Brian Ellis suggests:

What you are seeing is probably not oxidation but sulfidation. Trace
quantities of SO2 or H2S in the air will cause this. Other than the
aesthetic view, light sulfidation is harmless and reasonable flux
activity should be sufficient to handle it. The best way of prevention
is controlled atmosphere storage.

Doug chimes in:

I would agree with Brian.  Recently, Dave Hillman and I looked over an
immersion silver board that had black traces around the edges of the board,
and in one case, the whole back side of the board.  The board had been
wrapped with silver saver paper.  When I did some ion chromatography work
on the black residue, I found that it was high in both chloride and sulfur.
The silver saver paper was relatively low in both materials, but it had
been taped closed using normal masking tape and put inside a polyethylene
bag.  Analyzing the tape itself with a relatively benign extraction, we
found that it was loaded with both chloride and sulfur.  This was not very
surprising as most tape adhesives are loaded with these two material.  It
is my theory that the masking tape outgassed sulfur (or chloride) and so
tarnished the boards.  Unfortunately, I have not had the time (or funding)
to nail this down, but it does track with some of my experiences at CSL.

On the other hand, Dave analyzed the black material using SERA (where Dave
is the undisputed master) and SERA said the black material was silver
chloride.

This begs larger questions, what is your storage environment, what is your
storage material set (e.g. silver saver paper in polyethylene bags).

Doug Pauls
Rockwell Collins

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