WO1997045893A1

WO1997045893A1 - Anisotropic conductive film

Info

Publication number: WO1997045893A1
Application number: PCT/US1997/009422
Authority: WO
Inventors: Kazuo Ishibashi
Original assignee: The Whitaker Corporation
Priority date: 1996-05-31
Filing date: 1997-05-29
Publication date: 1997-12-04
Also published as: JPH09320345A

Abstract

The anisotropic conductive film (ACF) (1) comprises a sheet-like insulating adhesive film (2) and plural rows of electrically conductive particles (3). Any one electrically conductive particle (3a) is disposed equidistant from 6 surrounding adjacent electrically conductive particles (3b-3g). This particular configuration provides high density of electrically conductive particles (3) and a large current carrying capacity. The direction of each row of the electrically conductive particles (3) is slanted at a predetermined angle with respect to the normal to one side of the ACF (1) or the sides of the electrodes to be interconnected. This minimizes the number of the electrically conductive particles (3) on the sides of the electrodes and thus maintains large current carrying capacity.

Description

Anisotropic Conductive Film

The present invention is related to an anisotropic conductive film, more specifically, to the one having an improved arrangement of electrically conductive film in a plurality of rows on one surface of the film and with a predetermined angle with respect to the normal direction to one side of the film.

Anisotropic conductive films (referred to as ACF hereinafter) exhibiting electrical conductivity in the thickness direction of such films but insulating or non- conductive in the direction along the surface thereof have been used for interconnecting high density electrical conductors such as film carriers or FPCs (flexible printed circuits) and LCDs (liquid crystal display) devices. Such ACFs are interposed or sandwiched between opposed electrodes to be inter¬ connected. Heat and pressure are applied to the ACFs so that electrically conductive particles inside the ACFs are sandwiched between the opposed electrodes to cure the surrounding adhesive for establishing electrical connection. Since adjacent electrically connective particles are not in contact with one another, adjacent electrodes are insulated from each other. Electrically conductive particles (102) in an ACF (100) are generally disposed in a random manner (see JP- A-59-93323) or in a grid manner as shown in FIG. 6 (see JP-A-l-124977) . In the former case, the density of electrically conductive particles on each electrode is not uniform, thereby making it difficult to achieve reliable conductivity between opposed electrodes to be interconnected, and also assure good insulation between adjacent electrically conductive particles or electrodes due to non-uniform density of electrically conductive particles between adjacent electrodes. On the other hand, the latter case is better than the former in that predetermined current carrying capacity between opposed electrodes and good insulation between electrically conductive particles are assured.

Arrangement of electrically conductive particles in a grid manner provides the aforementioned advantages. However, the density of the electrically conductive particles is not high because there is only one particle per unit length which is equal to the center-line pitch of adjacent electrically conductive particles. This means that the ACF having electrically conductive particles in a grid pattern is not suitable to interconnect IC (integrated circuit) terminals and a circuit board requiring relatively large current carrying capacity.

Another problem of such ACF having electrically conductive particles in a grid pattern is that when the width of an electrode is equal to an integer number of times of the pitch P of the particles as shown in FIG. 7. Some electrically conductive particles 102 are located along sides 112 of the electrodes 110. Such peripheral particles or dots do not make complete electrical contact with the electrodes 110, thereby not substantially contributing to current carrying and thus decreasing the current carrying capacity.

It is, therefore, an object of the present invention to provide ACFs with high density of electrically conductive particles and a larger current carrying capacity.

It is another object of the present invention to provide the ACFs capable of avoiding reduced current carrying capacity due to relative position of such ACFs and electrodes to be interconnected.

The invention is directed to an anisotropic film comprising a sheet of insulated adhesive film having an electrically conductive particles disposed on one surface of the film. The conductive particles are disposed in a plurality of rows. Any one of the electrically conductive particles are disposed equidistance from 6 adjacent electrically conductive particles.

The invention is further directed to an anisotropic film comprising a sheet of insulative adhesive film of electrically conductive particles disposed on one surface of the film. The conductive particles are disposed in a plurality of rows. The rows of the electrically conductive particles are slanted at a pre¬ determined angle with respect to one edge of the film. Embodiments of the anisotropic conductive film (ACF) according to the present invention will be described in detail by reference to the accompanying drawings.

FIG. 1 is an enlarged plan view of a first embodiment of the ACF according to the present invention.

FIG. 2 is an enlarged plan view of a second embodiment of the ACF according to the present invention.

FIG. 3 is an enlarged plan view of a third embodiment of the ACF according to the present invention as placed on electrodes to be connected.

FIG. 4 are cross sectional views to show the manufacturing steps of the ACF according to the present invention. FIG. 5 is an isometric view of the wound ACF according to the present invention ready for delivery to end users.

FIG. 6 is an isometric view of a conventional ACF. FIG. 7 is a plan view to describe a problem associated with the conventional ACF.

In FIG. 1, the ACF 1 comprises an insulating adhesive film 2 made from an epoxy or acryl sheet and a large number of electrically conductive particles 3 made from metal such as nickel and the like on one surface of the film 2. The size of the ACF 1 depends on its particular applications but is typically 2 mm in vertical (Y) direction, 20-25 μm in thickness and 150 mm in horizontal (X) direction for a particular example of ACF 1 for PDP (plasma display panel) applications.

Each row of the electrically conductive particles 3 extends at a predetermined angle α with respect to the Y direction, normal to the longitudinal side 5 of the adhesive film 2. Also, the electrically conductive particles are arranged in a staggered manner between adjacent rows. In this particular embodiment, one particle 3a is surrounded by 6 adjacent particles 3b, 3c, 3d, 3e, 3f and 3g which are separated by an equal distance from the one particle 3a. In other words, the particles 3b through 3g are equidistance form the center particle 3a. Such arrangement of the electrically conductive particles is referred to as hexagonal arrangement in this specification for convenience.

According to the hexagonal arrangement, particles 3a, 3b and 3c are on 3 apexes of an equilateral triangle 6. This means that there is 1/2 electrically conductive particle in an area 0.253 of the equilateral triangle 6 having a unit length of one side thereof. The density of the particles 3 is about 1.15 which is about 15% higher than the density (1) of a typical grid arrangement. Additionally, as described in detail hereinafter, this particle density is the maximum as long as the distance between adjacent electrically conductive particles 3, 3 is constant.

Shown in FIG. 2 is a plan view of another embodiment of the ACF according to the present invention. Each row of the electrically conductive particles 13 extends in substantially orthogonal or normal with respect to the longitudinal direction of the adhesive film 12. However, staggered arrangement of adjacent rows of the electrically conductive particles is the same as in the first embodiment. The second embodiment of ACF 10 differs from the first embodiment of ACF 1 in that any one electrically conductive particle 13a is located at the center of a square defined by 4 electrically conductive particles 13b, 13c, 13d and 13e.

As a result, the density of the electrically conductive particles 13 in the square 16 of one side thereof as a unit length is 2, thereby doubling the density with respect to a simple grid arrangement. However, only restriction is that the distance between the closest electrically conductive particles, e.g., particles 13a, 13b is shorter than one side of the square 16. If the distance between the closest particles is set to the minimum unit length, the resulting density will be identical to that of the simple grid arrangement.

Now, shown in FIG. 3 is a plan view of the ACF 20 according to a third embodiment of the present invention superimposed with electrodes 8 of a PDP and the like. Although the electrically conductive particles 23 of the ACF 20 are arranged in a staggered manner, major differences from the first embodiment ACF 1 include that the Y coordinate of one electrically conductive particle 23f is the same as that of electrically conductive particles 23b which is 2 rows apart from the particle 23f for simplicity of patterning of the electrically conductive particles 23. Consequently, distance from the particle 23a to the surrounding 6 particles 23b,

23c, 23d, 23e, 23f and 23g are not exactly equal to one another, thereby it is not a precise hexagonal arrangement or configuration. However, since the shortest distance between particles 23a and 23b (or 23e) is not significantly different from the longest distance between the particles 23a and 23c (or 23f) , it is appreciated that a high density similar to the first embodiment can be achieved in the third embodiment.

Now, rows of the electrically conductive particles in the ACF 20 are slanted by a given angle α from the direction normal to the longitudinal side 25 of the adhesive film 22 while the longitudinal side 25 is perpendicular to the sides 8a, 8b of the electrodes 8. Thus, the rows of the electrically conductive particles slanted at the angle α with respect to the sides 8a, 8b of the electrodes 8. This means that when the particles 23b and 23f are on the sides 8a, 8b of the electrodes 8, the particles 23c and 23h in the same rows as the particles 23b and 23f are not located on the sides 8a, 8b, thereby ensuring that most of the electrically conductive particles contribute to full current carrying except a very few particles (23b, 23f, etc.) and thus avoiding significant reduction in current carrying capacity. This minimizes fluctuation in current carrying capacity of each electrode 8.

The angle α should be determined to minimize the number of the electrically conductive particles 23 to locate on the sides 8a, 8b of the electrodes 8 and depends on the outer diameter R of each electrically conductive particle 23 and the pitch L thereof in a row. It is desired that the electrically conductive particles 23c and 23h adjacent to and in the same row as the particles 23b, 23f on the sides 8a, 8b do not locate on the sides 8a, 8b. This means that the electrically conductive particles 23c, 23h should sift in the X direction equal to the outer diameter R of each particle 23. Accordingly, the angle α is preferably equal to or larger than the angle satisfying sin α = R/L. It should be noted, however, that any angle α larger than 0° will improve the above mentioned reduction in current carrying capacity even if the above preferred condition is not met. It is of course true that smaller than the preferred angle, the benefit of improving the current carrying capacity will be reduced. It is to be understood that the above discussions about the angle α applies not only to the first and second embodiments of the ACFs 1, 10 but also to any ACF of simple grid arrangement.

Attention must be paid, however, that the preferred angle α varies depending on the type of arrangements of the electrically conductive particles. For example, in the first embodiment ACF 1, the same condition appears at every 60° rotation and one of the sides is in parallel with the sides of the electrodes at the intermediate angle (30°). As a result improper conditions appear at every angle from 0° to 30° and a preferred range of the angle α is determined by the following mathematical expression :

(n-l)30° + sin^"1(R/L) α n30° - sin^"1(R/L) where, n=l,2,

On the other hand, in the second embodiment ACF 10, the same conditions appear at every 90° and one row of the electrically conductive particles is in parallel with the sides of the electrodes at the intermediate angle (45°) . Consequently, the range of preferred angle α is determined by the following mathematical expression:

(n-l)45° + sin"¹(R/L) α n45° - sin"¹(R/L) where, n=l,2, Now, reference is made to FIG. 4 illustrating steps of making the ACF according to the present invention. ACF l (10, 20) can be made from the following steps. Firstly, a liquid phase resist (resist to plating) is coated by a conventional spin coating technique on a substrate 30 comprising a glass substrate 31 and an electrically conductive layer 32 such as ITO and the like deposited on the surface of the glass substrate 31. A conventional photo-lithographic technique is used to bore predetermined openings 34 in the resist to form a resist pattern 33. A photo mask (not shown) made from glass to form the openings 34 has the same pattern as the aforementioned arrangement of the electrically conductive particles. Subsequently, an electro-plating is performed using the electrically conductive layer 32 as the cathode to form electrically conductive particles 3 of nickel and the like (see FIG. 4 (a)). At the next step, the resist pattern 33 is removed by acetone and the like (see FIG. 4 (b) ) . After washing, an epoxy or acrylic thermosetting resin adhesive film 2 is placed on the substrate 30 to transfer the electrically conductive particles 3 on the substrate 30 onto the adhesive film 2 by applying pressure and heat before completing the ACF 1 (see FIG. 4 (c) ) . Finally, the ACF 1 is peeled off the substrate 30 to obtain the descrete ACF 1 comprising the adhesive film 2 and the electrically conductive particles 3 (see FIG. 4 (d) ) . It is to be noted, here, that a gold plated layer may be formed on the spherical top surfaces 3h or bottom surfaces 3j of each electrically conductive particle 3, if necessary.

The ACF 1 made in the above mentioned steps has the electrically conductive particles 3 with the bottom surfaces 3j thereof exposed from one surface of the adhesive film 2 while the spherical top surfaces 3h of the particles 3 covered with the adhesive film 2 as shown in FIG. 4 (d) . When the ACF 1 is sandwiched between for example FPC electrodes (typically 18-35 μm in thickness) and LCD panel electrodes (typically 1 μm or thinner) and heated under sufficient pressure. Then, the adhesive will melt and flow between the electrodes and finally cured to achieve electrical and mechanical interconnection. In this particular example, the ACF 1 is oriented in such a manner that the surface having the exposed electrically conductive particles 3 contacts with the LCD panel electrodes and heat and pressure are applied from outside of the FPC for establishing interconnection. This helps to reduce irregularity in position of the electrically conductive particles caused by the flow of the adhesive during the interconnection process. One of the reasons is the frictional force between the bottom surfaces 3j of the electrically conductive particles 3 and the LCD panel electrodes. Another reason is that since heating is carried out from the FPC side (the upper side in FIG. 4(d)) , the adhesive covering the special surfaces 3h of the electrically conductive particles 3 (i.e., the upper portion of the adhesive film 2) is first melt to fill gaps between the electrodes to effect thermal setting before the lower part of the adhesive film 2 becomes essentially fluid. Shown in FIG. 5 is an isometric view of the anisotropic conductive film (ACF) according to the present invention in the form to ship to end users. For users' convenience, the ACF 1 is wound in a coil before delivery. The adhesive film 2 of the ACF 1 is so thin that a liner 4 made from a sheet of paper or plastic such as PET is attached to the rear surface having no exposed electrically conductive particles 3 to provide sufficient strength to the film 2. In this configuration, the combination of the ACF 1 and the liner 4 can be wound in a coil. The liner 4 is preferably included by using the adhesive film 2 having the liner 4 in the step of FIG. 4 (c) rather than attaching the liner 4 after making the ACF 1.

Although the preferred embodiments of the ACF according to the present invention have been described hereinbefore, the present invention should not be limited only to these embodiments. It is to be understood that various modifications can be made depending on particular needs. For example, the cross section of the electrically conductive particles may be oval or rectangular rather than circular.

The present invention can be implemented in the following embodiment. The ACF includes a sheet-like insulating adhesive film and plural rows of electrically conductive particles are disposed at equidistance from the surrounding 6 particles. The rows of the electrically conductive particles are slanted at a predetermined angle with respected to the normal line to one side of the film. The ACF has a high density of electrically conductive particles to increase a current carrying capacity and minimize the number of electrically conductive particles not contributing current carrying. This prevents the current carrying capacity from decreasing. The ACF as defined in claim l has any one electrically conductive particle surrounded by 6 equidistance adjacent particles. This provides the maximum density of the electrically conductive particles under the given minimum spacing between adjacent particles, thereby increasing the current carrying capacity.

Also, the ACF as defined in claim 2 has rows of the electrically conductive particles not parallel with the sides of the electrodes to be interconnected. This helps to minimize the number of particles not contributing to electrical conduction, thereby avoiding reduced current carrying capacity.

Claims

Claims :

1. An anisotropic film comprising a sheet of insulated adhesive film to having (2) having an electrically conductive particles (3) disposed on one surface of said film (2), said conductive particles (3), being disposed in a plurality of rows, characterized in that any one of said electrically conductive particles (3) is disposed equidistance from 6 adjacent electrically conductive particles(3) .

2. The anisotropic conductive film of claim l, wherein the electrically conductive particles are arranged in a hexagonal arrangement.

3. The anisotropic conductive film of claim 1, wherein rows of said conductive particles (3) are disposed at an angle to the edge of the isolated adhesive film (2) .

4. The anisotropic conductive film of claim 1, wherein conductive particles (3) are staggered from one row to another.

5. An anisotropic film comprising a sheet of insulative adhesive film (2) of electrically conductive particles (3, 13, 23) disposed on one surface of said film (2), said conductive particles (3, 13, 23) being disposed in a plurality of rows, characterized in that the rows of said electrically conductive particles (3, 13, 23) are slanted at a pre-determined angle with respect to one edge of said film (2) .

6. The anisotropic conductive film of claim 5, wherein the conductive particles are staggered from one row to another.

7. The anisotropic conductive film of claim 5, wherein the electrically conductive particles (3) are arranged in a hexagonal arrangement.

8. The anisotropic conductive film of claim 5, wherein the electrically conductive particles wherein any one of the electrically conductive particles (13) is located at the center of the square defined by four adjacent electrically conductive particles (13) .