Ingemar,

Jim Morris at Binghamton U. presented a very interesting paper at this
years InterPack Conf. on conduction mechanisms in adhesives and has looked
at how the conductor particle distribution and its ability to form a
percolating network is affected near an interface. The paper unfortunately
didn't make it into the proceedings, but the info may be available
elsewhere by now.

David Whalley

At 08:48 AM 9/3/99 +0200, you wrote:
>Many years ago I studied the very contacting surface between a silverepoxy
and different components, most of them semiconductor chips. I found that
the number of silver contacts were surprisingly few in the important
interface flake-to-object. The conductivity within the bulk silver epoxy
layer (20-50um thick is recommended)seemed to be without problems. We
did'nt make much science of the topic, just draw the logical conclusion
that any electrical conductivity problems that occur, are more likely to be
tracked to the few contacts in the nanometer interface flakes-to-object
rather within the bulk epoxy layer. This interface is exposed to a number
of phenomenons that can change the electrical behavior, as such water
vapour molucules, that can easily penetrate in the unperfect contact
between the silver epoxy and the object. Especially when you know what the
Angstrom-region looks like: a sponge! Even a good-looking semiconductor
chip gold backside is a sponge, in microscopical terms. Then imagine a
thickfilm o
>
>CU 
>
>/Ingemar Hernefjord
>Ericsson Microwave Systems
>
>
>
>-----Original Message-----
>From: Blomberg, Rainer (FL51) [mailto:[log in to unmask]]
>Sent: den 30 augusti 1999 18:19
>To: [log in to unmask]
>Subject: Re: [TN] Resisitivity change in conductive adhesives
>
>
>Cecilia,
>Use of silver-filled epoxy is not a simple matter.  I have experienced
>resistance changes also.  How much of a change are you experiencing and what
>cure is being used?
>My experience with silver-filled epoxy is first an increase in conductivity,
>not resistivity, across the first several cure cycles.  Literature on the
>subject agrees with our observation.  The change is greater or less
>depending on the curing time and temperature and cleanliness condition of
>the bonded surfaces.  In our case the interfacial bonds were made to
>gold-plated surfaces.  
>Over time, the resistance increased on some of our parts, but that is not
>normal.  Cleanliness and mixed condition of the material are of critical
>importance, as we later learned.  We had bond (electrical)failures over
>several months to a year time due to temperature cycling by virtue of
>subsequent board processing.  Good bonds remained low in resistance, bad
>ones increased from ohms to K ohms.  Just making a measurement is difficult
>too because there is both a surface and bulk resistance for the material.
>The edges of many silver flakes touching together provides the conductivity,
>but thermal cycling can degrade or break these contacts internally or at a
>weak bonding surface.  Repeated curing could be degrading the adhesion from
>repeated expansion and contraction.
>What can be done about it?  
>First check to see if the bond surface or the material has degraded. Do a
>bond shear-strength test.  Usually, a more compliant material rather than a
>rigid one will survive thermal cycling better.
>Check bulk resistivity of cured samples exposed to the same environment.
>The results will tell you where the problem is.  For us, it was a little of
>both.  We ended up using only fresh-mixed materials because the pre-mixed,
>frozen stuff would never work as well.  We also allowed the material to set
>at room temperature for at least one hour before elevated temperature cure.
>
>Hope this helps,
>Rainer Blomberg
>
>
>
>-----Original Message-----
>From: Cecilia Alkhagen (EMW) [mailto:[log in to unmask]]
>Sent: Monday, August 30, 1999 11:34 AM
>To: [log in to unmask]
>Subject: [TN] Resisitivity change in conductive adhesives
>
>
>Hello,
>
>Here is another question related to materials in the electronic
>industry...(I've got millions!)
>
>On using conductive adhesives in micro electronics we have found that
>sometimes the resistivity in conductive adhesives, epoxi and silver flakes,
>increases upon repeated curing/heating.
>
>A test was performed and a resistivity change was recorded upon repeated
>curing. Some time later the experiment was repeated, under exactly the same
>conditions as far as we know,  but no change in resistivity ocurred.
>
>Has anyone experienced the same?
>What can be done about it?
>Can it be avoided or is it a "naturally occuring" phenomena? 
>
>
>Cecilia Alkhagen
>******************************************
>Ericsson Microwave Systems AB
>Production Engineering Development
>Bergfotsgatan 2
>S-431 84 Mölndal
>
>Telephone +46 31 747 0000
>Direct       +46 31 747 0288
>Fax          +46 31 747 2635

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################