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From:
"Blomberg, Rainer (FL51)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 30 Aug 1999 12:18:43 -0400
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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

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