A few weeks ago I wrote asking for help on with a white residue problem.
Some technetters ask for a follow-up. Here it is.
Case: White residue appears on ref lowed SMT land patterns after
exposure to vapor cleaning. No white residue appears on wave soldered
land patterns. Even on mixed technology boards.
RMA type paste for SMT. Synthetic Activated flux applied as foam at
solder wave.
Vapor cleaner loaded with Vertrel XMS Plus (HFC, Trans Diclor, and
alcohol). Ultra sonic scrub rinse.
Prior to exposure to vapor process the flux residue is very thin clear
and almost invisible. After cleaning the residue appears as a white
powder where ever there was flux.
We tried a co-solvent Axerel 32 in the boil sump, with board immersion
instead of vapor cleaning. Rinse in Vertrel distillate with Ultrasonic
scrub. Slight improvement.

Result: We discovered that the residue was flux activators, not rosin.
We experimented with a few flux formulation until we found an old Alpha
Metals paste formulation that cleaned beautifully in the co-solvent
process. 40% Axerel 32 by volume in the boil sump. We have not found an
RMA paste that will clean completely in the traditional vapor process
with Vertrel XMS.

Did anyone ever hear of an SA solder paste formulation?

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