Note: Descriptions are shown in the official language in which they were submitted.
- 1141~28
Conductive Adhesive System Including
_Conductivity Enhancer
This invention relates to electrically
conductive epoxy adhesive systems and to electrical
devices and methods including the system. More
particularly, the present invention relates to an
electrically conductive epoxy adhesive system including
a conductivity enhancer.
In recent years, the use of metal filled
thermostat polymers as electrically conductive adhesives
has been extensive and has resulted in partial replacement
of the traditional eutectic bonding and thermocompression
techniques, particularly in the adhesion of miniature
devices such as light emitting diodes and integrated
circuits.
Typically, these thermoset polymers comprise
a mixture of an epoxy resin and a conductive particulate
component such as metal powder, silver flake, copper and
the like in combination with suitable curinq agents,
diluents and the like. At an early stage in the
development of these adhesive systems, workers in the art
determined that in order to maximize conductivity within
the system, it was necessary to promote dispersion of the
conductive component within the epoxy matrix. This end
was normally attained by coating the particles of the
conductive component with a thin film of an organic
material. Typical cf materials used for this purpose
i~4152~3
are fatty acids and compounds thereof, and it is found
that commercially available conductive fillers, such as
silver flake, typically have such coatings. These films
have been found to be highly effective in promoting
dispersion of the conductive component in the liquid
epoxy resin system and they often tend to render the
dispersion insulating before it is subjected to elevated
temperature cure. Accordingly, it is essential that in
the subsequent processing sequence, these films be
stripped in order to attain the desired conductivity.
This is normally effected by the heat associated with an
elevated temperature curing cycle. The specific
temperature levels are dependent upon the formulation
employed. Although these prior art conductive adhesive
systems have proven satisfactory, investigatory efforts
have continued with a view toward the development of
systems evidencing enhanced conductivity and yielding
conductivity levels comparable to those of prior art
systems at lower curing temperatures.
According to the present invention there is
provided a method for bonding one electrical assembly or
element to another or to a support member, comprising
the steps of applying an uncured epoxy material comprising
a conductive particulate component to at least one
surface, applying the surfaces to be bonded together
and curing the epoxy material, wherein the epoxy material
contains 0.1~ to 10~ by weight, based on the weight of
the epoxy resin, of a conductivity enhancer selected from
the group represented by
L ] 2
n
n being in the range of 2 to 10.
In the preferred embodiment of the invention,
the above objects are a-ttained by the use of a novel
conductivity enhancer. Compounds found to be particularly
suitable for the described purpose are the hydroxy-
terminated ethers, especially polyethylene-glycols,
1~
528
- 3
HOC~12 (C1120C~12 ) n ~120~1~ allcl the like, with n being in the
range 2 to 10. The upper limit of n is dictated by
practical considerations relating to solubility.
Epoxy resins employed in the adhesive systems
described are preferably selected from epoxy-novolac resins,
bisphenol A-epichlorohydrin resins, cycloaliphatic epoxies
and the like. The selection of a particular epoxy resin is
not critical, the only requirement being that it be
compatible with the ethers chosen.
In preparing the adhesive system, the epoxy resin is
initially mixed with a reactive viscosity reducer and a
curing agent, each of which is well known in the art. Then,
the conductivity enhancer described herein is added to the
resin system in an amount preferably ranging from 0.1 -
10 percent, by weight, based upon the weight of the resin.
These limits are dictated by practical considerations. Thus,
the use of conductivity enhancer in excess of the maximum
adversely affects the mechanical and thermal properties of
the system whereas less than the noted minimum tends to
yield an undesirable result. Then, the conductivity
component, typically in the form of metal powder or flakes
rangirg in particle size from 0.5-50 microns, is added to
the epoxy resin in an amount such that the conductivity
component comprises from 25-85~ by weight, based upon the
~5 total weight of the composition.
By "epoxy resin" we mean the epoxy without additives,
and by "epoxy material" we mean the epoxy resin plus
additives such as conductive particulate components, diluents,
curing agents and the like.
One example of the present invention is set forth
below.
_ample 1
An epoxy resin adhesive system was prepared comprising
the following components:
(a) 31~, by weight, o~ an epoxy novolac resin having
an average of 3.6 epoxy groups per molecule, a
viscvsity ranging from 20,000 to 50,000 cps at
50 C and an epoxy equivalent weight within the
range of 176-181,
114~5Z8
(b) 44%, by weight, resorcinol diglycidyl ether,
having a viscosity ranging from 300-500 cps
at 25 C and an epoxy equivalent weight of
approximately 127,
(c) 12.5%, by weight, tetraglycidyl ether of
tetraphenyl ethane,
(d) 12.5~, by weight, butanediol diglycidyl ether,
(e) 0.~275 moles per 100 grams of resin of
l-dimethylcarbamoyl-4-phenylimidazole,
and
(f) 1.4%, by weight, of a hydroxy terminated ether,
HOC~2(cH2OcH2)n--3c 2
To this mixture, silver flake having a surface area
ranging from 0.75 to 1.35m2/g containing a maximum of 0.02%
chloride and a maximum particle dimension ranging from about
0.5 to about 5 ~m was added. Electrical resistivity was
then measured using a stripe specimen. This involved the
use of a ~icroscope slide to which two copper pads spaced
25mm apart were bonded with an adhesive. Pressure sensitive
tape was laid lengthwise over the pads from end to end of
the slide. Two razor cuts, 2.5mm apart, were then made
lengthwise in the tape over the pads from end to end and
-the section of the tape so cut was peeled away. The
conductive adhesive was forced into the opening using the
edge of a microscope slide so that it was level with the top
surface of the remaining tape which was then peeled away.
There remained a stripe of adhesive bridging the copper pads
which, after curing at 175 C for 30 minutes, was 2.5mm wide
and approximately 0.038mm thick. The resistance was
computed from the voltage drop across the stripe at a current
of 100 milliamperes using a Keithley ~160B Digital Multime-ter
and a Keithley~ 227 constant current source. I~he resistivity
was as follows:
Resistivity, p(ohm-cm) (average of 2 specimens)
initial 1.80 x 10 ~
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-- 5
The foregoing procedure was repeated with the
exception that 2.4%, by weight, and 6.2%, by weight, of
hydroxy terminated ether were added. The resistivities
were 1.43 x 10 4 ohm-cm and 1.22 x 10 4 ohm-cm, respectively.
For comparative purposes, the foregoing procedure was
repeated without the presence of the hydroxy terminated
ether. The resistivity was 3.15 x 10 4 ohm-cm.
It is to be understood that the described
conductivity enhancers may be used as a conductivity re-
juvenant for conductive adhesives, which, as known, by
virtue of prolonged storage in thQ uncured state have lost
some of their conductivity attributes.
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