We have encountered the same problem.  The white residue seem to have been baked-on and the only way we could remove it was via burn-in (60 deg C overnight) process.  After that, we just defluxed the boards and rinse it with alcohol (a brush helps too).  It basically removed all moisture and turned the residue in to ash.

EG
-----Original Message-----
From:   Tegehall Per-Erik [SMTP:[log in to unmask]]
Sent:   Wednesday, May 26, 1999 8:07 AM
To:     [log in to unmask]
Subject:        Re: [TN] White Residue on solder joints

I have seen the type of flux residues you describe a number of times. In
most cases, it has turned out to be a lead salt, for example lead adipinate
formed when adipic acid in the flux (an activator) react with lead oxide on
the surface to be soldered. My experience is that these salts are very
difficult to dissolve whatever solvent you use. Since the residues are so
difficult to dissolve, they are probably rather harmless for most
applications (but I have no proof for that statement). However, if you will
conformally coat the assemblies, they may cause poor adhesion of the
coating. What puzzle me is that you say it was manifested a few days after
aqueous wash. It should have been there immediately after cleaning.

Per-Erik Tegehall
IVF
Sweden

> ----------
>
> Has anyone ever come across white residues that looks like snow cap
> covering the solder joints? The phenomenon only manifested  a few days
> after Aqueous wash and EDX captured high lead and oxygen. The white
> residues are so stubborn that they could not be dissolved in DI water and
> IPA. We use a water soluble paste and clean our assembly twice
>
> All inputs are welcome and appreciated.
>
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