TREATMENT OF SUBSTRATES

Abstract: A method of etching, plating or applying solder to a substrate by the steps of: (A) forming a patterned image of a cured resist material upon the substrate, and (B) etching, plating or applying solder to portions of the substrate not protected by the resist, in which method the patterned resist image is formed from a liquid coating comprising: (i) water as a liquid carrier; (ii) a curable component, affording a film-forming material on curing, dispersed or emulsified in the water; and (iii) a flexibilising agent comprising an elastomeric polymer (or precursor therefor), dispersed or emulsified in the water.

Description: TREATMENT OF SUBSTRATES This invention is concerned with improvements in and relating to the treatment of substrates. The invention is particularly concerned with the use of curable coating compositions especially, but not exclusively, for use in the manufacture of printed circuit boards and the like. Curable compositions are widely used in the electronics industry, in the manufacture of printed circuit boards and the like, as so-called "resists", that is compositions which are applied in patterned form to a substrate to facilitate and define the area of operation of, subsequent processes such as plating, etching, the application of solder etc. The composition may be applied to the substrate in patterned form by a suitable printing process, especially a screen printing process, and then cured on the substrate. Alternatively, a so-called "photoresist" may be employed, that is a photocurable composition which is applied over the surface of the substrate, imagewise exposed to appropriate radiation through a patterned mask to cure portions of the resist exposed to the radiation, and later "developed" (by removal of uncured portions of the material) to form a patterned image of cured material upon the substrate. Photocurable compositions generally comprise a photocurable material, that is a material capable of curing or polymerising upon exposure to the electromagnetic radiation, typically an ethylenically unsaturated or polyethylenically unsaturated material. Where, as is most common, curing is designed to be effected by exposure to ultraviolet light, the composition will also generally comprise a photosensitizer or photoinitiator, for initiating curing or polymerisation of the photosensitive material. In the past, the substrates upon which printed circuits were formed were relatively rigid, e.g. took the form of phenolic laminate boards. More recently, however, there have come into use flexible substrates, such as those made of polyimide resins. Clearly, for better compatibility between resin and flexible substrate, the latter, when cured, should, to some extent, be flexible. It has been proposed to improve the flexibility of cured resins by including a flexibilising material in the resist formulation. It has now been found, in accordance with the present invention, that a flexibilising component may be readily and effectively included in a coating composition, such as a resist composition, in the form of an emulsion or dispersion of the material in water, which water also serves as a carrier for the curable component of the composition. According to the invention, therefore, there is provided a method of etching, plating or applying solder to a substrate by the steps of: (A) forming a patterned image of cured resist mterial upon the substrate, and (B) etching, plating or applying solder to portions of the substrate not protected by the resist, in which method the patterned resist image is formed from a liquid coating composition comprising: (i) water as liquid carrier; (ii) a curable component, affording a film-forming material on curing, dispersed or emulsified in the water; (iii) a flexibilising agent comprising an elastomeric polymer (or precursor therefor) , dispersed or emulsified in the water. In accordance with a preferred embodiment of the invention, the patterned resist image is formed from a liquid photocurable composition by the steps of: (a) forming a layer of the composition upon the substrate, (b) imagewise exposing the layer to radiation whereby to cure portions of the layer exposed to that radiation; and (c) developing the exposed layer by removing unexposed portions of the layer with an appropriate solvent. By the term "dispersed" as used herein we intend to refer to discrete solid particles of a water-insoluble material suspended in the water with, or without, the aid of a dispersing agent. Typically such particles may have a particle size of from 0.1 to 20 μm, more preferably 0.15 to 5 μm. By the term "emulsified" as used herein we intend to refer to emulsions of water-imiscible liquid materials in the water, which liquid may comprise a resinous material (e.g. curable material or flexibilising agent) itself or a solution thereof in a water-immiscible solvent. The curable component of the compositions used in accordance with the invention (hereinafter, simply, the composition of the invention) may be any film-forming curable material but is preferably a photocurable material. We have, in our patent applications Nos. PCT/GB91/01046 and PCT/GB92/00451, proposed the use of liquid photocurable compositions in which the liquid carrier comprises or consists of water. Thus, in our application No. PCT/GB91/01046, we have proprosed the use of aqueous emulsions of liquid organic material comprising a photocurable component and, in our application no. PCT/GB92/00451, we have proposed the use of aqueous dispersions of solid organic material comprising a photocurable component. In our British patent application no. 9120695.3, we have proposed the use of two-part compositions affording an aqueous dispersion or aqueous emulsion of a photocurable composition. Basically, two methods are available for forming a dispersion or emulsion of flexibilising agent in the composition, namely: (a) dispersion of a cured (generally solid) flexibilising agent in the aqueous phase of the composition before or after incorporation of the curable resin component; or (b) dispersion of an uncured precursor for flexibiliser in the emulsion, followed by subsequent curing of that precursor to form cured flexibilising agent dispersed or emulsified in the composition. The first method can be simply carried out by mixing a suspension, e.g. a colloidal suspension, of the flexibilising agent into the aqueous phase of the composition (commonly in the form of a dispersion or emulsion of the film-forming curable resin) or by dissolving the flexibilising agent in a suitable water-immiscible solvent and emulsifying this solution with water and adding the emulsion to the composition. Alternatively a thermoplastic flexibilising agent may be melted, emulsified in molten form and the resultant emulsion then mixed into the composition once it has cooled and solidifed. The second method, in situ formation of the flexibilising agent, generally allows for a greater range of possibilities for producing the emulsified or dispersed flexibilising agent. Thus, for example, a moisture-curing material can be used which will cure, once the material has been emulsified, under the influence of the aqueous phase of the composition. Similarly, a two-pack flexibilising agent (typically comprising an oligomeric precursor together with a separate curing catalyst) can be added and then quickly emulsified (if it is a room temperature curing material) or emulsified and then heated to cure it (if it is an elevated temperature curing material) . A one-pack-heat curing flexibilising agent can also be emulsified and heated to give cured flexibilising agent particles in this way. In a further variant, there may be separately emulsified the oligomeric precursor and catalyst part of a two-pack flexibilising material, and the two then allowed to react during final processing of the coating composition, that is after drying. It is also possible to use a flexibiliser which is cured during initial U.V\ exposure of the resist. It is also possible to produce a solid suspension of a flexibilising agent by mixing two or more reactive oligomers together and emulsifying the mixture before reaction renders the composition solid. Finally, a flexibilising agent which is capable of reacting with polymeric curable material in the composition may also be emulsified and mixed into the composition to give a flexibilised material during the course of processing of the composition. A wide variety of flexibilising materials may be used in accordance with the invention. Thus, examples of pre-formed solid flexibilising agents include aqueous dispersions of elastomeric materials such as an aqueous dispersion of polychloroprene sold under the trade name Neoprene 115 (DuPont) . Similarly, solutions of pre-formed elastomeric materials (such as Neoprene AC or Neoprene D) in suitable water-immiscible solvents (such as toluene, xylene, or carbitol) or a solution of a silicone resin in xylene (SR 98 sold by General Electric Company) may be dispersed to give stable emulsions. An example of a solid flexibilising agent which may be first melted and then dispersed when hot is a polybutylene rubber sold under the trademark polybutene-H1900 (Amoco Chemical Company) . Similarly, a wide variety of precursors maybe used to form dispersions or emulsions of flexibilising agents in situ in a coating composition. Thus, for example moisture-curing flexibilisers which have proved suitable include moisture-curing silicone rubber such as that sold under the trade designation T84 (Wacker Chemicals) and a moisture-curing silicone sold under the trade name Dow Corning 1200. Two-pack flexibilisers which can be used may be either ambient temperature or high temperature curing systems. Examples of the former include the product sold under the trade Neoprene AF, a copolymer of chloroprene with methacrylic acid which is mixed with a metal oxide to induce cross-linking; the products sold under the trade names RTV 615 and RTV 41 (GEC) which are both silicone rubber compositions and the product RTV 6166 (GEC) which is silicone dielectric gel. Examples of thermally curing two-pack systems include solvent free silicone resins whose cure is accelerated using metal salts such as sold under the trade name Triplus 177 and 178 (GEC) , a silicone-based adhesive cured using benzyl peroxide such as sold under the trade name PSA 529 (GEC) , or the polychloroprene product Neoprene AD which may be cured using magnesium oxide. One-pack thermally curing flexibilising agents include the silicone material sold under the trade designations-SR 224 (GEC) which is a silicone insulating varnish and those sold under the trade designations ECC 440 and ECC 450 (GEC) which are elastomeric silicone materials. Such thermally curing flexibilising agents may be cured before inclusion in the final coating composition or, where the subsequent processing conditions of the composition are appropriate, curing of the flexibilising agent may take place during processing of the composition. It is also possible to separately emulsify a thermally curing flexibiliser precursor and catalyst therefor so that cross-linking does not occur until the composition is dried and the two materials are mixed. A suspension of a solid flexibilising agent can also be produced by mixing two or more reactive oligomeric precursors prior to emulsification and then converting the " liquid or semi-solid mixture to a flexibilising solid material by a chain extension or cross-linking reaction. Materials which have a low reactivity with, or solubility in, water are preferably used in order to ensure the desired degree of reaction within the organic phase, but it is possible to use water-reactive materials providing that they have a relatively low miscibility with water and that their reactivity with the other organic components is higher than that with water. This can be attained by judicious use of catalysts. Thus, polyurethane elastomer emulsions can be obtained by dispersing castor oil or polyester mixed with di- and polyisocyanates; epoxidised rubber materials can be cross-linked with reactive polyamides or acid functional resin. Flexibilising agents which can be emulsified and which may then react with the curable resin of the coating to become chemically bonded into the final film include a photoreactive polyisoprene sold under the trade Waycoat (Olin-Hunt Speciality Products Inc.); a monoepoxidised polybutene (sold under the designation E23 (Amoco Chemicals) or an epoxidised bis-phenol-A and rubber modified linseed oil/fatty acid material sold under the trade name XZ 86709 (Dow Chemicals Ltd) . The flexibilising agent suitably forms from 5 to 40% by weight, preferably from 10 to 25% by weight of the total solids in the composition. As will be appreciated, a wide variety of organic photocurable systems may be used in the compositions of the invention. Thus, for example, in the case of a composition in accordance with the invention taking the form of an aqueous emulsion of a liquid organic component, one general class of photocurable system comprises a curable reactive material (generally oligomeric or polymeric in nature) together with an initiator component which on exposure to the appropriate radiation, reacts with the curable component thereby to cross-link it or cure it. A particular embodiment of this sort of photocurable system comprises polyvinyl alcohol as curable component together with a diazo initiator. Another general class of photocurable system, which is also particularly suitable for use for compositions taking the form of suspensions or dispersions of solid powdered photocurable material, comprises an ethylenically unsaturated curable material (again generally oligomeric or polymeric in nature) together with a photoinitiator which, on exposure to the appropriate radiation, gives rise to free radicals which initiate polymerisation of the double bonds in the curable component. Examples of photocurable materials which may be used in this second general class of photocurable systems include include multifunctional acrylate oligomers such as pentaerythritol triacrylate, trimethylolpropane triacrylate and ethylene glycol polyacrylate. Other photosensitive materials are those which may be obtained by reacting multifunctional isocyanate compounds with ethylenically unsaturated compounds containing a group containing an active hydrogen atom such as a hydroxyl group or carboxylic acid group. Examples of suitable isocyanates include hexamethylene di-isocyanate, tolylene di-isocyanate or isophorone di-isocyanate, or di ers or trimers formed therefrom. Suitable unsaturated compounds containing active hydrogen include, for example, hydroxyl-ethyl acrylate, hydroxyethyl methacrylate, acrylic acid and methacrylic acid. A further class of UV sensitive curable materials are those formed by the reaction of poly-epoxy compounds (so-called "epoxy resins") with ethylenically unsaturated acids such as acrylic acid or methacrylic acid; which reaction products may simply be termed "epoxy acrylates". The epoxy compound may be a simple glycidyl ether such as ethylene glycol diglycidyl ether or phenyl glycidyl ether; or a bis-phenol A/epichlorohydrin adduct such as those sold under the trade name "EPI OTE". Further epoxy resins which may be employed are epoxy novolak resins (including epoxy phenyl novolak and epoxy cresol novolak resins) such as those sold under the trade name "QUATREX". The "epoxy acrylates" derived from such resins are particularly suitable for use as components in photoresists for preparing solder masks. In order to render the epoxy acrylate material soluble or developable by aqueous alkaline solutions, the epoxy acrylate resin may be reacted with one or more dicarboxylic acid anhydrides (serving to introduce free carboxyl groups into the final epoxy acrylate) . Suitable dicarboxylic acid anhydrides for this purpose include succinic, itaconic, maleic and phthalic anhydrides. A wide variety of photoinitiators are known for use in photocurable systems and examples of these include anthraquinones such 2-ethyl-anthraquinone, 2-methyl- anthraquinone and 1-chloro-anthraquinone; thioxanthones such as 2,4-dimethyl-thioxanthones, 2,4-diethyl- thioxanthones and 2-chloro-thioxanthones; ketals such as benzyl-dimethyl ketal and acetophenone-dimethyl-ketyl, benzophenones, and benzoin and ethers thereof. These photoinitiators can be alone or in admixture and may also be used together with photopolymerization accelerators such as benzoic acid type accelerators or tertiary amine accelerators. In addition to the photocurable system, the photocurable material may also contain other components such as inorganic fillers, pigments, rheological additives (flow aids and degassing agents) and thermal curing agents and surfactants. Surfactants or thickening agents may serve to assist in suspending the particulate material in the composition. Suitable surfactants include nonionic surfactants such as those sold under the trade names Synperonic PE/F 108, Pluronic 6800 and Disponsil 23. Suitable thickeners include acrylic polymers such as Acrysol RM 8 (Rohm & Haas) and FX1010 (Servo Delden B.V.) . Photocurable compositions in accordance with the invention may be employed simply as photocurable coating materials or, as indicated above, find particular application as photoresist materials for use in the electronics industry, for example in the formation or production of printed circuit boards or the like. Thus, the photocurable compositions of the invention are particularly suitable for use in a method of forming a patterned image upon a substrate which method comprises the steps of: (a) forming a layer of the liquid photocurable composition upon the substrate; (b) drying the layer of photocurable composition on the substrate; (c) imagewise exposing the dried layer to electromagnetic radiation to cure portions of the layer exposed to the radiation; and (d) "developing" the exposed layer by removing unexposed portions of the layer. The patterned layers or resists obtained by the above route may be used, as indicated above, as lithographic resists, plating resists, etch resists or solder masks. The liquid photocurable composition may be applied to the substrate by any suitable coating method such as, for example, by screen printing, curtain coating or, electrostatic spraying. The thickness of the resist layer will, of course, depend upon the intended end use but, in general, thicknesses of the order of 5 - 60 μm are generally suitable. In order that the invention may be well understood the following Examples are given by way of illustration only. In the Examples all percentages are by weight unless otherwise stated. EXAMPLE 1 A carboxylated epoxy cresol novolak acrylate was prepared by reacting an epoxy cresol novolak resin (Quatrex 3710, Dow Chemical Co.) (1 equivalent) with one equivalent of acrylic acid in ethyl ethoxy propionate solvent. The resultant acrylated resin was then adducted with a 50:50 mixture of maleic anhydride and tetrahydrophthalic anhydride sufficient to give an acid value of 70 mg/g KOH at the end of reaction. This material was then included in the following ink formulation: % carboxylated epoxy novolak acrylate 57 IRGACURE 907 (Ciba-Geigy) 4 Tri- (2)hydroxyethyl)isocyanurate triacrylate 6 Di-trimethylol-propane tetraacrylate 1 Barium sulphate 30 Atlas G1350 (ICI Speciality Chemicals) 1 Quantacure ITX (Ward Blenkinsop) This mixture was thoroughly mixed on a Diaf high speed dispersion stirrer to give an opaque straw coloured viscous liquid which was then emulsified using a Silverson High Speed mixer in the following mixture. % Ink (as above) 64 Poly(vinylpyrrolidone) 3 Acrysol RM8 (Rohm & Haas) 4 Phthalocyanin Green Pigment Dispersion l Distilled Water 28 This produced an opaque, light green coloured, stable emulsion which was mixed, by hand, with a latex of neoprene rubber (Neoprene 115 DuPont Chemicals) in the following ratio. % emulsion ink 85 Neoprene 115 15 This mixture was then screen printed onto an IPC solder resist test panel on polyimide flexible laminate. The film produced was dried at 90°C for 30 minutes to give a smooth, tack-free film. The resist film was then imaged, through suitable artwork, and that image developed using 0.6% w/w sodium carbonate solution. The film was then given a final bake of one hour at 150°C to yield a coating which displayed excellent adhesion to both copper tracks and polyimide substrate and resistance to creasing (i.e. it did not crack when bent over a 1 mm diameter mandrel) . It also showed good solder and solvent resistance properties. EXAMPLE II The basic ink emulsion produced in Example 1 (before addition of the Neoprene 115) was taken and T-84 (a moisture curing silicone rubber material (Wacker Chemicals) poured into it under high speed stirring in the ratio of 4 parts ink emulsion to 1 part of T-84. The material dispersed well to give a yellow green emulsion, which was draw down bar coated onto an etched copper printed circuit board on polyimide flexible laminate. The film was dried at 90°C for 30 minutes, exposed through suitable artwork and developed in 0.6% w/w sodium carbonate solution. The imaged film was then given a post bake of one-and-a-half hours at 140°C. The resulting film showed excellent solder, crease and solvent resistance and showed excellent resistance to ten day thermal shock testing (coating is subjected to constant cycling between -50°±140°C) without cracking or flaking. EXAMPLE III The following formulation was mixed together using a 3-roll mill. % Epoxy resin (DEN 438, Dow Chemicals) 47 Talc 24 Phthalocyanin Green 2 Silicone antifoam 1 Butyl cellosolve 23 Synperonic PE/108 3 This mixture was then emulsified using a Silverson high speed stirrer in the following formulation. % Ink (as above) 75 Distilled water 25 A thermal curing agent dispersion for use with the ink emulsion was manufactured by mixing the following materials. Epoxy resin (M51914, Ciba-Geigy) 34 Diamino diphenyl methane 26 Propylene glycol ethylether 32 Propylene glycol methyl ether acetate 8 The mixture was emulsified in the following formulation: % Thermal curing agent 73 Acrysol RM8 (Rohm & Haas) 7 Distilled water 20 These two emulsions were then mixed in a ratio of two parts ink to one part curing agent. A two-pack thermally curing silicone rubber compound (RTV 615 and catalyst, GEC) was emulsified in its mixed state in the following formulation. % RTV 615 45 Catalyst 5 Acrysol RM8 (Rohm & Haas) 6 Distilled water 44 This was then mixed in a ratio of four parts resist emulsion to one part rubber emulsion to give a thermally curing solder resist. The emulsion was screen printed onto an IPC solder resist test circuit on polyimide flexible laminate and cured by baking at 140°C for 30 minutes. The resultant film showed excellent solder, scratch and solvent resistance and was resistant to the 1 mm mandrel bend test. An unflexibilised sample of the resist processed in a similar fashion displayed cracking and loss of adhesion on this test. EXAMPLE IV The procedure of Example III was followed but the silicone rubber and cure catalyst were emulsified separately in the following formulation. % RTV615/catalyst 50 Acrysol RM8 6 Distilled water 44 The RTV615 emulsion was then mixed in the ratio 10:1 with the catalyst emulsion, the resulting mixed emulsion was added to the thermally curing solder resist emulsion described in Example III in the ratio of 4 parts resist emulsion to 1 part rubber emulsion. The material was processed as described in the Example III to yield a film with the same performance parameters as obtained in Example III. EXAMPLE V A solid carboxylated epoxy cresol novolak was prepared by reacting one equivalent of acrylic acid with one equivalent of epoxy cresol novolak (Quatrex 3410, Dow Chemicals) dissolved in toluene. Once acrylated the resin was carboxylated using maleic and tetrahydrophthalic anhydrides (in a 1:1 ratio) and the solvent was vacuum stripped to yield a solid resin with a melting point of between 95°-100°C and an acid value of 65 mg/gKOH. This resin was crushed and powdered and mixed in the following formulation. % Solid carboxylated epoxy cresol novolak 83 Irgacure 907 (Ciba-Geigy) 7 Quantacure ITX (Ward Blenkinsop) 2 Talc 7 Cure catalyst 1 The cure catalyst was made up as follows: % 2-ethyl-4-methyl imidazole 80 Dicyandiamide 20 This mixture was mixed into a homogenous solid, using a heated twin screw extruder, which was then ground to around 25 micron particle size using a blade grinder. The resultant green powder was then added to a solution of surfactant, thickener and humectant in distilled water following the formulation: % Powdered resist 49 Atlas G1350 (ICI Speciality Chemicals) 3 Acrysol RM8 (Rohm & Haas) 6 Poly (vinyl pyrrolidone) 2 Distilled water 40 The slurry was then ball milled for 12 hours to produce a suspension with solid particle size in the area of 5 microns. A solid cross-linking agent was then prepared by grinding a solid epoxy novolak material (Quatrex 2410, Dow Chemicals) down to a particle size of around 20 microns, suspending this in the formulation shown below and ball milling the slurry to give a stable suspension with particle size substantially below 10 microns. % Epoxy novolak resin 49 Atlas G1350 3 Acrysol RM8 6 Poly(vinyl pyrrolidone) 2 Distilled water 40 A flexible material was manufactured by melting a reactive polyamide (Versamid 100, Cray Valley) (2 parts) and mixing with XZ86709, A bis-phenol a epoxy linseed oil modified rubber (3 parts) , the mixture was further heated to make it liquid enough to emulsify. The material was then poured, with high speed stirring into the following formulation to give a stable off-white coloured emulsion. % Versamid/XZ mixture 26 Acrysol RM8 (Rohm & Haas) 8 Distilled water 66 The original resist formulation was mixed with the cross-linking suspension in the ratio of 1 part cross-linker to 4 parts resist emulsion and to this was added the flexibiliser suspension in the ratio 1 part flexibiliser to 3 parts mixed resist suspension. This mixture was screen printed onto an IPC solder resist test circuit on flexible polyimied laminate, dried at 90°C for 30 minutes, photoimaged through suitable art work, developed in 1% w/w% sodium carbonate solution and post-baked for one and a half hours at 150°C. The resulting solder resist coating showed good resolution of the art work pattern and excellent solder, flex and solvent resistance as well as good adhesion to the substrate. EXAMPLE VI A carboxylated epoxycresol novolak acrylate in ethyl ethoxy propionate (prepared as described in Example l) was dispersed by high speed stirring on a Silverson stirrer in the following formulation. % Carboxylated epoxy cresol novolak acrylate 79 Irgacure 907 (Ciba-Geigy) 4 Quantacure ITX (Ward Blenkinsop) 1 Di-trimethylolpropane triacrylate l Tris(2-hydroxyethyl)isocyanurate 7 Talc 4 Synperonic PE/108 (ICI Chemicals) 2 Methylene Blue Dye 2 The ink formulation was then emulsified in the following formulation: % Ink 44 FX1010 (Servo Delden B.V.) 2 Poly(vinylpyrrolidone) 2 Distilled water 52 A flexibilising emulsion for this ink was formulated using Waycoat, a UV sensitive polyisoprene rubber (Olin-Hunt) . % Waycoat (Olin-Hunt) 50 Acrysol RM8 8 Distilled Water 42 The resist emulsion was mixed with the flexibiliser emulsion in the ratio 7:3 by hand and the resulting light blue emulsion was screen printed onto precleaned two ounce copper clad FR4 laminate. The film was dried at 90°C for 15 minutes to produce a homogeneous tack-free film; which was photoimaged through artwork prepared for imaging a negative primary imaging resist. The circuit pattern was developed using 1% w/w sodium carbonate solution and the copper areas not protected by the resist were etched away using acid ferric chloride etchant. Once the circuit had been etched the resist was stripped to give a circuit which was precise reproduction of the art work pattern. During the etch process this formulation showed excellent resistance to the etch solution and good flexibility, none of the resist being chipped away or lost by the pressure of the etchant sprays even on very fine details.

Claims: CLAIMS : 1. A method of etching, plating or applying solder to a substrate by the steps of: (A) forming a patterned image of a cured resist material upon the substrate, and (B) etching, plating or applying solder to portions of the substrate not protected by the resist, in which method the patterned resist image is formed from a liquid coating comprising : - (i) water as a liquid carrier; (ii) a curable component, affording a film-forming material on curing, dispersed or emulsified in the water; and (iii) a flexibilising agent comprising an elastomeric polymer (or precursor therefor) , dispersed or emulsified in the water. 2. A method as claimed in claim 1 in which the patterned resist image is formed from a liquid photocurable coating compositon by the steps of: (a) forming a layer of the composition upon the substrate; (b) imagewise exposing the layer to radiation whereby to cure portions of the layer exposed to that radiation; and (c) developing the exposed layer by removing unexposed portion of the layer with an appropriate solvent.

Inventors: THATCHER WRENFORD JOHN, THATCHER, WRENFORD, JOHN
Applicants: COATES BROTHERS PLC, THATCHER WRENFORD JOHN
Application year: 1993
Main classification: H05K3
IPC codes: G03F7, H05K3