Bing,

I have experienced this problem with rigid boards and have found two possible
causes:

1. Polymerized flux residues adhering to the board surface.
2. Fillers used in soldermask.

Since I don't know what kind of flux you are using and whether or not the flex
areas that have the residue have soldermask, I can just tell you what we have
done for each case listed above.

If the residue is polymerized flux residue, the issue is strictly cosmetic. You
should not see a problem when testing for ionic contamination. If the cosmetic
appearance of the white residue is still an issue for you, then the choices are
to either change to a paste that uses an inorganic flux (which can't
polymerize) or to use a saponifier (basically, a detergent) in the wash.
Saponifiers are supplied by companies such as Alpha Metals. Saponifiers also
maintain the wash water at a constant pH, which helps to meet pH discharge
limits to your local sewage system.

If you are using soldermask, it could be that the wash temperature is driving
out the fillers used (soldermasks typically have quartz or talc filler
material). In that case, be sure the wash temp is less than 140F.

You also said that you use DI water in the wash and that it is changed once a
day. The wash should be a continuous cascade, counter-current, flow. That will
keep contaminates in the water from accumulating. We simply use softened city
water in the wash. DI water is a very aggressive solvent and may actually be
contributing to the problem.

Regards,

Denis Mori
Hewlett-Packard

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Subject: [TN] white residue

Hello netters
 We're using a with clean solder paste in the manufacturing of our
flex and I'm encountering  white residues  as seen after aqueous
cleaning. Can this be an effect of the flux content of the solder
paste that we are using? Or is it the cleanliness of the DI water
beign used by the aqueous cleaning machine (which is drained and
changed every end of the shift)? We tried reloading it in the aqueous
cleaner but it doesnt suffice cleaning the white residues. Will I be
needing another chemical or solvent to clean it up first before
reloading it in the aqueous machine?
By the way, did anybody among you have encountered this problem
before?
I need your help!

Thanks,
Bing

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