Hi Techies,
anyone who remember the mails that come after my lines?
If the guys at that time (97) aren't retired and are golfing and surfing
in Florida, I would like to chat about the headline issue, because we have seen some odd things likely. My worry is about human debris (spittle) and epoxy bleeding. What does saliva contain, and what's in the silver epoxy bleeding remnants? Silver migration took place between Au/Ag thickfilm conductors and also across MIS chips. In the later case, Ag migrated from the silver epoxy, across the MIS chip and climbed up on the gold bond wire. Yes,really weird.
I'm not so interested in 'qualified' guessings as in experiences from people around the TN. We have had a lot of guessings ourselves, but need corrective actions asap.
I could of cause call Mike Fenner, but he is only 0.1% of TN, so there is a chance that someone more are familiar with the sweet problem.
Wonder what will migrate out of this?
Ingemar Hernefjord
Ericsson Microwave Systems
PS. Unfortunately, I don't know who 'Steve' was. Can't be Steve Gregory,because he knows all secrets himself, and is teaching others.
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teve 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.
Steve
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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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Hi Steve,
you`re describing interesting phenomena. Unfortunally I don`t have a
global answer but one (perhaps weird) idea relating to your first
question, how migration can take place in the silver epoxy. Let try me
to explain my thoughts:
At first, conduction in silver filled epoxies is in principal done by
random contacts between the randomly placed silver particles. Lets
assume there are two adjacent conducting paths formed by particles
contacting each another, but not particles of the other path (the two
pathes are isolated from each other). One path is at +28 V, the other
one (probably short and located at the edge of the conductive glue) is
isolated. Now lets assume migration starts and dendrites grow from the
-5.5 V side towards the +28V side. Where will they reach the +28V side?
Perhaps by chance at first at the isolated path. This path then drops
down to -5.5V and you`ve got two very close conducting paths in the
epoxy with a potential difference of 33.5 V. Now one has: silver, high
field strength, some ionic impurities (which are resulting from the
chemicals used in the manufacturing of the epoxy), obviously moisture -
migration starts.
As I understand from your mail, you got a chip land covered with silver
filled die attach epoxy and some bondpads very close (146 micrometers
seems to be unusual close to me). The epoxy is sometimes squeezed out
over the chip land, thus reducing the distance furthermore. The first
idea to reduce migration issues is to get bigger distances.
If, as you wrote, migration under identical build up conditions in some
cases took place and in others didn´t, I would look carefully on
moisture issues which will be probably different in these cases.
By the way, if you can post pictures to your ftp-side, you should also
be able to include them in emails. But to not upset some technetters by
getting megabyte big mails, if you like, you could send them directly to
me. I would be interested in receiving them.
Greetings,
Ulrich Korndörfer
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