Rainer makes some good points below. Here are a few extra tit bits:
A post cure increase in volume conductivity (decrease in volume resistivity) is quite
normal and some manufacturers give data for this in their literature. It is usually
around no more than 5% tops, and is accompanied by optimising of other physical
properties such as bond strength over about 150 - 200 hours at room temperature. This
time can be shortened by re-heating. Insofar as joint properties are actually being
enhanced in this initial period most people do not find these slight changes to be
detrimental.
Subsequent decreases in overall joint conductivity are usually due to interactions at
the bond line (ie not in the body of the conductive epoxy) and are substrate
dependent. Some surfaces are more suited to epoxy bonding than others. If your
testing shows decreases in joint quality in time scales similar to the above then you
need to do some prompt investigations of the type outlined by Rainer.
When considering cure schedules it is important to recall that most suppliers' time
and temperature data is the minimum time and temperature at the bond line - this is
not necesarily the same as oven temp or time in oven - and it is permissible maybe
even desirable to increase the time at temperature to ensure more complete cross
linking. Again you will see this reflected in some supplier literature when giving
certain expected cured properties.
It is also good practise (considering bond quality only) to ramp to cure temperature
as quickly as possible and to choose a cure temperature which is above the designed
maximum operating temperature of your product.
Given proper handling and storage procedures there should be no difference between
the same product whether self or supplier mixed. In practise of course there may be
some departure from optimum for one presentation to another according to the
particular plant circumstances, and it could also be that some differences observed
between premixed and self mixed materials are attrituble to differences in
suitability for application between different supplier formulations.
Hope this helps, let me know direct if you need any further amplification.
Mike Fenner
-----Original Message-----
From: Blomberg, Rainer (FL51) <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 30 August 1999 17:24
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
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Direct +46 31 747 0288
Fax +46 31 747 2635
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