We have bare Polyimide multilayer boards using Arlon 35N.  We have been seeing a light powdery residue on the barrels and surfaces of the PTHs and on the substrate and soldermask.  Under high magnification they appear to be crystalline structures.  Cylindrical crystals in clumps.  Solvent cleaning, Axarel, Ethanol, Alcohol, all seem to partially dissolve them and then leave behind a film on the surface.  The film becomes insoluble.  The Undissolved crystals do not come off with any solvents and must be abraded from the surface.
A water and soap wash removed quite a bit of these crystals on a fresh board, but again any unremoved material was insoluble and needed abrasion.
Boards were from two different suppliers, one had a 45-99 date code another had a 32-00.  
Who can identify this material?  
Is this some byproduct of poor HASL flux cleaning?

Thanks for the input,
Richard MacCutcheon

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