Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 7~ 451,050 CAN/RRT
SHEET MATERIAL ADAPTED TO PROVIDE LONG LIVED STABLE
DHESIVE-BONDED ELECTRICAL CONNECTIONS
There is a need in the electronic equipment
industry for means for making convenient and secure
5 electrical connections to sets of small side-by-side
terminal pads, such as the terminal pads of a printed
circuit board or a liquid crystal display. A promising
technique for making such connections is taught in
laid-open United Kingdom patent application No. 2,048,582A,
which teaches an adhesive connector tape comprising a
flexible insulative sheet, a plurality of parallel,
separated, electrically conductive stripes carried on the
sheet, and an electrically conductive adhesive covering the
conductive stripes. Electrical connections can be made by
adhering an end of the tape against a set of terminal pads,
with individual stripes on the tape in alignment with
individual pads~
For satis~actory use of sheet material as
described, the electrically conductive adhesive in the
sheet material must achieve a low-resistance bond that is
stable for the length of time and under the operating
conditions that are expected for the sheet material.
Conventional electrically conductive adhesives have not
provided the needed degree of stability and low resistance.
Initial resistance is too high and/or resi~tance incr~ases
during use, to the extent that mechanical clamping
techniques are o~ten used to supplement the adhe~ive bond,
Summary of the Invention
The present lnvention provides sheet material
adapted to make adhesive-bonded electrical connections of
improved stability and low resistance. Briefly, this new
sheet material comprises
an adhesive layer which softens to an adhesive
condition upon heating to an elevated tempara-ture, and
subsequently hardens to exhibit a firm and substantially
nonflowable condition at room temperature; and
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a monolayer of discrete separated electrically
conductive elements distributed in the adhesive layer;
the elements having an average thickness greater
than the average ~hickness oE the adhesive layer, and the
~op edge of substantially each element being higher than at
least part oE the exterior surface of the adhesive layer
surrounding the element.
Most often, the described sheet material is
carried on or applied in use to a flexible backing. During
bonding of the sheet material to a substrate, the adhesive
around a conductive element is pressed lnto contact with
the substrate and forms an adhesive bond to ~he substrate~
The backing is also pressed toward the substrate and is
drawn around the individual conductive elements, which are
thicker than the adhesive layer and accordingly occupy a
greater height between the substra~e and the backing then
the adhesive layer occupies~ After hardening of the
adhesive, the hacking appears to be held in tension around
the conductive elements and to place the conductive element ~;~
20 under compression again3~ the substrate. With the adhesive ~j
layer in a firm and subs~an~ially nonflowable condition and
with the electrically conductive elements held against the
substrate, connections are formed that have low resi~tance
and maintain that low resistance over a long u~eful life.
Although the electrically conductive elements are
thicker on average than the average thickness of the
adhesive layer~ and the top edge of substantially each
element is higher than at least part oE the exterior
surface of the adhesive layer surrounding the element,
there is desirably a thin layer of adhesive material
covering the elements to electrically insulate them until
the tlme oF the bonding operation. ~uch a thin layer
preferably takes the form of a thin continuous electrically
insulating adhesive layer lying over the whole sheet
material as a blanket, and conforming to the protruding
electrically conductive element and to any adhesive layer
between the particles. But even with such an added layer
--3--
of insulating material, the top edge of the electrically
conductive elements is higher than the adhesive layer
between the particles, including any original adhssive
layer and the layer o insulating material applied over the
original adhesive layer, which becomes part of the complete
adhesive layer.
Typically, sheet material of the invention takes
the form of an elongated tape which is wound upon itself in
roll form for convenience in storage and use, Also, a
plurality of electrically conductive layers are typically
included as narrow parallel elongated stripes carried on
the backing under the adhesive layer, with the stripes
la~erally spaced fro~n one another an~1 extending the length
of the backing. Connections are thus conveniently made
between terminal substrates which comprise a plurality of
separated side-by-side terminal pads. Howevar, other
configurations of conductive stripes or paths besides
parallel stripes are u~ed in some embodiments of sheet
material of the invention Eor special applications.
The utility o~ sheet material of the invention
contrasts with previous experience with commercial
pressure-sensitive adhesive connector tape products of the
type described in U,S, Pat. 3,475,213. Those tapes use a
pressure-sensltive adhe~ive layer coated onto an
electrically conductive backin~, typically a ~etal foil~
with a monolayer of relatively large particles d,spersed in
the adhesive layer, The particles in thesa tapes were
substantially the same thickness as the adhesiv3 layer and
sometimes may have been more thick than the adhesive layer.
However, these tapes do not always achieve low-resistance
electrical connections unless clamps are used to hold the
tape against a sub~trate. Apparently, the force holding
the particle~ agains~ the substrate gradually decreases
after the tape has been adhered in place as a result oE
flow of the adhesive.
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rief Description of the_Drawin~s
Figure 1 is a sectional view through an
illustrative electrical connector tape of the invention;
~igure 2 is a drawing showing the illustrative
5 ~ape of Figure 1 adhered to a substrate; and
Figures 3 and 4 ar~ sectional views ~hrough
different illustrative electrical connector tapes of the
invention.
Detailed Description
The illustrative tape 10 shown in Figure 1
comprises a Elat flexible electrically insulating sheet or
backing 11, electrically conductive stripes 12, a layer of
adhesive material 13 coated over the conductive stripes,
alectrically conductive particles 14 distributed in the
adhesive layer, and a thin layer 15 of electrically
insulating material coated over the whole top surface of
the tape~ ~;
The flat electrically insulating sheet or backing
11 typically comprises a polymeric film, such as a film of
polyethylene terephthalate or polyimide, or a resin-
impregnated fibrous web. The backing should be flexible so
that it will conform around the electrically conductive
elements during a bonding operation and allow the adhesive
carried on the backing to contact the substrate to which a
bond is being mada~ Preferred backings have a flexibility
on the order o a 25- or 50-micrometer thick Eilm of
polyethylene terephthalate. However, less flaxible
backings can be used, generally by using greater pressure
during a bonding opera~ion and by using somewhat thicker
adheæive layers, so that the backing need not conEorm as
greatly as it does with thinner adhesive layers.
The electrically conductive stripes 12 typically
comprise a layer of metal, such as silver, gold, aluminum,
or copper, vapor-deposited on-to the Elat backing. Other
conductive layers can be used instead, so long as they
leave the backing sufEiciently flexible to generally
` ` ~.2~6~'7i~7'
conform around a conductive element during adhesion of
the sheet material to a substrate. Other useful conductive
layers include a metal foil (which may constitute the
whole backing or may be adhered to the backing with adhesive),
or a layer of metal sputtered onto the backing, or a layer
formed from a conductive coating composition or ink, typically
comprising a coating vehicle and conductors such as metal
or carbon particles.
The adhesive material 13 is a heat-activated
material which forms an adhesive bond during a heating
operation. During the heating operation the adhesive material
wets out a substrate to which adhesion is to be made.
Subsequently, either by cooling or reaction of the ingre-
dients, the adhesive hardens so that at room temperature
the shee-t material of the invention and conductive particles
are held in place with respect to an adherend. At this
point the adhesive material is either nontacky or poorly
tacky.
A preferred adhesive material, known as a "hot-
tackifying adhesive", is described in a copending applicationof Robert H. Stow, Canadian Patent Application Serial
No. 418,478, filed December 22, 1982. As described in
that application, the adhesive material is non-tacky or
poorly tacky at 20C, but becomes pressure-sensitive and
aggressively tacky when heated. Good bonds are immediately
formed at a tackifying temperature without any need for
crosslinking or other chemical reactions. The adhesive
~ material comprises an acrylic polymer or mixture of acrylic
polymers of at least one alkyl acrylate and/or methacrylate
ester monomer (here called "acrylic ester monomer"), and
differs from prior art adhesive materials in that:
1) acrylic ester monomer provides at least 50 mol
percent of the one or more acrylic polymers of the
adhesive layer,
2) said one or more acrylic polymers have a
T (glass transition temperature) or a weight-averaged
T of -10 to 80C,
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~.
~L26~ 7
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3) a layer of the adhesive material has
a~ a Probe Tack Valuel o~ less than 75 grams
of force (gf) at 20C,
b) Probe Tack Valuas of a~ least 75 gf over a
range o~ at least 50C, which values
remain substantially constant after 30
days at 40C, and
a) a Shear Value2 of at least 25 minutes at
65C, and
4) up to 50 mol percent of the one or more
acrylic polymers can be provided by copolymeriæable
monomer having a polar group, such as acrylic acid,
methacrylic acid, itaconic acid, maleic acid or
anhydride, the amides of said acids, acrylonitrile,
methacrylonitrile, and N-vinyl-2-pyrrolidone (the
notes are at the end o~ the speci~ication~.
The one or more acrylic polymers may be a
homopolymer o~ an acrylic ester monomer which pro~ides a Tg
within the range of -10 to 80C, e~g., methyl acrylate, or
a copolymer of acrylic ester monomer and copolymerizable
polar monomer having a Tg within that rangeO Useful
acrylic ester monomers which homopolymerize to a Tg of at
least -10 include methyl acrylate, methyl methacryla~e,
ethyl methacrylate, propyl methacrylates, butyl
methacrylates~ bornyl acryla~es, bornyl methacryla~es,
2 phenoxyethyl acrylate, 2-phenoxyethyl methacrylate, the
mono- and di- methyl and ethyl e~ters of itaconic acid, and
the mono- and di~ methyl and ethyl esters of maleic acld.
U~eful acrylic e~ter monomers which provide reduced Tg
include ethyl, butyl, and octyl acryla~es, and n-amyl,
hexyl and octyl methacrylate~. A copolymer of 43 mol
percent of methyl methacrylate, 53 mol percent of methyl
acrylate and 4 mol percent oE acrylamide had a Tg of abou~
50C, A copolymer of 73 mol percent of methyl
methacrylate, l9 mol percent oE methyl acrylate, 4 mol
percent of ethyl acrylate~ and 4 mol percent of acrylamide
had a Tg of about 79Co
~IL2()~
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The described hot tackifying adhesive material
become~ pressure-sensitive and aggressively tacky when
heated, typically for use in this invention to a temperW
ature of about 40C or above, and preferably 75C or above.
When later subjected to temperatures at or even above the
bonding temperature, ade~uate bonding strength may be
retained. Electrically conductive particles may be
dispersed into the adhesive material to form a conduc~ive
bond~ and the particles and adherend~ tend to be retained
in their bonded position by the firm adhesive material at
elevated temperature~ as well as room temperature.
Other copolymerizable monomers may also be
employed in small amounts without detracting from the value
of the acrylic copol~mer for the purposes taught in the
application, Among such copolymerizable monomers are
styrene, vinyl acetate and ~inyl chloride, each of which
can be used in amounts up to about 5 mol percent of the
total monomers.
Honds exhibiting the best durability during
prolonged exposure to high humidity (e.g., 95% RH) at
elevated temperatures (e.g., 80C) are obtained with hot
tacki~ying acrylic adhesives in which the acrylic polymer
has an interac~ed functionally reactive organosilane
coupling agent in an amount of at least 0,2 part per 100
parts hy weight of total monomer. ~est results are
attained at about 0.5 to 4 percent.
The organosilane may be interpolymerized with the
acrylic ester monomer, with or without other copolymeriz-
able monomers, or it may be reacted with Eunctional groups
on the backbone o~ an acrylic polymer. Either process
results in what is hereinafter called an "acrylic-silane
interpolymer,"
The organo~ilane has the general formula
R(4_n~SiXnt where X is a hydrolyzable group swch as ethoxy,
methoxy, or 2-methoxy-ethoxy; R is a monovalent organic
radical oE from 1 to 12 carbon atoms which contains a
functional organic group such as mercapto, epoxy, acrylyl,
methacrylyl, or amino; and n is an ~nteger of ~rom 1 to 3.
--8--
As is known in the art, the organosilane can
cause solutions of polymers to gel, so that it may be
dasirable to employ an alcohol or other known stabilizers.
When the organosilane is to be copolymerized with the other
monomer, a stabilizer should be selected that does not
interfere with the polymerization. Methanol is especially
useful and is preferably employed in amounts from about
twice to about four times the amount of the organosilane.
Other heat-activated adhesive materials that can
be used are hot-melt adhesive materials, which are
typically thermoplastic materials that soften to a flowable
state and then cool to form an adhesive bond, and reactive
compositions, such as epoxy-based adhesives. Sheet
material in which the adhesive is pressure-sensitive at
room temperature may also benefit from the present
invention, i.eO, by the use oE electrically conductive
elemants in a size relationship as taught herein with the
layer oE pressure-sensitive adhesive on a flexible backing,
especially under circumstances in which the bonded
electrical connection to be made with the sheet material
does not experience high ambient temperatures and stresses.
The conductive particles 14 in the illustrative
sheet ma~erial of the invention shown in Figure 1 are
flattened to a generally common thickness. For example, a
sieved batch of originally spherical particles may be
passed through nip rolls such as in a paint mill; see U.S.
Pat. 3,475l213. The flattened particles are e~peially
- desirable because they tenc] to lie on their flattened side,
and a high percentage of ths particles participate in
conducting electrically through the adhesive layer in an
adhesive bond. Spherical particles are also useful,
especially when screened within narrow size ranges so that
a high percentage of the particles are of about the same
size, The particles should be sufEiciently firm or rigid
so as to penetrate through the insulating layer 15 during a
bonding operation; but some de~ormation o~ the particles
may occur during the bonding operation, e.g., by pressure
- 9 -
against a rigid .substrate. The particles are usually
metal, preferably silver but alternatively copper or
aluminum (for which additives as described in U.S~ Pat.
3,475,213 are desirable to achieve compatibility), or
various other metals, metallized particles such as glass
beads, carbon particles, etc. Also, electrically
conductive elements may take the form of material embossed
Erom a conductive backing, such as the embossed protrusions
from a metal foil taught in U.S. Pat~ 3,497,383. Or small
particles clustered closely together may comprise an
electrically conductive element.
The particles can range in thickness from at
least 10 to 500 micrometers, though the preferred range Eor
presently contemplated products is about 20 to 100
micrometers, and the adhesive layer can range in thickness
from at least 6 to 45Q micrometers. (The average thickness
of the adhesive layer is determined by measuring the
approximate volume of adhesive material in the layer, and
dividing tha~ volume by the area of the sheet material.)
With presently contemplated typical si~es and densities of
electrically conductive elements, hackings, etc., good
adhesive bonds generally call for the average thickness of ~`
the adhesive layer to be no less than about sixty percent
(60%) of the average thickness of the electrically
conductivs elements. But lasting lo~-resistance electric
connections are achieved by making the adhesive layer
signiEicantly thinner than the electrically conductive
elements, i.e., wi~h an average thickness generally about
ninety percent (90~) or less of -the average thickness of
the conductive elements. Best results are obtained when
the average thicknes~ of the adhesive layer is about 70 ~o
30 percent of the average thickness of the electrically
conductive elements.
As a corollary to the above discussion, and as a
further contribution to lasting low-resistance electric
aonnections, the top edge of substantially all the
electrically conductive elements is higher than at least
- ~ o -
part of the adhesive layer surrounding the particles. That
is, the dimension 16 of substantially each particle 14 in
Figure l is greater than the dimension 17 of the adhesive
layer at at least some points on the exterior ~urface of
the adhesive surrounding the particle. Preferably, the
whole of substantially each particle is encircled by an
area of the adhesive layer that-is less high than the top
edge of the slectrically conductive element. Also, the
electrically conductive elements are preferably substan-
tially all separated on average by at least the averagediameter of the elements, and more typically four or five
times or more the average diameter, so as to allow the
backing to con~orm around the elements during a bonding
operation. On the other hand, the electrically conductive
elements preferably occupy at least 2 percent, and more
pre~erably at least 4 percent, o~ the area of the sheet
material.
The layer 15 o~ electrically insulating material
provides useful electrical insulation even though it should
be thinl on the order of 10 microme~ers in ~hickness over
the conductive particles 14 in a construction as shown in
Figure l. Resistances through the layer 15 to the
conductive particles of at least one megohm should be
achieved to obtain the desired insulation. Reslstance is
mea~ured by laying a test sample over a
one-csntimeter-square copper substrate, with the exterior
surface of the insulation layer of the sample against the
substrate, and laying a 500~9ram weight over the test
sample at room temperature~ Electrical connection has
prevlously been made betwean a metal conductor and the
conductive layer in the test sample, e.g., the stripes 12
in the sheet matsrial shown in Fi~ures 1 and 2, by heat and
p~essure, A voltage of 5 volts is applied to the metal
conductor, with the copper substrate maintained at ground~
and the resistance in the circuit measured.
The in~ulating layer 15 preferably compri~es the
same or a similar ma~erial as the adhesive material 13 in
g~
which the conductive particles 14 are dispersed. The
hot-tackifying adhesive taught in the previously mentioned
copending application of Robert H. Stow is a preferred
material, One advantage is tha~ it exhibits adhe~ive
character over a wide temperature interval so that adhesive
connections can be maintained even though the bond area has
not cooled to room temperature. In some cases the
insulating layer may comprise a diEferent variety of
hot-tackifying adhe.sive, such as a variety having a lower
glass transition temperature (Tg) than the adhesive
material in which conductive particles are dispersed. The
higher-Tg adhesive material offers greater firmness at room
temperaturel while the lower-Tg insulating layer flows
readily and assists in formation of a desired adhesive
bond. Other adhesive materials such as hot-melt adhesives
or reactive composltions may aliso be used.
After bonding to a substrate, as shown for the
substrate 18 carrying conductive pads 19 in Fiyure 2, the
contact surface o~ sheet material of the invention
generally follows the sur~ace of the substrate. (The
adhesive layer 13 and insulating layer 15 are shown to have
merged into one adhesive layer 13-lS). The terminal
substrates with which sheet material of the invention is
used may be planar, with terminal pads embedded in the
substrate and coplanar with the rest of the substrate~ in
which case sheet material of the invention forms a
generally planar full-area contact with the substrate.
PreEerably, however, the terminal pads are slightly raised.
As also shown in Figure 2, the side of the sheet
material 10 opposite from the substrate 18 is generally
contoured aEter a bonding operation, with the backing or
sheet 11 generally following the contour o~ the conductive
particles, and the backing typically feels rough2nQd by
this contouring~ Interestingly, the contoured surface can
he obtained even by pressing a smooth-surfaced rigid
bonding head against the back surface oE the backing or
sheet 11. ~pparently stresses are developed within the
-12-
sheet 11 during the bonding operation that force the sheet
upwardly into the spaces between the particles 14 toward
the sub~trate 18~ When the adhesive material 13-15
hardens, as by cooling, the backing 11 is held against the
s ~ubstrate and apparently holds the particles in compression
again3t the substra~e (al~hough the element is in
compression against tha substrate, it need not be in Airect
contact with the substrate, but may be separated from the
substrate by a thin layer oE adhesive material).
The smbodiment of sheet material shown in Figure
1 illustrate~ anothar desirable ~eature of ~heet material
of the invention. That is, it is desirable for the
adhesive surface of the sheet material to be profiled, with
at least part of the surface o~ the adhesive, Eor example,
the part that overlies spaces between the electrically
conductive stripes, being recessed below other areas of the
adhesive surface. Accordingly, some adhesive material in
the area of the conductive stripes can be displaced during
the bonding operation into the recessed areas between the
stripes, and the electrically conductive elements become
held in closer electrical association with the substrate.
Such displacement occurs in proportion to the degree of
flowability of the adhesive material and the degree of heat
and pressure applied to the adhesive material during the
bonding operation. A hot-tackifying adhesive material may
not flow extensively during a bonding operation, and as
shown in Figuxe 2, the flexible backing conforms to the
pro~iled thickness of the adhesive layer. Desirably the
recessed area of the adhesive layer are recessed at least
10 percent and preferably at least 25 percent, below the
average height of the non-recessed area oE the adhesive
layer, The insulating layer 15 in the embodiment of Figure
1, is of a rathar constant thickness and conforms to the
profile left ~y the protruding particles and adhesive
material 13,
In the finished bond the electrically conductive
elements occupy a sufficient proportion of the thickness o~
7 ~
-13-
the adhesive bond to allow any necessary diel~ctric break-
down through the adhesive material and achieve conduction
between the conductive stripe and a substrate to which the
sheet material is adhered. Since the electrically conduc-
tive elements occupy a minor proportion of the area in theplane of a bond~ they leave substantial area in which
adhesive contacts the adherend.
Together, the adhesive material and electrically
conductive elements provide an electrically conductive
adhesive layer which is conductive through the layer but
not laterally within the layer. As shown in Figure 3, in
some embodiments of the invention electrically conductive
adhesive ~0 extends over the whole surface of one side of
sheet material of the invention, thereby avoiding the
necessity for limited coating of the electrically conduc-
tive adhesive over only an electrically conductive stripe.
Since the electrically conductive adhesive is not conduc-
tive laterally, the adjacan~ stripes 21 remain electrically
isolated from one another. The conductive particles 22 in
the electrically conductive adhesive make connection only
through the adhesive layer Erom the electrically conductive
stripe 21 to a terminal pad with which the stripe is
aligned.
Another variety of sheet material of the inven-
~ion shown in Figure 4 includes an electrically conductive
layer 24 which ex~ends over the full extent of the sheet
material, Sheet material having such a layer is useful for
making ground connections, as between a metal chassis and a
part ~ounted on the chassis.
Sheet material of the invention, especially when
an elongated tape to be wound upon itselE in roll form,
preferably includes a low-adhesion backsize on the
non-adhesive side, or a release liner disposed over the
insulating layer, Also, primers may be applied to a
polymeric or metallic backing to promote adhesion to an
adhesive or insulating layer carried on the backing~
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~14-~
Sheet material of the invention is generally
applied by aligning an end of the tape over the desired
3ubstrate to which connection is to be made, pressing the
sheet material against the substrate, and at the same time
heating the sheet material. Transfer adhesive sheet
materials of the invention may be placed between deslred
adherends and a bonded electrical connection made by
applying heat and pressure. In such transfer adhesive
sheet materials electrically conductive elements may be
dispersed in an adhesive material which forms a support web
for the elements, and an insulating layer may be disposed
on one or both sides of the element-containing web and the
elemsnts may protrude from both sides of the web.
Alternatively, the material in which the elements is
dispersed is a non-adhesive polymeric film, and adhesion is
provided by the insulating layer. Similarly, the layer 13
in a product as shown in Figure l and 2 may be
non-adhesive, e.g., because of reaction to a durable, firm
state.
The invention will be further illustrated by the
following examples.
A film of polyethylene terephthalate 25
micrometers thick was vapor-coated on one surface through a
slotted mask to ~orm 875-micrometer-wide continuous stripes
of ~ilver spaced 875 micrometers apart. The stripes were
approximately 40Q anystroms ~0 nanometers) thick and had
an electrical resistance of 4 ohms per centimeter length.
Electrically conductive adhesive was prepared by mixing
94.9 volume-parts of acrylic terpolymer which comprised
10.4 weight-percent methyl methacrylate, 85.6 weight-
percent methyl acrylate, and 4 weight-percent acrylamide
dissolved in ethyl acetater and 5.1 volume~partæ oE silver
particles, The particles had been sieved through a
140-mesh screen (U,S, standard; 105 micrometer mesh size)
and retained on a 170 mesh screen (88 micrometers) and then
D71~7
-15-
passed through a roller mill to flatten the particles to
approximately 48 micrometers thickness. The mixture of
adhe~ive and particles was applied in registry over the
conductive stripes by coating through an apertured mask.
After drying, the adhesive terpolymer occupied a thickness
of approximately 20 micrometers.
An insulating layer o~ acrylic terpolymer com-
prising 40 weight-percent ethyl acrylats, 56 weight-
percent methyl acrylate, and 4 weight percent acrylamide
dissolved at about 25 weight percent solids in ethyl
acetate was then applied over the whole surface of the
sheet material by bar coating, ~hereby ¢overing the
adhesive-coated stripes and the film backing between the
stripes. A~ter drying, a rather con~stan~-thickness layer
approximately 10 micrometers thick was formed, as shown in
Figure 1. The ratio o~ the combined thickness of the
adhesive layer and insulating layer (30 micrometers) to the
average thickness of the particles was 62.5 percent.
The resistance through the layer as measured by
the method described above was ahout 1000 megohmsO For
comparison a similar tape wi~hout the insulating layer was
prepared and found to exhibit 10 ohms resistance.
One end of the tape of this example was adhered
to the electrically conduc~ive terminal pads of a printed
circuit test board by pressing the tape against the sub-
strat~ with a Eorce of 150 pounds per square inch tlO.5
kilograms per square centimeter) and heating the end of the
tape to a temperature of 170C~ Eor 5 seconds. After the
connection had been allowed to cool, the resistance at ~he
connectlon was measured by the four terminal re~istance
method and found to be 10 milliohms. The backing was
roughened in the manner shown in Figure ~. The peel
strength oE the bond to the substrate ~as also measured
according to ASTM D~1000 and ~ound to be 2.5 to 5 pound~
per inch width (c45 to .9 kilograms per centimeter).
~Z~ 7
-16-
Example 2
Two different tapes of the type described in
Example 1 wers prepared using particles having a flattened
thickness of approximately 40 micrometers, sufficient
adhesiva material in mixture with tha particlss to provide
an adhesive layer approximately 15 micrometers in
thickness, and insulating layers of two diferent
~hicknesses -- one (Example 2A) approximately 9 micrometers
and the other (Example 2B) approxima~ely 21 micrometers.
Ths ratio of the combined thickness of the adhesive layer
and insulating adhesive layers to the average particle
thickness was 60% Eor Example 2A and 90~ for Example 2B.
Pieces oP tape were cut to size and bonded ~etween the
conductive pads of a printed circuit board and a indium tin
oxide (ITO) vapor-coated surface on a glass test panel
using a pressure of 200 psi at 150C for five second~. The
multiple connections made by each tape to the ITO test
panel were monitored for individual contact resistances
using a four-wire ohms method. The test panel was cycled
between ~40C and 105C every four hours. Table I below
shows the results for the maximum contact resistance
obsarved during the reported test period. The data
demonstrates stable electrical per~ormance during the
stated thermal cycling for the construction uslng adhesive
thickness in the range of 60 percent of par~icle thickness,
and poor electrical perEormance when adhesive thickness is
90 percent o~ particle thickne~s. In othar tests, with
less temperature cycling and shorter times, tapes with a 90
percent adhesiva thickness to particle thickness ratio have
provided adequate stability.
^17
-17-
TABLE I
Effeets of Adhesive Thickness on Performance
in Thermal Age Testing
Ratio of Maximum Individual Conductor Bond
5 Example Adhfparticle R sistance to ITO Su
No ThicknQss(~) Initial 100 hours 1000 hours
2A 60 242 289 167
2B 90 267 >lO,000 ~10,000
1 The Probe Tack Value is determined a~ described in ASTM
D-2979 except in the following respec~s
l. To provide Probe Tack Values at variou~ tes~
temperatures, the probe and the annular
weight are heated to the test temperature,
except that the annular weight is never
heated above`220C.
2. The probe end is an annulus having inner and
outer diameters of 3083 and 5.10 mm.
3. The annular weight is 19.8 gramsO
4~ Ten-second dwell.
2 The Shear Value is determined by heating a bright
annealed stainles~ steel panel in an oven ~or 15 minute~ at
115Co above the weight-averaged Tg Of the adhesive
polymer. With the ~teel panel horizontal, part of a tape
1027 cm in width is adhered to ths steel panel using a
2~0~-kg hand roller conforming to Federal Standard 147,
giving 2 passes in each direction. The length of tape
adhering to the panel is trimmed to exactly 1.27 cm in
length and this assembly ls left at the bonding temparature
for 15 minutes longer. The plate is transEerred to an oven
having a shear stand which allows a 2 backward tilt of the
panel at its top tshear weight will Eorce tape toward panel
~lightly), After 15 minutes at 65C,, a one~kilogram
weigh~ is hung from ~he free end o the tape. The time
after which the weight falls is the 65C. Shear Value.
