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     Under a hood of a car....hmmmmmm.

     There is a huge sulfer source there, the battery. Most batteries are
     still lead-sulfuric acid batteries.

     I suggest you look there.

     Now how do you prevent it. In a previous incarnation, we had to use
     parylene conformal coating.


______________________________ Reply Separator _________________________________
Subject: [TN] Open circuits from Silver Sulfide Corrosion
Author:  [log in to unmask]>                David Weber <[log in to unmask] at
internet
Date:    9/23/98 9:08 PM


I need help tracing the possible root causes for a circuit failure from
the field. Does anyone have any experience with open circuit traces due
to a silver sulfide corrosion concentrated at the juncture tin-lead
solder joints and silver thick films? We are seeing this only on joints
that are low voltage potential. There is silver sulfide corrosion at
other randomn areas, but the heavy corrosion is primarily at just a few
points where the solder and silver meet. Under the microscope, it looks
like a black worm crawling along the interface between the solder joint
& the silver thick film. We pot the circuit board in silicone gel to
protect the circuit from moisture. So far, we haven't traced the source
of sulfur to any material that we use to make or to package the circuit
board.
The packaging does not contain cardboard, so sulfur from the packaging
is out. We meticulously clean the board after reflow soldering the
printed solder paste, so flux is out. Is it possible that we are getting
sulfur gas from the enviroment? The environment is automotive,
underhood. Is it possible that there is a galvanic action that is
concentrating the corrosion to just a few solder joints?
Thank you all for your time.

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