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White marks on computer cards PCBs

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LeonR:
I've noticed that for some time now (10+ years, lol) a lot of PC parts come with white smudges, especially on the PCBs back side. Ex.:



They don't seem to come off  - even 99.8% IPA + q-tips can only manage to scrub it off a little, and that's after some vigorous attempts.

It's clearly a problem with the ultrasonic bath those receive on the manufacturing process, but why exactly? Improper temperature/cleaning solvent/less time than required?

Also, most companies today use a matte varnish (conformal coating?). How do they compare to what the industry used ~20 years ago? Boards from the Socket 3/Super 7 era seemed to be more robust that what we have today.

edy:
I thought it was a problem during the silkscreening process... maybe dirty silkscreens or smudging before the ink cures. Is it always the same color as the silkscreen, or always white even if silksceen another color?

LeonR:
Huh. Never thought it could be a silking problem but I think you're right, since the decals are above the varnish coating.

tooki:
It's just flux residue from imperfect flux cleaning, especially of "no-clean" fluxes.

If you google "white flux residue" you will find dozens of pages discussing it, including white papers from solder manufacturers, etc.

tooki:
P.S. Try using 70% IPA. Often, the white residues are the parts of the flux that are not alcohol-soluble, which is why the IPA doesn't touch them. The bit of water in the 70% is enough to dissolve the water-soluble constituents, allowing them to be rinsed away.

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