Hello Technet,

We've got some expensive boards here with white residues after cleaning.
Most of it is invisible for the naked eye (conforming IPC class 3
standards), but each board has a  few small spots that are visible (if you
look really good).
Now the customer wants proof of what this residue is. Not because they can
see the residue, but because they know it's there.
These are double reflow boards, soldered with RMA solderpaste. After each
soldering step the boards are cleaned with Axarel 32 and rinsed with
deionised water.
Our ionograph shows that there is hardly any ionic contamination on these
products.

Some questions:
1)  When there's so little residue present, will it be possible to do an
analyses ?  To collect a few grams I'd have to scrape the residues of
hundreds of boards !
2)  What kind of analyses should be done ?
3)  Any recommendations on where to have the analyses done ? Locations in
Western Europe are prefered.

**Daan, I can almost guarantee that the white material you are seeing is
abietic acid, one of the constituent materials of a high solids (e.g. RMA)
flux.  I saw lots of white residue while at CSL.  It is generally a
cosmetic issue, but that never stopped spooking the clients (or their
clients).  There have been many industry studies demonstrating that these
white residues are harmless.  The only exception that I can think of is
when the white residues occur periodically, like at the knuckles of glass
bundles in the laminate.  Then you likely have a problem with a thin
buttercoat layer and fluids absorbed into the glass bundles.

What tests can you do?  Ion chromatography is one.  HPLC is is pretty good
for identifying residual rosin elements.  You might try a surface
reflectance FT-IR, but that signal can be confused with what you get from
the laminate.  These tests can be run on small samples.  I would not
recommend SEM-EDX.  Wrong tool for this residue.

In terms of who can do such testing for you in Europe, I would suggest
trying NPL in the United Kingdom (Chris Hunt or Alan Brewin) or IVF in
Sweden (Per-Erik Tegehall).

Doug Pauls
Rockwell Collins

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