There have been several posts recently asking about white residue on PC boards. 
It has been variously identified in the posts as lead oxide and here as
"proteinaceous material."  From past experience I doubt that it is either. 
White residue is often the result of delayed cleaning of printed circuit boards
after soldering using a rosin based flux.  There was a workshop on this problem
several years ago at China Lake.  The outcome of the workshop was that the exact
nature of the white residue remained an unknown but that it showed structural
similarities to the abietic and pimaric acid components of rosin based flux. 
These similarities can be seen using FTIR.  Typically, lead could be detected
chemically or by x-ray fluorescence.  The condition is agravated by prolonged
exposure to heat and by delayed cleaning.

A few words on FTIR.  While it is a wonderful technique for identifying pure
materials and if coupled other techniques such as gas chromatography can be used
effectively to identify the constituents of a solution.  It is virtually
impossible to identify the components of a complex solid solution using FTIR. 
What you can do is show that certain bands are similar to those obtained from
certain molecular groups such -OH or -COOH.  Unfortunately, there is enough
overlap of bands that if you have any preconceived notion of what should be in a
sample, you will probably think you see it.
_______________________________________________________________________________
Subject: White residue between soldered leads
From:    [log in to unmask] at Internet
Date:    5/7/96  2:31 PM

     I have a white residue problem that is becoming all to common. The
     white residue appears between and behind soldered leads on the
     polyimide substrate.  It does not appear on or inside of
     soldered-through holes.  On some boards I can find it between all the
     leads.  On others I see it only between a few leads.
     I have analyzed this residue many times with FTIR.  Everytime, I get
     proteinaceous matter (protein).  I see no evidence of rosin-based or
     water-soluble flux, but at times I see cellulose fibers mixed in with
     the proteinaceous matter.

     What is this stuff?

     We have considered:

     1. skin residue (dandruff)
     2. debris from camel hair brushes
     3. fungus

     There appears to be far too much residue to consider 1. and 2.
     The problem appears after the final cleaning/rinsing step.
     Any ideas would be greatly appreciated.

     Thank you.

     --Phil