Photopolymerizable compositions

Abstract: A photopolymerizable composition composed principally of an acrylic linear copolymer, an ethylenically-unsaturated compound and a photoinitiator and adapted to fabricate printed circuit boards. The ethylenically-unsaturated compound contains a specific acrylic ester. The photopolymerizable composition can be developed with an aqueous mild alkaline solution.

Description: The invention relates to a process for preparing a printed circuit board. Printed circuit boards are made using a photopolymerizable composition. Printed circuit boards are formed in the following manner: For example the copper surface of a copper coated substrate is first of all coated with a composition which is a solution of a photopolymerizable resin or a previously formed photopolymerizable film is laminated on the copper surface of a copper coated substrate, whereby a resist film is formed. This resist film is next exposed imagewise to radiation and thereafter developed with a solvent or an aqueous alkaline solution forming a resist pattern on the copper coated substrate. Copper surface areas not protected by a resist pattern are subjected to selective etching, metal plating or the like to fabricate a printed circuit board. To improve the working efficieny, air pollution problem and yield, dry film resists have recently been used. The dry film resists can be developed with an aqueous alkaline solution. There are flexible three-layered laminates formed of a base film layer, a photopolymerizable resin layer and a protective film. Photopolymerizable compositions that can be developed with an aqueous alkaline solution generally are composed of an acrylic copolymer containing carboxyl, amino or hydroxyl groups, a photopolymerizable monomer and a photoinitiator. Such photopolymerizable compositions are etched and removed with an aqueous strong alkaline solution. Known photoresist materials comprise a photosensitive material inserted between two resin films. In such three layered photoresist materials one of the resin films, the so called protective film, is stripped off to expose the photosensitive material upon use. After having been laminated on a substrate for printed circuits, example given a copper coated plate, the photosensitive material is imagewise exposed to radiation through the remaining resin film. This so called base film is then removed in the photosensitive layer developed by a suitable developer such as an aqueous alkaline solution, etc. Such a process is described in US-A-4157 261. The polymerized photoresist film must finally be removed by an organic solvent such as ethyl, ethyl ketone or trichloroethane. GB-A-2 171 107 discloses a similar photopolymerizable composition wherein the hardened photoresist is removed by an organic solvent. So the prior art teaches photopolymerizable compositions which in a polymerized state are etched and/or removed with aqueous strong alkaline solution and/or organic solvent. Further the etching of the known material was often very time consuming. As consequence, the copper wiring or solder plating layer provided over the copper wiring was easily corroded with the strongly alkaline etchant during the etching time so that the wiring pattern could be partly destroyed. If a solder plating was employed as an etching mask the copper surface area was corroded by the etchant, and the solder plating could not longer be used as an etching mask. It is therefore subject of the invention to provide a process for preparing printed circuit boards by means of photo-polymerizable compositions wherein the use of environmentally hazardous material may be avoided and corrosion problems can be avoided. The problem is solved by the invention as may be seen from claim 1. The above mentioned components will hereinafter be described in detail. [Component (a)] The component (a) of the composition used in the inventive process is an acrylic linear copolymer and can be obtained by copolymerizing two or more monomers. As exemplary monomers permitting such copolymerization, the following compounds may be mentioned: methyl acrylate; methyl mathacrylate; ethyl acrylate; ethyl methacrylate; butyl acrylate; butyl methacrylate; acrylic acid; methacrylic acid; 2-hydroxyethyl acrylate; 2-hydroxyethyl methacrylate; 2-hydroxypropyl acrylate; 2-hydroxypropyl methacrylate; glycidyl acrylate; glycidyl methacrylate; tert-butylaminoethyl acrylate; tert-butylaminoethyl methacrylate; 2,3-dibromopropyl acrylate; 2,3-dibromopropyl methacrylate; 3-chloro-2-hydroxypropyl acrylate; 3-chloro-2-hydroxypropyl methacrylate; tetrahydrofurfuryl acrylate; tetrahydrofurfuryl methacrylate; tribromophenyl acrylate; tribromophenyl methacrylate; acrylamide; methacrylamide; acrylonitrile; methacrylonitrile; styrene; α-methylstyrene; vinyltoluene; and butadiene. In the present invention, the component (a) may desirably have a weight average molecular weight of 30,000-200,000 and a glass transition point of 20-80°C. If the weight average molecular weight is smaller than the lower limit, the resulting resist pattern has insufficient strength. On the other hand, any weight average molecular weights greater than the upper limit lead to poor adhesion to the substrate. If the glass transition point is lower than the lower limit, a layer of the resulting photopolymerizable resin formed into a film may run out even at room temperature so that the photopolymerizable resin cannot be used as a dry film resist. If the glass transition point is higher than the upper limit, the resulting photopolymerizable resin cannot fill up concave portions of a rough substrate upon application of the photopolymerizable resin on the substrate. Any weight average molecular weight and glass transition point outside their respective ranges mentioned above are therefore not preferred. [Component (b)] The component (b) in the composition used in the process of this invention is an ethylenically-unsaturated compound which forms a polymer under the action of a photoinitiator. It contains, as an essential component, 10-50 wt.%, based on the total amount of the component (b), of an acrylic ester represented by the following formula: wherein X means a hydrogen atom or methyl group, Y denotes an alkylene group having 1-3 carbon atoms, and Z stands for a direct bond, an alkylene group having 1-8 carbon atoms or a phenylene or cyclohexylene group. The following compounds may be mentioned as illustrative examples of the acrylic ester, although not necessarily limited thereto: CH₂=CH-COO-CH₂-CH₂-OCO-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-CH₂-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₂-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₃-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₄-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₅-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₆-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₇-COOH; CH₂=CH-COO-CH₂-CH₂-OCO-(CH₂)₈-COOH; As an exemplary process for the synthesis of such an acrylic ester, the acrylic ester can be obtained as a dicarboxylate monoester by conducting an esterification reaction of an acrylic or methacrylic acid and an alkanediol to form a hydroxyalkyl acrylate or methacrylate and then subjecting the hydroxyalkyl acrylate or methacrylate to an esterification reaction with a dicarboxylic acid. Exemplary alkanediols useful as a reactant in the above process may include dihydroxymethane, ethylene glycol, propylene glycol, and 1,3-propanediol. Illustrative of the dicarboxylic acid may include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, terephthalic acid, and cyclohexanedicarboxylic acid. The above acrylic ester is used in combination with at least one of other ethylenically-unsaturated compounds to make up the component (b). As illustrative examples of such a compound, may be mentioned acrylic esters other than those described above, methacrylic esters, acrylic amides, methacrylic amides, allyl compounds, vinyl ether compounds, and vinyl ester compounds. As exemplary alcohols useful to form an acrylic or methacrylic ester, may be mentioned propyl alcohol, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, butanediol, and trimethylolethane. As exemplary acrylic amides and methacrylic amides, may be mentioned methylenebis(acrylamide), methylenebis(methacrylamide), polyamines such as ethylenediamine and hexamethylenediamine, and acid amides with acrylic acid or methacrylic acid. Illustrative allyl compounds may include the diallyl esters of phthalic acid, adipic acid, malonic acid and the like. Vinyl ether compounds are the polyvinyl ethers of polyhydric alcohols. As exemplary polyhydric alcohols, may be mentioned ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, neopentyl glycol, trimethylolpropane, pentaerythritol, butanediol, and trimethylolethane. As vinyl ester compounds, may be mentioned divinyl succinate, divinyl adipate and divinyl phthalate by way of example. The proportion of the ethylenically-unsaturated compound in the photopolymerizable composition may desirably range from 10 parts by weight to 100 parts by weight per 100 parts by weight of the component (a). [Component (c)] The component (c) in the composition used in the process of this invention is a photoinitiator capable of triggering the polymerization of the ethylenically-unsaturated compound, namely, the component (b) upon exposure to radiation. Illustrative examples of the component (c), may be mentioned anthraquinone and its derivatives such as 2-methylanthraquinone and 2-ethylanthraquinone; benzoine and its derivatives such as benzoine methyl ether; benzophenone; phenanthrene; and 4,4′-bis(dimethylamino)benzophenone. These photoinitiators may be used either singly or in combination. The photopolymerizable composition may desirably contain the photoinitiator in a proportion of from 1 part by weight to 15 parts by weight per 100 parts by weight of the component (a). [Optional Components] In addition to the essential components described above, the photopolymerizable composition may also contain one or more of heat polymerization inhibitor, dye, pigment, coatability improver, flame-retardant agent, flame-retardant aid and the like as optional components. They may be chosen in accordance with the same standard as for conventional photopolymerizable compositions. [Preparation Method and Application Method] The photopolymerizable composition can be prepared by either dissolving or dispersing the individual components in a suitable solvent. As typical examples of the suitable solvent, may be mentioned methyl ethyl ketone, ethyl acetate, toluene, methylene chloride and trichloroethane although not necessarily limited thereto. Upon application of the photopolymerizable composition, a copper-coated substrate is coated directly with a solution of the photopolymerizable composition in such a solvent as mentioned above, followed by drying to form a resist film. As an alternative, a polyester film as an exemplary base film is coated with the above solution, followed by drying to form a resist film. The resist film is laminated under heat on a copper-coated substrate. The resist film is exposed selectively to radiation and then developed to form a resist pattern on the substrate. Copper surface areas not protected by the resist pattern are subjected to selective etching, metal plating or the like to fabricate a printed circuit board. [Developer] The developer employed after the selective exposure of the photopolymerizable composition is an aqueous mild alkaline solution. As exemplary bases useful for the preparation of the aqueous mild alkaline solution, may be mentioned: alkali metal hydroxides, namely, lithium hydroxide, sodium hydroxide and potassium hydroxide; alkali metal carbonates and bicarbonates, namely, lithium carbonate and bicarbonate, sodium carbonate and bicarbonate, and potassium carbonate and bicarbonate; alkali metal phosphates such as sodium phosphate and potassium phosphate; alkali metal pyrophosphates such as sodium pyrophosphate and potassium pyrophosphate; primary amines such as benzylamine and butylamine; secondary amines such as dimethylamine, benzylmethylamine and diethanolamine; tertiary amines such as trimethylamine, triethylamine and triethanolaine; cyclic amines such as morpholine, piperazine, piperidine and pyridine; polyamines such as ethylenediamine and hexamethylenediamine; ammonium hydroxides such as tetramethylammonium hydroxide, tetraethylammonium hydroxide, trimethylbenzylammonium hydroxide, trimethylphenylammonium hydroxide; and sulfonium hydroxides such as trimethylsulfonium hydroxide, diethylmethylsulfonium hydroxide and dimethylbenzylsulfonium hydroxide. [Etchant] As a base useful for the preparation of an aqueous strong alkaline solution employed as an etchant, it is possible to use the same base as that employed for the preparation of a developer. DESCRIPTION OF THE PREFERRED EMBODIMENTS To illustrate the present invention in more detail, certain examples of this invention will hereinafter be described. Examples 1-8 & Comparative Examples 1-5: Photopolymerizable compositions of the below-described Formulation 1 were prepared respectively by using the ethylenically-unsaturated compounds shown in Table 1. Polyethylene terephthalate films of 25 µm thick was uniformly coated with the above photopolymerizable compositions respectively, followed by drying to obtain resist films of 30 µm thick. The films with the resist films formed from the respective photopolymerizable compositions were respectively laminated on copper-coated substrates whose surfaces had been roughened by oscillation buffing. The lamination was effected by bringing the resist film of each film into a contiguous relation with the roughened surface of the corresponding copper-coated substrate and laminating the film on the substrate by means of a hot roll laminator. The resist film of each substrate was then exposed, from the side of the polyethylene terephthalate film and through a negative film, to radiation of 90 mJ/cm² from a 3-kW ultra-high-pressure mercury vapor lamp ("HMW-201B" model; manufactured by ORK MFG. CO., Ltd.). After removing the polyethylene terephthalate film on the resist film, the resist film was developed for 80 seconds with a 1.5% aqueous sodium carbonate solution of 32°C in a spray developing machine, washed with water, and thereafter dried. The copper-coated substrate with the resist pattern formed thereon was then dipped in a plating bath of copper sulfate, so that the areas not covered by the resist pattern were subjected to electrolytic copper plating. At that time, the temperature of the plating bath was 60°C. The dipping time was 40 minutes. The copper-plated substrate was thereafter dipped at 25°C for 15 minutes in a solder plating bath to conduct solder plating. A 3% aqueous sodium hydroxide solution of 50°C was then sprayed at a pressure of 1.8 kg/cm² against the thus-plated substrate to remove the resist. The time required for the removal of the resist was measured. The results are shown in Table 2. After spraying the above-described aqueous solution of sodium hydroxide for a period twice as long as the time required for the removal, the state of corrosion of the plated solder layer was observed. The results are summarized in Table 3. Examples 9-16 & Comparative Examples 6-10: Photopolymerizable compositions of the below-described Formulation 2 were prepared respectively by using the ethylenically-unsaturated compounds shown in Table 4. In a similar manner as in Examples 1-6 and Comparative Examples 1-5 except for the use of the ethylenically-unsaturated compounds given in Table 4, resist patterns were formed on copper-coated substrates respectively and after conducting copper plating and solder plating, the resists were removed with a 3% aqueous sodium hydroxide solution of 50°C. The time required for the removal of each of the resists was measured. The results are shown in Table 5. After spraying the above-described aqueous solution of sodium hydroxide for a period twice as long as the time required for the lift-off of each of the resists, the state of corrosion of the corresponding plated solder layer was observed. The results are summarized in Table 6. Owing to the use of the specific ethylenically-unsaturated compound, the photopolymerizable composition allows to perform etching with a mild alkaline etchant in a short period of time so that the corrosion of the metal wiring pattern and solder plating layer can be minimized and the yield can hence be improved. Having described specific embodiments of our bearing, it is believed obvious that modification and variation of our invention is possible in light of the above teachings.

Claims: Process for preparing a printed circuit board comprising the steps: Image-wise exposing to radiation a layer or laminate provided with a resist film formed from a photopolymerizable composition disposed on a metal coated substrate; removal of nonexposed areas of the resist film formed from the photopolymerizable composition by an aqueous mild alkaline solution to obtain a resist pattern; selective metal etching and/or metal plating and/or solder plating; removal of the resist pattern by an aqueous strong alkaline solution, wherein the photopolymerizable composition consists essentially of an acrylic linear copolymer, a photoinitiator and an ethylenically unsaturated compound containing 10 to 50 wt.% of an acrylic ester represented by the following formula: wherein X is a hydrogen atom, or a methyl group; Y is an alkylene group with 1 to 3 carbon atoms; Z is a direct bond, an alkylene group with 1 to 8 carbon atoms, a phenylene or cyclohexylene group. Process according to claim 1, wherein Y is an ethylene or isopropylene group. Process according to claim 1 or 2, wherein Z is a direct bond or a methylene, ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, o-phenylene, p-phenylene or 1,2-cyclohexylene group. Process according to any one of the preceding claims, wherein the acrylic linear copolymer has a weight average molecular weight of 30,000 - 200,000 and a glass transition point of 20 - 80 °C. Process according to any one of the preceding claims, wherein the ethylenically unsaturated compound is contained in 10 - 100 wt.% of the acrylic linear copolymer. Process according to any one of the preceding claims, wherein the photoinitiator is contained in 1 - 15 wt.% of the acrylic linear copolymer. Process according to any one of the preceding claims, wherein the formation of the resist film comprises the steps: a) Dissolving the photopolymerizable composition, b) coating a substrate directly with the resulting solution, and c) drying the thus coated substrate. Process according to claim 7, wherein the substrate is a polyester film. Process as claimed in any of the preceding claims, wherein the radiation for photopolymerization is a UV radiation.

Inventors: HARADA KOUJI, OHTAWA SHIGERU, ONODERA JUNICHI, HARADA, KOUJI, OHTAWA, SHIGERU, ONODERA, JUNICHI
Applicants: TOKYO OHKA KOGYO CO LTD, TOKYO OHKA KOGYO CO., LTD.
Application year: 1989
Main classification: H05K3
IPC codes: C08F2, C08F246, G03F7, H05K3