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