A source for research in the matter is 'ASM Corrosion Handbook Nr 13',
1,000 pages or more. Also 'Electronic Packaging and Corrosion' by Morris
E. Nicholson (ISBN 0-87170-291-6). You will find something there, I'm
sure. / Ingemar Hernefjord
        Ericsson Microwave Systems


--------------------------------------------------------------------
> 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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