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BACKGROUND OF THE INVENTION
The present invention relates to an anisotropicelectroconductive adhesive
film which is electrically conductive only in the direction perpendicular
to the surface thereof and not in any direction parallel to the surface.
Solders and electroconductive adhesives have so far been used widely as
means of connecting large scale integrated circuits (LSI) or electrodes of
liquid crystal display elements (LCD) to printed circuit boards and of
bonding flexible flat cables (FFC) to external connecting terminals of
various devices.
However, with the miniaturization of electronic parts and the size
reduction of connecting circuits, it has become extremely difficult to
connect circuits to one another without causing short circuit between
neighboring parts of the circuits in particular when connecting terminals
or the like are aligned at fine pitches.
In addition, soldering has a drawback of low reliability in adhesion due to
hard and brittle properties thereof, that is, the liability of bonded
parts to be readily separated by a shock or the like.
Electroconductive adhesives, on the other hand, are expensive because
electroconductive fillers need to be incorporated to high concentrations
of at least 20% by volume into adhesives in order to obtain necessary
conductivity. Further, drawbacks of conductive adhesives are that
variation in the conductivity is caused by such a factor as the
precipitation of the filler during storage or difficulty in uniform
coating thereof and that environmental pollution due to the contained
organic solvent may occur.
An example of the attempts to eliminate these drawbacks is the method
disclosed in Japanese Patent Application Kokai (Laid-Open) No. 147,732/78,
which comprises applying a conductive adhesive prepared by incorporating 5
to 25% by weight of an electroconductive carbon black as an
electroconductive filler into a heat-sensitive adhesive and conducting an
electric current through the layer of the applied adhesive to generate
Joule's heat, thereby completing bonding.
According to this method, however, the upper limit of the
electroconductivity is 10.degree. .OMEGA.-cm.sup.-1, the appearance is
black in color, and the transparency is lost completely, since carbon is
used as the electroconductive filler. Moreover, the bonding operation
requires a long time ranging from 10 minutes to 10 hours and the
workability is inferior, since the adhesion is achieved with Joule's heat.
Adhesive tapes having electroconductive anisotropy are also known which are
prepared by dispersing metal particles as electroconductive filler
particles in a polymer binder (Japanese Patent Application Kokai
(Laid-Open) No. 101040/76). Such adhesive tapes, however, are inferior in
adhesive properties and transparency, since conductive fillers such as
metal particles need to be incorporated in large quantities of at least
10% by volume in order to secure the necessary electroconductivity.
Further, for circuit connecting purposes, there are used in practice
connecting materials having anisotropic-electroconductivity which
materials are produced by linear alignment of metal wires, carbon
filaments, or carbon particles in polymers, for example, silicone rubber.
However, these connecting materials have a number of drawbacks in that the
minimum separation width of the conductors or insulators is as large as
0.2 mm at best, thus the use of these materials for connecting fine
circuits is limited, compression pressure should always be applied in
order to maintain the connection in a good state and this requires a
special fixing means, which results in restricting equipment designs; and
the materials themselves are opaque so that the adjustment of relative
positions of circuits is complicated when the circuits are superposed and
connected together.
SUMMARY OF THE INVENTION
Thus, an object of the present invention is to provide an
anisotropic-electroconductive adhesive film which permits achieving with
simple operation the electroconductive connection between two groups of
connecting terminals which face each other and are each aligned at fine
pitches.
According to the invention, there is provided an
anisotropic-electroconductive adhesive film comprising an adhesive
component and electroconductive particles, the adhesive component
including 0.1 to 10% by volume of electroconductive particles having an
average particle size of 1 to 50 .mu.m with a ratio of the minimum
diameter to the maximum diameter of each particle being 0.5 to 1.0, and
the thickness of an adhesive layer is at least 110% of the average
particle size of the electroconductive particles and not more than 100
.mu.m.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the anisotropic-electroconductive
adhesive film according to the invention.
FIG. 2 is a perspective view showing the connected state of a group of
connecting terminals formed on a circuit board.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention is explained in detail referring to the drawings.
FIG. 1 shows the fundamental construction of the
anisotropic-electroconductive adhesive film according to the invention. As
shown in FIG. 1, the adhesive film comprises an adhesive layer 2 of
bonding material and electroconductive particles 3 dispersed therein. The
adhesive layer 2 is an electrical insulator and has functions of holding
electroconductive particles 3 in a dispersed state and adhering to a
printed circuit board or the like. The layer 2 undergoes plastic
deformation on applying pressure with or without heating, and this brings
electroconductive particles into contact one with another along the
direction of the pressure application so as to maintain electrical
conductivity permanently.
FIG. 2 shows the application of the invention wherein a connecting terminal
5 of a flexible printed circuit board 4 is connected electrically with a
connecting terminal 7 of a flexible flat cable 6 by using such an adhesive
film as shown in FIG. 1. In this case, the opposing pairs of connecting
terminals 5-7, 5'-7', . . . are each in an electrically conductive state
while the communications between the neighboring terminals 5 and 5', . . .
and between the neighboring terminals 7 and 7', . . . are each in an
electrically insulated state.
The adhesive film according to the invention has an anisotropy in
electrical conductivity between the directions perpendicular and parallel
to the surface thereof (hereinafter for convenience, the direction
perpendicular to the film surface is referred to as the "thickness
direction" and the directions parallel to the film surface as the "plane
direction"). The film has a resistivity of at least 10.sup.6 .OMEGA.-cm in
the plane direction and a resistivity of not more than 10.sup.3 .OMEGA.-cm
in the thickness direction, exhibiting thus a great anisotropy in
resistivity. The adhesive film has a small minimum separation width of
resistance both in the plane and the thickness direction, thus being good
in resolution. That is, the minimum insulation width in the film plane
direction is 0.05 mm. Moreover the application of the film can be
accomplished under both heat-sensitive and pressure-sensitive conditions.
Accordingly, the adhesive film offers good application workability and
strong adhesion as well as high electrical conductivity and additionally
has a considerably high transparency.
The above-mentioned features of the film are due to the high anisotropic
electrical conductivity that has become obtainable by the incorporation of
special electroconductive particles in small amounts and due to the
arbitrary compounding enabled thereby for the film formation. Conceivably
the shape of the electroconductive particles is the main cause of the fact
that the film exhibits a high anisotropic conductivity with small amounts
of electroconductive particles. In other words, the adhesive layer in
application becomes fluid with heat or pressure and each electroconductive
particle embedded in the surface layer of the film will be in one-point
contact with the surface of the conductor to be bonded, so that the
adhesive film sufficiently adapts itself to the adhesion surface of the
conductor and provides high bond strength. Electroconductive particles in
the surface layer take such configurations as to readily contact with
electroconductive particles present on the deeper side, forming links in
the thickness direction and as to have a little opportunity to contact
with other electroconductive particles in the plane direction, whereby the
anisotropic electroconductivity is obtained.
The following description refers to materials used in the invention.
The electroconductive particles used in the invention are metallic
particles of 1 to 50 .mu.m in diameter. The ratio of the smallest diameter
to the largest diameter of each particle is 0.5 to 1.0. The particles are
contained in the adhesive component in an amount of 0.1 to 10% by volume.
The maximum particle size is desirably 1 to 50 .mu.m, more preferably 1 to
10 .mu.m. When it is less than 1 .mu.m, a large amount of
electroconductive particles are required, which results in deteriorating
the bond strength greatly. When the maximum particle size is more than 50
.mu.m, it is impossible to obtain a smooth adhesive surface having good
affinity to an adhered from the viewpoint of thickness of the adhesive
film layer, which results in failing to give sufficient adhesiveness.
Further, insulating properties and resolution properties at electric
conduction are also undesirably worsened.
As regards the shape of the electroconductive particles, the ratio of the
smallest diameter to the largest of each particle (hereinafter referred to
as "the aspect ratio") is about 0.5 to 1.0 as stated above. If the aspect
ratio is out of the above range, the balance between the
electroconductivity and the adhesiveness will be disturbed. A typical
example of the particle shapes satisfying the above condition is spherical
or nearly spherical. However, the shape is not particularly limited so
long as the aspect ratio falls in the above range. The particles are
allowed to have projections or depressions on the surface.
The particle size herein means the average diameter of all the particles.
The shape and diameter of the particles are conveniently measured by way
of, for example, an electron microscope.
When the electroconductive particles are, for example, spherical, the
adhesive layer is fluidized by the heating or pressure at the time of
adhesion, which results in making it possible to contact one point of the
sperical particles with the conductor surface.
In contrast to this, flaky conductive particles, for example, are aligned
with longer axes thereof being directed in parallel to the adhesive
surface in the steady state and the adhered surface is therefore occupied
mostly with the electroconductive particles, thereby the adhesive
properties are lowered to a large extent.
The electroconductive particles can be produced by any possible method from
the following materials: Metals, for example, Ni, Fe, Cr, Co, AL, Sb, Mo,
Cu, Ag, Pt, and Au, and alloys and oxides of these metals. The above
materials may be used alone or in combination.
It is also possible to use metal-coated particles of non-conductive
materials, for example, glass and plastics.
Suitable content of conductive particles in the adhesive component layer is
0.1 to 10% by volume. The thickness of the adhesive film is at least 110%
of the average particle size of the electroconductive particles contained
and desirably not more than 100 .mu.m. With the particle content less than
0.1% by volume, no satisfactory electrical conductivity is obtained, and
when the content exceeds 10% by volume, the bond strength much lowers and
no sufficient transparency of the film is obtainable.
The following description refers to compounding for the adhesive film.
For selecting the polymer used for the adhesive, some considerations are
necessary depending upon the method of applying the adhesive film, in
other words, depending upon whether the film is of heat sensitive type or
of pressure sensitive type. The heat sensitive type of adhesive film is
applied by heating to soften and flow on the adherend surface, thereby
bonding the object. For the film of heat sensitive type, a polymer
relatively hard at ordinary temperatures is used. On the other hand, the
pressure sensitive type of adhesive film, which is applied with pressure
on an adherend, is relatively soft to such an extent that stickiness
thereof is perceptible even at ordinary temperatures.
As the heat sensitive type of adhesive film, various polymers can be
adapted while thermoplastic polymers that exhibit plasticity on heating
are usually employed as a main component. These polymers include, for
example, an ethylene-vinyl acetate copolymer, polyethylene,
ethylene-propylene copolymer, ethylene-acrylate copolymer, acrylic rubber,
polyisobutylene, atactic polypropylene, poly(vinyl butyral),
styrene-butadiene copolymer, polybutadiene, ethylcellulose, polyamide, and
polyurethane. These polymers may be used alone or as a mixture thereof.
As the pressure sensitive type of adhesive film, polymers exhibiting
stickiness even at room temperature can be employed. These polymers
include an acrylic rubber, natural rubber, silicone rubber,
polychloroprene, butadiene-styrene copolymer, ethylene-vinyl acetate
copolymer, polyisobutylene, and poly(vinyl ether) rubber. These polymers
also may be used alone or as a mixture thereof.
In addition to these polymers, tackifiers and conventional additives such
as plasticizers, crosslinking agents, and antioxidants can be used, if
necessary, for either the heat sensitive type or the pressure sensitive
type. Suitable tackifiers for use herein include, for example, rosin
family resins such as rosin, hydro-rosin, ester gum, and maleic
acid-modified rosin, petroleum resin, xylene resin, and cumarone-indene
resin. These tackifiers may be used alone or as a mixture thereof.
For compounding materials for the adhesive film, a mixture of the polymer
and additives, which are used as required, is either dissolved in a
solvent or fused to liquid form, and mixed with electroconductive
particles by a conventional method such as stirring. Thus, an
electroconductive adhesive composition is obtained.
In this case, a surfactant, for example, can be used, if required, as a
dispersing agent for the electroconductive particles.
The above electroconductive adhesive composition can be coated on paper,
plastic film, or the like, which is coated with a separator if necessary
for helping the later separation of the adhesive film, by means of a roll
coater or the like, followed by drying, or by hot melt coating to give an
electroconductive adhesive film. In the case of the heat sensitive type
that does not exhibit stickiness at room temperature, the adhesive layer
alone without using such a separator can be wound up into a roll or folded
up.
The thickness of the adhesive film is decided by considering the relation
to the diameter of the electroconductive particles used and
characteristics of the film.
That is, the thickness needs to be at least 110% of the particle size of
the electroconductive particles in order to hold the particles
satisfactorily in the adhesive. If the thickness is less than 110%, some
of the electroconductive particles will be protected not completely with
the adhesive, and in consequence will be oxidized or corroded, thereby the
electrical conductivity being deteriorated. Also from the viewpoint of
characteristics of the adhesive film, the thickness is required in the
range of preferably 5 to 100 .mu.m, particularly preferably 5 to 50 .mu.m.
When the thickness is less than 5 .mu.m, no sufficient adhesiveness is
obtained, and the thickness more than 100 .mu.m is impractical since a
large amount of electroconductive particles need to be mixed in order to
secure sufficient electrical conductivity with such a thick film.
One or both sides of the formed adhesive film may be covered with separator
films if necessary for the purpose of protecting from the adherence of
dust or the like. The adhesive film, when both sides thereof are covered
with separator films, can be continuously wound up into a roll or folded
up.
The thus obtained adhesive film has good transparency. Transparent adhesive
films are advantageous in that quality control in the production process
is easy to practice in the assembly of display devices or the like, such
structures thereof can be designed that the objects to bond can be seen
through in bonding operations, and the adjustment of relative positions
(hereinafter this adjustment is referred to as registration) for bonding
circuits is facilitated and thereby the film application can be automated.
For connecting a pair of circuits by using the adhesive film, the following
facilities and techniques can be used. In the case of the heat sensitive
type of adhesive film, the film is preliminarily attached onto all the
surfaces of connecting terminals on an adherend A, a separator, if present
on the film, is peeled off, the connecting terminals on the other adherend
B are placed on the film face to face, and the bonding is accomplished
with a hot press or heat roll.
In the case of the pressure sensitive type, conventional bonding techniques
such as compression between rolls can be adopted.
The anisotropic-electroconductive film according to the invention has a
total light transmittance of at least 40% as measured in accordance with
JIS K-6714. This is favorable since optical registration of the portions
to be bonded together is possible when at least one of the circuit boards
is a flexible printed circuit board that comprises a circuit formed on a
transparent film.
The invention is illustrated in further detail referring to the following
Examples. In the Examples, "parts" and "%" are by weight unless otherwise
noted; the compounding proportions for adhesives are expressed in terms of
solids; results obtained are all shown in Tables 1 and 2; referring to
metal particle sizes, the aspect ratio of the specimen was determined from
the respective mean values of the maximum diameters and minimum diameters
measured by a scanning electron microscope on 10 or more of the specimen
particles; and the particle size was represented by the mean value of said
maximum diameters.
EXAMPLES 1 AND 2
An ethylene-vinyl acetate copolymer (mfd. by Mitsui Polychemicals Co., Ltd.
under the tradename of Elvax 560; vinyl acetate content 4%, MI 3.5) and a
rosin-base tackifier (mfd. by The Japanese Geon Co., Ltd. under the
tradename of Quinton B-170; softening point 70.degree. C.) were each
dissolved in toluene, respectively, to a concentration of 20%.
Both the solutions (100 parts and 100 parts) were mixed, and in two
portions of this mixture were added silver-coated glass beads (mfd. by
Toshiba-Ballotini Co., Ltd. under the tradename of Silver Beads S-3000S-3;
spherical shape, particle size 45 .mu.m, aspect ratio 0.95) to different
concentrations. The mixtures were stirred to prepare adhesive compounded
liquids. Each compounded liquid was applied on a separator film (a
25-.mu.m thick poly(ethylene terephthalate) film surface-treated with a
silicone) by means of bar coater so as to give a dry thickness of 50
.mu.m. The coats were dried at 120.degree. C. for 3 minutes to remove
toluene to form adhesive films. A metal plate (SUS.430BA, 500 .mu.m thick)
and a copper foil 35 .mu.m thick were bonded together with each adhesive
film thus obtained. That is, each adhesive film was applied on one
SUS-430BA sheet, the separator film was peeled off and one copper foil was
bonded onto the stripped surface of the adhesive by using a pair of heat
rolls (heated to 120.degree. C.). Properties of the films and results of
the bonding are shown in Table 1.
From these results, it is proved that adhesive strength and
thickness-directional conductivity sufficient for practical use could be
obtained by simple bonding operation using the above films. In addition,
these films were found to have considerably high transparency, that is,
the total light transmittance was 40% or more, since contents of the
electroconductive particle were low.
COMPARATIVE EXAMPLE 1
An adhesive film was prepared in the same manner as in Examples 1 and 2
except that the content of the electroconductive particle was increased to
20% by volume.
In this case, the adhesiveness was by far inferior though the conductivity
was good. The total light transmittance of the adhesive film was 20% and
the low clarity was also ascertained by visual observation.
EXAMPLES 3 TO 5
Adhesive films were prepared by following the procedure of Examples 1 and
2, but the materials used and the thickness of the coats were varied and
the coating was carried out by means of a hot-melt applicator (without
using any solvent).
That is, a styrene-butadiene block copolymer (mfd. by Asahi Chemical
Industry Co., Ltd. under the tradename of Toughprene A, hereinafter
designated as SBR; MI 2.6) and an aromatic tackifier (mfd. by Mitsui
Petrochemical Industries, Ltd., tradename, Petrosin #150; softening point
150.degree. C.) were employed. The compounding ratio of the SBR to the
tackifier was 100 parts: 50 parts. The conductive particle used was a
nickel powder (mfd. by International Nickel Co. under the tradename of
Carbonyl 123; spherical shape, average particle size 4.5 .mu.m, fine
projections and depressions were present throughout the surface, aspect
ratio 0.70).
The SBR and the tackifier (the above compounding ratio) were fed into a
melt mixer attached to the hot-melt application and were melted by
heating. Said conductive particle was added to the melt, and coating films
of different thicknesses were prepared. The evaluation of the thus
prepared adhesive films was conducted in the same manner as in Examples 1
and 2 except that the application of films for bonding was carried out
with a hot press (temperature of object: 160.degree. C., pressure 2
kg/cm.sup.2, press time 5 seconds). All the obtained adhesive films showed
considerably high transparency.
Results of the evaluation, as shown in Table 1, indicated that all the
films had high adhesive strength and exhibited low resistance
characteristics.
The resistivities in the plane direction were higher by several orders than
those in the thickness direction, showing high anisotropy in
electroconductivity.
COMPARATIVE EXAMPLE 2
An adhesive film was prepared in the same manner as in Examples 3 to 5
except that the thickness of the coat was made 150 .mu.m. The resistivity
in the thickness direction was as high as 10.sup.8 .mu.-cm. The adhesive
film showed considerably low transparency.
CONPARATIVE EXAMPLE 3
An adhesive film was prepared by following the procedure of Example 4 but
using a flaky nickel powder (thickness 0.6 .mu.m, largest diameter ca. 40
.mu.m, aspect ratio 0.015).
The film showed a very low adhesive strength and a nearly isotropic
electroconductivity.
EXAMPLE 6
To a toluene solution of an adhesive compound of 100 parts of a natural
rubber (grade 1, masticated for 30 minutes) and 30 parts of a polyterpene
resin (softening point 100.degree. C.), was added 2% by volume of an
aluminum powder (mfd. by Toyo Aluminum Co., Ltd. under the tradename of
AC-2500; oval shape, particle size 30 .mu.m, aspect ratio 0.5), and the
mixture was well stirred.
In the same manner as in Examples 1 and 2, this mixture was applied on a
separator film of 100 .mu.m in total thickness which had been prepared by
coating a glassine paper substrate with a polyethylene layer and treating
the surface of the polyethylene layer with a silicone. The coated
separator was dried, giving an adhesive film 40 .mu.m thick.
Using this adhesive film, a 500-.mu.m thick stainless steel sheet (SUS
430BA) and a 35-.mu.m thick copper foil were bonded together by
compressing with a pair of rubber rolls.
In this case the adhesive film showed high stickiness at room temperature
and could be applied with simple operation. The resulting resistivity in
the thickness direction was as low as 10.sup.-2 .OMEGA.-cm.
The adhesive film was considerably transparent, that is, the total light
transmittance was 70%.
EXAMPLE 7
An adhesive compound composed of 100 parts of an acrylic polymer (mfd. by
Toagosei Chemical Industry Co., Ltd. tradename, Aron S-1511) and of 1 part
of a crosslinking agent (mfd. by Nippon Polyurethane Co., Ltd. under the
tradename of Colonate L) was mixed with 5% by volume of a silver powder
(supplied by Fukuda Metal Foil Co., Ltd. under the tradename of Silcoat
Ag. C-BOB; spherical shape, particle size 1.9 .mu.m, aspect ratio 0.5).
From this mixture an adhesive film 10 .mu.m thick was prepared and
evaluated in the same manner as in Example 6.
The film showed high stickiness and could be applied simply at room
temperature by using a pair of rubber rolls. The resulting resistivity in
the thickness direction was as low as 10.sup.-3 .OMEGA.-cm, and the
transparency was sufficient was as low as 10 for practical use.
In addition, the crosslinking agent incorporated into the adhesive gave
high cohesion, that is, high heat resistance to the adhesive film.
TABLE 1
__________________________________________________________________________
Content of Resistivity
electroconductive
Thickness
(.OMEGA.-cm)
Adhesive
Total light
Example
particles
of adhesive
Thickness
Plane strength
transmittance
No. (vol. %) film (.mu.m)
direction
direction
(kg/cm)
(%)
__________________________________________________________________________
Ex. 1 0.1 50 1 .times. 10.sup.2
3 .times. 10.sup.8
2.4 70
Ex. 2 10 50 5 .times. 10.sup.-2
4 .times. 10.sup.6
1.8 40
Com. Ex. 1
20 50 3 .times. 10.sup.-3
.sup. 0.1.times. 10.sup.-2
20
Ex. 3 2 5 4 .times. 10.sup.-3
5 .times. 10.sup.8
0.9 85
Ex. 4 2 30 2 .times. 10.sup.-1
7 .times. 10.sup.9
2.1 70
Ex. 5 2 100 6 .times. 10.sup.3
.sup. 3 .times. 10.sup.10
2.8 50
Com. Ex. 2
2 150 7 .times. 10.sup.8
.sup. 4 .times. 10.sup.11
3.2 30
Com. Ex. 3
2 30 2 .times. 10.sup.3
4 .times. 10.sup.3
0.2 40
Ex. 6 3 40 1 .times. 10.sup.-2
5 .times. 10.sup.9
0.8 70
Ex. 7 5 10 1 .times. 10.sup.-3
1 .times. 10.sup.3
0.4 60
__________________________________________________________________________
Test methods for the characteristics shown in Table 1 were as follows:
(1) Resistivity in the thickness direction: A stainless steel sheet
(SUS-430 BA) and an aluminum foil of 0.5 cm.sup.2 in surface area were
bonded together through an adhesive film specimen so that the edge effect
would be eliminated, the resistance of the adhesive film in the thickness
direction was measured, and the found value was converted into volume
resistivity.
(2) Resistivity in the plane direction: An adhesive film specimen was
bonded onto an insulating polyester film of 50 .mu.m in thickness, the
resistance of the film in the plane direction was measured at a specimen
width of 0.5 cm with the space between the electrodes being set to 0.5 cm,
and the found value was converted into volume resistivity.
For converting the found resistance into volume resistivity in the above
(1) and (2), the following equation was used:
.rho.=(SR/l)
wherein .rho.: volume resistivity (.OMEGA.-cm),
S: cross-sectional area of specimen (cm.sup.2),
R: found value of resistance (.OMEGA.), and
l: space between electrodes (cm).
(3) Adhesive strength: The 180.degree. -degree peeling method in accordance
with JIS Z-0237 was applied. The measurement was conducted at a peeling
speed 50 mm/min., 20.degree. C., and 65% RH by using a copper foil bent by
90.degree. degree.
(4) Total light transmittance: This was measured in accordance with JIS
K-6714 using a digital turbidimeter (Model NDH-20D of Nippon Denshoku
Kogyo K.K.).
EXAMPLES 8 TO 11 AND COMPARATIVE EXAMPLES 4 AND 5
The same ethylene-vinyl acetate copolymer and the same rosin-base tackifier
(softening point 70.degree. C.) as used in Example 1 were dissolved in
equal amounts in toluene to prepare solutions of concentration 20%. A
nickel powder (particle shape: spherical, average particle size: 4.5
.mu.m, aspect ratio: 0.70) having fine projections and depressions was
mixed in the proportions shown in Table 2 with the above solutions. The
mixtures were each ball-milled for 24 hours to give adhesive compounded
liquids. Each compounded liquid was applied on a 25-.mu.m thick polyester
film surface-treated with a releasing agent by means of a bar coater so as
to give a dry thickness of 30 .mu.m, and was dried at 120.degree. C. for 3
minutes to remove the toluene. In this way, adhesive films of different
electroconductive particle contents were obtained. Using each adhesive
film, two flexible polyester circuit boards (pitch of circuit lines: 0.1
mm, line width 0.05 mm) were coupled together by opposing the boards
through the adhesive film, registering the circuits, and bonding together
each pair of opposing lines by compressing a pair of heat rolls
(120.degree. C.). Characteristics of the thus coupled circuits are shown
in Table 2.
These results revealed that bond strength and thickness-directional
conductivity both sufficient for practical use could be obtained with
simple bonding operation using the films containing the conductive
particles in concentrations of 0.1 to 10.0% by volume.
In addition, these films were found to have considerably high transparency,
that is, the total light transmittance was 40% or more, since contents of
the conductive particles were low.
COMPARATIVE EXAMPLE 6
An adhesive film was prepared in the same manner as in Example 10 but using
a flaky nickel powder (particle shape: flaky, thickness 0.6 .mu.m, largest
diameter ca. 40 .mu.m, aspect ratio 0.015).
The film showed a very low bond strength and a nearly isotropic
electroconductivity.
EXAMPLE 12
To a toluene solution of an adhesive compound of 100 parts of a natural
rubber (grade 1, masticated for 30 minutes,) and 30 parts of a polyterpene
resin (softening point 100.degree. C.), was added 2% by volume of an
aluminum powder (particle shape: oval, particle diameter 10 .mu.m, aspect
ratio 0.5), and the mixture was well stirred.
In the same manner as in Example 8, this mixture was applied on a separator
film of 100 .mu.m in total thickness which had been prepared by coating a
glassine paper substrate with a polyethylene layer and treating the
surface of the polyethylene layer with a silicone. The coated separator
was dried, giving an adhesive film 20 .mu.m thick.
Using this adhesive film, two flexible circuit boards (pitch of circuit
lines: 0.2 mm, line width: 0.1 mm) were coupled together in the same
manner as in Example 8.
In this case, the adhesive film showed high stickiness at room temperature
and could be applied with simple operation. The resulting resistivity in
the thickness direction was as low as 10.sup.-1 .OMEGA.-cm.
The adhesive film was considerably transparent, that is, the total light
transmittance was 70%.
EXAMPLE 13
An adhesive compound composed of 100 parts of an acrylic polymer and of 1
part of an isocyanate type crosslinking agent was mixed with 10% by volume
of a silver powder (particle shape: spherical, particle size 0.5 .mu.m,
aspect ratio 0.9). From this mixture, an adhesive film 10 .mu.m thick was
prepared and evaluated in the same manner as in Example 12.
The film showed high stickiness and could be applied simply at room
temperature by using a pair of rubber rolls. The resulting resistivity in
the thickness direction was 10.sup.-3 .OMEGA.-cm, and the transparency was
sufficient for practical use.
In addition, the crosslinking agent incorporated into the adhesive gave
high cohesion, that is, high heat resistance to the adhesive film.
TABLE 2
__________________________________________________________________________
Electroconductive particles
Resistivity
Amount
Thickness
(.OMEGA.-cm)
Adhesive
Total light
Example
Particle
Aspect
added
of adhesive
Thickness
Plane
strength
transmittance
No. size (.mu.m)
ratio
(vol. %)
film (.mu.m)
direction
direction
(kg/cm)
(%)
__________________________________________________________________________
Com. Ex. 4
4.5 0.7 0.05 30 3 .times. 10.sup.6
5 .times. 10.sup.9
1.2 85
Ex. 8 " " 0.1 " 7 .times. 10.sup.2
1 .times. 10.sup.8
1.3 85
Ex. 9 " " 2.0 " 2 .times. 10.sup.-1
8 .times. 10.sup.7
1.0 69
Ex. 10
" " 5.0 " 5 .times. 10.sup.-2
4 .times. 10.sup.7
0.8 63
Ex. 11
" " 10.0 " 1 .times. 10.sup.-2
5 .times. 10.sup.6
0.6 44
Com. Ex. 5
" " 20.0 " 2 .times. 10.sup.-3
5 .times. 10.sup.0
0.2 30
Com. Ex. 6
40 0.015
5.0 " 2 .times. 10.sup.3
6 .times. 10.sup.3
0.1 41
Ex. 12
10 0.5 2.0 20 1 .times. 10.sup.-1
5 .times. 10.sup.7
0.7 70
Ex. 13
0.5 0.9 10.0 10 1 .times. 10.sup.-3
1 .times. 10.sup.7
0.3 45
__________________________________________________________________________
As described in detail hereinbefore, the anisotropic-electroconductive
adhesive film according to the present invention exhibits sufficient
anisotropic electroconductivity due to a specified electroconductive
particle mixed and has high bond strength since the content of the
particle is as low as 0.1 to 10% by volume.
Since the adhesive film is available in sheet or film form, a stable
electrical conductivity can be constantly obtained, bonding operation
therewith is simple and can be completed in a short time, thus a
considerable reduction in process time being possible. Moreover, there is
no hazard of environmental pollution since bonding operation with this
film is feasible without using any solvent. Furthermore, this film has
good transparency due to the low content of the electroconductive
particle. Accordingly, connection of circuits with visual observation is
possible with this film.
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