My experience is the "contamination with sulfur" usually comes from how you 
stored the board, did you interleaf with sulfur free paper, what did you do 
with the cardboard box, I believe upon investigation you will find part of 
making paper products(check sulfite paper pulp for info.) gives them 
inherent sulfur.  Manko in Solders and Soldering in the 1964 edition says" A 
word about sulfur is in order.  This contaminant which has a detrimental 
effect on wetting is reported to be dangerous at levels of 0.03%(in solder)" 
 In addition most sulfite have low solubility and once gotten are hard to 
get rid of.
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From: [log in to unmask]
To: [log in to unmask]
Subject: Sulphur contamination
Date: Thursday, April 25, 1996 10:43AM

I am interested in finding out potential causes of contamination of PCB,s
with sulphur. What typical fabrication or assembly processes may introduce
this contaminant to the board. What are the long term potential effects of
sulphur contamination and are there any studies I could reference.

Any suggestions or papers would be appreciated.