Rob said:
Does anyone have any idea on what the composition of the white residue is
that is left behind on PCBs when IPA is used to remove the flux residue. Is
is inert? / hygroscopic?
I have noticed it seems to be worse when methylated spirits is used in place
of IPA.
This problem is due to the nature of the flux.
Normally it consists of an alcohol mixture, the flux itself, and
an activator (an acidic salt usually). The alcohols used are
normally ethyl and isopropyl. They have different properties,
with respect to solving polar and nonpolar stuff. The activator
salt must be solved with a polar solvent, while the flux needs
a nonpolar solvent.
After having soldered a board, there are some residues of flux
that also contain small particles of the activator salt. As long
as you leave the board this way, these particles are covered by
the flux and do not cause any harm.
Now if you use ethylic alcohol to wash the flux off, the activator
salt does not get solved (since it needs a polar solvent), so it
stays on the board. That's the white residues you can see after
washing with an improper solvent.
Now these salt grains can catch humidity and start their acidic
work again, causing damage to the board.
To overcome this problem, use a solvent that has polar and
nonpolar components to wash the complete flux and activator
from the board. AFAIK, a 50:50 mixture of ethyl and isopropyl
is commonly used. Alternatively, you could wash in two steps:
remove the flux with alcohol, then wash off the activator with
water.
(Maybe I confused polar/nonpolar for the two alcohols, but
that's the principle.)
--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de
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