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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns adhesive tape, preferably pressure-sensitive
adhesive tape, the adhesive layer of which contains particles affording
electrical conductivity through the thickness of the layer while being
electrically insulating in lateral directions.
2. Description of the Related Art
As is pointed out in U.S. Pat. No. 4,548,862 (Hartman): "Modern electronic
devices are becoming so small and their electrical terminals are so
delicate and closely spaced that it is difficult and expensive to make
electrical connections by soldering or other established techniques. U.S.
Pat. No. 4,113,981 (Fujita et al.) uses an adhesive layer for individually
electrically interconnecting multiple pairs of arrayed electrodes. The
adhesive layer includes spherical electrically conductive particles of
substantially the same thickness as the adhesive, thus providing a
conductive path through each particle that bridges facing pairs of
electrodes. The particles are randomly distributed throughout the adhesive
layer, but the Fujita patent indicates that if the particles comprise less
than 30% by volume of the layer, they will be sufficiently spaced so that
the intervening adhesive will insulate against short circuiting between
laterally adjacent electrodes. Carbon powder, SiC powder and metal powder
are said to be useful.
"U.S. Pat. No. 3,475,213 (Stow) discloses a tape having an electrically
conductive backing and a pressure-sensitive adhesive layer which contains
a monolayer of electrically conductive particles that could be identical
to the adhesive layer of the Fujita patent if Fujita were to use a
pressure-sensitive adhesive" (col. 1, lines 15-38 of the Hartman patent).
The Stow patent says that the particles should "have a substantial
thickness slightly less than the thickness of the adhesive film" (col. 3,
lines 1-2), and "essentially none of the particles should extend above the
surface of the adhesive to preserve satisfactory adhesion values" (col. 3,
lines 39-41). The Stow patent indicates a preference for metal particles,
preferably flattened to appropriate thickness before being added to an
adhesive-coating mixture, while also suggesting the use of "[m]etallized
plastic or glass beads or spheres" and that "the particles can be metal
alloys, or composites in which one metal is coated on another" (col. 4,
lines 52-55).
The Hartman patent concerns a flexible tape that, like the adhesive layer
of the Fujita patent, can adhesively make individual electrical
connections between multiple pairs of electrode arrays without short
circuiting electrodes of either array by means of small particles that
form electrically conductive bridges extending through the thickness of
the layer. Each of the particles has a ferromagnetic core and an
electrically conductive surface layer such as silver.
U.S. Pat. No. 4,606,962 (Reylek et al.), like the above-discussed patents,
discloses a tape that provides electrical conductivity through the
thickness of an adhesive layer, and is especially concerned with
adhesively attaching a semiconductor die or chip to a substrate to
dissipate both heat and static electricity. The adhesive layer of the
Reylek tape, which preferably is heat-activatable, contains electrically
and thermally conductive particles that at the bonding temperature of the
adhesive are at least as deformable as are substantially pure silver
spherical particles. The thickness of the particles exceeds the thickness
of the adhesive between particles. When the particle-containing adhesive
layer is removed from the carrier layer of the transfer tape and
compressed between two rigid plates, the particles are flattened to the
thickness of the adhesive between particles, thus providing small, flat
conductive areas at both surfaces of the adhesive layer. The particles
preferably are substantially spherical and made of a metal such as silver
or gold or of more than one material such as "a solder surface layer and
either a higher melting metal core such as copper or a nonmetallic core"
(col. 4, lines 20-21).
The Reylek patent says: "To economize the use of the electrically
conductive particles, they may be located only in segments of the novel
adhesive transfer tape which are to contact individual electrical
conductors" (col. 2, lines 39-42). One technique suggested for so locating
the particles is outlined at col. 2, lines 42-55, where the patentee (1)
forms a viscous adhesive coating, (2) renders areas of the coating
substantially tack-free, (3) applies electrically conductive particles
that adhere only to the viscous portions of the coating, and then (4)
polymerizes the viscous areas to a substantially tack-free state. When the
viscous areas remaining after step (2) are small, the electrically
conductive particles are individually positioned in a predetermined
pattern.
SUMMARY OF THE INVENTION
The present invention provides pressure-sensitive adhesive tape that can be
used to make reliable electrical connections merely by applying ordinary
hand pressure. Like adhesive tapes of the Reylek patent, the adhesive
layer of the novel tape contains electrically conductive particles that
are equiax, i.e., particles having approximately the same thickness in
every direction. An equiax particle can be considered to have a diameter,
whether or not it is spherical. Like the electrically conductive particles
of the Reylek patent, those of the novel tape are of substantially uniform
diameter, preferably within the range of 25 to 150 .mu.m and about equal
to or somewhat larger than the adhesive thickness in order to conduct
electricity through the thickness of the adhesive layer. The particles
should be spaced laterally so that the adhesive layer is laterally
electrically insulating. As can be provided by the above-outlined
technique of the Reylek patent, the electrically conductive particles of
the novel tape are individually positioned in the adhesive layer in a
predetermined pattern. In doing so, substantially every particle is
partially uncoated at one face of the adhesive.
The tape of the invention differs from tapes of the Reylek patent in that
each equiax particle is hard relative to the electrodes or other
electrically conductive elements which it is to interconnect. In view of
Reylek's express teaching that the particles should be soft and readily
conformable in order to establish good electrical contact, it is
surprising that hard electrically conductive particles can be employed
effectively. Contrary to what might have been expected, however, it is now
believed that the hard particles are actually superior to soft particles,
since the mere application of hand pressure causes them to penetrate into
the conductive elements. To attain sufficient penetration, the equiax
particle should have a Knoop Hardness Value (as herein defined) of at
least 300. This penetration is believed to perform a sort of wiping
action, removing oxides and other surface contamination and thereby
creating a metallurgically and electrically superior connection.
Additionally, in presently preferred embodiments of the invention, the
hard particles are less expensive than Reylek's soft particles.
As demonstrated by electronmicrographs, each electrically conductive
particle of the novel tape forms a tiny crater in any electrode or other
conductive element of the substrate to which the tape is applied under
hand pressure. The formation of a crater assures good electrical
connection as well as good thermal conductivity when desired.
A pressure-sensitive adhesive tape of the invention can be a transfer tape
if it has a flexible backing, each surface of which is low-adhesion. The
transfer tape can be wound upon itself into a roll and used to provide
multiple electrical connections between electrodes on two substrates or
used between electrical components to provide grounding, static
elimination, and electromagnetic shielding in a variety of applications.
For such uses, the electrically conductive equiax particles can be widely
spaced laterally and still provide adequate electrical interconnection
between the adhered objects, because the individual positioning assures
the presence of an electrically conductive equiax particle wherever one is
needed. Wide lateral spacing also permits the formation of strong adhesive
bonds and minimizes the cost of the novel tape, even when the equiax
particles are expensive. Wide spacing also ensures against shorting when
the novel transfer tape is used for electrically interconnecting closely
spaced electrodes on two substrates.
Other useful backings include flexible webs to which the
particle-containing adhesive layer is permanently adhered, e.g., an
electrically conductive web such as a metal foil or an electrically
insulative web bearing electrically conductive elements such as parallel
metal strips. For example, a metal foil-backed pressure-sensitive adhesive
tape of the invention can be used to provide an electrical connection
across a seam between two abutting electrically conductive panels such as
may be used as part of an electromagnetically shielded enclosure.
The electrically conductive particles of the novel tape may have cores that
need not be electrically conductive when they have electrically conductive
surface layers. The Knoop Hardness Value of the core should be at least
300. An especially useful particle is a glass bead having an electrically
conductive surface layer such as a metallic coating that is preferably
only thick enough to provide the desired electrical conductivity, e.g.,
from 0.1 to 2 .mu.m. Other useful core materials that have a Knoop
Hardness Value of at least 300 include other ceramics, steel, nickel, and
work-hardened copper. Even when the conductive surface layer is quite
soft, typical electrodes and other electrical elements to be
interconnected should be penetrated by the particles upon applying hand
pressure whenever the thickness of the conductive surface layer is no more
than 10% of the diameter of the hard core.
To ensure good penetration of a particle, the thickness of a relatively
soft surface layer should be no more than 5% of the diameter of a hard
core. A gold surface layer may be economical only when its thickness is
about 0.1% or less of the thickness of the core of the particle. Other
useful metal surface layers include silver, copper, aluminum, tin and
alloys thereof.
When the particles are metal or have metallic surface layers, the novel
tape is both electrically and thermally conductive through its adhesive
layer and so is useful for applications requiring thermal conductivity.
A conventional adhesive tape having an exposed adhesive layer can be
converted into an adhesive tape of the invention by the sequential steps
of
1) attracting electrically conductive equiax particles only to separated
dots in a predetermined pattern on a carrier, and
2) while advancing the exposed adhesive layer in synchronism with the
carrier, pressing to transfer the attracted particles into the adhesive
layer in said pattern.
When the carrier is a rotating drum, this 2-step method can produce
electrically conductive tape of almost unlimited length, and the tape can
be wound up in roll form for convenient storage and shipment. In such a
roll of tape, the determined pattern of electrically conductive particles
repeats many times.
When the adhesive is a pressure-sensitive adhesive, the pressure applied in
step 2) can be very light, just enough to tack the particles to the
adhesive layer. If the adhesive layer is then wound with a low-adhesion
backing into roll form, the winding operation inherently forces the
conductive particles into the adhesive layer. Even when the particles are
forced below the face of the adhesive layer, a pressure-sensitive adhesive
does not flow to cover completely the embedded particles at that face.
DETAILED DESCRIPTION
For most applications of the novel tape, the breadth of the dots formed in
step 1,) of the above 2-step process should be small enough that only one
equiax particle is attracted to each position, but when each dot is large
enough to make it fairly certain that there will be a particle at every
dot, it can be expected that two or possibly three particles will be
deposited side by side at a few positions. When only one equiax particles
per dot is desired, each dot preferably is roughly circular and has a
diameter within the range of 30 to 90% of the diameter of the equiax
particles. When using spherical particles 55 .mu.m in diameter, good
results have been attained with dots 45 .mu.m in diameter. When the
diameter of the dots substantially exceeds the diameter of the equiax
particles, a monolayer of several particles may be attracted to each dot.
Preferably the dots and the particles they attract are as widely spaced as
possible while still providing the desired conductivity through the
adhesive layer. Wide spacing provides the strongest possible bonding as
well as economy, especially when the particles include expensive material
such as silver or gold. Wide spacing of the particles also ensures that
the adhesive layer of the novel tape is laterally nonconductive and can be
used for making electrical connections between two arrays of electrical
terminals without any danger of shorting closely spaced electrodes. To
this end, the electrically conductive equiax particles preferably occupy
no more than about 5% of the area of the adhesive layer of the novel tape,
and more preferably occupy from 0.05 to 1.0% of that area.
The electrically conductive particles may be arranged in rows extending
both longitudinally and laterally over the full area of the adhesive
layer, so that there is no need to align pieces of the tape with
substrates to which they are to be adhered. However, to form electrical
connections at specific locations, e.g., to pads of an integrated circuit
chip, the electrically conductive particles may be positioned only in
areas of the tape that will contact the pads, the other areas of the tape
preferably being substantially free from the particles. For this
application, it is sometimes desirable to locate a plurality of particles
at each pad-contacting area.
Preferably the thickness of the adhesive layer of the novel tape between
the electrically conductive particles is from 25 to 150 .mu.m. Adhesive
thicknesses above that range may be uneconomical, while thicknesses below
that range may not provide full contact between the adhesive layer and a
substrate which is not perfectly flat. When the adhesive is
pressure-sensitive and is coated from solution or emulsion, it is
difficult to obtain uniform coatings much greater than 50 .mu.m.
For most uses, the average diameter of the conductive equiax particles
should be from 5 to 50% greater than the thickness of the adhesive layer
between particles. When the novel adhesive tape is used for electrically
connecting two arrays of tiny metal electrodes or pads which are slightly
raised above the adjacent surfaces, the application of ordinary hand
pressure can cause the adhesive to flow into the depressions between the
electrodes, thus permitting the diameters of the particles to be somewhat
less than the original thickness of the adhesive layer.
The equiax particles of the novel tape preferably are substantially
spherical. This ensures that the pressure applied when bonding two
subtrates together is concentrated at very small areas of each particle.
When the spherical particles are about 50 .mu.m in diameter, hand pressure
may force them into solder-coated or soft-copper electrodes to about 10%
of their height, thus forming craters about 30 .mu.m in breadth and
extending across about 74 degrees of arc. Further penetration tends to be
limited, due to the rapidly increasing area of contact between the
spherical particle and electrode and possibly also due to dissipation of
the applied pressure into particle-free areas of the adhesive layer.
The carrier of step 1) of the above-outlined 2-step process can be provided
by a printing plate marketed by Toray Industries as "Toray Waterless
Plate." It has a flexible sheet of aluminum bearing a layer of
photosensitive material covered with a layer of silicone rubber. Upon
exposure to light through a half-tone screen, the silicone rubber of a
positive-acting plate causes the photosensitive material to bind itself
firmly to the silicone rubber in areas where the light strikes, after
which the silicone rubber in unexposed areas can be brushed off, leaving
the silicone rubber only in the predetermined pattern provided by the
light exposure. The printing plate is then wrapped onto a cylinder, and
the cylinder is rotated through a fluidized bed of electrically conductive
particles. The particles are attracted to the printing plate only where
the silicone rubber remains and are repelled by the ink-receptive areas.
Upon moving an adhesive tape in synchronism with the rotating printing
plate, the particles are picked up by and become embedded into the
adhesive layer in the pattern of the printing plate. That pattern repeats
a large number of times over a long length of the tape.
The adhesive of the novel tape preferably is a pressure-sensitive adhesive
that is aggressively tacky and so forms strong bonds on contact with
substrates such as printed circuit panels. The pressure-sensitive adhesive
may be substantially nontacky at room temperature if it becomes tacky at
an elevated temperature at which it is used to provide electrically
conductive connections.
Preferred pressure-sensitive adhesive tapes of the invention have silicone
pressure-sensitive adhesives which form exceptionally strong bonds to a
wide variety of surfaces, including low energy surfaces of materials that
are widely used in printed circuitry, e.g., polyethylene, polypropylene,
and poly(tetrafluoroethylene). Furthermore, the bonds remain intact when
exposed to large fluctuations in temperature. Especially good in these
respects are siloxane pressure-sensitive adhesives such as
poly(dimethylsiloxane) pressure-sensitive adhesive (Dow Corning DC 284)
and phenyl-containing siloxane pressure-sensitive adhesive (GE 6574).
These siloxane pressure-sensitive adhesives are so aggressively tacky that
when marketed as transfer tapes, specially prepared backings are required,
e.g., a biaxially oriented poly(ethyleneterephthalate) film, each surface
of which has been treated to make it low-adhesion.
Among other classes of adhesives that can be used in the novel tape are
thermoplastic adhesives (such as polyolefins, polyurethanes, polyesters,
acrylics, and polyamides) and thermosetting adhesives (such as epoxy
resins, phenolics, and polyurethanes).
When the adhesive of the novel tape is a silicone pressure-sensitive
adhesive and the moving carrier of the above-outlined two-step process is
a "Toray Waterless Plate," a transfer roll should be positioned between
the exposed adhesive layer and the silicone rubber of the printing plate
in carrying out the above-outlined 2-step process. This avoids the danger
of the adhesive transferring to the printing plate. The surface of the
transfer roll should be selected to cause the electrically conductive
particles to transfer from the silicone rubber of the printing plate,
while acting as a release surface in relation to the silicone
pressure-sensitive adhesive.
Because silicone adhesives are coated from solution or emulsion, it is
difficult to obtain uniform coatings greater than about 50 .mu.m in
thickness. Where thicker pressure-sensitive adhesive coatings are desired,
it may be desirable either to apply multiple layers of the adhesive or to
photopolymerize an adhesive in situ. For example, monomeric mixtures of
alkyl acrylates and copolymerizable monomers such as acrylic acid can be
copolymerized by exposure to ultraviolet radiation to a pressure-sensitive
adhesive state.
The use of a printing plate mounted on a cylinder in the above-outlined
2-step process results in a seam that may produce discontinuities in the
pattern of electrically conductive particles embedded into the adhesive
layer. A conventional adhesive tape that has an exposed adhesive layer can
be converted into an adhesive tape of the invention, having no seam in its
pattern, by sequentially coating onto a cylinder formulations that provide
a cylindrical printing plate, preferably including a silicone rubber
layer. Preferred sequential coating formulations are those of U.S. Pat.
No. 3,511,178 (Curtin).
If a seam in the pattern is not objectionable, step 1) of the
above-outlined 2-step process for making an adhesive tape of the invention
can use a carrier provided by the steps of
a) coating one face of a sheet of metal foil with rubber and the other face
with a photoresist,
b) exposing the photoresist to light in a predetermined pattern,
c) removing areas of the photoresist corresponding to said pattern, and
d) removing the metal foil in said areas to expose the rubber in said
predetermined pattern.
Step d) is preferably followed by step
e) removing the remaining photoresist.
Knoop Hardness Value
The hardness of the core of a tiny electrically conductive particle can be
measured after hot-pressing the particle into a fused resin while it is
being thermoset and then polishing to remove about half of the particle.
See "Metals Handbook", American Society for Metals, 8th Ed., Vol. 8, pages
117-118. The hardness then is measured by ASTM Test Method E384-84 using
the Knoop indenter.
The Drawing
The invention may be more understandable by reference to the drawing, FIGS.
1-3 of which are schematic, wherein:
FIG. 1 shows a method of printing electrically conductive particles onto
the adhesive layer of a transfer tape to provide a preferred electrically
conductive adhesive tape of the invention;
FIG. 2 is an enlarged fragmentary cross section through the tape of FIG. 1
before it has been wound up into a roll;
FIG. 3 is an enlarged fragmentary cross section through a piece of the tape
of FIG. 1 by which an electrical connection has been made adhesively; and
FIG. 4 is a photomicrograph of the exposed surface of a pressure-sensitive
adhesive tape made as shown in FIG. 1.
In FIG. 1, a printing plate 10 has been attached to a rotating cylinder 12
with its outer surface developed to leave roughly circular rubber dots 13.
The cylinder is rotated through a fluidized bed of electrically conductive
spherical particles 14 of uniform diameter somewhat larger than the
diameter of the dots. Each of the particles has a glass bead core 17
having a thin electrically conductive surface layer 18 as seen in FIG. 2.
While the cylinder is rotating, the particles are attracted to the rubber
dots, and excess particles are removed by suction at 15, leaving only one
particle at almost every dot. The adhered particles are transferred to the
surface of a rubber-covered roll 16. Moving in synchromism with the roll
16 is a pressure-sensitive adhesive tape 20 having a flexible backing 22
bearing an exposed adhesive layer 24 facing downwardly. The adhesive layer
is pressed against the roll 16 by a nip roll 26, thus tacking the
attracted particles 14 to the adhesive layer 24. Upon winding the
resulting tape 28 into a roll, the electrically conductive particles 14
become embedded into the adhesive layer. Because both faces of the
flexible backing 22 have low-adhesion surfaces, the tape 28 can later be
unwound for use.
Shown in FIG. 3 are two substrates 30 and 31 which have been adhered
together with a piece of the pressure-sensitive adhesive tape 28 of FIG.
2. The illustrated fragment of the resulting composite shows two of the
particles 14 electrically connecting two pads or electrodes 32 and 33 on
each of the substrates 30 and 31, respectively. Upon application of
ordinary hand pressure to the composite, the particles 14 have formed
shallow craters in each of the electrodes.
EXAMPLE 1
A negative-acting "Toray Waterless Plate" was exposed to a half-tone screen
having 200 lines per inch (79 lines per cm) of circular dots spaced
equally in both directions. After being developed, the face of the Toray
printing plate had a uniform pattern of silicone rubber dots, each about
60 .mu.m in diameter and on 125-.mu.m centers. The imaged plate was then
flooded with an excess of silvered glass beads of substantially uniform
diameter (about 50 .mu.m), having a silver surface layer of 0.5 .mu.m
thick. Excess beads were removed by turning the plate over and tapping it.
As viewed with a microscope, the spherical particles were held by the
silicone rubber dots of the plate in substantially the pattern of the
half-tone screen. There was only one spherical particle at almost every
dot, but at a few dots were 2 or 3 particles side by side and at an
occasional dot there was no particle.
Onto the particle-bearing plate was laid a piece of a pressure-sensitive
adhesive transfer tape, the adhesive layer of which was a 96/4 copolymer
of isooctyl acrylate/acrylamide polymer about 40 .mu.m thick, on a
silicone-precoated, biaxially oriented poly(ethyleneterephthalate) film
that was about 30 .mu.m thick. After pressing the tape against the plate
using a small hand-held printer roller, the tape was lifted from the
plate. In its exposed adhesive-coated face were embedded the particles
that had been held by the plate.
The resulting particle-bearing adhesive transfer tape was used to adhere a
flexible printed circuit (on polyimide film obtained from E.I. du Pont) to
a rigid printed circuit board. At the face of each of the flexible and
rigid printed circuits was a series of parallel solder-coated copper
electrode strips, each 1.4 mm in width and spaced 1.1 mm apart. With the
electrode strips of the printed circuits aligned, they were bonded
together by a piece of the adhesive transfer tape which was 0.635 cm wide
in the lengthwise direction of the electrode strips. The bonding was made
in a press at 150.degree. C. for 6 seconds under a pressure of about 1400
kPa.
After cooling, testing with a Simpson continuity meter showed that a good
electrical connection had been established between each pair of the
electrodes and that no shorting had occurred between adjacent electrodes.
EXAMPLE 2
A negative-acting "Toray Waterless Plate" 47.8 cm in length and 12 cm in
width was developed to have a uniform pattern of roughly circular silicone
rubber dots in rows extending longitudinally and transversely. The dots
were uniformly about 60 .mu.m in diameter on 0.29-mm centers. The
developed plate was mounted on a cylinder with its length in the
circumferential direction, leaving a seam of about 1 mm. Using apparatus
as shown in FIG. 1, the cylinder was rotated in a fluidized bed of
particles, each having a glass bead core about 60 .mu.m in diameter and a
silver surface layer about 0.5 .mu.m thick. Particles carried by the Toray
plate to the transfer roll were pressed into the adhesive layer of a
transfer tape. Its adhesive was a phenyl-containing siloxane
pressure-sensitive adhesive (GE 6574), 50 .mu.m thick, and its backing was
60 .mu.m biaxially oriented poly(ethyleneterephthalate) film, each surface
of which had been provided with a low-adhesion backsize coating of a
perfluoropolyether polymer as disclosed in U.S. Pat. No. 4,472,480
(Olson). The resulting electrically conductive adhesive transfer tape was
wound upon itself into a roll for storage. After being unwound, a
photomicrograph was made of the face of the tape, a fragment of which is
shown in FIG. 4 of the drawing. The full photomicrograph showed only one
particle at almost every dot and two side-by-side particles at a few dots.
A highlight at the top of each particle suggests that each particle is
partially uncoated.
EXAMPLES 3 and 4
Two electrically conductive pressure-sensitive adhesive tapes were made as
in Example 2 except that the silicone rubber dots were about 100 .mu.m in
diameter. The dots were on 1.27-mm centers when making the tape of Example
3 and on 0.63-mm centers when making the tape of Example 4. Each of these
tapes was tested for 180.degree. peel adhesion in comparison to a
"control" tape which was identical except having no particles. The
180.degree. peel adhesion was measured from glass under ASTM D330-81,
Method B, except using an "I-mass" adhesion tester 3M-90 in a controlled
atmosphere of 22.degree. C. and 50% relative humidity. Initial adhesion
measurements were made within five minutes after adhering a tape to the
glass at 22.degree. C. Some of the specimens were placed in an oven at
70.degree. C. for either 3 or 5 days and then held at the aforementioned
controlled atmosphere overnight before testing. Results reported in Table
I include testing of both the face side (FS) and the back side (BS). Also
reported in Table I are tests for electrical resistance at 22.degree. C.,
50% RH, between two electrodes, each about 2.5 cm square, adhered to each
other by one of these tapes.
TABLE I
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180.degree. Peel Adhesion (N/dm)
After 3 days
After 5 days
Electrical
Tape of
Initial at 70.degree. C.
at 70.degree. C.
Resistance
Example
FS BS FS BS FS BS (ohms/cm.sup.2)
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3 13.1 17.1 15.1 18.0 | | |
