JP4864545B2 - Thermosetting solder resist composition for flexible substrate, flexible substrate and method for producing flexible substrate - Google Patents

Description

  The present invention relates to a solder resist ink used for a flexible substrate requiring flexible flexibility, for example, a flexible substrate used as a solder resist ink for manufacturing a printed wiring board of a small electronic device such as a mobile notebook computer or a mobile phone. The present invention relates to a solder resist composition.

  Solder resist used in the manufacture of flexible printed wiring boards and tape carrier packages is a type that can be applied with an adhesive after punching a polyimide film called a coverlay film with a mold that matches the pattern. There are a type in which an ultraviolet curable type, thermosetting type solder resist ink or liquid polyimide ink is formed by screen printing to form a coating film having liquid crystal, and a liquid photo solder resist ink type in which a flexible film is formed.

  The coverlay film has a problem that it cannot follow a copper foil and cannot form a highly accurate pattern. On the other hand, in the ultraviolet curable solder resist ink and the liquid photo solder resist ink, the adhesiveness with the polyimide of the base material is poor, and sufficient flexibility cannot be obtained. Moreover, since the curing shrinkage of the solder resist ink and the cooling shrinkage after curing are large, there is a problem that warpage occurs. In the liquid polyimide ink, soluble aromatic polyimide is used (see Patent Document 1), but there are problems that it is expensive and blurring occurs during printing, and sufficient workability cannot be obtained.

  Further, as a conventional thermosetting solder resist ink, one using a melamine resin or the like has been proposed (for example, see Patent Document 2). However, these are cured only by the dehydration reaction on the surface of the coating film, and flexibility is obtained by having an uncured portion inside the coating film. For this reason, if a solder heat resistance test etc. are repeated, it will harden completely and a coating film will become brittle. In addition, since an acid catalyst is used as a curing catalyst, there is a problem that electrical characteristics and the like are inferior.

  In particular, due to the recent rapid reduction in thickness, the core material of the build-up substrate used for mobile phones and the like has become thinner, and the warpage due to the hardening shrinkage of the solder resist has become an issue to be solved.

Furthermore, a substrate used for a mobile phone tends to accumulate heat generated from a light emitting diode or a display device used for illumination of a push button, and heat dissipation characteristics are required.
JP-A-1-121364, claims, etc. JP-A-50-69563, claims and specification

  The present invention has been made to solve the above-mentioned problems, and its purpose is tough and flexible even when crosslinking is performed with a firm curing reaction without leaving an uncured part, and heat resistance is also improved. Further, it is to provide a solder resist composition for a flexible substrate that is resistant to cracking and can maintain flexibility even when the filler filling rate is increased in order to increase thermal conductivity.

(1) A hydrogenated biphenyl type epoxy resin (A), a rubbery compound (B) having a functional group that reacts with an epoxy group, and aluminum oxide particles (C) , wherein the rubbery compound (B) is It is a carboxyl group-terminated butadiene acrylonitrile and is blended so that the carboxyl group that reacts with the epoxy group is 0.5 to 3.0 equivalents per equivalent of the epoxy group of the epoxy resin blended in the solder resist composition. A thermosetting solder resist composition for a flexible substrate.

(2) The hydrogenated biphenyl type epoxy resin (A) has an average hydrogenation rate of 5 to 95%. The thermosetting solder resist composition for a flexible substrate as described in (1).

(3) Further, the average water containing the biphenyl type epoxy resin (D) and based on the sum ((A) + (D)) of the hydrogenated biphenyl type epoxy resin and the biphenyl type epoxy resin (D) The thermosetting solder resist composition for flexible substrates according to (1) or (2), wherein the addition ratio is 5 to 95%.

(4) The hydrogenated biphenyl type epoxy resin (A) contains 2.5 to 950 parts by mass of a biphenyl type epoxy resin with respect to 50 parts by mass of any one of (1) to (3) The thermosetting solder resist composition for flexible substrates as described.

(5) In any one of (1) to (4) , 60 to 95 parts by volume of aluminum oxide particles (C) are contained in 100 parts by volume of the cured product of the solder resist composition for flexible substrates. The thermosetting solder resist composition for flexible substrates as described.

(6) The aluminum oxide particles (C) are spherical aluminum oxide particles having a thermal conductivity of 15 W / m · K or more and a particle size of 30 μm or less, according to any one of (1) to (5) Thermosetting solder resist composition for flexible substrates.

(7) The aluminum oxide particles (C) are 1/2 to 1 of the average particle diameter of the first aluminum oxide particles with respect to 100 parts by mass of the first aluminum oxide particles having a particle diameter of 20 to 5 μm. The thermosetting solder resist composition for flexible substrates according to any one of (1) to (6) , wherein 20 to 100 parts by mass of second aluminum oxide particles in the range of / 10 are blended .

(8) A solder resist composed of the thermosetting solder resist composition for flexible substrate described in any one of (1) to (7) is formed on a substrate, and 100 parts by volume of the cured product of the solder resist. A flexible substrate comprising 60 to 95 parts by volume of aluminum oxide particles.

(9) A method for producing a flexible substrate, comprising applying a solder resist comprising a thermosetting solder resist composition for a flexible substrate described in any one of (1) to (7) on a substrate.

  In the present invention, the “average hydrogenation rate” is a value obtained by adding all hydrogen to the aromatic ring of the epoxy resin used as a raw material and setting the addition amount (M) to be a cyclohexane ring as 100%. From (X), it was determined as (X) / (M).

  The average hydrogenation rate was determined from the absorbance at 276 nm with a UV spectrophotometer.

  Moreover, the average hydrogenation rate in the mixture is the value (X) that is actually added to the aromatic ring of all epoxy resins as raw materials by adding all hydrogen to 100% as the addition amount (M) that becomes a cyclohexane ring. Therefore, it was determined as (X) / (M).

  The “average particle diameter” refers to the average particle diameter of fine particles obtained by observing a mixture of primary particles and secondary particles (aggregated particles) with a transmission electron microscope (TEM) in an arbitrary range.

  “Spherical shape” means a state in which a shape is rounded as a result of heat treatment after pulverizing mineral or synthetic product. It does not mean a true sphere.

  In general, a rubbery compound (providing flexibility of a cured solder resist) and a liquid bisphenol A type epoxy resin are compatible but have poor heat resistance. Biphenyl type epoxy resin (exactly bixylenol type epoxy resin) is excellent in heat resistance, but it is a powdery epoxy resin with high crystallinity, and a uniform cured product cannot be obtained even if dispersed uniformly. As a result, a cured product having poor flexibility is obtained.

  On the other hand, in the present invention, hydrogenated biphenyl type epoxy resin is used instead of these resins. The hydrogenated biphenyl type epoxy resin is excellent in heat resistance and has low crystallinity, so that it has good compatibility with the rubbery compound and a cured product excellent in flexibility can be obtained. Since a cured product with excellent flexibility is obtained in this way, even if the filler filling rate is increased to increase the thermal conductivity (improving heat dissipation characteristics), it is difficult to break and maintain flexibility. it can.

  The composition according to the present invention contains a hydrogenated biphenyl type epoxy resin, a rubber having a functional group that reacts with the epoxy resin, and aluminum oxide particles, and further contains a biphenyl type epoxy resin as necessary.

(1) Hydrogenated biphenyl type epoxy resin (A)
Any hydrogenated biphenyl type epoxy resin can be used as the hydrogenated biphenyl type epoxy resin. For example, as described in JP-A-11-335439, biphenol compounds (biphenol, 2,2′-dimethylbiphenol, bixylenol, 2,2′-diallylbiphenol, 2,2 ′, 6,6 ′ Hydrogenated biphenyl type epoxy resin obtained by reacting haepihalohydrin after hydrogenation of tetraallylbiphenol or the like) using boron trifluoride or boron trifluoride complex as a catalyst can be used. However, when an acidic phenolic hydroxyl group is changed to a neutral alcoholic hydroxyl group in this way, not only β-cleavage but also α-cleavage occurs in the reaction with epihalohydrin, and a compound that does not cyclize to the epoxy group is generated. The obtained reaction product is a compound containing a relatively large number of halogen atoms in the molecule.

  In contrast, for example, by a method as described in JP-A-2000-226380, a ruthenium-supported catalyst or the like is selectively used only on the benzene ring of a biphenyl type epoxy resin or a bixylenol type epoxy resin. The epoxy resin obtained by hydrogenation is preferable because it has few halogen atoms in the molecule unlike the above method and has excellent electrical characteristics. As a commercial item, Japan Epoxy Resin Co., Ltd. brand name YL6800 etc. are mentioned.

  The average hydrogenation rate of the hydrogenated biphenyl type epoxy resin according to the present invention is to maintain the hydrogenated biphenyl type epoxy resin in a low crystallinity state (in other words, to maintain compatibility with the rubber-like compound). And maintaining good heat resistance. Specifically, the average hydrogenation rate is 5 to 95%, preferably 25 to 75%, more preferably 35 to 65%. When the average hydrogenation rate is lower than the lower limit, the crystallinity becomes strong and the compatibility with the rubbery compound is lowered. Similarly, when it is higher than the upper limit value, the crystallinity becomes strong, the compatibility with the rubbery compound is lowered, and the heat resistance is lowered. In addition, hydrogenated biphenyl type epoxy resins with low crystallinity are generally liquid at room temperature, but hydrogenated biphenyl type epoxy resins compatible with rubber-like compounds must be liquid at room temperature. is not.

  The crystallinity of the hydrogenated biphenyl type epoxy resin is not uniquely determined only by the hydrogenation rate. Other conditions such as substituents (allyl groups, etc.) attached to the raw material biphenyl compound are also hydrogenated biphenyl type. Since these are important factors for determining the crystallinity of the epoxy resin, these are appropriately set in consideration of compatibility with the rubber-like compound.

  For example, the number average molecular weight of the hydrogenated biphenyl type epoxy resin is preferably 300 to 1,000, particularly preferably 380 to 450. By making it within this range, crystallinity can be reduced, compatibility with the rubbery compound is improved, and heat resistance is also excellent.

(2) Other epoxy resins such as biphenyl type epoxy resin (C) In the present invention, well-known and commonly used epoxy resins such as bisphenol A type epoxy are used as long as they do not adversely affect the coating film, such as inhibiting the flexibility of the cured product. Resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, brominated bisphenol A type epoxy resin, hydrogenated bisphenol A type epoxy resin, biphenol type epoxy resin, bixylenol type epoxy resin, phenol novolac type epoxy resin, cresol novolak type Glycidyl ether compounds such as epoxy resin, brominated phenol novolac epoxy resin, bisphenol A novolac epoxy resin; terephthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, dimer Glycidyl ester compounds such as acid diglycidyl ester; triglycidyl isocyanurate, N, N, N ′, N′-tetraglycidylmetaxylenediamine, N, N, N ′, N′-tetraglycidylbisaminomethylcyclohexane, N, Glycidylamine compounds such as N-diglycidylaniline, alicyclic epoxy compounds, epoxidized polybutadiene, and the like can be blended.

  Examples of the biphenyl type epoxy resin (generic name for biphenol type epoxy resin, bixylenol epoxy resin, etc.) include trade name YX4000 manufactured by Japan Epoxy Resin Co., Ltd.

  When the biphenyl type epoxy resin is blended, the average hydrogenation rate based on the sum of the hydrogenated biphenyl type epoxy resin and the biphenyl epoxy resin is a state in which the hydrogenated biphenyl type epoxy resin has low crystallinity (that is, rubber In a state excellent in compatibility with the glassy compound) and maintaining good heat resistance. Specifically, the average hydrogenation rate is 5 to 95%, preferably 25 to 75%, more preferably 35 to 65%. When the average hydrogenation rate is lower than the lower limit, the crystallinity becomes strong and liquefaction becomes difficult. Similarly, when it is higher than the upper limit, the crystallinity becomes strong and liquefaction becomes difficult. In addition, heat resistance is reduced.

  The blending ratio of the biphenyl type epoxy resin is 2.5 to 950 parts by mass, preferably 10 to 300 parts by mass, particularly preferably 30 to 50 parts by mass of the hydrogenated biphenyl type epoxy from the viewpoint of heat resistance improvement and crystallinity. It is good to set it to -100 mass parts.

  The number average molecular weight of the biphenyl type epoxy resin is preferably 300 to 1,000, particularly 380 to 450. By making it within this range, crystallinity can be reduced, compatibility with the rubbery compound is improved, and heat resistance is also excellent.

(3) Rubber-like compound having a functional group that reacts with an epoxy group (B)
The rubbery compound is a component that imparts flexibility to the cured solder resist.

  Examples of the functional group that reacts with the epoxy group include a carboxyl group, an acid anhydride group, a hydroxyl group, and an amino group. Examples of the rubber-like compound having these functional groups include maleated polybutadiene obtained by reacting polybutadiene and maleic anhydride, and further reacting a primary alcohol with the maleated polybutadiene to provide maleated polybutadiene having a free carboxyl group. Examples include half esters, carboxyl group-terminated butadiene acrylonitrile, amino group-containing butadiene acrylonitrile.

  Among these, carboxyl group-terminated butadiene acrylonitrile called CTBN is particularly preferable. The carboxyl group-terminated butadiene acrylonitrile preferably has a molecular weight of 2000 to 5000. Examples of commercially available products include Hiker CTBN2000 × 162, CTBN1300 × 31, CTBN1300 × 8, CTBN1300 × 13, and CTBNX1300 × 9 manufactured by Ube Industries.

  The blending ratio of these rubber-like compounds (B) is an epoxy resin (hydrogenated biphenyl type epoxy resin and other additives added as necessary) from the viewpoint of electrical insulation and flexibility of the cured product. Functional group that reacts with the epoxy group (carboxyl group in the case of carboxyl-terminated butadiene acrylonitrile) 0.5 to 3.0 equivalents, preferably 0.8 to 2.0 equivalents per equivalent of epoxy group It is good that it is blended.

(4) Aluminum oxide particles The aluminum oxide particles (C) having a thermal conductivity of 15 W / m · K or more and a particle size of 30 μm or less, preferably 20 μm or less are used. The lower limit of the particle size is preferably 0.01 μm. If it is smaller than 0.01 μm, the viscosity of the composition becomes too high, so that it is difficult to disperse and application to an object to be coated becomes difficult. If it is larger than 30 μm, cueing to the coating film occurs and the sedimentation speed is increased, so that the storage stability is deteriorated.

  Aluminum oxide particles (C) having a thermal conductivity of 15 W / m · K or more can be obtained by setting the purity to 92% or more.

  The blending amount of the aluminum oxide particles is 60 to 95 parts by volume, preferably 70 to 90 parts by volume in 100 parts by volume of the solder resist cured product in consideration of heat dissipation characteristics and flexibility. Good part.

  Furthermore, the aluminum oxide particles can be made to have a higher packing by blending two or more types having an average particle size with a particle size distribution that results in the closest packing, and the storage stability and thermal conductivity are improved. Preferred from both sides. For example, a particle size distribution that provides close-packing is 1/100 of the average particle size of the first aluminum oxide particles with respect to 100 parts by mass of the first aluminum oxide particles in the range of 20 to 5 μm. 20-100 mass parts of 2nd aluminum oxide particles in the range of 2/1/10 are mix | blended.

  As typical commercial products of spherical aluminum oxide particles (C) used in the present invention, DAW-05 (manufactured by Denki Kagaku Kogyo Co., Ltd., average particle size 5 μm), DAW-10 (manufactured by Denki Kagaku Kogyo Co., Ltd., average particle size) Diameters 10 μm), AS-40 (manufactured by Showa Denko KK, average particle size 12 μm), AS-50 (manufactured by Showa Denko KK, average particle size 9 μm), and the like.

(5) Other compounding components The solder resist composition for a flexible substrate according to the present invention may contain an organic solvent for adjusting the composition and adjusting the viscosity. Examples of the organic solvent include ketones such as methyl ethyl ketone and cyclohexanone; aromatic hydrocarbons such as toluene, xylene, and tetramethylbenzene; cellosolve, methyl cellosolve, butyl cellosolve, carbitol, methyl carbitol, butyl carbitol, propylene glycol Glycol ethers such as monomethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol diethyl ether, tripropylene glycol monomethyl ether; 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 DOO, esters such as propylene carbonate; octane, aliphatic hydrocarbons decane; petroleum ether, petroleum naphtha, and organic solvents such as petroleum-based solvents such as solvent naphtha used. These organic solvents can be used alone or in combination of two or more.

  The blending amount of these organic solvents is not particularly limited, but in order to ensure the coating properties and the film thickness of the cured coating film, it is 50% by mass or less, preferably 30% by mass or less in the composition. is there.

  In the solder resist composition for a flexible substrate according to the present invention, a well-known and commonly used inorganic filler or organic filler can be used as long as it does not adversely affect the coating film.

  Examples of these fillers include inorganic fillers such as barium sulfate, talc, silica, aluminum oxide, and aluminum hydroxide, and organic fillers such as benzoguanamine resin. The particle size of such a filler is 20 μm or less, preferably 10 μm or less, from the viewpoint of maintaining good electrical characteristics. If the particle size is larger than the above range, the surface state of the coating film is deteriorated by the filler protruding from the coating film, moisture and the like enter through the filler interface, and electrical characteristics are deteriorated.

  The blending amount of these fillers is 50% by mass or less, preferably 30% by mass or less in the composition. When the amount is more than the above range, warpage due to curing shrinkage of the coating film is reduced, but flexibility is impaired, which is not preferable.

  The solder resist composition for a flexible substrate according to the present invention is a known and commonly used phthalocyanine blue, phthalocyanine green, iodin green, disazo yellow, crystal violet, titanium oxide, carbon black, naphthalene black, etc. Colorants, hydroquinone, hydroquinone monomethyl ether, t-butylcatechol, pyrogallol, phenothiazine and other well-known and conventional thermal polymerization inhibitors, finely-silica, organic bentonite, montmorillonite and other well-known and conventional thickeners, silicone-based, fluorine-based, high Well-known and conventional additives such as molecular-type antifoaming agents and / or leveling agents, imidazole-based, thiazole-based, triazole-based silane coupling agents, and the like can be blended.

(5) Solder resist coating method using this composition and properties of the cured film obtained The solder resist composition for a flexible substrate according to the present invention as described above is usually applied to a flexible printed wiring board on which a circuit is formed. Coating is performed by a screen printing method using a 250 mesh screen plate making. The coated substrate is heated to a temperature of 120 to 200 ° C., for example, and cured for 20 to 120 minutes. If necessary, two-stage curing may be performed by changing the temperature and time.

  The cured coating film thus obtained is free from warping due to curing shrinkage and cooling shrinkage, and adheres to the substrate, flex resistance, folding resistance, flexibility, plating resistance, PCT resistance, solder heat resistance, Excellent electrical insulation.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In the following description, “part” means part by mass unless otherwise specified.

  The thermosetting compositions of Examples and Comparative Examples were pattern printed on a circuit-formed FR-4 substrate by screen printing so that the dried coating film was about 30 μm, and cured at 170 ° C. for 60 minutes.

(Evaluation)
(1) Solvent resistance The obtained substrate was immersed in propylene glycol monomethyl ether acetate for 30 minutes, dried, and then subjected to a peel test using a cellophane adhesive tape to evaluate peeling and discoloration of the coating film.

○: No peeling or discoloration ×: Separation or discoloration (2) Heat resistance Rosin flux on a substrate obtained by the same method as solvent resistance using the thermosetting compositions of Examples and Comparative Examples Is applied for 30 seconds in a solder bath at 260 ° C., washed and dried with propylene glycol monomethyl ether acetate, then peel-tested with a cellophane adhesive tape, and the maximum number of times the film does not peel off.

(3) Pencil hardness Using a thermosetting composition of Examples and Comparative Examples, sharpen a B to 9H pencil core so that the tip is flat on a substrate obtained by the same method as the solvent resistance. Pressing at an angle of 45 °, the maximum hardness of the pencil where the copper foil under the coating was not visible was recorded.

(4) Electrical insulation The thermosetting compositions of Examples and Comparative Examples were patterned so that the dried coating film was about 30 μm by screen printing on an FR-4 substrate on which comb-shaped electrodes of IPC standard 8 patterns were formed. Printed and cured at 170 ° C. for 60 minutes. The insulation resistance value between the electrodes of the obtained substrate was measured at an applied voltage of 500V.

(5) Flexibility The thermosetting compositions of Examples and Comparative Examples were each printed on the entire surface of a Kapton material (thickness 50 μm) by screen printing and cured at 170 ° C. for 60 minutes (dry film thickness 20 μm). The bending resistance of the cured coating film was evaluated according to the following criteria under conditions of 1/8 inch diameter and 10 cycles in accordance with IPC-SM-849B TM2.4.29.

○: The cured coating film has no cracks Δ: The cured coating film has some cracks ×: The cured coating film has cracks Synthesis Example 1
A flask equipped with a thermometer, a stirrer, a dropping funnel and a reflux condenser was charged with 214.3 parts of carbitol acetate and 24.0 parts of azobisisobutyronitrile and heated to 70 ° C. under a nitrogen atmosphere. . Thereto was added dropwise a mixed monomer of 18.0 parts of acrylic acid, 162.5 parts of i-butyl methacrylate, and 319.5 parts of 2-ethylhexyl methacrylate over 3 hours. The mixture was further stirred and reacted for 4 hours, and the polycarboxylic acid resin solution had a non-volatile content of 70%, a solid content acid value of 28.1 mgKOH / g, a mass average molecular weight of 16,000, and a calculated Tg value of FOX = 9.7 ° C Got. Hereinafter, this polycarboxylic acid resin solution is referred to as A varnish.

The components and evaluation of each composition are summarized in Table 1 below.

  As clearly shown in Table 1, the solder resists of Examples 1 and 2 are excellent in flexibility, solvent resistance, heat resistance, pencil hardness, electrical insulation, heat dissipation, and for flexible substrates. It turns out that it has the outstanding characteristic.

  On the other hand, the comparative example 1 mix | blends a biphenyl type epoxy resin and liquid CTBN with aluminum oxide (alumina), without mix | blending hydrogenated biphenyl type epoxy resin. In Comparative Example 1, the biphenyl type epoxy resin had poor blending due to crystallinity.

  Comparative Example 2 is an example in which aluminum oxide was blended without blending hydrogenated biphenyl type epoxy resin, biphenyl type epoxy resin, or rubber. It can be seen that this product is inferior in flexibility, solvent resistance and heat resistance.

  Comparative Example 3 has the same blending components as Comparative Example 2, but the amount of aluminum oxide blended is larger than that of Comparative Example 2. In this example, ink could not be formed.

Claims (9)

It contains a hydrogenated biphenyl type epoxy resin (A), a rubbery compound (B) having a functional group that reacts with an epoxy group, and aluminum oxide particles (C), and the rubbery compound (B) has a carboxyl group terminal. It is characterized by being blended so that the carboxyl group that reacts with the epoxy group is 0.5 to 3.0 equivalents per equivalent of epoxy group of the epoxy resin blended with butadiene acrylonitrile and in the solder resist composition. A thermosetting solder resist composition for flexible substrates. The thermosetting solder resist composition for a flexible substrate according to claim 1, wherein the hydrogenated biphenyl type epoxy resin (A) has an average hydrogenation rate of 5 to 95%. Furthermore, the biphenyl type epoxy resin (D) is contained, and the average hydrogenation rate when the sum ((A) + (D)) of the hydrogenated biphenyl type epoxy resin and the biphenyl type epoxy resin (D) is used as a standard is It is 5-95%, The soldering resist composition for flexible substrates of Claim 1 or 2 characterized by the above-mentioned. The flexible substrate according to any one of claims 1 to 3, wherein 2.5 to 950 parts by mass of a biphenyl type epoxy resin is contained with respect to 50 parts by mass of the hydrogenated biphenyl type epoxy resin (A). Thermosetting solder resist composition. The cured product 100 parts by volume of the flexible substrate for a solder resist composition, for a flexible substrate according to any one of claims 1-4, characterized by containing 60 to 95 parts by volume of aluminum oxide particles (C) Thermosetting solder resist composition. The aluminum oxide particles (C) are spherical aluminum oxide particles having a thermal conductivity of 15 W / m · K or more and a particle size of 30 µm or less, and the thermosetting for a flexible substrate according to any one of claims 1 to 5. Type solder resist composition. The aluminum oxide particles (C) are ½ to 1/10 of the average particle diameter of the first aluminum oxide particles with respect to 100 parts by mass of the first aluminum oxide particles having a particle diameter of 20 to 5 μm. The thermosetting solder resist composition for a flexible substrate according to any one of claims 1 to 6 , wherein 20 to 100 parts by mass of second aluminum oxide particles within the range are blended. A solder resist made of the thermosetting solder resist composition for a flexible substrate according to any one of claims 1 to 7 is formed on a substrate, and aluminum oxide particles are contained in 100 parts by volume of the cured product of the solder resist. Contains 60 to 95 volume parts of a flexible substrate. A method for producing a flexible substrate, comprising: applying a solder resist comprising the thermosetting solder resist composition for a flexible substrate according to any one of claims 1 to 7 on the substrate.