Dave

I remember seeing a very similar problem on a low voltage ceramic hybrid a
few years ago but unfortunately it was a different company so I don't have a
lot of information but you may want to try:-

1. Cleaning the furnace muffle. If your muffle is as dirty as mine was it
could well be a source of sulfur.

2. Check to see if any of the operators smoke. The sulfur may be introduced
from their hands.

3. I seem to remember that we stopped using clear silicon gel (a 2 part Dow
Corning material) on these hybrids so it maybe worthwhile trying to build
and test a few circuits without encapsulant.

Hope this is some help


===== Original Message from TechNet@PAT [log in to unmask] (TechNet E-Mail
Forum.) at 24/09/98 02:08
>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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Mark Austin (Process Engineer)
ACW Technology Ltd
Hylton Road
Petersfield
GU32 3XX

TEL - (01730) 300000
FAX - (01730) 266045

DDI - (01730) 304401
MA @ ACW.CO.UK

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