To really answer your question, we need to know a lot more about your
process, including the type of paste, the solder mask and various other
factors. In my book, I identified five different basic causes of "white
residues", each subdivided into a number of other causes, over 20 in all
and this list is by no means exhaustive.
However, there is one possible cause that I have seen in both Thailand
and Malaysia and that is lack of appropriate humidity control in the
workshops. Many solder pastes absorb atmospheric humidity like
gangbusters on the printer and during all the wait periods up to reflow.
If this happens, then some of the components will hydrolyse, producing
the effect you see: it can also happen even after reflow. This is a real
problem in the tropics and even in humid temperate climates. Close
humidity control to <40% RH is a must.
Hydrolysed compounds are generally very difficult to remove. Certainly,
IPA, which is a poor solvent at the best of times, is not going to look
at them. On top of this, "no-clean" fluxes are not even meant to be
removed; they are designed for a minimum of low-activity residues, not
for cleaning. If you really must clean, then you will probably have to
use an aggressive saponifier, which ain't at all a pleasant job unless
you have adequate equipment for it.
However, there are other possible causes: for example, if the solder
mask is under cured, the surface may be attacked by the paste chemistry,
revealing fillers.
Brian
Somtawin Pornpottanamas wrote:
> Has anyone ever seen white contaminate ( like a crystal) on assembly
> boards , between IC lead? I found white residue on some board after
> reflow . I try to clean by used IPA, flux removal solvent and also DI
> water. But could not remove it. I also found that after we used the
> solvent to clean, the white residue increase more.
>
> Our process used no clean . What is this residue? What type of solvent
> that should be use to clean flux residue when we have to rework or
> Dedug?
>
>
>
> Thanks & Regards,
>
>
>
> Somtawin Pornpottanamas
>
> Material Engineer
>
> Kimball Electronics (Thailand)
>
> Tel. 038-401566 ext. 211
>
> Fax. 038-495029
>
>
>
>
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