All,

Background;

I'm being consumed by a hybrid failure suffered by one of my
subcontractors. The cause of failure is silver bridging two gold
conductors metallized on an alumina substrate. In DC mode, one conductor
is at -5.5V, the other at +28V. The conductor spacing is ~146 microns.

The silver is from an epoxy die attach on the +28V conductor. The silver
bridging the conductors is a combination of deposited (workmanship) and
dendrite (electrochemical) formation. The deposited silver is half on
the -5.5V wire-bond land and half bridging the alumina. Dendrites form
the rest of the bridge across the alumina. Dendrites are also observed
*IN* the silver epoxy die attach.

Now for the question;

1)Understanding the conditions necessary for silver (electrochemical)
migration, how could silver dendrites form *IN* the silver epoxy? There
shouldn't be any potential difference *IN* silver epoxy.

2)How could the +28V silver migrate out of the epoxy die attach across
the +28V gold metallization then across the alumina? The silver die
attach was completely within the +28V gold metallization. Again, no
potential difference to "steer" migration. Oddly, an identical config in
the same hybrid did have +28V silver epoxy squeeze out into the alumina
bridging 1/3 the way to -5.5V. No silver migration observed there.

If any one can help, I can be reached @ steve.r.anderson @ trw.com
(remove the spaces before & after the @).

I can post optical & SEM/EDX images to our ftp site but they will be
removed after 24 hrs.

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