Jaqueline There are so many variables in the electronic assembly manufacturing process involving more than 12 different chemistries, that it is impossible to easily identify and determine the nature and reason of such residues. GEC Hirst Research in the UK, had identified more than 16 different white powder residues! What are they? How are they caused?....Consider the following, and I believe I may be shot at... The "rosin" particles that may be present in certain types of flux, are usually based upon resins that are derived from trees. As such they are seasonal and their quality varies from year to year, season to season. When these particulates are exposed to a more efficient solvent, as was the case when DuPont first introduced an alcohol/azeotrope of 113 (Freon); then the "cleansing" action was greatly improved. However, in so doing, the alcohol (having a higher KB factor or solvency capacity)partially dissolved such particles that were trapped in the "sponge" like surface of the board. Take a look at your board under a scanning electron microscope and you will see what I mean. So... the particle was rendered a different light refractive condition which showed up as white and proved that your 113 "Freon" type solvent alone, was a lousy cleaning solvent and that the alcohol was actually doing a better cleaning job....but there were occasional "white residues". Modern process chemistries may be similarly affected or may exhibit OTHER forms of white residue... the essential message being: DO Not clean No-Clean! There is no such thing as no residue flux - there is fluxless soldering, but that is another technology. Thus, if you leave residues, they WILL react unpredicatbly maybe, with different types of cleaning chemistry - alcohol, methanol, glycol based??? Who knows for sure? HOWEVER, a simple suggestion, check with your flux/paste supplier, and get them or others, to supply a Water SOLUABLE (not washable) alternative and you should find your problem has gone away. I hope I did not bore you... Regards Graham Naisbitt | At this moment in time we are using water soluble solder paste on our PCBs these are cleaned using water only, agitated with ultrasonics. After drying a white/grey residue is left around solder joints. Reflow profile is fine and a reliable wash procedure is followed. Advice would be appreciated.
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