Inge,
Can you do ionic extraction of your cured Ag epoxy? If it is higher
than 40 microgram... Hmmm, what I can say...if it is 40<x<100.... Not
too bad, you should look other place. If it is more than 100
microgram...oh, boy, you have fun...(don't forget, you need add "+" and
"-" together...Don't forget the organic complex either...hehehe). Now,
let me ask you, did you cured stuff properly? (how many % cure? Within
the 24 hr application of the stuff?).
I am no Steve....Don't ask me more...
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar
>Hernefjord (EMW)
>Sent: Monday, February 10, 2003 9:56 AM
>To: [log in to unmask]
>Subject: [TN] Back to 97, silver migration
>
>
>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.
>
>------------------------------------------------------------------
>
>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
>
>
>
>------------------------------------------------------------------
>
>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]
>
>----------------------------------------------------------------------
>
>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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