues had any
kind of chemical activity in the softened condition.

Think of it this way:  At what temperature does solder reflow?  Kind of
depends on the alloy, right?

Some residues will soften under high heat and may creep.  It depends on
whether the residue acts more like a thermoplastic or more like a
thermoset.  The latter will resist heat better.  If your application has a
high amount of heat, or point heat sources, then LSFs may not be good for
that application, or topical cleaning may be needed for that area.  In my
experience, you either want to leave a LSF residue totally alone, or
totally remove it.  Doing a half-ass job of cleaning results in an amalgam
of cleaner, flux, and interactive products.  Not a pretty sight and often
detrimental.

If you have conformal coating over a residue that will soften and creep, I
would suspect you would have adhesion problems and blister formation.  The
same problem arises when solder mask is put over a melting metal.  The
metal melts and moves, wrinkling or blistering the mask.  Same with
conformal coat.

Doug Pauls

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