Keith,

There are a number of different sources for sulfur or sulfate in the PWB
manufacturing area.  Metal etching steps which use sulfuric acid or which use
a sulfonated buffering compound may result in sulfate residues.  Sulfates can
come from contact with normal brown papers and cardboards, since sulfuric
acid is used in their manufacture.  The most common source of sulfates is tap
water rinsing during board fabrication.  Almost all tap water has sulfate and
hard water areas (e.g. the Dakotas) have high levels of sulfate.  You would
have to review each manufacturing history to determine the most probable
source of sulfate.

What are the effects of sulfate?  Like most residues, it depends on how much.
 Tap water rinsing generally results in sulfate levels in the 0-3 microgram
per square inch range (as determined by ion chromatography).  We have not
seen degradation on boards with these low levels of sulfate, and treat it
more as a process indicator.  Generally, we see low levels of sulfates on
bare boards (cleaned with tap water) that disappear by the time they get to
the end of the assembly process (volatilization).  Most often, when we see
high levels of sulfate on a bare board, we also see high levels of chloride
as well, indicating a poor or ineffective cleaning process after HASL.

Theoretically, like the halides, sulfates can cause corrosion and metal
migration when present in high enough amounts.  How high is the borderline
amount?  It is difficult to say, since different assemblies have differing
sensitivities to various residues.  Sulfate can combine with unprotected
copper (coper sulfate) and can corrode silver alloys.  I don't have any
papers that I can reference.  Most of my experience comes from the analysis
we do here.

Doug Pauls
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CSL, Inc.