Steve wrote:
>
> 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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Hi Steve-

Silver is one of the few metals that will form "single crystal"
wire-like filaments when subjected to stress.  In fact, in the
literature, it was reported back in the '30 forming filaments on the
same conductor/plated surface.  I suspect the epoxy is expanding due to
thermal stress which stresses the silver and then the silver forms the
filaments.

Silver has always been a reliability concern inelectronic assemblies.

Hope this helps.

Ralph
--
Ralph Hersey

Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN: 510.454.9805
FAX: 510.454.9805
e-mail: [log in to unmask]

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