GB2273707A - Photocurable resins - Google Patents

Abstract

A process for the preparation of an ethylenically unsaturated resin, suitable for use as a component of a photoresist composition, comprises the steps of: (A) reacting a phenol with a less than stoichiometric amount of formaldehyde to form a novolak resin; (B) reacting the novolak resin with a glycidyl ester of an ethylenically unsaturated monocarboxylic acid; and (C) reacting the reaction product from (B) with a dicarboxylic acid anhydride. In the manufacture of printed circuit boards the resin is used in a liquid carrier, in dissolved or suspended form, together with a photoinitiator.

Description

Photocurable resins This invention is concerned with improvements in and relating to the production of photocurable resins suitable for use, for example, in curable compositions which may be used in the manufacture of printed circuit boards and the like.

Photocurable 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, eventually in patterned form, to a substrate to facilitate and define the area of operation of subsequent processes such as electroplating, etching, the application of solder, etc. Such a photocurable composition, or "photoresist", 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.Photoresist compositions generally comprise a photocurable material, that is a material capable of curing or polymerising upon exposure to electromagnetic radiation, typically an ethylenically unsaturated or polyethylenically unsaturated material. Where, as is most common, curing is 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.

Resist materials for use in defining the area of application of molten solder are commonly termed "solder resists". One class of photocurable resins used in such solder resists are those obtained by reacting an epoxidised phenol or cresol novolak resin with an ethylenically unsaturated monocarboxylic acid such as acrylic acid or methacrylic acid. The resulting ethylenically unsaturated resin may then be rendered alkali soluble by further reaction with a dicarboxylic acid anhydride such as maleic anhydride, tetrahydrophthalic anhydride, itaconic anhydride or hexahydrophthalic anhydride. Reaction of the dicarboxylic anhydride with hydroxyl groups in the epoxy acrylate resin introduces free carboxylic acid groups into the polymer chain, thereby rendering it soluble in aqueous alkaline solutions.

In practice, reaction of the ethylenically unsaturated acid with the epoxy novolak does not completely consume all the epoxy groups. As a result the final resin contains free epoxy groups, in addition to carboxylic acid groups. The presence of these two sorts of groups gives rise to problems of stability since, with the action of time and/or temperature, the acid groups can react with the epoxy groups causing chain extension and hence, possibly, gelation of the resin. Accordingly resists produced from such resins are susceptible to overdrying. When overdried, the resist film becomes insoluble in aqueous alkaline solutions and, therefore, incapable of being developed by an alkaline solution, i.e. the resist is virtually incapable of being processed.

It is an object of the present invention to provide an improved process for the production of ethylenically unsaturated resins suitable for use, inter alia, in solder resists.

Basically, the invention provides a process for the preparation of an ethylenically unsaturated resin, which process comprises the steps of: (A) reacting a phenol with formaldehyde to form a novolak resin; (B) reacting the novolak resin with a glycidyl ester of an ethlylenically unsaturated monocarboxylic acid; and (C) reacting the reaction product from step (B) with a dicarboxylic acid anhydride.

In the first stage of the process in accordance with the invention, a phenol is reacted with formaldehyde.

By the term "phenol", we intend to refer to any monohydric aromatic phenolic compound but especially preferred phenols are phenol itself and cresols. The molar ratio of formaldehyde to phenol should be less than 1:1 and, in the case of phenol, it is preferably 0.4:1 to 0.9:1 and, in the case of cresol, it is preferably 0.2:1 to 0.8:1. These ratios give products, from phenol, having ball and ring melting points in the range of 60 to 1200C and products, from cresol, having ball and ring melting points of from 50 to 140 C. The reaction of the phenol with the formaldehyde is suitably carried out by heating the phenol with the formaldehyde, preferably in the presence of an acidic catalyst, e.g.

oxalic acid, and maintaining the reaction mixture at elevated temperature until the free formaldehyde level reaches zero. Conveniently any unreacted phenol and water is then removed from the reaction mixture under reduced pressure and/or at elevated temperature.

The novolak resin so obtained is then dissolved in a suitable solvent (e.g. propylene glycol diacetate, propylene glycol methylether acetate, ethyl ethoxy propionate or isopropyl cellosolve acetate) and reacted with the glycidyl ester of an ethylenically unsaturated monocarboxylic acid, typically glycidyl acrylate or glycidyl methacrylate. This second stage of the process of the invention may be carried out, at least initially, in the absence of catalyst but, in the later stages, in order to drive the reaction to completion it is preferred that a catalyst be employed. Suitable catalysts include aqueous solutions of caustic soda, caustic potash or tertiary amines such as triethylamine, dimethyl aniline, tributylamine and trimethylamine.

In the final stage of the process of the invention the reaction product from the second stage [step (B) ] is reacted with a dicarboxylic acid anhydride (such as those mentioned above) whereby to render the final product soluble in aqueous alkaline solutions.

The resinous material produced in accordance with the invention is, as noted above, suitable for use in photocurable resist materials, especially solder resists. These compositions may contain, in addition to the photocurable resin, photoinitiators such as anthraquinones (e.g. 2-ethyl-anthraquinone, 2-methylanthraquinone and l-chloro-anthraquinone), thioxanthones (e.g. 2,4-dimethyl-thiaxanthone, 2,4-diethyl thioxanthone and 2-chloro-thiaxanthone), ketals (e.g.

benzyl-dimethyl ketal and acetephonone-dimethyl-ketal), benzophenones and benzoin and ethers thereof. These photoinitiators can be used alone or in admixture and may also be used together with photopolymerization accelerators such as bezoic acid type accelerators or tertirary amine accelerators.

In addition to the photocurable material, the resists may also contain other components such as inorganic fillers, pigments, rheological additives (flow aids and degassing agents) and thermal curing agents.

The photoresist composition should also comprise a liquid vehicle in which the photocurable resin is dissolved or suspended. Photoresists containing photocurable resins produced in accordance with the invention are particularly suitable for use in forming a patterned image upon a substrate, generally a printed circuit board, which process comprises the steps of: (a) forming a layer of liquid photocurable composition upon the substrate; (b) drying the layer of photocurable composition on the substrate (by removal of liquid vehicles therefrom); (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 the exposed portions of the layer by the action of an aqueous alkaline solution.

The patterned layers or resists obtained by the above route may be used, as indicated above, as resists, plating resists, etch resists or, in particular, solder masks.

The liquid photoresist composition may be applied to the substrate by any suitable coating method such as, for example, by screen printing, curtain coating or electrostatic spraying. Thickness of the resist layer will, of course, depend upon the intended end use but, in general, thicknesses of the order of from 20 to 80 Fu 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 cresol novolak resin was prepared by charging a formaldehyde solution with phenol (in a ratio of formaldehyde:phenol of 0.50) into a reactor. 0.5 by weight of oxalic acid was then added and the mixture heated, under stirring and an inert atmosphere, to 800C, at which point the temperature rose to 1000C. The reaction mixture was held at this temperature for a further 45 minutes after which time the formaldehyde level (determined by standard titrimetric methods) was found to be zero. The reaction mixture was then held at 1000C for 4 hours before any unreacted phenol and water from the formaldehyde solution were vacuum stripped to leave a solid dark brown resin with a ball and ring melting point of 760C and a cone and plate viscosity of 14.8 poise at 1000C.

The solid resin was dissolved in propylene glycol methyl ether acetate (to give a non-volatiles content of 40%) by heating to 100 C. Hydroquinone (0.13% by weight of bath) and glycidyl methacrylate (49.0% by weight of batch) were then charged to the reaction flask and air and nitrogen sponge applied. The reaction mixture was held at 1100C until the epoxy value (determined by standard titrimetric method) had fallen to 16 mg KOH/g.

At this point a strong aqueous solution of caustic soda was added (to give 0.08% sodium hydroxide by weight of the batch) and the batch temperature raised to 1300C and held there until anepoxy value of less than 9 mg KOH/g had been achieved.

The resultant methacrylate resin was then cooled to 900C and sufficient tetrahydrophthalic anhydride added to give the finished resin an acid value of 80 mg KOH/g.

The carboxylated phenolic novolak methacrylate resin was then mixed in the following formulation, using a Silverson high speed stirrer.

Resin (as described above) 52.0 Cellulose acetate butyrate 11.9 Tris (2-hydroxyethyl)isocyanurate triacrylate 6.5 Pigment dispersion (Phthalocyanine green) 2.0 Flow aid (Modaflow, Monsanto Inc.) 1.5 Talc 15.5 Levelling additive (Byk-055, Byk Chemicals) 1.5 Propylene Glycol Methyl ether acetate 5.0 Xanthone ITX (Ward Blenkinsop) 0.7 Irgacure 907 (Ciba-Geigy) 3.5 100. O The resultant, green solder resist ink was screen printed onto etched IPC test circuit boards, dried, photoimaged through suitable artwork, developed using a weak solution of sodium carbonate in water, and given a final drying at 1400C for 1 hour 30 mins before being given a final W cure.

The resultant solder resist film showed good cross-hatch adhesion, pencil hardness, solvent resistance and solder resistance.

EXAMPLE 2 A carboxylated phenolic novolak methacrylate was prepared as described in Example 1. This resin was then dispersed in the following formulation.

Resin (as described above) 68.7 Irgacure 908 (Ciba-Geigy) 7.6 Xanthone ITX (Ward Blenkinsop) 1.3 Pigment dispersion (phthalocyanine green) 4.0 Flow aid (Modaflow, Monsanto Inc.) 1.6 Levelling additive (Byk-055, Byk Chemicals) 1.3 Talc 13.1 2-ethyl-4-methyl imidazole 0.8 N-methylpyrrolidone 0.8 Dipropylene glycol methyl ether 0.2 Dicyandinimide 0.2 Cellulose acetate butyrate 0.1 Silica 0.4 100.0 An epoxy hardening formulation was made up as follows: Quatrex 2410 epoxy resin (Dow Chemicals) 32.9 Ditrimethylolpropane triacrylate 2.4 Tris(2-hydroxymethyl)isocyanurate triacrylate 13.6 Talc 27.0 Propyleneglycol diacetate 24.1 100.0 This hardener was mixed with the ink described above in the ratio of 1 part hardener to 2 parts ink and was screen printed through a 43T screen onto an IPC test circuit boad.The resist film was dried, photoimaged through suitable art-work, developed in dilute aqueous sodium carbonate and given a final post bake at 1400C for 1 hour 30 mins.

The film cured film showed good resistance to solvent rub and pencil hardness tests as well as good x-hatch adhesion and solder resistance.

EXAMPLE 3 A cresol-novolak resin was produced by reacting cresol with formaldehyde in a ratio of 0.55:1 (formaldehyde:cresol) using 0.5 by weight of oxalic acid as catalyst. The materials were heated to 1000C under a nitrogen atmosphere at which point the temperature of the batch rose to 1100C. The batch was held at this temperature until its free formaldehyde content had dropped to zero; it was then held at l100C for a further 3 hours. At the end of this time the water from the formaldehyde solution and any reacted cresol were vacuum stripped to leave a brown, solid resin with a ball and ring melting point of 1090C and a cone and plate viscosity of 47 poise at 1500C.

The resin described above was dissolved in ethyl ethoxy propionate to yield a solution with a non-volatiles content of 47. To this solution was added hydroquinone (0.13% by weight of batch), glycidyl methacrylate (18.8% by weight of batch) and glycidyl acrylate (11.9% by weight of batch). The mixture was then heated to 1300C under stirring and nitrogen and air sparging and held at this temperature until its epoxy value (determined by standard titrimetric methods) had fallen below 9 mg KOH/g.

The bath was then cooled to 800C and sufficient maleic anhydride added to give a finished resin with an acid value of 40 mg KOH/g.

The carboxylated cresol-hovolak acrylate resin described above was used to produce a photoimageable primary etch resist by high speed stirring in the following formulation: Resin (as described above) 76.6 Irgacure 908 (Ciba Geigy) 3.7 Xanthone ITX (Ward Blenkinsop) 1.0 Di-trimethylol propane trioacrylate 0.4 Methylene blue 0.3 Ethylene glycol 1.1 Tris(2-hydroxyethyl)isocyanurate triacrylate 7.7 Flow aid (Modaflow, Monsanto Inc.) 1.5 Talc 7.7 100.0 The blue ink was roller coated onto scrubbed, plain copper laminate boards and dried to give an even film of around 10 microns thickness. This film was photoimaged and developed in a dilute aqueous solution of sodium carbonate. The board areas not protected by the resist were then etched away using acid cupric chloride solution, the remaining areas were then stripped from the board using caustic soda solution. The circuit formed showed a true reproduction of the art work used with no shorts or mousebites.

Claims (5)

CLAIMS:

1. A process for the preparation of an ethylenically unsaturated resin, which processses comprises the steps of: (A) reacting a phenol with a less than stoichiometric amount of formaldehyde to form a novolak resin; (B) reacting the novolak resin with a glycidyl ester of an ethylenically unsaturated monocarboxylic acid; and (C) reacting the reaction product from (B) with a dicarboxylic acid anhydride.

2. A process as claimed in claim 1 in which the phenol is phenol itself and the molar ratio of formaldehyde to phenol is from 0.4:1 to 0.9:1.

3. A process as claimed in claim 1 in which the phenol is cresol and the molar ratio of formaldehyde to cresol is from 0.2:1 to 0.8:1.

4. A photocurable composition comprising a resin obtainable by a process as claimed in any one of claims 1-3, a liquid carrier in which the resin is dissolved or suspended, and a photoinitator.

5. The use of a composition as claimed in claim 4 as a solder resist in the manufacture of printed circuit boards.