Is the rubber real rubber or plastic? Many plastics are degrade 
     slightly and release sulfuric acid. (Paper does this also, look at any 
     old newspaper. The yellow color is from the acid.)
     
     There are also might be some unbound sulfur in the rubber, left from 
     the vulcanization process.


______________________________ Reply Separator _________________________________
Subject: Copper Sulphide corrosion
Author:  <[log in to unmask]> at internet
Date:    5/21/97 2:07 PM


Hi
     
We have seen failures on a bought-in assembly due to growth of copper 
sulphide on the copper traces of a flexible PCB.  The failure occurs at the 
base of SnPb solder joints, where the copper traces are not covered by 
photoresist.  The whole area of exposed copper and solder joints is covered 
by a silicone based potting material.
     
Unfortunately our supplier is not being very helpful, so I am looking for 
clues as to what is happening and how to stop it myself.
     
The source of sulphur is not known, but there are various rubber components 
within the immediate area.
     
Does anyone have any experience of this type of corrosion system?  If so, I 
would appreciate a discussion about the factors involved - in particular, 
the role of the potting compound and whether an alternative passivating 
material would do a better job.
     
Thank you
     
     
     
John Loveluck
Group Quality Manager
Tadpole Technology plc
Cambridge Science Park
Cambridge CB4 4RB
Tel: (+44)1223.428200
Fax: (+44)1223.428201
     
email: [log in to unmask]
     
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