JP7727751B2 - Curable resin composition, cured product, printed wiring board, and method for producing printed wiring board - Google Patents

Description

The present invention relates to a curable resin composition, particularly a curable resin composition suitable for use in forming solder resist. Furthermore, the present invention also relates to a printed wiring board using the curable resin composition and a method for producing the same.

The adhesion of solder to solder resist that occurs during the flow soldering process in the manufacture of printed wiring boards can cause defects such as solder bridges (solder shorts), which can ultimately lead to defective printed wiring boards. In recent years, as the wiring patterns on printed wiring boards have become increasingly fine, solder bridges have become particularly likely to occur, and are considered to be one of the factors hindering the efficient manufacture of printed wiring boards.

It has been known that giving a solder resist a matte (low gloss) surface can prevent solder from adhering to the solder resist. One method for achieving a matte surface has been to incorporate a component known as a matting agent as a filler into the resin composition used to form the solder resist. For example, Patent Document 1 proposes forming a solder resist with a matte surface using a resin composition containing finely divided aluminum silicate as a filler, which has a matting effect. Other known matting agents include ultrafine anhydrous silica, talc, large-particle fused silica, clay, and aluminum hydroxide.

With the recent rise in the trend toward carbon neutrality, electrification of social infrastructure and daily necessities is progressing, increasing the need for electrically powering large machines such as automobiles. Many of these social infrastructures and daily necessities require operation in harsh environments and high voltages and currents. Therefore, components for operating these machines, such as printed wiring boards, and ultimately the solder resists that make up printed wiring boards, must be able to meet these needs. For example, Patent Document 2 proposes an electrically insulating resin composition that can form a solder resist that has excellent heat resistance and moisture resistance, higher voltage resistance (dielectric breakdown voltage) than conventional products, and can be used stably for long periods of time, even in harsh environments.

Japanese Patent Application Publication No. 9-157574 Japanese Patent Application Laid-Open No. 2005-314554

However, it has traditionally been difficult for a solder resist to maintain low gloss over time, thereby achieving high levels of solder adhesion resistance over time, and high voltage resistance (dielectric breakdown voltage). Therefore, a technical challenge exists to provide a solder resist that maintains low gloss over time, thereby achieving high levels of solder adhesion resistance over time, and high voltage resistance, as well as a resin composition for forming such a solder resist.

In addition, since it is expected that printed wiring boards will be required to carry higher voltages and currents in the future than at present, it is also a technical challenge to provide a solder resist with voltage resistance equivalent to or greater than that of current printed wiring boards, and a resin composition for forming such a solder resist.

The present invention therefore aims to provide a curable resin composition capable of forming a solder resist that maintains low gloss over time and thereby achieves high levels of solder adhesion resistance and voltage resistance, both of which have previously been difficult to achieve. It also aims to provide a cured product of the cured resin composition, and a printed wiring board comprising the cured product. Another aim of the present invention is to provide a method for producing a printed wiring board comprising a solder resist that maintains low gloss over time and thereby achieves high levels of solder adhesion resistance and voltage resistance, using the curable resin composition.

As a result of extensive research, the inventors have discovered that the above-mentioned problems can be solved by adjusting the oil absorption of barium sulfate to 40-70 mL/100 g in a curable resin composition containing a curable resin, barium sulfate, and a thermosetting catalyst. The present invention is based on this discovery. Specifically, the gist of the present invention is as follows:

[1] A curable resin composition comprising a curable resin, barium sulfate, and a thermosetting catalyst,
A curable resin composition characterized in that the oil absorption of the barium sulfate is 40 to 70 mL/100 g.
[2] The curable resin composition according to [1], wherein the barium sulfate comprises barium sulfate surface-treated with silica.
[3] The curable resin composition according to [1] or [2], wherein the curable resin comprises one or more thermosetting resins selected from the group consisting of epoxy compounds, polyfunctional oxetane compounds, and oxazoline compounds.
[4] The curable resin composition according to any one of [1] to [3], wherein the curable resin comprises a carboxyl group-containing photosensitive resin having one or more ethylenically unsaturated groups in one molecule.
[5] The curable resin composition according to [4], further comprising a photopolymerization initiator.
[6] The curable resin composition according to any one of [1] to [5], which is used for forming a solder resist.
[7] The curable resin composition according to [6], wherein the solder resist is a matte solder resist.
[8] A cured product of the curable resin composition according to any one of [1] to [7].
[9] A printed wiring board comprising the cured product according to [8].
[10] A method for producing a printed wiring board provided with a solder resist, the method comprising a step of forming a solder resist by curing the curable resin composition according to any one of [1] to [7].

The present invention provides a curable resin composition capable of forming a solder resist that maintains low gloss over time and thereby achieves high levels of both solder adhesion resistance and voltage resistance over time, a cured product thereof, and a printed wiring board comprising the cured product. Furthermore, it is possible to provide a method for manufacturing a printed wiring board comprising a solder resist that maintains low gloss over time, solder adhesion resistance, and voltage resistance over time, using the curable resin composition.

[Curable resin composition]
The curable resin composition of the present invention contains a curable resin, barium sulfate, and a thermosetting catalyst as essential components. The curable resin composition of the present invention can be suitably used to form a solder resist, particularly a matte solder resist, during the production of printed wiring boards, particularly for printed wiring boards that require high-voltage, large-current conduction. By containing barium sulfate having an oil absorption of 40 to 70 mL/100 g, the curable resin composition of the present invention can maintain a low gloss over time, thereby imparting high levels of solder adhesion resistance and voltage resistance over time to the solder resist that is the cured product of the curable resin composition.

The reason why it has been difficult to maintain a low gloss over time in solder resists and thereby achieve high levels of both solder adhesion resistance and voltage resistance over time is believed to be as follows. A common method for achieving a matte (low gloss) solder resist is to incorporate a matting agent as a filler into the resin composition used to form the solder resist. When commonly used matting agents such as silica are used, air bubbles adhere to the particles, resulting in the presence of air bubbles in the resin composition. Consequently, air bubbles remain in the solder resist formed by curing the resin composition. Air bubbles adhering to silica and other materials are difficult to separate and remove; for example, even when a large amount of antifoaming agent is incorporated into the resin composition, a large number of air bubbles remain in the resin composition. Similarly, when commonly used matting agents such as aluminum hydroxide and aluminum silicate are used, air bubbles adhere to the particles, resulting in a large number of air bubbles remaining in the solder resist. The presence of bubbles in a solder resist significantly impairs the voltage resistance of the solder resist, and therefore it is thought that when a conventional matting agent is used, it has not been possible to achieve a high level of both a matte finish (low gloss) in the solder resist and the resulting solder adhesion resistance, as well as voltage resistance.

On the other hand, in the curable resin composition of the present invention, by using barium sulfate with a specific oil absorption amount as the matting agent, adhesion of air bubbles to the barium sulfate matting agent is suppressed, and air bubbles are prevented from remaining in the solder resist formed by curing the curable resin composition. As a result, it is believed that the solder resist can maintain a low gloss over time and thereby be endowed with high levels of solder adhesion resistance and voltage resistance over time.

The curable resin composition of the present invention preferably has a 60° gloss of 20 or less, more preferably 15 or less, and even more preferably 10 or less when cured and formed immediately after preparation, and having a film thickness of 25 μm. Furthermore, the curable resin composition of the present invention can form a cured product that maintains a low gloss comparable to that of the curable resin composition immediately after preparation, even after storage at 30°C for six months after preparation. In other words, the curable resin composition of the present invention has a 60° gloss of 20 or less, more preferably 15 or less, and even more preferably 10 or less when cured and formed immediately after storage at 30°C for six months after preparation, and having a film thickness of 25 μm. The gloss of the cured product of the curable resin composition can be measured as a 60° specular reflectance using a gloss meter (Micro Trigloss) manufactured by BYK-Chemie, for a cured product (cured coating film) of the curable resin composition formed by applying the curable resin composition to the entire surface of a 1.6 mm thick copper FR-4 substrate by screen printing so that the film thickness after curing is 25 μm, and then heating the composition at 150°C for 30 minutes using a hot air circulation drying oven.

The curable resin composition of the present invention preferably has a withstand voltage value of 5 kV (5 kV/0.05 mm) or more, more preferably 5.5 kV (5.5 kV/0.05 mm) or more, and even more preferably 6 kV (6 kV/0.05 mm) or more when cured and having a film thickness of 50 μm. The withstand voltage value of the curable resin composition can be measured by applying a voltage increase of 0.5 kV/second in AC mode using a Kikusui Electronics Co., Ltd. TOS5101 withstand voltage tester on a substrate having a cured product (cured coating) with a film thickness of 50 μm. The voltage increase is measured as the voltage value at which the cured product breaks down when the cured product breaks down.

Each component of the curable resin composition of the present invention will be described in detail below.
(curable resin)
The curable resin composition of the present invention contains a curable resin. The curable resin is not particularly limited as long as it is a resin that is cured by the action of heat, light, or the like. Specifically, the curable resin may be a thermosetting resin, a photocurable resin, or the like. One type of curable resin may be used alone, or two or more types may be used in combination. For example, a thermosetting resin or a photocurable resin may be used alone, or these may be used in combination.

The curable resin composition of the present invention can contain a thermosetting resin as the curable resin. The inclusion of a thermosetting resin is expected to improve the heat resistance of the curable resin composition. A single thermosetting resin may be used alone, or two or more may be used in combination. Any known thermosetting resin can be used. Examples of known thermosetting resins that can be used include amino resins such as melamine resins, benzoguanamine resins, melamine derivatives, and benzoguanamine derivatives; isocyanate compounds; blocked isocyanate compounds; cyclocarbonate compounds; epoxy compounds; oxetane compounds; oxazoline compounds; episulfide resins; bismaleimides; and carbodiimide resins. Each of these thermosetting resins may be monofunctional or polyfunctional. Among these thermosetting resins, thermosetting resins having a cyclic ether group or a cyclic thioether group in the molecule are preferred. Examples of such thermosetting resins include epoxy compounds having an epoxy group in the molecule, oxetane compounds having an oxetanyl group in the molecule, oxazoline compounds having an oxazoline group in the molecule, and episulfide resins having a thioether group in the molecule. The thermosetting resin preferably contains one or more compounds selected from the group consisting of epoxy compounds, polyfunctional oxetane compounds, and oxazoline compounds, and particularly preferably contains an epoxy compound. By incorporating these compounds as the thermosetting resin in the curable resin composition, it is possible to impart particularly excellent heat resistance, chemical resistance, and adhesion to the curable resin composition.

Epoxy compounds include epoxidized vegetable oils; bisphenol A type epoxy resins; hydroquinone type epoxy resins; bisphenol type epoxy resins; thioether type epoxy resins; brominated epoxy resins; novolac type epoxy resins; phenol novolac type epoxy resins; biphenol novolac type epoxy resins; bisphenol F type epoxy resins; hydrogenated bisphenol A type epoxy resins; glycidylamine type epoxy resins; hydantoin type epoxy resins; alicyclic epoxy resins; trihydroxyphenylmethane type epoxy resins; bixylenol type or biphenol type epoxy resins; or mixtures thereof; bisphenol S type epoxy resins; bisphenol A novolac type epoxy resins; tetraphenylolethane type epoxy resins; heterocyclic epoxy resins; diglycidyl phthalate resins; tetraglycidyl xylenoylethane resins; naphthalene group-containing epoxy resins; epoxy resins having a dicyclopentadiene skeleton; glycidyl methacrylate copolymer epoxy resins; cyclohexylmaleimide and glycidyl methacrylate copolymer epoxy resins; epoxy-modified polybutadiene rubber derivatives; CTBN-modified epoxy resins, etc., but are not limited to these. These epoxy resins may be used alone or in combination of two or more.

As the oxetane compound, a polyfunctional oxetane compound having two or more oxetanyl groups in one molecule is preferably used. Examples of polyfunctional oxetane compounds include bis[(3-methyl-3-oxetanylmethoxy)methyl]ether, bis[(3-ethyl-3-oxetanylmethoxy)methyl]ether, 1,4-bis[(3-methyl-3-oxetanylmethoxy)methyl]benzene, 1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene, (3-methyl-3-oxetanyl)methyl acrylate, and (3-ethyl-3-oxetanyl)methyl acrylate. Examples of suitable oxetane compounds include polyfunctional oxetanes such as acrylate, (3-methyl-3-oxetanyl)methyl methacrylate, (3-ethyl-3-oxetanyl)methyl methacrylate, and oligomers or copolymers thereof, as well as ethers of oxetane alcohols with novolak resins, poly(p-hydroxystyrene), cardo-type bisphenols, calixarenes, calixresorcinarenes, or hydroxyl group-containing resins such as silsesquioxane. Other examples include copolymers of unsaturated monomers having an oxetane ring with alkyl (meth)acrylates.

Examples of oxazoline compounds include compounds containing two or more oxazoline groups per molecule. Examples of such oxazoline compounds include oxazoline group-containing polymers, such as polymers of oxazoline group-containing monomers and copolymers of oxazoline group-containing monomers with other monomers. Examples of oxazoline group-containing monomers include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, 2-isopropenyl-2-oxazoline, and 2-isopropenyl-4,4-dimethyl-2-oxazoline.

Episulfide resins include, for example, compounds in which the oxygen atoms constituting the epoxy group in any epoxy compound are replaced with sulfur atoms. Examples of epoxy compounds include those similar to those described above. Conventional methods can be used to replace the oxygen atoms constituting the epoxy group in an epoxy compound with sulfur atoms.

Examples of amino resins such as melamine derivatives and benzoguanamine derivatives include methylolmelamine compounds, methylolbenzoguanamine compounds, methylolglycoluril compounds, and methylolurea compounds.

The isocyanate compound may be a polyisocyanate compound. Examples of polyisocyanate compounds include aromatic polyisocyanates such as 4,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, naphthalene-1,5-diisocyanate, o-xylylene diisocyanate, m-xylylene diisocyanate, and 2,4-tolylene dimer; aliphatic polyisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate, methylene diisocyanate, trimethylhexamethylene diisocyanate, 4,4-methylenebis(cyclohexyl isocyanate), and isophorone diisocyanate; alicyclic polyisocyanates such as bicycloheptane triisocyanate; and adducts, biuret compounds, and isocyanurates of the above-listed isocyanate compounds.

The blocked isocyanate compound can be an addition reaction product of an isocyanate compound and an isocyanate blocking agent. Examples of isocyanate compounds that can react with an isocyanate blocking agent include the polyisocyanate compounds described above. Examples of isocyanate blocking agents include phenol-based blocking agents, lactam-based blocking agents, active methylene-based blocking agents, alcohol-based blocking agents, oxime-based blocking agents, mercaptan-based blocking agents, acid amide-based blocking agents, imide-based blocking agents, amine-based blocking agents, imidazole-based blocking agents, and imine-based blocking agents.

The amount of thermosetting resin in the curable resin composition is not particularly limited as long as the effects of the present invention are achieved, and can be, for example, 3 to 50 mass % in solid content terms relative to the total mass of the curable resin composition.

The curable resin composition of the present invention can contain a photocurable resin as the curable resin. Known and commonly used photocurable resins can be used as the photocurable resin. Among these, from the viewpoints of photocurability and development resistance, photocurable resins that can be cured by a radical addition polymerization reaction with active energy rays are preferred, and photocurable resins having ethylenically unsaturated groups in the molecule are more preferred. The ethylenically unsaturated groups are preferably derived from acrylic acid, methacrylic acid, or derivatives thereof. One type of photocurable resin may be used alone, or two or more types may be used in combination. A carboxyl group-containing photosensitive resin, as described below, is preferably used as the photocurable resin.

Photocurable resins containing ethylenically unsaturated groups in the molecule include well-known and commonly used photopolymerizable oligomers and photopolymerizable monomers. Examples of photopolymerizable oligomers include unsaturated polyester oligomers and (meth)acrylate oligomers. Examples of (meth)acrylate oligomers include epoxy (meth)acrylates such as phenol novolac epoxy (meth)acrylate, cresol novolac epoxy (meth)acrylate, and bisphenol-type epoxy (meth)acrylate, as well as urethane (meth)acrylate, epoxy urethane (meth)acrylate, polyester (meth)acrylate, polyether (meth)acrylate, and polybutadiene-modified (meth)acrylate.

On the other hand, a monomer having an ethylenically unsaturated group is preferably used as the photopolymerizable monomer. Examples of such photopolymerizable monomers include commonly known polyester (meth)acrylates, polyether (meth)acrylates, urethane (meth)acrylates, carbonate (meth)acrylates, and epoxy (meth)acrylates. Specific examples include alkyl acrylates such as 2-ethylhexyl acrylate and cyclohexyl acrylate; hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate and 2-hydroxypropyl acrylate; mono- or diacrylates of alkylene oxide derivatives such as ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol; acrylamides such as N,N-dimethylacrylamide, N-methylolacrylamide, and N,N-dimethylaminopropylacrylamide; aminoalkyl acrylates such as N,N-dimethylaminoethyl acrylate and N,N-dimethylaminopropyl acrylate; hexanediol, trimethylolpropane, pentaerythritol, ditrimethylolpropane, dipentaerythritol, and trishydroxyethyl isocyanurate. Polyhydric acrylates such as polyhydric alcohols or their alkylene oxide adducts or ε-caprolactone adducts; phenols such as phenoxy acrylate and bisphenol A diacrylate or their alkylene oxide adducts; glycidyl ether acrylates such as glycerin diglycidyl ether, trimethylolpropane triglycidyl ether, and triglycidyl isocyanurate; and, without limitation, acrylates obtained by directly acridating polyols such as polyether polyols, polycarbonate diols, hydroxyl-terminated polybutadienes, and polyester polyols, or by urethane acrylate via diisocyanates, melamine acrylates, and at least one of the methacrylates corresponding to the acrylates can be appropriately selected and used. The photopolymerizable monomers may be used alone or in combination of two or more. Furthermore, the photopolymerizable monomer can also be used as a reactive diluent. The term "photopolymerizable monomer" refers to a compound that is a monomer, particularly among photocurable resins.

The amount of photocurable resin in the curable resin composition is not particularly limited as long as the effects of the present invention are achieved, and can be, for example, 10 to 50 mass % in solid content terms relative to the total mass of the curable resin composition.

The curable resin may preferably contain a carboxyl group-containing resin in order to impart alkaline developability to the curable resin composition. As the carboxyl group-containing resin, various conventionally known resins having a carboxyl group in the molecule may be used.
Specific examples of the carboxyl group-containing resin include the following compounds (which may be either oligomers or polymers):

(1) A carboxyl group-containing resin obtained by copolymerizing an unsaturated carboxylic acid such as (meth)acrylic acid with an unsaturated group-containing compound such as styrene, α-methylstyrene, lower alkyl (meth)acrylate, or isobutylene.

(2) Carboxylic acid-containing urethane resins obtained by the polyaddition reaction of diisocyanates such as aliphatic diisocyanates, branched aliphatic diisocyanates, alicyclic diisocyanates, and aromatic diisocyanates with carboxyl-containing dialcohol compounds such as dimethylolpropionic acid and dimethylolbutanoic acid, and diol compounds such as polycarbonate polyols, polyether polyols, polyester polyols, polyolefin polyols, acrylic polyols, bisphenol A alkylene oxide adduct diols, and compounds having phenolic hydroxyl groups and alcoholic hydroxyl groups.

(3) Carboxylic acid group-containing photosensitive urethane resins obtained by the polyaddition reaction of diisocyanates with bifunctional epoxy resins such as bisphenol A type epoxy resins, hydrogenated bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, bixylenol type epoxy resins, and biphenol type epoxy resins, and monocarboxylic acid compounds having ethylenically unsaturated groups such as (meth)acrylic acid, partially acid anhydride-modified products thereof, carboxyl group-containing dialcohol compounds, and diol compounds.

(4) A carboxyl group-containing photosensitive urethane resin that is (meth)acrylated at the terminal by adding a compound having one hydroxyl group and one or more (meth)acryloyl groups in the molecule, such as a hydroxyalkyl (meth)acrylate, during the synthesis of the resin (2) or (3).

(5) A carboxyl group-containing photosensitive urethane resin that has been (meth)acrylated at the end by adding a compound having one isocyanate group and one or more (meth)acryloyl groups in the molecule, such as an equimolar reaction product of isophorone diisocyanate and pentaerythritol triacrylate, during the synthesis of the resin (2) or (3).

(6) A carboxyl group-containing photosensitive resin obtained by reacting a difunctional or more polyfunctional (solid) epoxy resin with (meth)acrylic acid and adding a dibasic acid anhydride to the hydroxyl groups present in the side chains.

(7) A carboxyl group-containing photosensitive resin obtained by reacting a polyfunctional epoxy resin in which the hydroxyl groups of a bifunctional (solid) epoxy resin have been further epoxidized with epichlorohydrin with (meth)acrylic acid, and then adding a dibasic acid anhydride to the resulting hydroxyl groups.

(8) A carboxyl group-containing polyester resin obtained by reacting a dicarboxylic acid such as adipic acid, phthalic acid, or hexahydrophthalic acid with a bifunctional oxetane resin, and then adding a dibasic acid anhydride such as phthalic anhydride, tetrahydrophthalic anhydride, or hexahydrophthalic anhydride to the resulting primary hydroxyl groups.

(9) A carboxyl group-containing photosensitive resin obtained by reacting an epoxy compound having multiple epoxy groups per molecule with a compound having at least one alcoholic hydroxyl group and one phenolic hydroxyl group per molecule, such as p-hydroxyphenethyl alcohol, and an unsaturated group-containing monocarboxylic acid, such as (meth)acrylic acid, and then reacting the alcoholic hydroxyl groups of the resulting reaction product with a polybasic acid anhydride, such as maleic anhydride, tetrahydrophthalic anhydride, trimellitic anhydride, pyromellitic anhydride, or adipic acid.

(10) A carboxyl group-containing photosensitive resin obtained by reacting a compound having multiple phenolic hydroxyl groups in one molecule with an alkylene oxide such as ethylene oxide or propylene oxide, reacting the resulting reaction product with an unsaturated group-containing monocarboxylic acid, and then reacting the resulting reaction product with a polybasic acid anhydride.

(11) A carboxyl group-containing photosensitive resin obtained by reacting a compound having multiple phenolic hydroxyl groups in one molecule with a cyclic carbonate compound such as ethylene carbonate or propylene carbonate, reacting the resulting reaction product with an unsaturated group-containing monocarboxylic acid, and then reacting the resulting reaction product with a polybasic acid anhydride.

(12) A carboxyl group-containing photosensitive resin obtained by adding a compound having one epoxy group and one or more (meth)acryloyl groups in one molecule to any of the resins (1) to (11).
In this specification, (meth)acrylate is a general term that refers to acrylate, methacrylate, and mixtures thereof, and the same applies to other similar expressions.

In addition, the photosensitive resin can be a resin synthesized according to the procedure described in the examples below.

The acid value of the carboxyl group-containing resin is preferably in the range of 30 to 150 mgKOH/g, and more preferably in the range of 50 to 120 mgKOH/g. If the acid value of the carboxyl group-containing resin is 30 mgKOH/g or higher, alkaline development can be carried out appropriately. On the other hand, if the acid value is 150 mgKOH/g or lower, the exposed areas do not dissolve in the developer, allowing the exposed and unexposed areas to be dissolved and peeled apart separately in the developer, making it easier to draw a normal resist pattern, which is preferable.

The weight-average molecular weight of the carboxyl group-containing resin varies depending on the resin skeleton, but is generally in the range of 2,000 to 150,000, and preferably 5,000 to 100,000. A weight-average molecular weight of 2,000 or higher results in good moisture resistance of the coating film after exposure, no film loss during development, and excellent resolution. On the other hand, a weight-average molecular weight of 150,000 or lower results in good developability and good storage stability. The weight-average molecular weight can be measured by gel permeation chromatography (GPC).

(barium sulfate)
The curable resin composition of the present invention contains barium sulfate having an oil absorption of 40 to 70 mL/100 g. By including barium sulfate having such a relatively high oil absorption, the curable resin composition can provide a solder resist, which is a cured product of the curable resin composition, with a low gloss level over time, which in turn can provide high levels of solder adhesion resistance and voltage resistance over time.

The oil absorption of barium sulfate is preferably 45 to 70 mL/100 g, more preferably 45 to 65 mL/100 g, and even more preferably 50 to 60 mL/100 g. The oil absorption of barium sulfate can be measured in accordance with the method specified by Japanese Industrial Standards (JIS K5101-13-1).

Barium sulfate may be untreated or chemically and/or physically treated. Examples of such treated barium sulfate include surface-treated barium sulfate. Examples of materials used to surface-treat barium sulfate include inorganic substances such as silica and alumina. It is particularly preferable to use barium sulfate that has been surface-treated with silica (also known as "multi-treatment processing"). Multi-treatment refers to a process in which a very thick layer of the surface-treating material (e.g., silica, alumina, etc.) is attached to the surface of the barium sulfate. Commercially available barium sulfate that has been surface-treated with silica includes, for example, PFS-701 manufactured by Ishihara Sangyo Kaisha, Ltd.

Barium sulfate, particularly barium sulfate surface-treated with silica, has superior matting performance compared to conventional matting agents. By incorporating a small amount of silica-surface-treated barium sulfate into a curable resin composition, a high matting effect can be achieved. This minimizes the problem of air bubbles being introduced into the curable resin composition, which can be a problem when incorporating a matting agent, and the residual air bubbles in the solder resist formed by curing the curable resin composition. As a result, the decrease in the voltage resistance of the solder resist can be suppressed. Furthermore, since the viscosity of a resin composition generally increases when a matting agent is incorporated into the resin composition, the use of barium sulfate, particularly barium sulfate surface-treated with silica, which can achieve a high matting effect with a small amount of addition, can minimize the increase in viscosity of the curable resin composition, thereby minimizing the decrease in printability of the curable resin composition on a substrate. Furthermore, barium sulfate, particularly barium sulfate surface-treated with silica, has high acid resistance (acid resistance). Therefore, by blending barium sulfate as a matting agent in the curable resin composition, the curable resin composition has high acid resistance, and as a result, has high resistance (tin plating resistance) to tin plating or the like that is carried out under strong acidic conditions.

In the curable resin composition, the amount of barium sulfate with an oil absorption of 40 to 70 mL/100 g is not particularly limited as long as the effects of the present invention are achieved. For example, the amount is preferably 30 to 74 mass%, more preferably 40 to 70 mass%, and even more preferably 45 to 67 mass%, calculated as solid content, relative to the total mass of the curable resin composition.

(Thermosetting catalyst)
The curable resin composition of the present invention contains a thermosetting catalyst. Examples of the thermosetting catalyst include imidazole derivatives such as imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 4-phenylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole, and 2-phenyl-4,5-dihydroxymethylimidazole; amine compounds such as dicyandiamide, benzyldimethylamine, 4-(dimethylamino)-N,N-dimethylbenzylamine, 4-methoxy-N,N-dimethylbenzylamine, and 4-methyl-N,N-dimethylbenzylamine; hydrazine compounds such as adipic acid dihydrazide and sebacic acid dihydrazide; and phosphorus compounds such as triphenylphosphine. Commercially available examples include 2MZ-A, 2MZ-OK, 2PHZ, 2PHZ-PW, 2P4BHZ, and 2P4MHZ (all trade names of imidazole-based compounds) manufactured by Shikoku Chemicals Corporation, and U-CAT 3513N (trade name of a dimethylamine-based compound), DBU, DBN, and U-CAT SA 102 (all bicyclic amidine compounds and salts thereof) manufactured by San-Apro Co., Ltd. However, the curing catalyst is not limited to these, and any curing catalyst that promotes the reaction of at least one of an epoxy group and an oxetanyl group with a carboxyl group may be used, and they may be used alone or in combination of two or more. Also usable are S-triazine derivatives such as guanamine, acetoguanamine, benzoguanamine, melamine, 2,4-diamino-6-methacryloyloxyethyl-S-triazine, 2-vinyl-2,4-diamino-S-triazine, 2-vinyl-4,6-diamino-S-triazine·isocyanuric acid adduct, and 2,4-diamino-6-methacryloyloxyethyl-S-triazine·isocyanuric acid adduct. Preferably, these compounds that also function as adhesion promoters are used in combination with a heat curing catalyst. The heat curing catalyst may be used alone or in combination of two or more.

The amount of thermosetting catalyst to be added is preferably 2 to 30 parts by mass, more preferably 5 to 25 parts by mass, per 100 parts by mass of thermosetting resin.

(Photopolymerization initiator)
When the curable resin composition of the present invention contains a photosensitive resin, the curable resin composition preferably contains a photopolymerization initiator. Any known photopolymerization initiator can be used. The photopolymerization initiator may be used alone or in combination of two or more.

Specific examples of photopolymerization initiators include bis-(2,6-dichlorobenzoyl)phenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-2,5-dimethylphenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-4-propylphenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-1-naphthylphosphine oxide, bis-(2,6-dimethoxybenzoyl)phenylphosphine oxide, and bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl Phosphine oxide, bisacylphosphine oxides such as bis-(2,6-dimethoxybenzoyl)-2,5-dimethylphenylphosphine oxide and bis-(2,4,6-trimethylbenzoyl)-phenylphosphine oxide; 2,6-dimethoxybenzoyldiphenylphosphine oxide, 2,6-dichlorobenzoyldiphenylphosphine oxide, 2,4,6-trimethylbenzoylphenylphosphine acid methyl ester, 2-methylbenzoyldiphenylphosphine oxide, pivaloylphenylphosphine monoacylphosphine oxides such as phosphinic acid isopropyl ester and 2,4,6-trimethylbenzoyldiphenylphosphine oxide; ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate, 1-hydroxy-cyclohexyl phenyl ketone, 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]phenyl}-2-methyl-propan-1-one, 2-hydroxy- hydroxyacetophenones such as hydroxy-2-methyl-1-phenylpropan-1-one; benzoins such as benzoin, benzil, benzoin methyl ether, benzoin ethyl ether, benzoin n-propyl ether, benzoin isopropyl ether, and benzoin n-butyl ether; benzoin alkyl ethers; benzophenones such as benzophenone, p-methylbenzophenone, Michler's ketone, methylbenzophenone, 4,4'-dichlorobenzophenone, and 4,4'-bisdiethylaminobenzophenone; acetophenone , 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 1,1-dichloroacetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-1-propanone, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-(dimethylamino)-2-[(4-methylphenyl)methyl)-1-[4-(4-morpholinyl)phenyl]-1-butanone, N,N-dimethylaminoacetophenone acetophenones such as thioxanthone, 2-ethylthioxanthone, 2-isopropylthioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2-chlorothioxanthone, 2,4-diisopropylthioxanthone, etc.; anthraquinones such as anthraquinone, chloroanthraquinone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 1-chloroanthraquinone, 2-amylanthraquinone, 2-aminoanthraquinone, etc.; acetophenone dimethyl ketals such as ketal and benzyl dimethyl ketal; benzoic acid esters such as ethyl-4-dimethylaminobenzoate, 2-(dimethylamino)ethyl benzoate, and p-dimethylbenzoic acid ethyl ester; oxime esters such as 1,2-octanedione, 1-[4-(phenylthio)phenyl]-, 2-(O-benzoyloxime), ethanone, 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl]-, 1-(O-acetyloxime); bis(η5-2,4-cyclopentadien-1-yl)-bis(2, Examples of suitable phenyl disulfides include titanocenes such as bis(cyclopentadienyl)-bis[2,6-difluoro-3-(2-(1-pyrrol-1-yl)ethyl)phenyl]titanium and bis(cyclopentadienyl)-bis[2,6-difluoro-3-(2-(1-pyrrol-1-yl)ethyl)phenyl]titanium; alkylphenones such as 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholinophenyl)-butan-1-one; phenyl disulfide 2-nitrofluorene, butyroin, anisoin ethyl ether, azobisisobutyronitrile, and tetramethylthiuram disulfide.

A photoinitiator aid or sensitizer may be used in combination with the above-mentioned photopolymerization initiator. Examples of photoinitiator aids or sensitizers include benzoin compounds, anthraquinone compounds, thioxanthone compounds, ketal compounds, benzophenone compounds, tertiary amine compounds, and xanthone compounds. Thioxanthone compounds, such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2-chlorothioxanthone, 2-isopropylthioxanthone, and 4-isopropylthioxanthone, are particularly preferred. The inclusion of a thioxanthone compound can improve deep section curing properties. While these compounds may be used as photopolymerization initiators, it is preferable to use them in combination with a photopolymerization initiator. Furthermore, one photoinitiator aid or sensitizer may be used alone, or two or more may be used in combination.

These photopolymerization initiators, photoinitiator assistants, and sensitizers absorb specific wavelengths, which can reduce sensitivity in some cases and act as UV absorbers. However, they are not used solely for the purpose of improving the sensitivity of the curable resin composition. By absorbing light of specific wavelengths as needed, they can increase the photoreactivity of the surface, change the line shape and openings of the resist to vertical, tapered, or reverse tapered, and improve the precision of the line width and opening diameter.

(filler)
The curable resin composition of the present invention may contain fillers other than the above-mentioned barium sulfate as needed to increase the physical strength of the coating film, as long as the effects of the present invention are not impaired. Known inorganic or organic fillers can be used as such fillers, with various silicas (e.g., finely divided silica, spherical silica, etc.), hydrotalcite, and talc being particularly preferred. Furthermore, metal oxides and metal hydroxides such as aluminum hydroxide can also be used as extender pigment fillers to achieve a white appearance and flame retardancy.

(organic solvent)
The curable resin composition of the present invention may contain an organic solvent for the purpose of adjusting the viscosity when preparing the curable resin composition or when applying it to a substrate or film. Examples of the organic solvent include ketones such as methyl ethyl ketone and cyclohexanone; aromatic hydrocarbons such as toluene, xylene, and tetramethylbenzene; glycol ethers such as cellosolve, methyl cellosolve, butyl cellosolve, carbitol, methyl carbitol, butyl carbitol, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol diethyl ether, diethylene glycol monomethyl ether acetate, and tripropylene glycol monomethyl ether; esters such as ethyl acetate, butyl acetate, butyl lactate, cellosolve acetate, butyl cellosolve acetate, carbitol acetate, butyl carbitol acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, and propylene carbonate; aliphatic hydrocarbons such as octane and decane; and petroleum-based solvents such as petroleum ether, petroleum naphtha, and solvent naphtha. These organic solvents may be used alone or in combination of two or more.

Volatilization and drying of organic solvents can be carried out using a hot air circulation drying oven, IR oven, hot plate, convection oven, etc. (using a heat source equipped with a steam-based air heating system, a method in which hot air inside the dryer is brought into countercurrent contact, or a method in which hot air is blown onto the support from a nozzle).

(Other ingredients)
The curable resin composition of the present invention may further contain, as necessary, components such as colorants, elastomers, mercapto compounds, urethanization catalysts, thixotropic agents, adhesion promoters, block copolymers, chain transfer agents, polymerization inhibitors, copper inhibitors, antioxidants, rust inhibitors, thickeners such as organic bentonite and montmorillonite, at least one of silicone-based, fluorine-based, and polymer-based antifoaming agents and leveling agents, imidazole-based, thiazole-based, and triazole-based silane coupling agents, and flame retardants such as phosphinates, phosphate ester derivatives, and phosphorus compounds such as phosphazene compounds. These may be known in the field of electronic materials.

The curable resin composition of the present invention may be used in the form of a dry film or in liquid form. When used in liquid form, it may be a one-component or two-component or more-component composition.

[Cured product]
The cured product of the present invention is obtained by curing the curable resin composition of the present invention described above, and maintains a low gloss over time, thereby achieving high levels of both solder adhesion resistance and voltage resistance over time. The cured product of the present invention is, for example, a solder resist, particularly a matte solder resist, having a thickness of 1 to 150 μm.

[Printed wiring board]
The printed wiring board of the present invention has a cured product obtained from the curable resin composition of the present invention described above. In the method for producing a printed wiring board of the present invention, for example, the curable resin composition of the present invention is adjusted to a viscosity suitable for the coating method using an organic solvent, and applied to a substrate by a method such as dip coating, flow coating, roll coating, bar coating, screen printing, or curtain coating, and then the organic solvent contained in the composition is volatilized and dried (pre-dried) at a temperature of 60 to 100°C to form a tack-free resin layer.

The above-mentioned substrates include printed wiring boards and flexible printed wiring boards with circuits already formed using copper or other materials, as well as copper-clad laminates for high-frequency circuits made from materials such as paper phenol, paper epoxy, glass cloth epoxy, glass polyimide, glass cloth/non-woven cloth epoxy, glass cloth/paper epoxy, synthetic fiber epoxy, fluororesin/polyethylene/polyphenylene ether, polyphenylene oxide/cyanate, etc., including copper-clad laminates of all grades (FR-4, etc.), as well as metal substrates, polyimide film, polyethylene terephthalate film, polyethylene naphthalate (PEN) film, glass substrates, ceramic substrates, wafer plates, etc.

The volatilization drying carried out after applying the curable resin composition of the present invention can be carried out using a hot air circulation drying oven, IR oven, hot plate, convection oven, etc. (a method in which hot air in the dryer is brought into countercurrent contact using a heat source equipped with a steam air heating method, or a method in which hot air is blown onto the support from a nozzle).

After forming a resin layer on a substrate, it is selectively exposed to active energy rays through a photomask with a predetermined pattern formed on it, and the unexposed areas are developed with a dilute alkaline aqueous solution (e.g., a 0.3 to 3% by weight aqueous solution of sodium carbonate) to form a pattern in the cured product. The cured product is then irradiated with active energy rays and then heat-cured (e.g., 100 to 220°C), or is irradiated with active energy rays after heat-curing, or is heat-cured alone for final curing (main curing), forming a cured coating film with excellent properties such as adhesion and hardness.

The exposure device used for the active energy ray irradiation may be any device equipped with a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a metal halide lamp, a mercury short arc lamp, or the like, and capable of irradiating ultraviolet rays in the range of 350 to 450 nm. Direct imaging devices (e.g., laser direct imaging devices that directly draw images with a laser based on CAD data from a computer) can also be used. The lamp or laser light source for the direct imaging device may have a maximum wavelength in the range of 350 to 450 nm. The exposure dose for image formation varies depending on factors such as the film thickness, but can generally be in the range of 10 to 1,000 mJ/ ^cm2 , and preferably 20 to 800 mJ/ ^cm2 .

Developing methods include dipping, showering, spraying, and brushing, and developers include alkaline aqueous solutions such as potassium hydroxide, sodium hydroxide, sodium carbonate, potassium carbonate, sodium phosphate, sodium silicate, ammonia, and amines.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples. In the examples, "parts" and "%" are all by mass unless otherwise specified.

[Preparation of photocurable resin solution]
A photocurable resin (aromatic ring-containing carboxyl group-containing resin) solution used in this example was prepared according to the following procedure.
To 600 g of diethylene glycol monoethyl ether acetate, 1070 g of orthocresol novolac epoxy resin (EPICLON N-695, manufactured by DIC Corporation), 360 g (5.0 mol) of acrylic acid, and 1.5 g of hydroquinone were added, and the mixture was heated at 100 ° C. while stirring until homogeneous. Next, 4.3 g of triphenylphosphine was added to the resulting solution, and the mixture was heated to 110 ° C. for 2 hours to react. The temperature was then raised to 120 ° C. and the reaction continued for another 12 hours to obtain a reaction solution. To the resulting reaction solution, 415 g of aromatic hydrocarbon (Solvesso 150, manufactured by Ando Parachemie Co., Ltd.) and 456.0 g (3.0 mol) of tetrahydrophthalic anhydride were added, and the reaction was continued for 4 hours at 110 ° C. After cooling, a photosensitive carboxyl group-containing resin solution having an aromatic ring with an acid value of 89 mg KOH/g and a solids content of 65% by mass was obtained.

[Preparation of Curable Resin Composition]
The components shown in Table 1 below were mixed in the amounts shown in the table, pre-mixed using a mixer, and then kneaded using a three-roll mill to prepare the curable resin compositions of Examples 1 to 6 and Comparative Examples 1 to 6. Details of each component in Table 1 are as follows.
Thermosetting resin: phenol novolac epoxy resin (N-770, manufactured by DIC Corporation)
Photocurable resin (photosensitive resin): A carboxyl group-containing resin solution having an aromatic ring prepared by the method described above, the amount is calculated as solid content. Photocurable resin (photopolymerizable monomer): Dipentaerythritol hexaacrylate (manufactured by Nippon Kayaku Co., Ltd.).
Blue colorant: organic colorant phthalocyanine blue
Yellow colorant: organic colorant chromophthalo yellow Antifoaming agent: polydimethylsiloxane (KS-66, manufactured by Shin-Etsu Chemical Co., Ltd.)
Photopolymerization initiator 1: bis(η5-2,4-cyclopentadien-1-yl)-bis(2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl)titanium (Irgacure 784, manufactured by BASF Japan Ltd.)
Photopolymerization initiator 2: 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholinophenyl)-butan-1-one (Omnirad 379EG, manufactured by IGM Resins)
Heat curing catalyst 1: 2-phenyl-4,5-dihydroxymethylimidazole (2PHZ-PW, manufactured by Shikoku Chemicals Corporation)
Heat curing catalyst 2: melamine Aluminum silicate-based matting agent: calcined kaolin (Satenton 5HB, manufactured by BASF Japan Ltd.)
Aluminum hydroxide-based matting agent: Aluminum hydroxide (Higilite H-42, manufactured by Showa Denko K.K.)
Filler anti-settling agent: organic bentonite (BENTONE® 38, manufactured by Elementis Specialties)
Finely divided silica-based matting agent: finely divided silica (ACEMATT (registered trademark) 82, manufactured by EVONIK)
Silica-alumina surface-treated barium sulfate (BARIACE® B-30, manufactured by Sakai Chemical Industry Co., Ltd.) (oil absorption: 18 mL/100 g)
Silica surface-treated barium sulfate (highly weather-resistant matte material PFS701, manufactured by Ishihara Sangyo Kaisha, Ltd.) (oil absorption: 56 mL/100 g)
Organic solvent 1: Diethylene glycol monoethyl ether acetate (carbitol acetate)
Organic solvent 2: petroleum-based solvent (SOLVESSO 150, manufactured by Ando Parachemie Co., Ltd.)

[Evaluation of Printing Properties]
Each of the curable resin compositions of the Examples and Comparative Examples was printed on an IPC standard B pattern substrate using a printing machine (manufactured by Ceria Corporation) with a Tetron 100 mesh screen. After curing each curable resin composition, the appearance of the coating film (cured coating film) was visually observed, and the printability of the curable resin composition was evaluated according to the following criteria. The evaluation results are shown in Table 1.
◯: Good printability (good leveling)
△: Acceptable printability (mesh patterns are visible)
×: Unacceptable printability (pinholes present, and the base is visible)

Furthermore, the curable resin compositions of the Examples and Comparative Examples were stored at 30°C for six months after preparation, and then similar tests were carried out to evaluate the printability over time according to the same criteria. The evaluation results are shown in Table 1.

[Preparation of Evaluation Substrates (Examples 1 and 2 and Comparative Examples 1 and 2)]
Each of the curable resin compositions of Examples 1 and 2 and Comparative Examples 1 and 2 was applied to the entire surface of a 1.6 mm thick copper FR-4 substrate by screen printing so that the film thickness after curing would be 25 μm, and the substrate was heated at 150°C for 30 minutes using a hot air circulation drying oven to prepare evaluation substrates having a cured product (cured coating film) of each curable resin composition.

[Preparation of Evaluation Substrates (Examples 3 to 6 and Comparative Examples 3 to 6)]
Each curable resin composition of Examples 3 to 6 and Comparative Examples 3 to 6 was applied to a 1.6 mm thick copper FR-4 substrate by screen printing so that the film thickness after drying was 25 μm, and the substrate was dried at 80 ° C. for 30 minutes using a hot air circulation drying oven and allowed to cool to room temperature. The substrate was exposed to 500 mJ / cm ² using an exposure device equipped with a high-pressure mercury lamp (short arc lamp), and developed by spraying a 1 mass% aqueous sodium carbonate solution at 30 ° C. at a spray pressure of 0.2 MPa for 90 seconds. Next, the substrate was irradiated with ultraviolet light using a UV conveyor oven at an integrated exposure dose of 1000 mJ / cm ² , and then heated at 160 ° C. for 60 minutes using a hot air circulation drying oven to prepare a cured product (cured coating film) of each curable resin composition for evaluation.

[Evaluation of Glossiness]
The gloss of the cured product of each curable resin composition of the Examples and Comparative Examples formed on each evaluation substrate was measured as 60° specular reflectance using a Micro Trigloss glossmeter manufactured by BYK-Chemie. The gloss evaluation was performed on five evaluation substrates, and the average gloss of the five evaluation substrates was rounded to one decimal place and expressed as a number to one decimal place. Based on the obtained values, the gloss was evaluated according to the following criteria. The gloss values and evaluation results are shown in Table 1.
⊚: The gloss level is less than 10, which is extremely low.
Good: The gloss level is 10 or more and less than 20, and is sufficiently low.
×: The gloss level is 20 or more, and does not have a low gloss level.

Furthermore, the same tests were performed on the evaluation substrates prepared after storing each of the curable resin compositions of the Examples and Comparative Examples at 30°C for six months after preparation, and the gloss over time was evaluated according to the same criteria. The gloss over time values and evaluation results are shown in Table 1.

[Evaluation of solder adhesion resistance]
Each evaluation board was subjected to solder flow in the atmosphere without application of flux using a flow soldering device at a solder temperature of 260°C, double wave, and a conveyor speed of 1.4 m/min. The state of solder adhesion on the surface of the cured product of the curable resin composition on each evaluation board was then visually observed, and the solder adhesion resistance was evaluated according to the following criteria. The evaluation results are shown in Table 1. In this evaluation, evaluation boards were prepared in the same manner as in the preparation of the above evaluation boards, except that a copper circuit pattern board was used instead of the solid copper board, pattern printing was used instead of the full-surface screen printing in Examples 1 and 2 and Comparative Examples 1 and 2, and pattern exposure was used instead of the exposure in Examples 3 to 6 and Comparative Examples 3 to 6.
Good: No solder adhesion was observed on the surface of the cured product, and the solder adhesion resistance was good.
×: Thorny or spider web-like solder adhesion was observed, and the solder adhesion resistance was poor.

Furthermore, the curable resin compositions of the Examples and Comparative Examples were prepared and stored at 30°C for six months, after which similar tests were conducted on the evaluation substrates produced, and the solder adhesion resistance over time was evaluated according to the same criteria. The evaluation results are shown in Table 1.

[Evaluation of voltage resistance]
For each evaluation substrate, a voltage was increased at 0.5 kV/sec in AC mode using a 10 mm diameter electrode using a TOS5101 withstand voltage tester manufactured by Kikusui Electronics Co., Ltd., to measure the voltage value (breakdown voltage) at which the cured product of the curable resin composition on each evaluation substrate experienced dielectric breakdown. The evaluation of dielectric breakdown was performed on three evaluation substrates, and the average value of the dielectric breakdown voltages of the three evaluation substrates was rounded to one decimal place and expressed as a number to one decimal place. Based on the obtained values, evaluation was performed according to the following criteria. The dielectric breakdown voltage values and the evaluation results of dielectric breakdown are shown in Table 1. The values of dielectric breakdown voltage (kV/0.05 mm) in Table 1 are the dielectric breakdown voltage values of each evaluation substrate having a cured coating film 25 μm (0.025 mm) thick converted to 0.05 mm.
Good: The dielectric breakdown voltage is 5 kV/0.05 mm or more, and the dielectric breakdown voltage is extremely high.
×: The breakdown voltage is less than 5 kV/0.05 mm, and the voltage resistance is insufficient.

[Evaluation of acid resistance]
Each evaluation substrate was tin-plated using a commercially available electroless tin plating bath under conditions of tin 1±0.2 μm. After plating, the cured product of the curable resin composition on each evaluation substrate was visually observed, and then tape peeling was used to check for peeling of the cured product. Acid resistance was evaluated according to the following criteria. The evaluation results are shown in Table 1. In this evaluation, evaluation substrates were prepared in the same manner as in the preparation of the above evaluation substrates, except that a copper circuit pattern substrate was used instead of the solid copper substrate, pattern printing was used instead of the full-surface coating by screen printing in Examples 1 and 2 and Comparative Examples 1 and 2, and pattern exposure was used instead of the exposure in Examples 3 to 6 and Comparative Examples 3 to 6.
Good: Neither peeling nor whitening of the cured product was observed, and the cured product had sufficiently high acid resistance.
×: Peeling and whitening of the cured product were observed, and the acid resistance was insufficient.

The evaluation results shown in Table 1 indicate that when the curable resin compositions of each Example are used, a solder resist can be formed that maintains low gloss over time and thereby achieves high levels of solder adhesion resistance and voltage resistance. Furthermore, it can be seen that when the curable resin compositions of each Example are used, a solder resist with high acid resistance can be formed. That is, a curable resin composition containing a curable resin, barium sulfate, and a thermosetting catalyst, in which the barium sulfate has an oil absorption of 40 to 70 mL/100 g, can be used to form a solder resist that maintains low gloss over time and thereby achieves high levels of solder adhesion resistance and voltage resistance, and that also has high acid resistance. On the other hand, when the curable resin compositions of each Comparative Example are used, it was found that maintaining low gloss over time and thereby achieving high levels of solder adhesion resistance and voltage resistance are not compatible.

Claims (9)

A curable resin composition comprising a curable resin, barium sulfate, and a thermosetting catalyst,
the curable resin comprises one or more thermosetting resins selected from the group consisting of epoxy compounds, polyfunctional oxetane compounds, and oxazoline compounds;
A curable resin composition characterized in that the oil absorption of the barium sulfate is 40 to 70 mL/100 g. The curable resin composition according to claim 1, wherein the barium sulfate comprises barium sulfate surface-treated with silica. The curable resin composition according to claim 1, wherein the curable resin comprises a carboxyl group-containing photosensitive resin having one or more ethylenically unsaturated groups per molecule. The curable resin composition according to claim 3 , further comprising a photopolymerization initiator. The curable resin composition according to claim 1, which is used to form a solder resist. The curable resin composition according to claim 5 , wherein the solder resist is a matte solder resist. A cured product of the curable resin composition described in claim 1. A printed wiring board comprising the cured product according to claim 7 . A method for manufacturing a printed wiring board provided with a solder resist, the method comprising the step of forming the solder resist by curing the curable resin composition described in claim 1.