ELECTRICALLY CONDUCTIVE PASTES

Abstract: Pastes for use in the production of a highly electrically conductive thick film cured by ultraviolet radiation with the primary application of screen printing interconnective patterns suitable for electronic circuits comprise a chemical which contracts on polymerisation and a metal powder pigment in the form of spherical particles. The particles advantageously comprise nickel coated with gold or silver. Metal spheres enable radiation reflected from their surfaces to penetrate the matrix to promote efficient polymerisation. The sphere size is advantageously linked to the thickness of the screen. For example, the sphere size is 4 to 15 mu m for a screen thickness of 25 mu m.

Description: ELECTRICALLY CONDUCTIVE PASTES The present invention relates to pastes for use in the production of a highly electrically conductive thick film cured by uv radiation with the primary application of screen printing interconnective patterns suitable for electronic circuits. The field is currently served by thermally cured pastes or inks which have a high content of silver flake. Whilst the conductivity is good being as low as 0.05m cm, the films suffer from two disadvantages. Firstly, containing such a high level of silver flake they are expensive and secondly, being thermally cured, the process is long, typically 6 to 10 minutes. This curing time makes for slow processing with the associated costs and the energy costs are also high. The current invention sets out to reduce the costs of the metal pigment or * filler and to produce a uv - cured matrix which has a curing time typically of seconds rather than minutes. Attempts to produce a uv-cured ink with flake shaped pigments have not met with success. It is believed that the geometrical stacking of plates prevents the penetration of uv radiation between the flakes and hence satisfactory polymerisation is not achieved. According to the present invention there is provided a paste comprising at least one chemical which contracts on polymerisation together with metal powder pigment substantially of a spherical geometry with an outer surface of a metal which either does not oxidise or produces a very thin stable oxide. Advantageously, the metal of the pigment is ferromagnetic, preferably nickel powder. The size of the spheres advantageously matches the thickness and definition requirements of the screen printing process. For 25μm thick films a particle size distribution from 4μm to 15μm has been found to be successful. The provision of a metal surface which, when oxidised, produces a very thin stable oxide enables electrons to tunnel through the dielectric layer between the metal spheres. Coatings which do not oxidise, the noble metals such as gold (Au) for example, are, of course, very suitable. Silver (Ag) produces a thin stable oxide and .is most suitable. Copper (Cu) on the other hand does not have a suitable oxide. Copper spheres are attractive on a resistivity and cost basis although adhesion of other metal coatings may in some cases be difficult. On the other hand adhesion of silver (Ag) to nickel (Ni) is good and this has been shown to be a successful combination, namely that of silver (Ag) coated nickel (Ni) spheres. The use of nickel (Ni) spheres in conductive thick film sheets has a further major advantage, in that the ferromagnetic nature of Ni reduces the wavelength of any electromagnetic waves propagating through the film. The film, therefore, has a smaller skin depth and hence attenuates an electromagnetic wave more effectively. The attenuation coefficient, a, of an electromagnetic wave of angular frequency, w, through a film of resistivity, p, and permeability, μ, is given by a = (wμ/p) As the relative permeability of pure Ni is about 250, then a film largely composed of Ni spheres will be expected to attenuate up to about 15 times more effectively than a non-ferromagnetic film of the same resistivity. The pigment spheres are advantageously blended with a binding polymer and a photointiator to promote the uv curing of the polymer. Further, a diluent is preferably added to generate the appropriate rheology suitable to screen printing. A viscosity in the range of 5 to 150 poise is appropriate. Finally, the formulation benefits from the addition of small amounts of three further chemicals to improve adhesion, abrasion resistance and to minimise bubble formation. The requirements of the basic polymer is important in that for good results the material must contract on curing, such that the pigment sphere are compressed together in the final film so promoting good electrical contact from one sphere to the next. In this respect a satisfactory polymer has been found to be urethane aerylate. In order that the invention may be more clearly understood, one embodiment will now be described by way of example. The embodiment comprises the following formulation (as a percentage-by weight) Silver coated nickel spheres 4 to 15μm 44.15g Urethane Acrylate 1.25g Acrylic monomer viscosity modifier 3.35g Photoinitiator 0.8g Additives 0.5g This produces a film with a resistivity of 0.3m Lcm, after curing for a few seconds in uv radiation from a medium pressure Hg lamp with a power of 125W at a distance of about 20cms from the sample. The major uv emission from the medium power lamp is at a wavelength of 365nm. Typical chemicals in the formulation could be:- Photomer 6140 - Aliphatic Polyester Urethane Acrylate dissolved in Tripropylene glycol diacrylate. * Viscosity modifier 4237 - Tripropylene glycol diacrylate both from Harcros Chemicals UK Ltd. * Photoinitiator 4265 combination of Acrylic Keytone with Lucirin TPO, available from Ciba Geigy, Basle, Switzerland. * The additives. Antibubble - blend of surface active agents in solution including Lecithin and polyvinyl alcohol, Lanco flow - Acrylic copolymer and Lanco-Wax TS1778 - modified micronised PTFE wax are available from Capricorn Chemicals Ltd. These are used respectively to prevent bubble formation, to increase adhesion and to improve the abrasion resistance. A post cure treatment is found to significantly reduce the resistivity. It is known that uv radiation promotes polymerisation to about 95%. Further polymerisation can be achieved by a number of methods such as:- * Heat treatment * Infra Red heating * RF heating * Electron beam irradiation * Higher energy uv radiation from a low pressure Hg lamp with a major emission at 254nm. Baking the films in an oven for 6 hours at 100°C reduces the resistivity from 0.3m-&cm to 0.05ΠLΩ.CΠI. The use of metal spheres overcomes the problem of uv radiation penetration experienced with the prior art flakes by enabling the radiation reflected from their surfaces to penetrate the matrix and promote efficient polymerisation. It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims: CLAIMS 1. A paste comprising at least one chemical which contracts on polymerisation together with metal powder pigment substantially of a spherical geometry with an outer surface of a metal which either does not oxidise or produces a very thin stable oxide. 2. A paste as claimed in claim 1, in which the metal powder pigment is fluromagnetic. 3. A paste as claimed in claim 1 or 2, in which the metal powder pigment is nickel. 4. A paste as claimed in any of claims 1 to 3, in which the particle size of the metal powder pigment is linked to the thickness.of the screen printing process in which the paste is to be used. 5. A paste as claimed in claim 4, in which the particle size lies in the range 4μm to 15um for a screen thickness of 25μm. 6. A paste as claimed in any preceding claim, in which the surface of the metal is a coating of a noble metal. 7. A paste as claimed in claim 6, in which the noble metal is gold. 8. A paste as claimed in claim 6, in which the noble metal is silver. 9. A paste as claimed in any preceding claim in which the metal powder pigment is blended with a binding polymer and a photoinitiator to promote the curing of the polymer. 10. A paste as claimed in any preceding claim, in which a diluent is added having a viscosity in the range of 5 to 150 poise. 11. A paste as claimed in any preceding claim including chemicals to improve adhesion, abrasion resistance and to minimise bubble formation. 12. A paste formulated in accordance with the example described in the specification.

Inventors: HAMPSHIRE MICHAEL JOHN, SLIFKIN MICHAEL ARTHUR, HAMPSHIRE, MICHAEL, JOHN, SLIFKIN, MICHAEL, ARTHUR
Applicants: FIRST CLASS SECURITIES LIMITED, HAMPSHIRE MICHAEL JOHN, SLIFKIN MICHAEL ARTHUR
Application year: 1993
Main classification: H01B1
IPC codes: H01B1, H05K1