Heat-curable resin composition, cured product, and printed wiring board

Abstract

[Problem] To provide a heat-curable resin composition that achieves both high heat conductivity and dielectric breakdown resistance, that can form a cured product which has a highly smooth surface, and that has excellent storage stability at normal temperatures. [Solution] An epoxy resin, a thermally conductive filler, a phosphate ester-based wetting dispersant, and a urea-based rheology modifier are added to a thermosetting resin, the content of the phosphate ester-based wetting dispersant and the content of urea-based rheology modifier are adjusted to be in respective specific ranges, and a thixotropic index at 25°C is adjusted to be not less than 1.0 but less than 1.4.

Description

Thermosetting resin composition, cured product, and printed wiring board

[0001] The present invention relates to a thermosetting resin composition, and particularly to a thermosetting resin composition suitably used for forming a high thermal conductivity and high withstand voltage insulating layer. Further, the present invention also relates to a cured product of the thermosetting resin composition and a printed wiring board including the cured product.

[0002] In recent years, with the high-performance improvement of various electrical products, the problem of heat control has become prominent. In particular, with the spread of electric vehicles (EVs), since a large amount of electric power is consumed, heat control can be said to be one of the examples where it is extremely important. While EVs attract attention as a sustainable means of transportation, they are equipped with advanced technologies such as high-output motors and rapid charging systems, and efficient energy conversion is required for these functions. In this process, power semiconductors play an essential role, but these devices have a problem that while flowing a high current, an enormous amount of heat is generated at the same time. For example, in the inverters and chargers of EVs, since power semiconductors are used as switching elements, a large amount of energy can be converted instantaneously. On the other hand, the amount of heat generation also increases, and if the heat generation becomes excessive, it can cause performance degradation and failure of the device. Therefore, efficiently cooling the power semiconductor itself or the printed wiring board on which the power semiconductor is mounted is extremely important for maintaining the safety and reliability of EVs.

[0003] Conventionally, in order to cool a printed wiring board on which a power semiconductor is mounted, a technique has been adopted in which a heat sink is attached to the printed wiring board via a thermally conductive adhesive or thermal grease to dissipate the generated heat. Also, a technique using a resin composition in which a thermally conductive filler is highly filled in the insulating layer of the printed wiring board has been proposed. However, when highly filling the thermally conductive filler in the insulating layer in this way, problems such as a decrease in the breakdown voltage resistance and a decrease in the adhesion to the heat sink due to the easy occurrence of unevenness on the surface of the insulating layer can occur.

[0004] Further, for the insulating layer of a printed wiring board on which a power semiconductor is mounted, in addition to high thermal conductivity and breakdown voltage resistance, high smoothness of the insulating layer surface is required.

[0005] To solve these problems, Patent Documents 1 and 2 propose a technique for achieving both high thermal conductivity and dielectric breakdown resistance in a thermosetting resin composition by highly filling epoxy resin with filler, and for forming an insulating layer of a printed circuit board using such a thermosetting resin composition. In particular, Patent Document 2 proposes a technique for improving the smoothness of the insulating layer, improving the adhesion of the insulating layer to the heat sink, and improving thermal conductivity by adjusting the thixotropic index (Ti value) of the thermosetting resin composition to a specific range.

[0006] However, depending on the Ti value, filler sedimentation may occur during the curing of the thermosetting resin composition, and if a thick resin-rich layer with a low content of thermally conductive fillers is formed on the surface of the cured product, the relative thermal conductivity may decrease. Furthermore, if the Ti value is low, filler sedimentation is more likely to occur, raising concerns that the storage stability of the thermosetting resin composition at room temperature may deteriorate. In addition, for thermosetting resin compositions with a short pot life, long-term storage is often required in a refrigerated or frozen environment rather than at room temperature. However, when storing thermosetting resin compositions in a refrigerated or frozen environment, thawing of the thermosetting resin composition is required before use, which takes time. Therefore, from the viewpoint of ease of handling and work efficiency, it is preferable to have a thermosetting resin composition that is stable for long-term storage at room temperature.

[0007] Furthermore, Patent Document 3 proposes a technique for suppressing filler sedimentation, preventing non-uniformity of the cured product, and improving mechanical strength by incorporating a urea-based compound and a wetting dispersant in specific proportions into the resin composition. However, in order to achieve good adhesion between the cured product and the heat sink of a resin composition highly filled with thermally conductive filler, it is desirable not to completely suppress the sedimentation of the thermally conductive filler, but rather to allow a slight skin layer to form on the surface of the cured product. Therefore, depending on the degree to which filler sedimentation is suppressed, it may not be possible to achieve sufficiently good adhesion between the cured resin composition and the heat sink.

[0008] Japanese Patent Publication No. 2024-122722, Japanese Patent Publication No. 2024-122724, Japanese Patent Publication No. 2016-60826

[0009] Under these circumstances, a continuing technical challenge exists to provide a thermosetting resin composition that achieves both high thermal conductivity and dielectric breakdown resistance, and furthermore, forms a cured product with excellent adhesion to the cooling environment due to its highly smooth surface, while also exhibiting excellent storage stability at room temperature.

[0010] Therefore, the present invention aims to provide a thermosetting resin composition that can form a cured product having both high thermal conductivity and dielectric breakdown resistance, and further having a highly smooth surface, and that also has excellent storage stability at room temperature. Furthermore, another object of the present invention is to provide a cured product of such a thermosetting resin composition, and a printed circuit board equipped with the cured product.

[0011] As a result of diligent research, the present inventors have found that the above-mentioned problems can be solved by adjusting the content of the phosphate ester wetting and dispersing agent to 6 to 12 parts by mass per 100 parts by mass of epoxy resin, and the content of the urea-based rheology modifier to 1 to 12 parts by mass per 100 parts by mass of epoxy resin, in a thermosetting resin composition containing an epoxy resin, a thermally conductive filler, a phosphate ester wetting and dispersing agent, and a urea-based rheology modifier, so that the thixotropic index of the thermosetting resin composition at 25°C is 1.0 or more and less than 1.4. The present invention is based on this finding. That is, the gist of the present invention is as follows.

[0012] [1] A thermosetting resin composition comprising an epoxy resin, a thermally conductive filler, a phosphate ester wetting and dispersing agent, and a urea-based rheology modifier, wherein the content of the phosphate ester wetting and dispersing agent is 6 to 12 parts by mass per 100 parts by mass of the epoxy resin, the content of the urea-based rheology modifier is 1 to 12 parts by mass per 100 parts by mass of the epoxy resin, and the thixotropic index of the thermosetting resin composition at 25°C is 1.0 or more and less than 1.4. [2] The thermosetting resin composition according to [1], which does not contain a solvent. [3] The thermosetting resin composition according to [1] or [2], wherein the ratio of the mass of the urea-based rheology modifier to the mass of the phosphate ester wetting and dispersing agent in the thermosetting resin composition is 0.2 to 1.5. [4] The thermosetting resin composition according to any one of [1] to [3], wherein the thermally conductive filler comprises aluminum oxide. [5] The thermosetting resin composition according to any one of [1] to [4], wherein the content of the thermally conductive filler is 700 parts by mass or more per 100 parts by mass of the epoxy resin. [6] A cured product obtained by curing the thermosetting resin composition according to any one of [1] to [5]. [7] A printed circuit board comprising the cured product according to [6].

[0013] According to the present invention, it is possible to provide a thermosetting resin composition that can achieve both high thermal conductivity and dielectric breakdown resistance, and can form a cured product having a highly smooth surface. Furthermore, according to the present invention, it is possible to provide a cured product of such a thermosetting resin composition, and a printed circuit board equipped with the cured product.

[0014] [Thermosetting Resin Composition] According to one aspect of the present invention, a thermosetting resin composition (hereinafter also referred to as "the thermosetting resin composition of the present invention") is provided. The thermosetting resin composition of the present invention contains an epoxy resin, a thermally conductive filler, a phosphate ester wetting and dispersing agent, and a urea-based rheology modifier as essential components. The thermosetting resin composition of the present invention contains an epoxy resin and a thermally conductive filler, and by further adjusting the content of the phosphate ester wetting and dispersing agent and the urea-based rheology modifier to a predetermined range, and by adjusting the thixotropic index (Ti value) at 25°C to a predetermined range, the storage stability of the thermosetting resin composition at room temperature can be made excellent, and furthermore, a high level of thermal conductivity and dielectric breakdown resistance can be achieved in the cured product, and furthermore, high smoothness can be imparted to the surface of the cured product.

[0015] The thermosetting resin composition of the present invention has a Ti value of 1.0 or more and less than 1.4 at 25°C, preferably 1.0 to 1.3, more preferably 1.1 to 1.3, and even more preferably 1.2 to 1.3. In particular, the thermosetting resin composition of the present invention, having a Ti value of 1.0 or more at 25°C within the above range allows for appropriate sedimentation of thermally conductive fillers during the thermosetting of the thermosetting resin composition, enabling the formation of a resin-rich skin layer on the surface of the cured product. Furthermore, a Ti value of 1.0 or more at 25°C suppresses deterioration of storage stability due to sedimentation of thermally conductive fillers in the thermosetting resin composition. Moreover, a Ti value of less than 1.4 at 25°C allows the applied thermosetting resin composition to exhibit appropriate fluidity, enabling the acquisition of a smooth surface without impairing leveling properties. Furthermore, having a Ti value of 1.0 or more at 25°C within the above range suppresses excessive sedimentation of thermally conductive fillers. Excessive sedimentation of thermally conductive fillers can impair the storage stability of thermosetting resin compositions at room temperature. Therefore, suppressing excessive sedimentation of thermally conductive fillers can improve the storage stability of thermosetting resin compositions at room temperature.

[0016] The Ti value of a thermosetting resin composition can be determined by measuring the viscosity of the thermosetting resin composition when it is rotated at 1 rpm and 5 rpm at 25°C using a cone plate type viscometer TV-30 manufactured by Toki Sangyo Co., Ltd., and calculating the ratio of the viscosity when it is rotated at 1 rpm to the viscosity when it is rotated at 5 rpm (viscosity when rotated at 1 rpm / viscosity when rotated at 5 rpm).

[0017] The thermosetting resin composition of the present invention preferably has a viscosity at 25°C of 250 to 900 Pa·s, more preferably 350 to 800 Pa·s, and even more preferably 450 to 750 Pa·s. Having a viscosity within the above range allows the thermosetting resin composition to be applied well to the substrate, and more specifically, prevents the layer of the thermosetting resin composition from becoming excessively thick or thin when applied to the substrate. Furthermore, having a viscosity of 900 Pa·s or less prevents air (bubbles) from entering the layer of the thermosetting resin composition when applied to the substrate, thereby suppressing a decrease in dielectric breakdown resistance caused by such bubbles in the layer of the thermosetting resin composition.

[0018] The viscosity of a thermosetting resin composition can be determined by using a cone-plate viscometer TV-30 manufactured by Toki Sangyo Co., Ltd., and rotating the composition at 25°C and a rotation speed of 5 rpm.

[0019] The thermosetting resin composition of the present invention preferably has a cured product with a thermal conductivity of 2.5 W / m·K or higher, more preferably 2.7 W / m·K or higher, even more preferably 3.0 W / m·K or higher, and particularly preferably 3.2 W / m·K. Having the thermal conductivity of the cured product within the above range allows for efficient cooling of the printed circuit board containing the cured product.

[0020] The thermal conductivity of the cured product of a thermosetting resin composition can be measured as follows. First, the thermosetting resin composition is applied to rolled copper foil using a T-100B screen plate to print a coating film with a dry film thickness of 70 μm. Next, the substrate with the printed coating film is left to stand horizontally and defoamed at 23°C for 10 minutes for leveling. Then, it is left to stand in a hot air circulating box-type drying oven and pre-cured at 100°C for 20 minutes, followed by post-curing at 160°C for 30 minutes to heat-cur the thermosetting resin composition and form the first layer of cured coating film. A second layer of coating film is printed on top of the first layer of cured coating film using the same procedure to print a second layer of coating film with a dry film thickness of 70 μm. After leveling and pre-curing using the same procedure as the first layer, it is post-cured at 160°C for 60 minutes to form the second layer of cured coating film. Next, the thermal diffusivity of the film-like cured material obtained by peeling off the rolled copper foil is measured using a periodic heating method with a load of 5 kgf / cm³ using an FTC-1 manufactured by ULVAC, Inc. The specific heat capacity of the cured material is measured using a differential scanning calorimetry (DSC) manufactured by Perkin-Elmer, with a heating rate of 20°C / min, a helium gas flow rate of 20 ml / min, and a sample volume of 15 mg. The density of the cured material is measured at room temperature (25°C) using the water displacement method. Based on the measured thermal diffusivity (α), specific heat capacity (c), and density (ρ) of the cured material, the thermal conductivity (λ) of the cured material is calculated using the following formula: Thermal conductivity (λ) = Thermal diffusivity (α) × Specific heat capacity (c) × Density (ρ)

[0021] The components of the thermosetting resin composition of the present invention will be described in detail below. (Epoxy Resin) The thermosetting resin composition of the present invention contains an epoxy resin as an essential component. The epoxy resin can be used without limitation as long as it has two or more epoxy groups in one molecule. For example, bisphenol A type epoxy resin, bisphenol F type epoxy resin, hydrogenated bisphenol A type epoxy resin, brominated bisphenol A type epoxy resin, bisphenol S type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, bisphenol A novolac type epoxy resin, biphenyl type epoxy resin, naphthol type epoxy resin, naphthalene type epoxy resin, dicyclopentadiene type epoxy resin, triphenylmethane type epoxy resin, alicyclic epoxy resin, aliphatic chain epoxy resin, phosphorus-containing epoxy resin, anthracene type epoxy resin, norbornene type epoxy resin, adamantane type epoxy resin, fluorene type epoxy resin, aminophenol type epoxy resin, aminocresol type epoxy resin, alkylphenol type epoxy resin, etc. The epoxy resins mentioned above may be used individually or in combination of two or more types.

[0022] Furthermore, the thermosetting resin composition of the present invention may also contain an epoxy resin having a bisphenol-type skeleton. Examples of epoxy resins having a bisphenol-type skeleton include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol E (AD) type epoxy resin, and bisphenol S type epoxy resin, but among these, bisphenol A type epoxy resin, bisphenol F type epoxy resin, and bisphenol E (AD) type epoxy resin are preferred. The epoxy resin having a bisphenol-type skeleton can be used in any form, such as liquid, semi-solid, or solid, but among these, liquid form is preferred from the viewpoint of filling properties. Note that liquid form means a liquid state that is fluid at 20°C.

[0023] These epoxy resins having a bisphenol-type skeleton may be used individually or in combination of two or more types. However, from the viewpoint of achieving excellent filling properties and thus having a better effect on the properties after curing, it is preferable to use a combination of two or more epoxy resins selected from bisphenol A type epoxy resin, bisphenol F type epoxy resin, and bisphenol E (AD) type epoxy resin. Examples of commercially available products include ZX-1059 manufactured by Nippon Steel Chemical & Material Co., Ltd., jER® 828, jER® 834, jER® 1001 (bisphenol A type epoxy resin), jER® 807, jER® 4004P (bisphenol F type epoxy resin) manufactured by Mitsubishi Chemical Corporation, and R710 (bisphenol E type epoxy resin) manufactured by Air Water Inc.

[0024] Furthermore, the thermosetting resin composition of the present invention may also contain a polyfunctional epoxy resin. Examples of commercially available polyfunctional epoxy resins include EP-3300E, a hydroxybenzophenone-type liquid epoxy resin manufactured by ADEKA Corporation; jER® 630, a para-aminophenol-type liquid epoxy resin manufactured by Mitsubishi Chemical Corporation; ELM-100, a para-aminophenol-type liquid epoxy resin manufactured by Sumitomo Chemical Co., Ltd.; jER® 604, a glycidylamine-type epoxy resin manufactured by Mitsubishi Chemical Corporation; Epotote YH-434, a Nippon Steel Chemical & Material Corporation; SumiEpoxy® ELM-120, a glycidylamine-type epoxy resin manufactured by Sumitomo Chemical Co., Ltd.; DEN-431, a phenol novolac-type epoxy resin manufactured by Dow Chemical Company; and Celoxide 2021P, an alicyclic epoxy resin manufactured by Daicel Corporation. These polyfunctional epoxy resins may be used individually or in combination of two or more types.

[0025] The epoxy resin content in the thermosetting resin composition is not particularly limited as long as the effects of the present invention are achieved, but is preferably 5 to 20% by mass, more preferably 10 to 20% by mass, based on the total mass of the thermosetting resin composition.

[0026] (Curing Agent) The thermosetting resin composition of the present invention may contain a curing agent for curing the thermosetting resin. As the curing agent, known curing agents commonly used for curing thermosetting resins, particularly the epoxy resins described above, can be used. Specific curing agents include, for example, amines, imidazoles, polyfunctional phenols, acid anhydrides, isocyanates, imidazole latent curing agents such as imidazole adducts, and polymers containing these functional groups. Examples of amines include dicyandiamide and diaminodiphenylmethane. Examples of imidazoles include alkyl-substituted imidazoles and benzimidazole. Examples of polyfunctional phenols include hydroquinone, resorcinol, bisphenol A and its halogen compounds, and novolac and resol resins, which are condensates of these with aldehydes. Examples of acid anhydrides include phthalic anhydride, hexahydrophthalic anhydride, methylnadic anhydride, and benzophenonetetracarboxylic acid. Examples of isocyanates include tolylene diisocyanate and isophorone diisocyanate. These isocyanates can also be masked with phenols or the like. The curing agent may be used alone or in combination of two or more types.

[0027] Of the curing agents described above, amines and imidazoles can be suitably used from the viewpoint of adhesion to conductive and insulating parts, storage stability, and heat resistance. Specifically, adduct compounds of aliphatic polyamines such as alkylenediamines having 2 to 6 carbon atoms, polyalkylene polyamines having 2 to 6 carbon atoms, and aromatic ring-containing aliphatic polyamines having 8 to 15 carbon atoms, or adduct compounds of alicyclic polyamines such as isophorone diamine and 1,3-bis(aminomethyl)cyclohexane, or mixtures of the above-mentioned aliphatic polyamine adduct compounds and the above-mentioned alicyclic polyamine adduct compounds can be suitably used. In particular, curing agents mainly composed of xylylenediamine or isophorone diamine adduct compounds are preferred.

[0028] Preferred adduct compounds for aliphatic polyamines are those obtained by adding an aryl glycidyl ether (particularly phenyl glycidyl ether or tolyl glycidyl ether) or an alkyl glycidyl ether to an aliphatic polyamine. Preferred adduct compounds for alicyclic polyamines are those obtained by adding an n-butyl glycidyl ether, bisphenol A diglycidyl ether, or the like to an alicyclic polyamine.

[0029] Examples of aliphatic polyamines include alkylenediamines with 2 to 6 carbon atoms, such as ethylenediamine and propylenediamine; polyalkylene polyamines with 2 to 6 carbon atoms, such as diethylenetriamine and triethylenetriamine; and aromatic ring-containing aliphatic polyamines with 8 to 15 carbon atoms, such as xylylenediamine.

[0030] Examples of alicyclic polyamines include isophoronediamine, 1,3-bis(aminomethyl)cyclohexane, bis(4-aminocyclohexyl)methane, norbornenediamine, 1,2-diaminocyclohexane, and laromine.

[0031] Examples of imidazoles include reaction products of epoxy resins and imidazoles. Specific examples of imidazoles include 2-methylimidazole, 4-methyl-2-ethylimidazole, 2-phenylimidazole, 4-methyl-2-phenylimidazole, 1-benzyl-2-methylimidazole, 2-ethylimidazole, 2-isopropylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, and 1-cyanoethyl-2-undecylimidazole.

[0032] Of the curing agents mentioned above, from the viewpoint of maintaining the storage stability of the thermosetting resin composition, the composition may contain at least two of the curing agents, one of which may be an imidazole. Furthermore, from the viewpoint of suppressing cracks and delamination, it is preferable to include at least one of polyamines and indazole latent curing agents. In addition, if imidazoles are included, it is preferable to include two or more types of imidazoles.

[0033] The curing agent content is preferably 1 to 35 parts by mass, more preferably 4 to 30 parts by mass, per 100 parts by mass of thermosetting resin, from the viewpoint of storage stability, viscosity ratio R, and ease of adjusting the pencil hardness measured by a pencil hardness test in accordance with JIS-K5600-5-4:1999 at a state of heating at 100°C for 160 minutes, to a more appropriate range. Furthermore, when imidazoles and other curing agents are used in combination, the content ratio of imidazoles to other curing agents is preferably 1:99 to 99:1, more preferably 10:90 to 90:10, by mass.

[0034] (Thermal Conductivity Filler) The thermosetting resin composition of the present invention contains a thermal conductivity filler. Any particles capable of conducting heat can be used as the thermal conductivity filler without particular limitations. By incorporating a thermal conductivity filler into the thermosetting resin composition, it is expected that the thermal conductivity of the thermosetting resin composition will be improved. The thermal conductivity filler may be used alone or in combination of two or more types. By including a thermal conductivity filler in the thermosetting resin composition, the thermal conductivity efficiency of the thermosetting resin composition can be improved and the specific gravity can be adjusted.

[0035] The type of thermally conductive filler is not particularly limited as long as the effects of the present invention are achieved, but examples include aluminum oxide (alumina), aluminum nitride, boron nitride, silicon nitride, silicon carbide, magnesium oxide, zinc oxide, and diamond. Aluminum oxide is particularly preferred as the thermally conductive filler. In particular, by using spherical aluminum oxide, the increase in viscosity of the thermosetting resin composition when high-filling is achieved can be suppressed. Preferred commercially available aluminum oxides include Denka Spherical Alumina DAW-03, DAW-07, and ASFP-20 manufactured by Denka Co., Ltd.

[0036] The thermal conductivity of the thermally conductive filler is not particularly limited as long as the effects of the present invention are achieved, but is preferably 30 W / m·K or higher.

[0037] The particle size of the thermal conductive filler is not particularly limited as long as the effects of the present invention are achieved, but its average particle size (d50) is preferably 0.01 to 30 μm, more preferably 0.01 to 20 μm. The average particle size (d50) of the thermal conductive filler refers to the average particle size including not only the particle size of primary particles but also the particle size of secondary particles (aggregates). An average particle size (d50) of 0.01 μm or more suppresses excessive viscosity of the thermosetting resin composition, resulting in high dispersibility and high coatability to the coated object. On the other hand, an average particle size of 30 μm or less reduces the likelihood of particles emerging from the coating film and results in a sufficiently slow sedimentation rate, thus providing high storage stability. The average particle size (d50) of the thermal conductive filler can be measured by laser diffraction using a Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd.

[0038] It is preferable to use a combination of two or more average particle sizes of thermally conductive fillers that have a particle size distribution such that a close-packed structure is formed when compounded into a thermosetting resin composition. In a thermosetting-focused composition, a close-packed structure of thermally conductive fillers can lead to a higher packing density, which in turn can improve thermal conductivity and storage stability.

[0039] The content of the thermally conductive filler in the thermosetting resin composition is not particularly limited as long as the effects of the present invention are achieved, and can be appropriately set according to the type of thermally conductive filler and the various properties of the thermally conductive filler as described above. For example, the content of the thermally conductive filler can be 700 parts by mass or more per 100 parts by mass of the epoxy resin described above. Preferably, it is 700 to 1600 parts by mass, more preferably 700 to 1300 parts by mass, and even more preferably 800 to 1100 parts by mass per 100 parts by mass of epoxy resin. By setting the content of the thermally conductive filler to 700 parts by mass or more per 100 parts by mass of epoxy resin, the thermally conductive filler can be in sufficient contact with the thermosetting resin composition, and the cured product of the thermosetting resin composition can have a high thermal conductivity. On the other hand, by setting the content of the thermally conductive filler to 1600 parts by mass or less per 100 parts by mass of epoxy resin, a thermosetting resin composition with fluidity can be obtained.

[0040] (Phosphate Ester Wetting and Dispersing Agent) The thermosetting resin composition of the present invention contains a phosphate ester wetting and dispersing agent as an essential component. The phosphate ester wetting and dispersing agent is a wetting and dispersing agent having a phosphate ester bond, and promotes high-filling by improving the wetting and dispersibility of thermally conductive fillers into the thermosetting resin composition. In the thermosetting resin composition of the present invention, surface smoothness can be improved by increasing the wetting properties of the thermosetting resin and the thermally conductive filler. The phosphate ester wetting and dispersing agent may be used alone or in combination of two or more types.

[0041] Examples of phosphate ester-based wetting and dispersing agents include phosphate ester copolymers, and commercially available examples include BYK-W9010, W9011, W9012, and BYK-W111 manufactured by BIC Chemie Japan Co., Ltd.

[0042] The content of the phosphate ester wetting and dispersing agent in the thermosetting resin composition is 6 to 12 parts by mass, preferably 7 to 12 parts by mass, and more preferably 8 to 10 parts by mass, per 100 parts by mass of the epoxy resin. When the content of the phosphate ester wetting and dispersing agent per 100 parts by mass of epoxy resin is 6 parts by mass or more, the thermal conductive filler is sufficiently wetted and dispersed, and can be easily pasteurized by stirring. On the other hand, when the content of the phosphate ester wetting and dispersing agent per 100 parts by mass of epoxy resin is 12 parts by mass or less, excessive settling of the thermal conductive filler can be suppressed, and uniform dispersion of the thermal conductive filler can be promoted.

[0043] (Urea-based rheology modifier) ​​The thermosetting resin composition of the present invention contains a urea-based rheology modifier as an essential component. The urea-based rheology modifier is a urea-based rheology modifier having a urea bond, and can suppress the sedimentation of thermally conductive fillers by forming a three-dimensional structure in the resin composition. One type of urea-based rheology modifier may be used alone, or two or more types may be used in combination.

[0044] Examples of commercially available urea-based rheology modifiers include BYK-410, 411, 420, etc. manufactured by BYK Chemie Japan Co., Ltd.

[0045] The content of the rheology modifier in the thermosetting resin composition is in the range of 1 to 12 parts by mass, preferably 2 to 11 parts by mass, more preferably 3 to 10 parts by mass, and even more preferably 4 to 9 parts by mass, based on 100 parts by mass of the above-described epoxy resin.

[0046] In the thermosetting resin composition of the present invention, the ratio of the mass of the urea-based rheology modifier to the mass of the phosphate ester-based wetting and dispersing agent (mass of urea-based rheology modifier / mass of phosphate ester-based wetting and dispersing agent) is preferably 0.2 to 1.5, more preferably 0.2 to 1.4, even more preferably 0.3 to 1.2, and particularly preferably 0.5 to 1.0. When the ratio of the mass of the urea-based rheology modifier to the mass of the phosphate ester-based wetting and dispersing agent is 0.2 or more, sedimentation of the thermally conductive filler occurs during the thermosetting of the thermosetting resin composition, and a resin-rich skin layer is formed on the surface of the cured product, but it is possible to suppress the skin layer from becoming excessively thick. For example, it is possible to suppress the skin layer from having a film thickness of 5% or more of the film thickness of the thermosetting resin composition layer. On the other hand, when the ratio of the mass of the urea-based rheology modifier to the mass of the phosphate ester-based wetting and dispersing agent is 1.5 or less, it is possible to suppress the formation of a skin layer on the surface of the cured product and the impairment of the smoothness of the coating film, and further to suppress the decrease in the dielectric breakdown resistance.

[0047] (Solvent) The thermosetting resin composition of the present invention may contain a solvent or may not contain a solvent, but preferably does not substantially contain a solvent. When the thermosetting resin composition contains a solvent, its viscosity can be lowered and the printability can be improved. However, when the volatilization of the solvent in the deep part of the coating film is insufficient when the thermosetting resin composition is thermally cured, bubbles may be generated, which may cause a decrease in the dielectric breakdown voltage. Therefore, the thermosetting resin composition of the present invention does not contain a solvent from the viewpoint of improving the dielectric breakdown resistance.

[0048] "Substantially solvent-free" means that, with the exception of a very small amount of solvent contained in the components used in preparing the thermosetting resin composition (i.e., solvents that are inevitably included when preparing the thermosetting resin composition), the composition does not contain solvents (solvents are not actively incorporated). Specifically, "substantially solvent-free" means that the solvent content in the thermosetting resin composition is 1% by mass or less, preferably 0.5% by mass or less, more preferably 0.1% by mass or less, and particularly preferably 0% by mass.

[0049] (Other components) The thermosetting resin composition of the present invention may further contain, if necessary, one or more components of the following: a colorant, an adhesion promoter (adhesion imparting agent), a polymerization inhibitor, an antioxidant, and a rust inhibitor. These can be those known in the field of electronic materials.

[0050] [Cured Product] According to another aspect of the present invention, a cured product obtained by curing the thermosetting resin composition of the present invention (hereinafter also referred to as "the cured product of the present invention") is provided. Since the cured product of the present invention has high dielectric breakdown resistance, it can continuously exhibit high insulation even in printed circuit boards where high voltage and high current currents are required. Furthermore, since the cured product of the present invention has high thermal conductivity, it can be efficiently cooled by contact with the cooling environment surrounding the printed circuit board (for example, a cooling device, a housing or other component cooled by the cooling device). Moreover, since the cured product of the present invention has a highly smooth surface, it can come into good contact with the cooling environment as described above, and can be cooled even more efficiently.Therefore, the thermosetting resin composition of the present invention can be suitably used to form an insulating layer in printed circuit boards where high voltage and high current currents are required.

[0051] The thickness of the cured product of the present invention is not particularly limited as long as the effects of the present invention are achieved, but for example, it is 30 to 200 μm. Furthermore, the cured product of the present invention may or may not be homogeneous throughout. By having a thickness of the cured product within the above range, the cured product can exhibit sufficient dielectric breakdown resistance and sufficient thermal conductivity.

[0052] The cured product of the present invention preferably has a skin layer near its surface. The presence of a skin layer in the cured product results in a highly smooth surface. Here, the skin layer refers to a layer such that the arithmetic mean roughness Ra of the cured product's surface is less than 1.0 μm, and which substantially does not contain thermally conductive fillers with a particle size of 1 μm or larger. While it is preferable that the product is completely free of thermally conductive fillers with a particle size of 1 μm or larger, this does not exclude the possibility of thermally conductive fillers with a particle size of 1 μm or larger being present, provided that the arithmetic mean roughness Ra of the cured product's surface is less than 1.0 μm. The presence or absence of a skin layer in the cured product can be determined by observing the cross-section of the cured product using an optical microscope and measuring the particle size of the thermally conductive fillers. A cured product having such a skin layer can consequently have a smooth surface. On the other hand, the skin layer is a resin-rich layer with a low content of thermally conductive fillers and does not have high thermal conductivity. Therefore, from the viewpoint of increasing the overall thermal conductivity of the cured product, the skin layer is preferably thin, preferably in the range of 1 to 10 μm, and more preferably in the range of 1 to 5 μm.

[0053] The cured product of the present invention preferably has an arithmetic mean surface roughness Ra of 0.1 to 1.5 μm and a maximum height roughness Rz of 1.5 to 10 μm. Because the Ra and Rz of the cured product's surface are within the above ranges, surface contact with the heat sink is possible, resulting in better heat conduction and more efficient cooling of the component.

[0054] The Ra and Rz of the cured material can be measured as follows. First, the thermosetting resin composition is applied to a buffed copper substrate using a T-100B screen plate to print a coating film with a dry film thickness of 70 μm. Next, the substrate with the printed coating film is left to stand horizontally and defoamed at 23°C for 10 minutes for leveling. Then, it is left to stand in a hot air circulating box-type drying oven and pre-cured at 100°C for 20 minutes, followed by post-curing at 160°C for 60 minutes to thermoset the thermosetting resin composition and prepare a test substrate. Next, the Ra (arithmetic mean roughness) and Rz (maximum height roughness) of the surface of the cured material on the obtained test substrate are measured using a surface roughness measuring instrument SE600 manufactured by Kosaka Laboratory Co., Ltd., in accordance with the method compliant with JIS B 0601:2013.

[0055] [Printed Wiring Board] According to another aspect of the present invention, a printed wiring board comprising the cured product of the present invention (hereinafter also referred to as "the printed wiring board of the present invention") is provided. In the method for manufacturing the printed wiring board of the present invention, for example, the thermosetting resin composition according to the present invention is applied to a substrate by a method such as screen printing, and then the thermosetting resin composition is thermally cured while the substrate is placed horizontally to form a cured coating film. The thermal curing of the thermosetting resin composition can be, for example, by heating at a temperature of 130 to 200°C for 30 to 180 minutes. In one preferred embodiment, the thermal curing of the thermosetting resin composition is performed by a step cure including pre-curing and post-curing. Specifically, after applying the thermosetting resin composition, it is heated at a temperature of 60 to 100°C (pre-curing) to reduce the apparent viscosity and promote defoaming leveling. Then, a cured coating film is formed by thermal curing by heating at a temperature of 140 to 180°C. The pre-cure time mentioned above can be, for example, 10 to 60 minutes, and the post-cure time can be 30 to 120 minutes.

[0056] Examples of the above-mentioned substrates include printed circuit boards and flexible printed circuit boards with circuits pre-formed using copper, paper phenol, paper epoxy, glass cloth epoxy, glass polyimide, glass cloth / nonwoven fabric epoxy, glass cloth / paper epoxy, synthetic fiber epoxy, copper-clad laminates using materials such as fluororesin / polyethylene / polyphenylene ether, polyphenylene oxide / cyanate, and others such as metal substrates, polyimide films, polyethylene terephthalate films, polyethylene naphthalate (PEN) films, glass substrates, ceramic substrates, and wafers.

[0057] The thermosetting (step curing) performed after applying the thermosetting resin composition according to the present invention can be carried out using a hot air circulation drying oven, an IR oven, a hot plate, a convection oven, or the like.

[0058] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, the numerical values ​​for each component all refer to parts by mass unless otherwise specified. [Preparation of Thermosetting Resin Compositions] The components shown in Table 1 below were mixed in the amounts shown in the table, pre-mixed using a stirrer, and then kneaded using a three-roll mill to prepare the thermosetting resin compositions of Examples 1 to 6 and Comparative Examples 1 to 6. Details of each component in Table 1 are as follows. Epoxy resin 1: ZX-1059 manufactured by Nippon Steel Chemical & Material Co., Ltd. Epoxy resin 2: jER (registered trademark) YED216D manufactured by Mitsubishi Chemical Corporation Adhesion agent: Silane coupling agent KBM-403 (chemical name: 3-glycidoxypropyltrimethoxysilane) manufactured by Shin-Etsu Chemical Co., Ltd. Antifoaming agent: Oil compound type antifoaming agent KS-66 manufactured by Shin-Etsu Chemical Co., Ltd. Hardener: Imidazole epoxy resin hardener Curazole 2MZA-PW manufactured by Shikoku Chemicals, Inc. Urea-based rheology modifier 1: Urea-based rheology modifier BYK-410 manufactured by BIC Chemie Japan Co., Ltd. Urea-based rheology modifier 2: Urea-based rheology modifier BYK-411 manufactured by BIC Chemie Japan Co., Ltd. Phosphate ester wetting and dispersing agent 1: Wetting and dispersing agent BYK-W9010 manufactured by BIC Chemie Japan Co., Ltd. Phosphate ester wetting and dispersing agent 2: Wetting and dispersing agent BYK-111 manufactured by BIC Chemie Japan Co., Ltd. Phosphate ester wetting and dispersing agent 3: Wetting and dispersing agent BYK-W9011 manufactured by BIC Chemie Japan Co., Ltd. Thermal conductive filler 1: Denka spherical alumina ASFP-20 manufactured by Denka Co., Ltd. Thermal conductive filler 2: Denka spherical alumina DAW-03 manufactured by Denka Co., Ltd. Thermal conductive filler 3: Denka spherical alumina DAW-07 manufactured by Denka Co., Ltd. Note that with the compositions of Comparative Examples 1 and 2, it was not possible to stir and mix the components, and therefore a thermosetting resin composition could not be obtained. In Comparative Example 1, no phosphate ester wetting and dispersing agent was included, and in Comparative Example 2, the amount of phosphate ester wetting and dispersing agent included was insufficient, so it is thought that in both cases the epoxy resin did not sufficiently wet the surface of the thermal conductive filler.

[0059]

[0060] [Measurement of Viscosity and Ti Value of Thermosetting Resin Compositions] The viscosity of each thermosetting resin composition in the examples and comparative examples was measured using a cone plate viscometer TV-30 manufactured by Toki Sangyo Co., Ltd. at 25°C and rotational speeds of 1 rpm and 5 rpm. The viscosity when rotating at a rotational speed of 5 rpm was defined as the viscosity of each thermosetting resin composition. The ratio of the viscosity when rotating at a rotational speed of 1 rpm to the viscosity when rotating at a rotational speed of 5 rpm (viscosity when rotating at a rotational speed of 1 rpm / viscosity when rotating at a rotational speed of 5 rpm) was defined as the Ti value of each thermosetting resin composition. The viscosity and Ti value of each thermosetting resin composition are shown in Table 1.

[0061] [Preparation of Substrates for Measuring Skin Layer Thickness] Each thermosetting resin composition from the Examples and Comparative Examples was printed onto a buffed copper substrate using a T-100B screen plate to achieve a dry film thickness of 100 μm. The substrate was then left to stand horizontally and defoamed at 23°C for 10 minutes for leveling. Finally, it was heated in a hot air circulating box-type drying oven at 100°C for 30 minutes and then at 150°C for 30 minutes to cure. After curing, it was left at room temperature. Next, a second layer of coating was printed using a T-100B screen plate to achieve a dry film thickness of 70 μm. The substrate was then left to stand horizontally and defoamed at 23°C for 10 minutes for leveling. Finally, it was heated in a hot air circulating box-type drying oven at 100°C for 30 minutes and then at 150°C for 90 minutes to cure, thereby preparing a substrate for measuring the thickness of the skin layer.

[0062] [Measurement and Evaluation of Skin Layer Thickness] The cross-section of each substrate for measuring the skin layer thickness was polished, and the skin layer visible between the layers was observed and measured using a VHX-7000 digital microscope manufactured by Keyence Corporation. The skin layer has a high resin content and is lightly colored, so it transmits ambient light and the edges are difficult to see. Therefore, when measuring the thickness, two layers were formed, the cross-section of the coating film was observed, and the thickness of the skin layer between the first and second layers was measured. Furthermore, based on the measurement results, the thickness of the skin layer was evaluated according to the following criteria. The measurement results and evaluation results are shown in Table 1. ○: Skin layer thickness is 5 μm or less. ×: Skin layer thickness is greater than 5 μm or 0 μm (no skin layer).

[0063] The results shown in Table 1 indicate that the cured products of each of the thermosetting resin compositions in Examples 1 to 6 all have a skin layer of appropriate thickness.

[0064] [Preparation of substrates for measuring dielectric breakdown voltage and surface roughness] Each thermosetting resin composition from the examples and comparative examples was printed onto a buffed copper substrate as a first layer coating using a T-100B screen plate to achieve a dry film thickness of 100 μm. After being left to stand horizontally and defoamed at 23°C for 10 minutes, the substrates were heat-cured in a hot air circulating box-type drying oven at 100°C for 30 minutes and then at 150°C for 90 minutes to prepare substrates for measuring dielectric breakdown voltage and surface roughness.

[0065] [Measurement of Dielectric Breakdown Voltage and Evaluation of Dielectric Breakdown Resistance] For each substrate used to measure dielectric breakdown voltage, a TOS5101 dielectric strength tester manufactured by Kikusui Electronics Co., Ltd. was used. The voltage was increased to 0.5 kV / sec in AC mode using a 10 mm diameter electrode, and the voltage value at which the cured product of the curable resin composition on each substrate underwent dielectric breakdown (dielectric breakdown voltage) was measured. Dielectric strength evaluation was performed at three points for each evaluation substrate, and the average value of the three dielectric breakdown voltages was rounded to one decimal place by rounding the second decimal place. Based on the obtained values, dielectric breakdown resistance was evaluated according to the following criteria. The dielectric breakdown voltage values ​​and the evaluation results of dielectric breakdown resistance are shown in Table 1. Note that each value of dielectric breakdown voltage (kV / 0.1 mm) in Table 1 is the dielectric breakdown voltage value of each substrate with a cured coating film of 70 μm (0.07 mm) thickness converted to 0.1 mm. ○: The dielectric breakdown voltage is 6.5 kV / 0.1 mm or higher, indicating extremely high dielectric breakdown resistance. ×: The dielectric breakdown voltage is less than 6.5 kV / 0.1 mm, indicating insufficient dielectric breakdown resistance.

[0066] The results shown in Table 1 indicate that the cured products of each of the thermosetting resin compositions in Examples 1 to 6 all exhibit excellent dielectric breakdown resistance. Therefore, it can be said that thermosetting resin compositions containing epoxy resin, a thermally conductive filler, a urea-based rheology modifier, and a phosphate ester-based wetting dispersant, and adjusted to have a thixotropic index of 1.0 or higher and less than 1.4 at 25°C, can form cured products with excellent dielectric breakdown resistance.

[0067] [Measurement of Surface Roughness and Evaluation of Surface Smoothness] For each substrate used for surface roughness measurement, Ra (arithmetic mean roughness) and Rz (maximum height roughness) were measured in accordance with JIS B 0601:2013 using a surface roughness measuring instrument SE600 manufactured by Kosaka Laboratory Co., Ltd. Based on the obtained values, surface smoothness was evaluated according to the following criteria. The numerical values ​​of surface roughness Ra / Rz and the evaluation results of surface smoothness are shown in Table 1. The evaluation results are shown in Table 1. ○: Ra is less than 1.0 μm and Rz is less than 5 μm, indicating excellent surface smoothness. ×: Ra is 1.0 μm or more and Rz is 5 μm or more, indicating insufficient surface smoothness.

[0068] The results shown in Table 1 indicate that the cured products of each of the thermosetting resin compositions in Examples 1 to 6 all exhibit excellent surface smoothness. Therefore, it can be said that thermosetting resin compositions containing epoxy resin, a thermally conductive filler, a urea-based rheology modifier, and a phosphate ester-based wetting and dispersing agent, with a thixotropic index of 1.0 or higher and less than 1.4 at 25°C, can form cured products with excellent surface smoothness.

[0069] [Preparation of Substrates for Thermal Conductivity Measurement] Samples for measuring the thermal conductivity of cured thermosetting resin compositions from the examples and comparative examples were prepared according to the following procedure. First, the thermosetting resin composition was applied to rolled copper foil using a T-100B screen printing plate to a dry film thickness of 70 μm, and the coating was printed. Next, the substrate with the printed coating was left to stand horizontally and defoamed at 23°C for 10 minutes for leveling. Then, it was left in a hot air circulating box-type drying oven and pre-cured at 100°C for 20 minutes, followed by post-curing at 160°C for 30 minutes to thermally cure the thermosetting resin composition and form the first layer of cured coating. A second layer of coating was printed on top of the first layer of cured coating using the same procedure to a dry film thickness of 70 μm. After leveling and pre-curing in the same procedure as for the first layer, the second layer was post-cured at 160°C for 60 minutes to form a cured coating. The rolled copper foil was then peeled off to obtain a film-like cured material with a total thickness of 140 μm.

[0070] [Measurement of Thermal Conductivity and Evaluation of Thermal Conductivity] The thermal conductivity of the cured product of the thermosetting resin composition was measured according to the following procedure. First, the thermal diffusivity of the cured product was measured using an FTC-1 manufactured by ULVAC, Inc. under a load of 5 kgf / cm². ３ The thermal diffusivity (α), specific heat capacity (c), and density (ρ) of the cured material were measured using a periodic heating method. Next, the specific heat capacity of the cured material was measured using a differential scanning calorimetry (DSC) manufactured by Perkin-Elmer, with a heating rate of 20°C / min, a gas flow rate of 20 ml / min of helium, and a sample volume of 15 mg. Then, the density of the cured material was measured at room temperature (25°C) using the water displacement method. Based on the measured thermal diffusivity (α), specific heat capacity (c), and density (ρ) of the cured material, the thermal conductivity (λ) of the cured material was calculated using the following formula: Thermal conductivity (λ) = Thermal diffusivity (α) × Specific heat capacity (c) × Density (ρ)

[0071] Based on the obtained thermal conductivity (λ) values, the thermal conductivity was evaluated according to the following criteria. The thermal conductivity values ​​and evaluation results are shown in Table 1. ○: Thermal conductivity is greater than 3.0 W / m·K, indicating excellent thermal conductivity. △: Thermal conductivity is greater than 2.5 W / m·K and less than or equal to 3.0 W / m·K, indicating good thermal conductivity. ×: Thermal conductivity is less than or equal to 2.5 W / m·K, indicating insufficient thermal conductivity.

[0072] The results shown in Table 1 indicate that the cured products of each of the thermosetting resin compositions in Examples 1 to 6 all exhibit excellent thermal conductivity. Therefore, it can be said that thermosetting resin compositions containing a thermosetting resin, a thermally conductive filler, a urea-based rheology modifier, and a phosphate ester-based wetting and dispersing agent, with a thixotropic index of 1.0 or higher and less than 1.4 at 25°C, can form cured products with excellent thermal conductivity.

[0073] [Evaluation of Storage Stability] Each thermosetting resin composition of the examples and comparative examples was stored at room temperature (25°C) after preparation, and the presence or absence of sedimentation and aggregation of components was visually confirmed. Based on the confirmed sedimentation and aggregation of components, the storage stability was evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: No sedimentation and aggregation of components occurred for 72 hours or more after the start of storage, indicating excellent storage stability. ×: Sedimentation and aggregation of components occurred in less than 72 hours after the start of storage, indicating insufficient storage stability.

[0074] The results shown in Table 1 indicate that each of the thermosetting resin compositions in Examples 1 to 6 exhibits excellent storage stability under normal temperature conditions. Therefore, it can be said that thermosetting resin compositions containing a thermosetting resin, a thermally conductive filler, a urea-based rheology modifier, and a phosphate ester-based wetting and dispersing agent, with a thixotropic index of 1.0 or higher and less than 1.4 at 25°C, can form cured products with excellent storage stability.

Claims

1. A thermosetting resin composition comprising an epoxy resin, a thermally conductive filler, a phosphate ester-based wetting and dispersing agent, and a urea-based rheology modifier, wherein the content of the phosphate ester-based wetting and dispersing agent is 6 to 12 parts by mass per 100 parts by mass of the epoxy resin, the content of the urea-based rheology modifier is 1 to 12 parts by mass per 100 parts by mass of the epoxy resin, and the thixotropic index of the thermosetting resin composition at 25°C is 1.0 or more and less than 1.

4.

2. The thermosetting resin composition according to claim 1, which does not contain a solvent.

3. The thermosetting resin composition according to claim 1, wherein the ratio of the mass of the urea-based rheology modifier to the mass of the phosphate ester-based wetting dispersant in the thermosetting resin composition is 0.2 to 1.

5.

4. The thermosetting resin composition according to claim 1, wherein the thermally conductive filler contains aluminum oxide.

5. The thermosetting resin composition according to claim 1, wherein the content of the thermally conductive filler is 700 parts by mass or more per 100 parts by mass of the epoxy resin.

6. A cured product obtained by curing the thermosetting resin composition described in claim 1.

7. A printed circuit board comprising the cured product described in claim 6.

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