Anisotropic electrical connection

A method of forming an anisoptropic electrical connection between conductive elements (22, 24) having an oxide layer (28) is disclosed. The method comprises the use of an adhesive (30) including carbon fibres (32) and metallic particles (34). On the application of pressure, the carbon fibres (32) penetrate the oxide layer (28), whilst the metallic particles (34) deform such that a good electrical connection is made between overlying elements (22, 24).

This invention relates to anisotropic electrical connection, and in 
particular to an adhesive for the anisotropic electrical connection of 
electrically conductive elements. 
More specifically, the invention relates to the electrical interconnection 
of aligned conductive elements mounted respectively on first and second 
substrates, without electrical interconnection between adjacent conductors 
on each substrate. 
This is a common requirement, for example, in the production of LCDs or 
other display panels. In this case, one substrate may be the glass display 
panel, the electrically conductive elements mounted thereon commonly being 
formed of a transparent oxide of indium and tin (ITO). The required 
interconnection is that of the ITO elements to a circuit board for 
supplying the required driving voltages. 
One well established technique for achieving this interconnection is the 
use of a substrate of thin flexible material in the form of a strip 
carrying tracks of metal or conductive ink. The strip is bonded to the 
glass substrate at one end, and to the circuit board at the other end. 
Alternatively, the circuit board may be incorporated on the strip. 
Electrical interconnection is achieved by the use of an adhesive substance 
which forms an anisotropic connection in bonding. The substance comprises 
an adhesive polymer in which metallic particles are suspended. 
This known method of connecting ITO conductors is shown diagrammatically in 
FIG. 1 of the accompanying drawings. The adhesive 2 is sandwiched between 
the end of the flexible strip 4 and the edge of the glass display panel 6, 
with the conductive elements 8, 10 aligned. Heat and pressure are applied 
to the joint, such that the metallic particles 12 deform between the 
aligned conductors 8, 10 on the panel 6 and the flexible substrate 4 
forming an electrically conductive pathway therebetween. The adhesive is 
then cured or set by heat to maintain the conductive pathways. Since the 
metallic particles 12, are small compared to the gap between the 
conductive elements 8, 10 on each substrate 4, 6 electrical connection is 
not made between adjacent conductors 8 or 10. 
With the fast rate of addressing which is possible with FLCD 
(ferro-electric liquid crystal display) technology, it is preferable to 
use low resistive conductive elements or electrodes on the display 
substrate. This can be achieved by adding a much narrower metal conductor 
to each ITO element to increase its conductivity, or by replacing the ITO 
elements with metallic elements where transparency is not required, for 
example in printer applications or outside of the active display area. 
Aluminium is suitable for this purpose due to its high electrical 
conductivity. However, connection of aluminium conductors using the method 
described above has been found to be unreliable. This appears to be due to 
the non-conductive oxide which forms on the surface of aluminium, the 
metallic particles merely being squashed when the substrates are pressed 
together in bonding thus distributing the load over a larger area, and not 
being capable of fracturing the oxide layer in order to make reliable 
electrical contact with the conductor. 
It is an object of this invention to mitigate this problem. 
From one aspect the present invention consists in a method of forming an 
anisotropic electrical connection between first and second substrates each 
having a plurality of adjacent conductive elements mounted thereon, at 
least some of the conductive elements having a non-conductive coating, the 
method comprising the steps of: aligning the substrates such that at least 
some of the conductive elements on the first substrate overlie a 
respective conductive element on the second substrate, and sandwiching 
between the substrates a substance comprising an insulating adhesive and a 
plurality of electrically conductive particles, some of the particles 
being capable of penetrating the conductive elements having a 
non-conductive coating by piercing the coating, and some of the particles 
being capable of deforming between the overlying elements, on the 
application of pressure between the substrates, and applying pressure 
between the substrates such that the substance forms an electrical 
connection between the overlying elements whilst isolating the adjacent 
elements from each other. 
From another aspect the invention consists in an electrically 
interconnected assembly comprising first and second substrates each having 
a plurality of adjacent conductive elements mounted thereon, at least some 
of the conductive elements having a non-conductive coating, and being 
aligned such that at least some of the conductive elements on the first 
substrate each overlie a respective conductive element on the second 
substrate, and a substance including an adhesive bonding the substrates 
together, the substance forming an electrical connection between the 
overlying elements whilst isolating the adjacent elements from each other, 
and the substance including electrically conductive particles, some of 
which have pierced the non-conductive coating and penetrated the 
conductive element and some of which have deformed between the overlying 
elements. 
The elements having a non-conductive coating may be of a metal which forms 
a stable oxide, for example, aluminium. Particles capable of piercing 
aluminium oxide have been found to include carbon fibres, disilicides, 
titanium diboride and tungsten whiskers. 
The deformable particles may be metallic; for example, gold plated nickel. 
The adhesive base is conveniently a polymer base, or may be, for example, 
of chemically bonded ceramics such as magnesium-aluminium phosphates. 
From yet another aspect, the invention consists in a substance comprising 
an adhesive polymer base, carbon fibres and metallic particles.

Referring to FIG. 2, it is required to connect first aluminium conductive 
elements 22 at the edge of a first substrate, which forms a glass display 
panel 20 of an FLCD screen, to a circuit board for supplying driving 
voltages. This is achieved by joining the elements 22 to second conductive 
elements 24, for example, of copper, gold-plated copper, silver or 
carbon-loaded inks, which are carded by a second substrate 26 in the form 
of a flexible strip. It is necessary to connect each first element 22 to a 
respective overlying second element 24 without connecting the adjacent 
first elements 22 together, or connecting the adjacent second elements 24 
together. 
An anisotropically electrically conductive adhesive substance 30, which 
conveniently is supplied in the form of a film, is sandwiched between the 
substrate 20, 26, and heat and pressure are applied to the joint. 
Referring to FIG. 2, the adhesive substance 30, which may comprise a 
polymer base, includes carbon fibres 32, and metallic particles 34, such 
as gold coated nickel particles. 
The aluminium elements 22, 24, on the first substrate 20 have a 
non-conductive oxide coating 28. When the joint is made, the carbon fibres 
which are captured in the gaps 36 between respective overlying conductors 
22, 24 pierce the oxide coating 28 and penetrate the bodies of the 
aluminium elements 22. The metallic particles, however, are squashed 
between the elements 22, 24. 
The particles 32, 34 are in sufficient quantity that an electrical 
connection is made between overlying conductors 22, 24, but no such 
connection is made between adjacent conductors 22 or 24. For instance, in 
the gap 36 between overlying conductors 22, 24 come carbon fibres 32 may 
penetrate the oxide layers 28 of the first element 22, and also contact 
the second element 24. Alternatively some fibres 32 which penetrate a 
first element 22 may contact a metallic particle 34, which contacts the 
second element 24. 
With conductive elements having a pitch of about 1 mm, and a depth of about 
0.0002 mm, carbon fibres having an average length of about 0.05 mm have 
been found to be suitable.