US20060237321A1

US20060237321A1 - Method of forming conductive tracks

Info

Publication number: US20060237321A1
Application number: US11/400,928
Authority: US
Inventors: John Fyson; Peter Hewitson
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 2005-04-22
Filing date: 2006-04-10
Publication date: 2006-10-26
Also published as: GB0508188D0

Abstract

A patterned electrical conductor is obtained by exposing, to a desired conductive pattern, a photosensitive element comprising a support and a photosensitive material coated thereon which is sensitive to the wavelength of exposing radiation and capable of providing a latent image upon exposure, developing the exposed element to form a developed metal (e.g. silver) image being capable of conducting on having a voltage applied across it, and electroplating the metal image with a second metal (e.g. silver) to form a conductive pattern having improved conductivity, without the need for an intermediate physical development or electroless plating step.

Description

FIELD OF THE INVENTION

The present invention relates to formation of conductive materials as conductive tracks for and in electronic circuit boards and devices utilising such conductive tracks. The invention is particularly concerned with the improvement of conductivity of conductive silver tracks obtained by a photographic method, such as on a flexible support.

BACKGROUND OF THE INVENTION

In the imaging, lighting, display and electronics industries, it is predicted that in order to meet consumer demands, and fuelled by industry competitiveness, electronics products will be required to be increasingly durable, thin, lightweight and of low cost. In a growing market where consumers are demanding more from portable electronic devices and displays such as mobile phones, laptop computers, etc., flexible displays and electronics have the potential to eliminate the rigid constraints of traditional flat panel displays and electronics products. The goal in displays and electronics is to produce thin, lightweight, flexible devices and displays with achievable power requirements at a minimal cost.
Traditionally electronic devices requiring multiple layers of circuits have been fabricated using multiple circuit boards, with circuitry formed on one or both sides thereof, which may be bonded together and connected to one another by drilling holes (or vias) in the circuit boards which are filled with conductive material. To make such multiple layer circuit boards, a copper coated insulating board made of a composite material is treated with a light-sensitive material, known as a photoresist, which is imaged with the pattern of the desired electronic circuit, typically by exposing the photoresist through a photomask. The resist is affected by the exposure such that the exposed and non-exposed parts can be differentiated in terms of ease or method of removal. The imaged resist is then treated to remove the resist in an image-wise manner to reveal bare copper. The bared copper is then etched away and then the remaining resist removed to reveal a copper track on the insulating board. A second board may be made in a similar way with its own circuit pattern and the two boards bonded together and optionally connected by drilling vias as mentioned above.
The process of making electronic circuit boards such as this can be quite laborious and involves several sequential steps.
It is desirable to provide a solution to improve the efficiency of the electronic circuit manufacturing process and to enable electronic circuits to be generated on flexible supports to meet the predicted growth in demand for flexible circuits and flexible and thin devices. A number of attempts to provide new manners of manufacturing electronic circuits have been previously disclosed, but the processes are often lengthy and laborious.
U.S. Pat. No. 3,839,038 describes a method that can be modified to make conducting tracks more or less continuously on a flexible support using imaging methods to lay down metals, particularly silver. The tracks made in this way have resistance that is too high for some purposes. This method creates a non-conducting image.
U.S. Pat. No. 6,706,165 describes a way of making metallic structures, which are presumably conducting by forming a silver image which is then grown in an electroless-plating bath to make it conductive and then electroplating this grown image to form the conducting metal structure. This process is relatively laborious and complicated. GB-A-0585035 describes a process for making conducting tracks, including an electroless plating process, which may or may not be followed by an electroplating step.
U.S. Pat. No. 3,223,525 describes a method of manufacturing, by photographic means, external electrically conductive noble-metal patterns on non-conductive supports. In the described method, a non-conductive support is treated with a light sensitive compound such as silver halide, exposed to light to produce a silver or mercury germ image, which is then treated with a stabilised physical developer for a prolonged period of time whereby the internal image is made to grow out beyond the surface of the support to become an external image having resistance of less than 10⁴ohms per square.
U.S. Pat. No. 3,647,456 relates to a method of making electrically conductive silver images with the object of providing such electrically conductive silver images having high spatial resolution, which conducting silver image may be advantageously utilised in printed circuit techniques thereby eliminating the need for an aluminium layer in photoresists and establishing a silver pattern directly upon a wafer. There is described the use of a coating of silver bromide emulsion comprising cadmium iodide on a substrate to produce a latent image on the substrate, developing the latent image using a high resolution developer to provide a silver image and heating the silver image at a temperature of from 200° C. to 450° C. to render the silver image electrically conductive.
The various alternative methods of generating conductive circuit patterns illustrated in the above-referenced documents each has advantages as described therein, but they do not provide a more efficient and improved method of manufacturing conductive tracks.

PROBLEM TO BE SOLVED BY THE INVENTION

It is desirable to provide a method of forming conductive tracks which is more efficient and involves fewer steps in fabrication as compared with traditional printed circuit board manufacture.
It is still further desirable to provide a method capable of forming conductive tracks or conductive areas having gaps with very high resolution to meet the demands of increasingly complex circuitry of high-tech devices.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a process for preparing a patterned electrical conductor comprising the steps of providing a photosensitive element comprising a support substrate and a photosensitive material coated onto the support, said photosensitive material being capable of providing a latent image on exposure to sensitising radiation, exposing the photosensitive element to sensitising radiation according to a desired conductive track pattern to form a latent image on the photosensitive element, developing the latent image to form a developed image formed by a first metal corresponding to the desired conductive track pattern, said image formed by the first metal being capable of conducting when a voltage is applied across it, and electroplating said developed metal image with a plating of a second metal to improve the conductivity of said developed metal image to form a conductive track according to the desired pattern.
In a second aspect of the invention, there is provided a patterned electrically conductive element comprising a conductive track pattern on a support substrate, said element being obtainable by the above process.

ADVANTAGEOUS EFFECT OF THE INVENTION

The process of preparing a patterned electrical conductor according to the invention provides a quicker, more efficient method of manufacturing conductive tracks, which may be formed on a flexible support and which does not require a physical development step. A further advantage of the process is that the conductive pattern is reflective when viewed from the emulsion side of the support, which is beneficial in certain display applications. The method may be utilised to form conductive tracks having tightly controlled conductivity, track width and gap width according to the desired utility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a standard 2-electrode electrochemical cell for use in the electroplating step of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention involves photographically generating a desired pattern capable of conducting when a voltage is applied across it, whereby the photographically generated pattern can be subjected to electroplating with a metal to improve the conductivity of the photographically generated pattern by a desired amount, without the necessity of an intermediate step to make the photographically generated pattern sufficiently conductive for electroplating, such as by electroless plating (i.e. physical development).
Preferably, therefore, the process of the invention does not involve an electroless plating or physical development step.
An advantage of electroplating the sufficiently conductive photographically generated pattern to form the conductive pattern having the desired conductivity, as compared with physical development, for example, is that only those areas of the pattern capable of conducting will be plated, whereas by electroless plating, even spurious photographically generated specks of metal will be plated, thereby increasing the risk of shorting the circuit or limiting the resolution of the conductive pattern formed. Furthermore, by electroplating the photographically generated image, it is possible to obtain a reflective upper surface on the conductive pattern, which is advantageous in certain display applications.
The electroplating step can be controlled by various parameters, such as the concentration of the electroplating solution used, the voltage applied across the photographically generated conductive metal pattern, the duration of the electroplating step, the choice of metal or metal salt utilised in the plating solution and the choice of additives to the solution to maximise efficiency, to ensure that an even and equal coating of the plating metal is applied across the photographically developed conductive pattern and to control the resultant conductivity of the pattern depending upon the intended application of the conductive pattern. It is possible to plate the photographically generated pattern with two or more metals, either simultaneously or preferably successively. For example, it may be desirable to plate the photographically generated image with copper and then a thin plating of silver to provide improved conductivity and a reflective surface, whilst minimising the amount of silver used.
By photographically generated image, it is meant an image formed by imagewise exposure of a photosensitive material, which may be a photosensitive metal salt dispersed in a polymer material such as gelatin or other suitable hydrophilic polymer, to form a latent image (i.e. a germ or nucleus of metal in each exposed grain of metal salt), and development of the latent image to form a metal image corresponding to the desired pattern, the step of developing the latent image consisting of the catalytic reduction of the metal salt, in those particles of the metal salt having a latent metal image, to the metal by a development composition. In order to be capable of being electroplated, the metal image formed must be capable of conducting when a voltage is applied across it. To this end, the photosensitive element utilised in this invention must be suitably formulated to enable the image to be at least slightly conductive after conventional development, such as, in the case of silver halide as the photosensitive metal salt, by utilising a low-gelatin silver halide emulsion. Suitable such low-gelatin emulsions are described in U.S. Pat. No. 5,512,415.
The first metal, being the metal that forms the photographically generated metal image, may be any suitable metal obtainable through imagewise reduction of a suitable photosensitive metal salt.
The second metal, which according to the process of the invention is plated onto the developed image of the first metal, may be any suitable conductive metal which is capable of being plated onto a conductive pattern by electrochemical methods, typically applying a voltage across the conductive patterns in the presence of a solution of a salt of the second metal.
The second metal may be, for example, silver, gold, zinc, chromium, lead, copper or nickel. Preferably the second metal is silver or copper and more preferably silver.
The electroplating step of the process is achieved by providing a plating solution in contact with the photographically generated pattern whilst applying a voltage across the photographically generated pattern through the solution, by making the photographically generated pattern the negatively-charged electrode (referred to as the cathode in electrochemistry) in an electrochemical cell with a positively-charged electrode (the anode). The plating solution utilised according to the process of the invention may be, for example, depending on the identity of the second metal, a solution of a silver thiosulfate complex, e.g. Na₃Ag(S₂O₃)₂, wherein silver is the second metal; a solution of copper sulfate optionally with or without a polyethylene glycol PEG 200, wherein copper is the second metal; nickel sulfate, i.e. NiSO₃, in the presence of boric acid, wherein nickel is the second metal; or zinc sulfate, ZnSO₄, wherein zinc is the second metal. Preferably the plating solution has an equivalent concentration of the second metal of from 0.01 to 2 molar, more preferably 0.03 to 0.5 molar and still more preferably 0.05 to 0.2 molar. Boric acid to control pH and/or PEG as a throwing agent may optionally be added to any of the plating solutions utilised.
As mentioned above, the silver is preferably the second metal, so a solution of a silver salt or complex is preferably used. The silver salt is preferably a silver thiosulfate complex, e.g. Na₃Ag(S₂O₃)₂, and can be formed by making a solution of silver chloride, sodium sulfite and ammonium thiosulfate. Preferably, the silver-plating solution has an equivalent concentration of silver of from 0.01 to 2 molar, more preferably 0.03 to 0.5 molar and still more preferably 0.05 to 0.2 molar. The low equivalent concentration of silver in the plating solution enables the plating process to be controlled, allowing even plating across the patterned conductor and minimising the build-up of plating metal close to the electronic contacts.
The formulation of metal salts for use in the plating solution may be adapted from any suitable plating solution formulation, a useful source of known plating solution formulations includes “Modern Electroplating” 4^thEdn, Ed. M. Schlesinger, M. Pacinovic, published by Wiley.
Preferably, the developed silver image has a surface conductivity of 40 ohms/square or less. The voltage applied across the patterned conductor is preferably up to 2 V, more preferably up to 1 V.
Preferably, the method of the invention is capable of providing a plated conductive element having a track width resolution of 20 μm or less, more preferably 10 μm or less and still more preferably up to 5 μm. Preferably, it is capable of provide a similar gap width resolution.
As mentioned above, the photosensitive material coated on a support substrate utilised according to the process of the present invention may be any suitable photosensitive material capable of providing a photographically generated metal image according to a desired pattern, which can have a voltage applied across it for the purpose of electroplating.
In a preferred embodiment of the invention, the photosensitive material is a silver halide emulsion in a hydrophilic colloid. The hydrophilic colloid may be gelatin or a gelatin derivative, polyvinylpyrrolidone or casein and may contain a polymer. Suitable hydrophilic colloids and vinyl polymers and copolymers are described in Section IX of the Research Disclosure referred to below. The preferred hydrophilic colloid is gelatin.
The silver halide may be, for example, silver chloride, silver bromide, silver chlorobromide, silver bromoiodide, etc. Preferably, the silver halide emulsion is a high contrast silver halide emulsion, which is suitable for use in the graphic arts and in manufacturing printed circuit boards, for example, to which the present invention is particularly applicable. The silver halide emulsion is preferably a chlorobromide emulsion, preferably comprising at least 50 mol % silver chloride, more preferably 60-90 mol % silver chloride and most preferably 60-80 mol % silver chloride. The remainder of the silver halide is preferably substantially made up of silver bromide and more preferably comprises a small proportion (e.g. up to 1 or 2%) of silver iodide.
Preferably, the photosensitive material is a high silver/low gelatin photosensitive material, so that after conventional development, it is sufficiently conductive to enable direct electroplating of the metal pattern formed. In this regard, a preferred ratio of gelatin to silver in the photosensitive layer is in the range of from 0.1 to 0.7, more preferably from 0.2 to 0.6.
The silver halide emulsion may be sensitised to any suitable wavelength of the exposing radiation, as desired, but is preferably sensitised to light of the wavelengths emitted by solid state diode red light sources commonly used in imagesetters and photoplotters. Preferably, the silver halide emulsion is sensitised to light in the range 600-690 nm.
The amount of sensitising dye used in the silver halide emulsion is preferably in the range of 50 to 1000 mg per mol equivalent of silver (mg/Agmol), more preferably 100 to 600 mg/Agmol and still more preferably 150 to 500 mg/Agmol. Optionally, depending upon the photographic utility intended for the silver halide emulsion (and the grain size), it is most preferable to incorporate the sensitising dye into the silver halide emulsion in an amount of from 300 to 500 mg/Agmol.
The emulsions employed in the photographic materials described herein, and the addenda added thereto, the binders, supports, etc., may be as described in Research Disclosure Item 36544, September 1994, published by Kenneth Mason Publications, Emsworth Hants, PO10 7DQ, UK.
The silver halide emulsion may be coated onto any suitable support, which is preferably a transparent support, such as an Estar® polyethylene-terephthalate support.
The photographic materials may also contain an overcoat hydrophilic colloid layer, which may also contain a vinyl polymer or copolymer located as the last layer of the coating (furthest from the support). It may contain one or more surfactants to aid coatability and may also contain some form of matting agent. The vinyl polymer is preferably an acrylic polymer and preferably contains units derived from one or more alkyl or substituted alkyl acrylates or methacrylates, alkyl or substituted acrylamides, or acrylates or acrylamides containing a sulfonic acid group.
The photographic materials described herein preferably include an antihalation layer that may be on either side of the support, preferably on the opposite side of the support from the emulsion layer. In a preferred embodiment, an antihalation dye is contained in the hydrophilic colloid underlayer. The dye may also be dissolved in or dispersed in the underlayer. Suitable dyes are listed in the Research Disclosure above.
The silver halide emulsions may be prepared by any common method of grain growth, preferably using a balanced double run of silver nitrate and salt solutions using a feedback system designed to maintain the silver ion concentration in the growth reactor. Dopants may be introduced uniformly from start to finish of precipitation, or may be structured into regions or bands within the silver halide grains. Dopants, for example osmium dopants, ruthenium dopants, iron dopants, rhenium dopants or iridium dopants, for example cyanoruthenate dopants, may be added. Preferably a combination of osmium and iridium dopants is used, and preferably wherein the osmium dopant is an osmium nitrosyl pentachloride (especially in combination with a red-sensitising trinuclear merocyanine dye). Such complexes may alternatively be utilised as grain surface modifiers in the manner described in U.S. Pat. No. 5,385,817. Chemical sensitisation may be carried out by any of the known methods, for example with thiosulfate or other labile sulfur compound and with gold complexes. Preferably, the chemical sensitisation is carried out with thiosulfate and gold complexes.
After addition of the sensitising dye at a level to give the emulsion the desired sensitivity, antifoggants and stabilisers may be added as is known in the art. Antifoggants that may be useful include, for example, azaindenes such as tetraazaindenes, tetrazoles, benzotriazoles, imidazoles and benzimidazoles. Specific antifoggants that may be used include 5-carboxy-2-methylthio-4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene, 1-(3-acetamidophenyl)-5-mercaptotetrazole, 6-nitrobenzimidazole, 2-methylbenzimidazole and benzotriazole.
Nucleators and, preferably, development boosters may be used to give ultra-high contrast, for example combinations of hydrazine nucleators such as those disclosed in U.S. Pat. No. 6,573,021, or those hydrazine nucleators disclosed in U.S. Pat. No. 5,512,415 at col. 4, line 42 to col. 7, line 26, the disclosures of which are incorporated herein by reference. Booster compounds that may be present in the photographic material (or alternatively, in the developer solution used) include amine boosters that comprise at least one secondary or tertiary amino group and have an n-octanol/water partition coefficient (log P) of at least 1, preferably at least 3. Suitable amine boosters include those described in U.S. Pat. No. 5,512,415, col. 7, line 27 to col. 8, line 16, the disclosure of which is incorporated herein by reference. Preferred boosters are bis-tertiary amines and bis-secondary amines, preferably comprising dipropylamino groups linked by a chain of hydroxypropyl units, such as those described in U.S. Pat. No. 6,573,021. Any nucleator or booster compound utilised may be incorporated into the silver halide emulsion, or alternatively may be present in a hydrophilic colloid layer, preferably adjacent the layer containing the silver halide emulsion for which the effects of the nucleator are intended. They may, however, be distributed between or among emulsion and hydrophilic colloid layers, such as undercoat layer, interlayers and overcoat layers.
Preferably, a photosensitive silver halide material such as that described in U.S. Pat. No. 5,589,318 or U.S. Pat. No. 5,512,415 is utilised.
The preferred photographic materials are particularly suitable for exposure by red or infra-red laser diodes, light emitting diodes or gas lasers, e.g., a helium/neon laser.
The light-sensitive silver halide contained in the photographic material may be processed following exposure to form a visible image by associating the silver halide with an aqueous alkaline medium in the presence of a developing agent contained in the medium or in the material itself. The photographic material may be processed in conventional developers to obtain very high contrast images. When the material contains an incorporated developing agent, it can be processed in the presence of an activator, which may be identical to the developer in composition but lacking a developing agent.
The developers are typically aqueous solutions although organic solvents, such as diethylene glycol, can also be included to facilitate the solution of organic components. The developers contain one or a combination of conventional developing agents, such as for example, a polyhydroxybenzene such as dihydroxy-benzene, aminophenol, a paraphenylenediamine, ascorbic acid, erythorbic acid and derivatives thereof, pyrazolidone, pyrazolone, pyrimidine, dithionite and hydroxylamine.
It is preferred to employ hydroquinone and 3-pyrazolidone developing agents in combination or an ascorbic acid-based system. An auxiliary developing agent exhibiting super-additive properties may also be used. The pH of the developers can be adjusted with alkali metal hydroxides and carbonates, borax and other basic salts. It is a particular advantage that the use of nucleators as described herein reduces the sensitivity of the photographic material to changes in this developer pH.
To reduce gelatin swelling during development, compounds such as sodium sulfate can be incorporated into the developer. Chelating and sequestering agents, such as ethylenediamine tetraacetic acid or its sodium salt, can be present. Generally any conventional developer can be used in the practice of this invention. Specific illustrative photographic developers are disclosed in the Handbook of Chemistry and Physics, 36^thEdition, under the title “Photographic Formulae” at page 30001 et seq. and in “processing Chemicals and Formulas”, 6^thEdition, published by Eastman Kodak Company (1963).
The support substrate may be any suitable support substrate and may be rigid or flexible, transparent or opaque. Suitable support substrates include, for example, PET (polyethylene terephthalate), cellulose triacetate, PEN (polyethylene naphthalate) and glass.
The substrate upon which the photosensitive material utilised according to the process of the present invention may be coated depends upon the intended utility. The substrate may be rigid or flexible but is preferably flexible. For example, suitable substrates include rigid, glass-reinforced epoxy laminates, metal pads and semiconductor components, adhesive-coated polymer substrates, printed circuit board (PCB) substrates including polymer based PCBs, ceramic substrates, polymer tapes (e.g. dielectric green tape for multi-layer ceramic devices), paper, gloss art paper, bond paper, semi-synthetic paper (e.g. polyester fibre), synthetic paper (e.g. Polyart™), resin-coated paper, polymer substrates and composite materials.
Suitable polymers for use as polymer substrates include polyethylene, polypropylene, polyester, polyamide, polyimide, polysulfone and mixtures thereof. The substrate, especially a polymer substrate, may be treated to improve adhesion of the silver halide emulsion to the substrate surface. For example, the substrate may be coated with a polymer adhesive layer or the surface may be chemically treated or subjected to a corona treatment.
For coating onto a substrate in the manufacture of flexible electronic devices or components, the support is preferably flexible, which aids rapid roll-to-roll application.
The patents and publications referred to herein are incorporated by reference in their entirety.
The invention will now be described with reference to the following Examples and Fig., which are not however to be construed as limiting the scope thereof.

EXAMPLES

Example 1

A 100 μm PET support coated with an emulsion layer of 0.18 μm chemically sensitised silver chlorobromide (30% bromide) cubes with a silver laydown of 3.6 g/m²and a gelatin laydown of 1.6 g/m²was overcoated with a layer of gelatin plus surfactant to give 0.3 g/m²in this layer. There was no hardener added to the coating.

This layer was fogged to white light and processed as follows:



Developer	120 s	at 20° C. with nitrogen burst agitation
Fixer	30 s	at 20° C. with continuous air agitation
Wash in	120 s	at 15-20° C. with continuous air
running water		agitation
Dry at room
temperature

using the following processing formulations:



Developer

Sodium metabisulfite	24	g
Sodium bromide	4	g
Benzotriazole	0.2	g
1-Phenyl-5-mercaptotetrazole	0.013	g
Hydroquinone (photograde)	25	g
4-Hydroxymethyl-4-methyl-1-phenyl-3-pyrazolidone	0.8	g
Potassium sulfite,	35	g
Potassium carbonate,	20	g
Water to	1	litre

pH adjusted to 10.4 with 50% potassium hydroxide



Fixer

Ammonium thiosulfate	200	g
Sodium sulfite	20	g
Acetic acid	10	ml
Water to	1	litre

The step of electroplating the exposed and developed photosensitive element is described herein with reference to FIG. 1. A 100 ml beaker (35) was stood on a magnetic stirrer (10). A 20 mm long magnetic stirrer (20) was placed in the beaker (35) and the beaker (35) centred on the stirrer (10). Into the beaker was placed a 10×100 mm 100 μm silver foil positively-charged electrode (anode) (40), held in place with pegs (not shown), and a 15×100 mm strip of the exposed and processed photosensitive element (70) as negatively-charged electrode (cathode). The electrodes (40) and (70) were connected to a constant current power supply (100) through an ammeter (90) using crocodile clips (50) and (80) as connectors, the voltage being monitored with voltmeter (60). 75 ml of plating solution (30) was poured into the beaker (35) before a run which covered both electrodes to give a submerged area of about 500 mm².
The plating solution was as follows:

Silver plating solution

Sodium thiosulfate 0.2 molar

Sodium sulfite 0.02 molar

Silver chloride 0.1 molar
The plating was carried out at 21° C. with the current set as 10 mA and the average voltage applied to the electrode was recorded. After 20 min. the current flowing through the cell was switched off, the now-plated photographic coating washed under the tap for 2 min. and then allowed to dry at room temperature. The surface of the coating was matt silver and when viewed from the back the coating was black.
The resistance of the coating was measured before and after plating.
The results were

Before plating 30 ohm/square

After plating 0.011 ohm/square

showing that the resistance had been reduced almost 3000 fold.

Example 2

Example 1 was repeated except that copper was plated onto the imaged silver. The positively charged electrode (anode) (40) was replaced by a piece of copper foil and the following plating solutions (70) were used in separate experiments.

Copper plating solution 1

Copper (II) sulfate 20 g

Water to 1 litre



Copper plating solution 2

	Copper (II) sulfate	20 g
	Polyethylene glycol mw 200	20 g
	Water to	1 litre

Again, the plating was for 20 min. at 10 mA for both solutions. The resistances of the electroplated images were

Copper plating solution 1 1.8 ohm/square

Copper plating solution 2 1.1 ohm/square
Again, the electroplating reduced the resistance but not as effectively as did silver plating. It appeared that the polyethylene glycol (commonly added to copper-plating solutions) improved the plating.

Example 3

Example 1 was repeated except that nickel was plated onto the image silver. The positively-charged electrode (anode) (40) was replaced by a piece of nickel sheet and the following plating solution (70) was used.

Nickel plating solution

Nickel (II) sulfate 20 g

Boric acid 20 g

Water to 1 litre

pH found to be 4.44
The plating was for 10 min. at 10 mA. The resistance of the electroplated images was 4 ohm/square.

Example 4

Example 1 was repeated except that zinc was plated onto the image of silver. The positively-charged electrode (anode) (40) was replaced by a zinc stick and the following plating solution (70) was used.

Zinc plating solution

Zinc sulfate 12.5 g

Water to 1 litre
The plating was for 10 min. at 10 mA. The resistance of the electroplated image was 3 ohm/square.

Example 5

Example 1 was repeated except that the film was replaced with one that was hardened. The coating was identical to the one described, except that 2% (of total gel by weight) bis vinylmethylsulfone was added at the coating stage.
The solutions and the plating conditions were the same as in Example 1. The final resistance of the plated coating was 2.5 ohm/square. Although the resistance of the coating had been reduced in comparison with the coating which had not been electroplated, the resistance of the unhardened coating was much lower than in Example 1.
The invention has been described in detail with particular reference to preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention

Claims

1. A process for preparing a patterned electrical conductor comprising the steps of

providing a photosensitive element comprising a support substrate and a photosensitive material coated onto said support, said photosensitive material being capable of providing a latent image on exposure to sensitising radiation;

exposing said photosensitive element to sensitising radiation according to a desired conductive track pattern to form a latent image on said photosensitive element;

developing said latent image to form a developed image formed by a first metal corresponding to the desired conductive track pattern, said image formed by said first metal being capable of conducting when a voltage is applied across it; and

electroplating said developed metal image with a plating of a second metal to improve the conductivity of said developed metal image to form a conductive track according to the desired pattern.

2. The process of claim 1, wherein said photosensitive material comprises a silver halide emulsion in gelatin and said first metal is silver.

3. The process of claim 1, wherein said electroplating step comprises applying a voltage across said developed metal image in the presence of a solution of a salt of said second metal.

4. The process of claim 3, wherein said second metal is selected from silver, gold, zinc, lead, copper or nickel.

5. The process of claim 4, wherein said second metal is silver.

6. The process of claim 3, wherein said electroplating step comprises applying a voltage across said developed metal image in the presence of a solution of a silver thiosulfate complex.

7. The process of claim 6, wherein said silver thiosulfate solution is present in a concentration of from 0.01 to 0.1 molar.

8. A patterned electrically conductive element comprising a conductive track pattern on a support substrate, said element being obtainable by the process of

providing a photosensitive element comprising said support substrate and a photosensitive material coated onto said support, said photosensitive material being capable of providing a latent image on exposure to sensitising radiation;

9. The element of claim 8, wherein said photosensitive material comprises a silver halide emulsion in gelatin and said first metal is silver.

10. The element of claim 9, wherein said photosensitive material comprises a silver halide emulsion in gelatin, wherein said silver halide is present in an amount of at least 1 g/m².

11. The element of claim 8, wherein said second metal is selected from silver, gold, zinc, lead, copper or nickel.

12. The element of claim 11, wherein said second metal is silver.