GB1587416A - Anisotropically electroconductive sheets - Google Patents

Description

(54) ANISOTROPICALLY ELECTROCONDUCTIVE SHEETS (71) We, TORAY INDUSTRIES INC., a body corporate organised and existing under the laws of Japan, of 2 Nihonbashi-Muromachi 2-chome, Chuo-ku, Tokyo, 103 Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to anisotropically electroconductive sheets and processes for producing them. More particularly, the invention relates to elastomeric sheets which are electroconductive in their thickness direction (i.e. the direction of their smallest dimension, the Z or normal direction) and non-conductive in the plane direction (i.e. the direction of their greatest dimension, the XY direction), and to processes for producing such sheets.

The anisotropically electroconductive sheets may be used as connectors for electronic circuits, since they can connect many electrodes which are independently located on directly opposite sides of the sheets. They are especially useful as connectors in electronic calculators, cameras, wrist watches, etc., where circuit elements occupy very limited spaces.

Anisotropically electroconductive sheets have been made of sheets which comprise alternate layers of conductive and non-conductive silicone rubber; sheets having metal particles dispersed in an elastomer; and as described in the U.S. Patent Specification No.

2,189,340 made by orientating conductive fibres in a transparent glass or rosin matrix.

According to the invention there is provided a process for producing an anisotropically electroconductive sheet which comprises: (a) a dispersing magnetic electroconductive fibrous wires (as herein defined) in a non-electroconductive elastomer-forming liquid matrix; (b) forming the dispersion into a sheet; (c) applying a magnetic field in a direction substantially perpendicular to the sheet to orientate the wires in a direction perpendicular to the surface of the sheet direction and gather them into bundles; and (d) hardening the matrix to form an elastomeric sheet, (e) the length of the wires being in the range 0.9 to 2.0 times the thickness of the product sheet, at least a substantial proportion of the wires having a length greater than the thickness of the product sheet so as to be individually capable of forming electrical contact on each side of the sheet.

The term magnetic electroconductive fibrous wires as used herein refers to metal wire and/or metal plated fibres having both magnetic and electroconductive properties. The wires may be of a pliant type resembling textile fibres or be more bending-resistant wires resembling electrical wiring strands. Preferably the dispersion is initially formed into a sheet having a thickness greater than the thickness of the product sheet and the sheet is then compressed after orientating the wires. The magnetic field may be formed using plates having a multiplicity of magnetic material protrusions arranged so as to concentrate the magnetic field. The dispersion may be formed into a sheet by casting the dispersion into a space having a pair of surface spaced apart initially at a distance greater than the desired thickness of the product sheet.

Matrix material forming the elastomer sheet should have enough fluidity for appropriate dispersion and orientation of the conductive wires in manufacture. The matrix material should be hardened so as to make it solid at room temperature. The term "hardening" covers both chemically curing to solidify the matrix material as well as solidifying by cooling a molten matrix material. When the electomeric sheet is used as a connector, close contact can be realized because of the elasticity of the sheet, even if the terminals have rough surfaces or foreign particles on their surfaces. Therefore, rubber-like materials such as silicones or polyurethanes are preferably used, which may be available as liquid monomers or prepolymers.

The materials which may be used as the magnetic electroconductive fibrous wires include metals such as iron, nickel, cobalt or their alloys, which have magnetic and electroconductive properties. Fibres such as copper wire, aluminium wire, carbon fibre, glass fibre and synthetic fibres which are plated with a magnetic metal such as nickel, can also be used. The diameter of the wires is preferably less than 50 microns.

The length of the wires influences the stability of conductivity. The length of the wires should be from 0.9 to 2.0 times the thickness of the product sheet, preferably from 1.0 to 1.5 times, and more preferably less than 1.1 times the thickness of the sheet. If the length is shorter than 0.9 times the sheet, the conductive efficiency becomes extremely poor as the ability to form electrical contact is very restricted. If the length is longer than 2 times the sheet, the wires may become entangled and adversely affect the insulation of adjacent wire bundles. If a substantial proportion of the fibres have a length just equal to or shorter than the thickness of the sheet, the fibres may sometimes or somewhere be covered by a thin layer of the matrix material. This causes high unstable contact resistivity, requiring a high holding pressure and causing the end of the wires to sink into the matrix when the sheet expands thermally at high temperatures. At least some of the wires must have a length within the ranges set out so that current can be conducted across the sheet thickness by the wires but some shorter, wires may be present, although they do not enhance conductivity.

When the wire content is excessive the wires may entangle during their orientation. This causes irregular conductive paths which reduce the conductive performance of the sheet.

The wires are preferably added at from 0.01 to 5.0 volume percent, more preferably from 0.05 to 1.0 volume percent of the matrix. The content of the wires also may be adjusted with respect to the length of the wires. This relationship is given by the following formula.

Y S 200 X wherein X is the thickness of the sheet (mm) and within 0.1 and 2.0 mm, and Y is the number of the efficient wires in 1 sq mm of the sheet. For example, when the individual wire is 0.5 mm long and 6 microns diameter, and the sheet is 0.5 mm thick, the maximum content of the wires is 1.13 the volume percent according to the formula.

With regard to the pattern arrangements of bundles of wires many various patterns of lattice, ring and other forms can be utilized.

The anisotropically conductive sheet of this invention has conductive paths which comprise more than one conductive wire arranged in bundles. This ensures reliability of the connection, because even if one of the wires conducts poorly, the others can conduct the electricity. The bundles of wires can be arranged in any pattern which is desired for their ultimate application.

The electroconductive sheet of the invention may be produced as follows. A certain amount of magnetic electroconductive fibrous wires of a length 0.9 to 2.0 times the thickness of the product sheet are mixed with the starting liquid or prepolymer of the matrix elastomer. This mixture is shaped into a sheet and then the cut wires are oriented in a direction normal to the sheet by the action of a magnetic field. The mixture is kept in this state until the starting liquid or prepolymer of the matrix elastomer has been hardened sufficiently and is solidified.

One method of shaping the mixture into a sheet is to feed the mixture into a cavity surrounded by sheets of thin polyester film with a ring-shaped spacer betwen the two sheets of film. Another method is to feed the mixture directly into a mould cavity formed by a set of electromagnetic plates with a frame between the plates. In this latter case, the surface of the pattern engraved plates is preferably thinly coated with non-magnetic and chemically inert material so that the product can be easily removed from the mould whereby the surface of the product is smooth.

One problem which arises in producing this conductive sheet as hereinbefore described, is the entanglement of the wires. The entanglement of the wires not only reduces the desired wire orientation but also cause the appearance of undesirable electroconductivity in the plane of the sheet. This electroconductivity can be a fatal defect of the conductive sheet when used as a fine connector for small circuitry. By maintaining the thickness of the starting mixture thicker than the final sheet during the orientation the problem of entanglement may be reduced. After the orientation of wires is complete, the thickness of the starting mixture is slowly reduced to the thickness of the final sheet and then the starting mixture is solidified. During this period, the orientation of the wires is maintained in the thickness direction. This method is effective not only for the prevention of entanglement, but also for improving the wire orientation. The length of wires varies to some extent.

Therefore, the thickness of the sheet should preferably be brought finally to the length of the comparatively shorter wires in order to raise the ratio of the wires that project completely through the sheet.

However, if the orientation is started after fixing the thickness to just the length of the shorter wires, the comparatively longer wires stop their orientation halfway and form undesirable inclined conductive paths. As hereinbefore mentioned, if the orientation is started while the starting mixture is thicker than the final sheet, then after the orientation has been completed and the thickness of the mixture is reduced, the longer wires do not create inclined paths although they may form curved paths or protrude out of the surface of the conductive sheet. In this case both ends of each wire are located on the same line normal to the sheet so that the conductive paths are kept substantially perpendicular.

In the above mentioned process, it is desirable to cover both surfaces that contact the mixture with a soft and chemically inert layer. If these surfaces are hard, the conductive wires may be bent when the distance between the two surfaces become narrower than the length of these wires. When these surfaces are covered with a soft material, even if the distances between the surfaces become smaller than the length of these wires, the ends of the wires project and become embedded into the soft layer to a certain extent and the wires are not deformed. We can apply this method to obtain the vertically conductive sheet as shown in Figure 2 of the accompanying drawings in which the ends of the wires 3 project above the surfaces of the elastic sheet 2.

The applied soft layers which contact the mixture may be made of chemically inert material so that they do not react with the matrix liquid nor attach to the matrix sheet. The rheological property and the thickness of the soft layer needed depends on the moulding temperature, the diameter, and the length and rigidity of the wires.

It is necessary that the surface layer be at least soft enough to permit the wires to project into the layer at least 1 deep before the wires begin to bend. Many kinds of elastomers may be utilized, for example, polybutadiene, nitrile-butadiene rubber, styrene-butadiene rubber, and certain kinds of plastics such as polyethylene, which become soft enough at low temperatures. These soft layers are usually coated on the surface of the retainer of the starting mixture. Also, a soft film may be used which is merely laid on the hard surface.

Another and more effective method to prevent entanglement of the wires is to utilize a special kind of magnetic pretreatment. That is, the sheet is subjected to a magnetic field which is inclined from the perpendicular before the final orientation. One can get better results by moving the degree of inclination of the magnetic field. For example, changing the degree of inclination from +45 to 450 repeatedly, gives good results. The reason for this effect is believed to be as follows. The wires which are attached to the surface of the retainer cavity during feeding of the starting mixture are not affected by a magnetic field perpendicular to the sheet. Therefore, they remain on the surface of the sheet. By means of a magnetic field, inclined to the perpendicular those wires are forced to move and become orientated in the direction of the magnetic field.

As to the entanglement, the wires oriented in an inclined direction have more back and forth freedom of movement than the wires that are perpendicularly oriented in the sheet.

This freedom helps to diminish the entanglement. Changing the degree of inclination of the magnetic field causes the same effect as shaking the wires and has a good effect on loosening the entanglement of the wires. The degree of inclination of the magnetic field to the perpendicular is not restricted but an inclination of more than 5", preferably more than 10 , is desirable.

By providing the mould with a magnetic plate having many regularly arranged surface protrusions a regularly distributed pattern may be obtained conveniently such as for example, that shown in Figure 1 of the accompanying drawings. The magnetic force is stronger in the region near those protrusions than in the region of the dent or valley.

Therefore, the ferromagnetic wires which are distributed randomly in the matrix liquid are gathered at the nearest protrusion and then they are orientated in a direction perpendicular to the sheet. The pattern of the arrangement of the conductive wires is decided by the pattern of the arrangement of the protrusions on the surface of the magnetic plate. Several methods including an etching process and a mechanical process maybe used to form the pattern on the plate. The surface of the magnetic plate is made from a ferromagnetic material. It is possible to make the surface mechanically smooth by filling the hollow defining the protrusions with non-magnetic material.

The elastomer is used as the matrix. Therefore, when used as a connector that is held between two electric conductive boards, the matrix is compressed and the reliability of the connection is increased by the reaction force. This reliability is not affected in any major degree by vibration or temperature because of the elastic resilience of the matrix.

The conductive sheet can be used with many types of electrical contact configurations; the conductivity is not dependent on the pressure exerted on the sheets; the sheets are soft and their performance is not unduly affected by the presence of foreign particles.

Using this invention a conductive sheet can be made without entanglement and with both ends of each wire protruding out of the surface. A high reliability of electrical connection may result.

The invention is more particularly described with reference to the drawings in which: Figure 1 shows a perspective view of a conductive sheet produced according to the invention Figure 2 shows the cross section of the sheet of Figure 1 taken along line II-II; Figure 3 shows the conductive sheet used as a connector for electronic circuit elements; Figure 4 is a conceptional plane view showing a pattern arrangement of the conductive wires; Figures 5 to 8 show further patterns of the conductive wires; and Figure 9 is a conceptional plane view showing an example of a pattern arrangement of the terminals of an electronic circuit element.

As shown in Figures 1 and 2, an anisotropically electroconductive sheet 1 consists of an elastomeric sheet 2 as matrix and fibrous electroconductive wires 3 dispersed and arranged in the elastomer sheet 2. The elastomer sheet 2 consists of electrically non-conductive material. The fibrous conductive wires 3 are orientated in a direction perpendicular to the surfaces of elastomer sheet 2 independently from each other. More than one of the conductive wires 3 may be gathered in one spot 6 and many of these spots 6 may be formed in a regular pattern.

Figure 4 is an example of a pattern in which the spots 6 are arranged hexagonally. The length of the conductive wire 3 is slightly longer than the thickness of elastomer sheet 2 so that both ends of each wire 3 are exposed on the surface of the sheet 2. As noted from the aforementioned composition, the conductive wires 3 provide conduction in the thickness direction and the elastomer sheet 2 provides electric insulation in the plane direction. With reference to Figure 3, the conductive sheet 1 can be used as a connector between electronic circuit elements by inserting the sheet 1 between circuit elements 4, fixed with a little holding pressure and connected to terminals 5.

With reference to Figures 5 to 9, different patterns of wire bundles may be used besides the hexagonal pattern shown in Figure 1. Figures 5 to 8 show some examples of such patterns Figure 5 shows a rectangular form, Figure 6 shows a ring form, Figure 7 shows a spiral form and Figure 8 shows a rhomboid form. The pattern shown in Figure 5 is especially useful for connection of circuit elements which have the terminal pattern shown in Figure 9.

The invention is illustrated by the following Examples.

Example 1 A low temperature vulcanizable liquid silicone rubber is mixed with 0.5 volume percent of cut stainless steel fibres which have a diameter of 12 Il, an average length of 0.49 mm and standard deviation of length of 0.02 mm. The mixture is degassed in vacuum and then fed into a cavity formed by two sheets of 50 F thick polyester film with an aluminium spacer of 0.49 mm thickness between the sheets. The mixture volume is controlled so as to form the mixture into a sheet with the following thicknesses: case (A) 0.47 mm; case (B) 0.70 mm and case (C) 0.70 mm.

One set of a plane steel mould comprising two plates is prepared. Each plate of the mould is attached face to face with a pole of an electromagnet so that each plate forms a magnetic pole and a magnetic field is created in the cavity between the two plates in a direction normal to their surfaces. Then the mould is heated to 60"C.

A set of polyester films (a) containing the starting mixture of 0.47 mm thickness is placed into the cavity and then the mould is closed as far as permitted by the spacer. Thereafter, the electromagnet is activated to create a 3,000 Gauss vertical magnetic field in the cavity.

After 2 hours, a sheet of anisotropically electroconductive silicone rubber (A), is taken out from the mould.

A set of polyester films (b) containing the starting mixture and of 0.70 mm thickness is also put in the cavity and the mould is closed to the last 1.0 mm. The electromagnet is then activated to create a 3,000 Gauss of magnetic field through the sheet for 5 minutes.

Thereafter, in the same magnetic field the mould is slowly closed until stopped by the spacer. After two hours the sheet of anisotropically electroconductive silicone rubber (B) is taken out of the mould.

A set of polyester films (c) containing a starting mixture with a 0.70 mm thickness is also placed into the cavity and the mould is closed to the last 1.0 mm. Thereafter, the electromagnet is stored to create a 3,000 Gauss of magnetic field through the sheet for 2 hours. The sheet of anisotropically electroconductive silicone rubber (C) is then taken out of the mould.

As shown in Table 1, there are many differences between the three sheets. Sheet (B) is best. This illustrates the importance of: (1) the comparatively high thickness of the starting mixture during the magnetically induced orientation, and (2) controlling the thickness to the length of fibres after orientation and before curing.

The results are shown in Table 1 as follows.

TABLE I Sheet (A) Sheet (B) Sheet (C) Appearance (entanglement of fibres) Many Nothing Nothing Surface conductivity (distances of electrodes Irregularly No No 0.6 mm) conductive conductivity conductivity Resistances in the direction of thickness 0.02 Qcm 0.015 Qcm 1.3 Qcm Example 2 Cut fibres of graphite fibre metallized with 0.2 thick Ni, which have a diameter of 61l, an average length of 0.50 mm and a standard deviation of length of 0.02 mm are mixed with a low temperature vulcanizable silicone rubber liquid and the mixture is degassed in vacuum.

The volume fraction of cut fibres is 0.5 volume percent.

A controlled amount of the mixture is fed into a cavity made of two sheets of polyester films with a 0.5 mm thick aluminium spacer between the films so that the thickness of the mixture becomes 0.60 mm. Two sets (d) and (e) are also prepared.

The starting material of set (d) is treated in the same way as (b) in Example 1 and a silicone rubber sheet (D) is formed.

As to the starting material of set (e), a special pretreatment is performed. Two pieces of steel plates having a wavy surface on one side are prepared. The pitch and the height of the waves are 12 mm and 6 mm respectively and each wave has a straight edge. The plates are attached to the opposite poles of an electromagnet. Two plates are set face to face with their wavy surfaces at a distance of 5 mm from each other, and the wave of the two plates are pitched so that the tops of the waves of one plate face the bottoms of the waves of the other plate. The maximum vertical magnetic field at the same distance from the two plates is 2,000 Gauss.

The starting material is set (e) is held on a plane of an aluminium plate of 1 mm thickness and is moved in the centre of these magnetic plates to and fro so as to cross the waves repeatedly. The entanglement of the fibres completely disappears in about 10 minutes.

Thereafter, it is treated exactly the same way as (d) so as to form a silicone rubber sheet (E).

The effect, as shown in Table 2 of this magnetic pretreatment, is enormous.

TABLE 2 Sheet (D) Sheet (E) Appearance Back to grey Almost transparent (naked eye) because of fibres entangled Observation by Laying fibres on 0.0 - 0.1/cm2 microscope the surface of sheet 15 - 20/cm2 Conductivity in surface (distances of electrode Irregularly 0.6 mm) conductive No conductivity Resistances in the direction of thickness 0.23 Q cm 0.20 Q cm Example 3 Cut steel fibres having a diameter of 12y, an average length of 0.49 mm and a standard deviation of length of 0.01 mm, are non-electrically metallized with 0.6 thickness of Ni and additional 0.08 thickness of gold. These fibres are mixed with a low temperature vulcanizable liquid silicone rubber and the mixture is degassed in vacuum. The volume fraction of the fibres in the mixture is 0.12 volume percent.

A controlled amount of the mixture is fed into a cavity made between two sheets of 100 It thick polyester film and containing a 0.47 mm thick aluminium spacer, is fed so that the thickness of the mixture becomes 0.60 mm.

Two sets of the starting mixture were prepared In one set, polyester film, coated with a 30 thick nitrile-butadiene rubber is used (t). In the other set, a conventional polyester film is used (g). These two sets are treated in the same manner as (b) in Example 1 and two kinds of conductive sheets (F) and (G) are obtained.

As shown in Table 3, sheet (F), wherein each end of the conductive fibres protrudes out of a matrix surface, is obviously superior to sheet (G) which has no fibre protrusion.

The effect of the protrusion of both fibre ends of the conductive phase is obvious.

TABLE 3 Sheet (F) Sheet (G) Shape df conductive Straight. Average Bending. Protrusion fibre length of protrusion from the matrix is is 12 It not clearly observed.

Normal direction resistance and pressure at the electrodes.

Pressure resistance resistance 1 kg/cm2 0.8 Q 3.2 Q 3 kg/cm2 0.7 Q 1.3 Q 7 kg/cm2 0.7 Q 0.9 Q 13 kg/cm2 0.7 Q 1.0 Q Area of electrode: 1 mm2 Normal direction resistance and temperature.

Temperature resistance resistance 22"C 0.7 Q 0.9 Q 50"C 0.7 Q 1.7 Q 75"C 0.7 Q 18 Q 100"C 0.8 Q 100 Q < 125"C 1.5 Q 100 Q < Area of electrode: 1 mm2 Pressure: 7 kg/cm2 Example 4 Cut stainless steel fibres having a diameter of 12 , an average length of 0.49 mm, and standard deviation of length of 0.0/mm is mixed with a low temperature vulcanizable liquid silicone rubber and the mixture is degassed in a vacuum. The volume fraction of the fibre is 0.13 volume percent.

This mixture is fed into a cavity formed by two sheets of 50 thick polyester film having an aluminium spacer of 0.49 mm thick between the sheets. The mixture,is controlled so that the thickness of the mixture between the polyester films becomes 0.60 mm.

A plane steel mould is prepared and the two plates of the mould are attached to electromagnetic poles in the same manner as described in Example 1. However, this time the surface of one plate is engraved so that there are many cylindrical protuberances of 0.1 mm height and 0.2 mm diameter arranged hexagonally with a 0.5 mm pitch. This means that the distances from every one point to the neighbouring six points are all equal to 0.5 mm. The two plates of the mould are separately attached to both magnetic poles so that the two plates form the electromagnetic plates and the lower plate has the engraved surface at the top.

The mould was heated at 60"C. The mixture sheet was put on the engraved surface and the gap between the two plates was closed to 0.9 mm. Then, the electromagnet was activated to create 3,000 Gauss magnetic field in the gap. Then the mould was again slowly closed until it was stopped by the spacer. After 40 minutes, a half vulcanized mixture sheet was taken out and further heated 15 minutes in an oven at 1500C to complete the vulcanization.

In this conductive sheet, the conductive fibres are all oriented in a direction normal to the surfaces of the sheet and every fibre is arranged to the point of hexagonal pattern corresponding with the protruberance on the plate with most points having 3 to 5 fibres that belong to the same point.

WHAT WE CLAIM IS: 1. A process for producing an anisotropically electroconductive sheet which comprises: (a) dispersing magnetic electroconductive fibrous wires (as herein defined) in a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 3 Sheet (F) Sheet (G) Shape df conductive Straight. Average Bending. Protrusion fibre length of protrusion from the matrix is is 12 It not clearly observed. Normal direction resistance and pressure at the electrodes. Pressure resistance resistance 1 kg/cm2 0.8 Q 3.2 Q 3 kg/cm2 0.7 Q 1.3 Q 7 kg/cm2 0.7 Q 0.9 Q 13 kg/cm2 0.7 Q 1.0 Q Area of electrode: 1 mm2 Normal direction resistance and temperature. Temperature resistance resistance 22"C 0.7 Q 0.9 Q 50"C 0.7 Q 1.7 Q 75"C 0.7 Q 18 Q 100"C 0.8 Q 100 Q < 125"C 1.5 Q 100 Q < Area of electrode:

1 mm2 Pressure: 7 kg/cm2 Example 4 Cut stainless steel fibres having a diameter of 12 , an average length of 0.49 mm, and standard deviation of length of 0.0/mm is mixed with a low temperature vulcanizable liquid silicone rubber and the mixture is degassed in a vacuum. The volume fraction of the fibre is 0.13 volume percent.

This mixture is fed into a cavity formed by two sheets of 50 thick polyester film having an aluminium spacer of 0.49 mm thick between the sheets. The mixture,is controlled so that the thickness of the mixture between the polyester films becomes 0.60 mm.

A plane steel mould is prepared and the two plates of the mould are attached to electromagnetic poles in the same manner as described in Example 1. However, this time the surface of one plate is engraved so that there are many cylindrical protuberances of 0.1 mm height and 0.2 mm diameter arranged hexagonally with a 0.5 mm pitch. This means that the distances from every one point to the neighbouring six points are all equal to 0.5 mm. The two plates of the mould are separately attached to both magnetic poles so that the two plates form the electromagnetic plates and the lower plate has the engraved surface at the top.

The mould was heated at 60"C. The mixture sheet was put on the engraved surface and the gap between the two plates was closed to 0.9 mm. Then, the electromagnet was activated to create 3,000 Gauss magnetic field in the gap. Then the mould was again slowly closed until it was stopped by the spacer. After 40 minutes, a half vulcanized mixture sheet was taken out and further heated 15 minutes in an oven at 1500C to complete the vulcanization.

In this conductive sheet, the conductive fibres are all oriented in a direction normal to the surfaces of the sheet and every fibre is arranged to the point of hexagonal pattern corresponding with the protruberance on the plate with most points having 3 to 5 fibres that belong to the same point.

WHAT WE CLAIM IS: 1. A process for producing an anisotropically electroconductive sheet which comprises: (a) dispersing magnetic electroconductive fibrous wires (as herein defined) in a

non-electroconductive elastomer-forming liquid matrix; (b) forming the dispersion into a sheet; (c) applying a magnetic field in a direction substantially perpendicular to the sheet to orientate the wires in a direction perpendicular to the surface of the sheet direction and gather them into bundles and; (d) hardening the matrix to form an elastomeric sheet; (e) the length of the wires being in the range of 0.9 to 2.0 times the thickness of the product sheet, at least a substantial proportion of the wires having a length greater than the thickness of the product sheet so as to be individually capable of forming electrical contact on each side of the sheet.

2. A process according to claim 1 in which the dispersion is initially formed into a sheet having a thickness greater than the desired thickness of one product sheet and the sheet is then compressed after orientating the wires.

3. A process according to claim 2 in which the dispersion is formed into a sheet by casting the dispersion into a space having a pair of surfaces spaced apart initially by a distance greater than the desired thickness of the product sheet.

4. A process according to any of claims 1 to 3 in which a magnetic field is formed initially in a direction inclined to the perpendicular direction to the step of applying the magnetic field in the perpendicular direction.

5. A process according to any of claims 1 to 4 in which the sides of the sheet are covered with a soft material for embedding protruding ends of the wires during the orientation.

6. A process according to any of claims 1 to 5 in which the liquid matrix is a prepolymer of a polyurethane or silicone polymer.

7. A process according to any of claims 1 to 6 in which the electroconductive wires comprise a magnetic iron, nickel or cobalt metal or a magnetic alloy predominantly composed of at least one of the metals.

8. A process according to any of claims 1 to 6 in which the electroconductive wires comprise a filament-like substance plated with a magnetic metal, and the filament-like substance is copper wire, aluminium wire, carbon fibre, glass fibre or synthetic fibre.

9. A process according to any of claims 1 to 8 in which the magnetic field is formed using plates having a multiplicity of magnetic material protrusions arranged so as to concentrate the magnetic field.

10. A process for producing an anisotropically electroconductive sheet according to claim 1 substantially as described in any of the Examples.

11. An anisotropically electroconductive sheet produced by a process according to any of claims 1 to 10.