JP7510241B2 - Manufacturing method of hardened sheet - Google Patents

Description

The present invention relates to a thermosetting sheet containing a plurality of boron nitride particles. The present invention also relates to a method for producing a cured sheet by thermally curing the thermosetting sheet.

In recent years, electronic and electrical equipment has become smaller and more powerful. This has resulted in higher packaging density for electronic components, and there is an increasing need to dissipate heat generated by electronic components. Heat generation is a problem that is directly linked to the reliability of devices and equipment, and so finding ways to dissipate heat as quickly as possible has become an urgent issue.

One method to solve the above problems is to use highly thermally conductive ceramic substrates such as alumina substrates and aluminum nitride substrates as heat dissipation substrates on which power semiconductor devices are mounted. However, these substrates have problems such as difficulty in multi-layering, poor workability, and high cost.

On the other hand, thermally conductive compositions containing thermally conductive fillers are used to dissipate heat. The following Patent Documents 1 to 3 disclose fillers that can be used as thermally conductive fillers.

The following Patent Document 1 discloses a thermally conductive sheet in which boron nitride powder is dispersed as a thermally conductive filler in an organic matrix material. The boron nitride powder contains 1 to 20% by weight of secondary aggregate particles with a particle size of 50 μm or more.

The following Patent Document 2 discloses a thermally conductive sheet containing 30% by volume or more and 80% by volume or less of scaly boron nitride having an average long diameter of 3 μm or more and 50 μm or less in a matrix resin. In an X-ray diffraction diagram obtained by irradiating the thermally conductive sheet with X-rays in the thickness direction, the intensity ratio (I _<002> /I _<100> ) of the diffraction peak of the <002> plane to the <100> plane may be 6 or more and 200 or less.

The following Patent Document 3 discloses an insulating heat dissipation sheet in which boron nitride is dispersed in rubber or resin. The boron nitride is obtained by granulating boron nitride in a specific amount of a silane coupling agent having an amino group while fluidizing the boron nitride in a fluidized bed. The insulating heat dissipation sheet may have a peak intensity ratio I ₀₀₂ /I ₁₀₀ , which is the ratio of the intensity I ₀₀₂ of the 002 diffraction line to the intensity I ₁₀₀ of the 100 diffraction line in an X-ray diffraction diagram obtained by irradiating X-rays in the sheet thickness direction, of 6 to 80.

JP 2003-60134 A JP 2011-142129 A JP 2013-220971 A

Compositions containing conventional boron nitride particles, such as those described in Patent Documents 1 to 3, can increase thermal conductivity to a certain extent.

However, in compositions containing conventional boron nitride particles such as those described in Patent Documents 1 to 3, even if the thermal conductivity can be increased to a certain extent, the voltage resistance may not be sufficiently high.

With conventional compositions containing boron nitride particles, it is difficult to achieve both high thermal conductivity and high voltage resistance.

The object of the present invention is to provide a thermosetting sheet that can improve both thermal conductivity and voltage resistance. It is also an object of the present invention to provide a method for producing a cured sheet that can improve both thermal conductivity and voltage resistance.

The inventors conducted extensive research to solve the above problems and discovered that the ratio between the degree of orientation S1 of the boron nitride particles in the first cured sheet and the degree of orientation S2 of the boron nitride particles in the second cured sheet is important in order to achieve both high thermal conductivity and high voltage resistance, leading to the completion of the present invention. The first cured sheet is obtained by curing the first cured sheet at 110°C for 30 minutes. The second cured sheet is obtained by curing the first cured sheet at 195°C for 60 minutes at a pressure of 12 MPa.

Furthermore, as a result of extensive research into solving the above problems, the inventors discovered that in order to achieve both high thermal conductivity and high voltage resistance, the ratio between the degree of orientation S1 of the boron nitride particles after pre-curing and the degree of orientation S2 of the boron nitride particles after post-curing after pre-curing is important when obtaining a cured sheet, and thus completed the present invention.

According to a broad aspect of the present invention, there is provided a thermosetting sheet comprising a thermosetting compound, a thermosetting agent, and a plurality of boron nitride particles, in which the degree of orientation S1 of the boron nitride particles in a first cured sheet obtained by curing at 110°C for 30 minutes and the degree of orientation S2 of the boron nitride particles in a second cured sheet obtained by curing the first cured sheet at 195°C for 60 minutes at a pressure of 12 MPa satisfy the relationship of the formula: 1.05 x S1 ≦ S2 ≦ 3.0 x S1.

According to a broad aspect of the present invention, there is provided a method for producing a cured sheet, comprising the steps of pre-curing a thermosetting sheet containing a thermosetting compound, a thermosetting agent, and a plurality of boron nitride particles to obtain a first cured sheet, and post-curing the first cured sheet to obtain a cured sheet, in which the pre-curing and post-curing are performed so that the degree of orientation S1 of the boron nitride particles in the first cured sheet and the degree of orientation S2 of the boron nitride particles in the resulting cured sheet satisfy the relationship of the formula: 1.05 x S1 ≦ S2 ≦ 3.0 x S1.

In a particular aspect of the present invention, the boron nitride particles include aggregates of boron nitride particles.

In a particular aspect of the present invention, the content of the boron nitride particles in the thermosetting sheet is 10% by volume or more and 80% by volume or less.

In a particular aspect of the present invention, the thermosetting sheet contains an insulating filler that is not boron nitride particles.

In a particular aspect of the present invention, the total content of the boron nitride particles and the insulating filler in the thermosetting sheet is 20% by volume or more and 80% by volume or less.

In a particular aspect of the present invention, the content of the boron nitride particles in the thermosetting sheet is 50% by volume or more out of a total of 100% by volume of the boron nitride particles and the insulating filler.

The thermosetting sheet according to the present invention includes a thermosetting compound, a thermosetting agent, and a plurality of boron nitride particles. In the present invention, the degree of orientation S1 of the boron nitride particles in the first cured sheet obtained by curing at 110°C for 30 minutes and the degree of orientation S2 of the boron nitride particles in the second cured sheet obtained by curing the first cured sheet at 195°C and a pressure of 12 MPa for 60 minutes satisfy the relationship of the formula: 1.05 x S1 ≦ S2 ≦ 3.0 x S1. The thermosetting sheet according to the present invention has the above configuration, and therefore can improve both thermal conductivity and voltage resistance.

The method for producing a cured sheet according to the present invention includes a step of pre-curing a thermosetting sheet containing a thermosetting compound, a thermosetting agent, and a plurality of boron nitride particles to obtain a first cured sheet. The method for producing a cured sheet according to the present invention includes a step of post-curing the first cured sheet to obtain a cured sheet. In the method for producing a cured sheet according to the present invention, pre-curing and post-curing are performed so that the degree of orientation S1 of the boron nitride particles in the first cured sheet and the degree of orientation S2 of the boron nitride particles in the obtained cured sheet satisfy the relationship of the formula: 1.05 x S1 ≦ S2 ≦ 3.0 x S1. Since the method for producing a cured sheet according to the present invention has the above configuration, it is possible to increase both thermal conductivity and voltage resistance.

FIG. 1 is a cross-sectional view showing a schematic diagram of a cured product of a thermosetting sheet according to one embodiment of the present invention.

The present invention is described in detail below.

The thermosetting sheet according to the present invention and the thermosetting sheet used in the method for producing the cured sheet according to the present invention contain (A) a thermosetting compound, (B) a thermosetting agent, and a plurality of (C) boron nitride particles. The thermosetting sheet can be used as a thermally conductive sheet. The plurality of boron nitride particles may contain an aggregate of boron nitride particles (also called an aggregate of (C1) boron nitride particles or an aggregate (C1)).

In the thermosetting sheet according to the present invention, the first cured sheet obtained by curing at 110°C for 30 minutes can be called, for example, a B-stage sheet. In the thermosetting sheet according to the present invention, the second cured sheet obtained by curing the first cured sheet at 195°C and a pressure of 12 MPa for 60 minutes can be called, for example, a C-stage sheet.

In the method for producing a cured sheet according to the present invention, the first cured sheet obtained by pre-curing can be called, for example, a B-stage sheet. In the method for producing a cured sheet according to the present invention, the cured sheet (second cured sheet) obtained by post-curing the first cured sheet can be called, for example, a C-stage sheet.

In the thermosetting sheet according to the present invention, the degree of orientation S1 of the boron nitride particles in the first cured sheet obtained by curing at 110°C for 30 minutes and the degree of orientation S2 of the boron nitride particles in the second cured sheet obtained by curing the first cured sheet at 195°C and a pressure of 12 MPa for 60 minutes satisfy the relationship of the following formula (1).

In the method for producing a cured sheet according to the present invention, pre-curing and post-curing are performed so that the degree of orientation S1 of the boron nitride particles in the pre-cured first cured sheet and the degree of orientation S2 of the boron nitride particles in the cured sheet (second cured sheet) obtained by post-curing the first cured sheet satisfy the relationship of the following formula (1).

1.05 x S1 ≤ S2 ≤ 3.0 x S1 ... formula (1)

The present invention has the above-mentioned configuration, and therefore can improve both thermal conductivity and voltage resistance. The present invention can achieve both high thermal conductivity and high voltage resistance. In particular, the inventors have found that by controlling the degree of orientation S1 in the first cured sheet and the degree of orientation S2 in the second cured sheet, which is a thermosetting sheet that has been cured more than the first cured sheet, to satisfy the above-mentioned relationship, it is possible to achieve both high thermal conductivity and high voltage resistance.

From the viewpoint of effectively increasing thermal conductivity and voltage resistance, the orientation degree S2 is preferably 1.07 x orientation degree S1 or more, and preferably 2.3 x orientation degree S1 or less.

Specifically, the orientation degree S1 and the orientation degree S2 are measured as follows:

The X-ray diffraction patterns of the first and second cured sheets are measured using an X-ray diffractometer (Rigaku Corporation's "SmartLab Multipurpose"). Measurements are performed in steps of 0.02° in the 2θ scan range of 20° to 60° under conditions of 45 kV and 200 mA with CuKα radiation. The counting time is 2°/min. From the obtained diffraction patterns, the diffraction peak intensity I(002) corresponding to the (002) plane of the boron nitride particles at 26.7° and the diffraction peak intensity I(100) corresponding to the (100) plane of the boron nitride particles at 41.6° are calculated. The ratio of the diffraction peak intensity I(002) calculated using the first cured sheet to the diffraction peak intensity I(100) is the degree of orientation S1, and the ratio of the diffraction peak intensity I(002) calculated using the second cured sheet to the diffraction peak intensity I(100) is the degree of orientation S2.

Since it is easy to satisfy the above-mentioned relationship between the orientation degree S1 and the orientation degree S2, the orientation degree S1 of the boron nitride particles in the first hardened sheet is preferably 5 or more and preferably 60 or less.

In the thermosetting sheet of the present invention, the above orientation degrees S1 and S2 can be controlled by, for example, selecting the type of boron nitride particles.

In the method for producing a hardened sheet according to the present invention, the above orientation degrees S1 and S2 can be controlled by selecting the type of boron nitride particles and controlling the temperature and pressure during hardening.

When the thermosetting sheet of the present invention is actually used, the pre-curing conditions do not have to be 110°C for 30 minutes. When the thermosetting sheet of the present invention is actually used, the post-curing conditions do not have to be 195°C for 60 minutes at a pressure of 12 MPa.

In the manufacturing method of the cured sheet according to the present invention, the pre-curing conditions do not have to be 110°C for 30 minutes. In the manufacturing method of the cured sheet according to the present invention, the post-curing conditions do not have to be 195°C for 60 minutes at a pressure of 12 MPa.

The heating temperature during the post-curing is preferably 120°C or higher, and preferably 250°C or lower. When the first cured sheet is post-cured, pressure may be applied while heating, or pressure may be applied once and then heating may be performed. In addition, as a method for applying temperature and pressure, a conventional press machine having a heating function may be used, or an isotropic pressure device capable of applying pressure isotropically may be used. The pressure during the post-curing is preferably 1.0 MPa or higher, more preferably 2.0 MPa or higher, and preferably 30 MPa or lower, more preferably 25 MPa or lower. In addition, a vacuum state may be used when applying pressure.

From the viewpoint of effectively increasing both thermal conductivity and voltage resistance, it is preferable that the boron nitride constituting the boron nitride particles (C) be hexagonal boron nitride.

In order to obtain a more excellent effect of the present invention, it is preferable that each of the non-aggregated (C) boron nitride particles and the (C) boron nitride particles constituting the (C1) aggregate has a scale-like shape. In each of the non-aggregated (C) boron nitride particles and the (C) boron nitride particles constituting the (C1) aggregate, the ratio of the average major axis to the average thickness is preferably 2 or more, more preferably 3 or more, and is preferably 200 or less, more preferably 100 or less.

From the viewpoint of effectively increasing the thermal conductivity and controlling the compressive strength within an appropriate range, the average primary particle diameter of the multiple (C) boron nitride particles (non-aggregated (C) boron nitride particles and (C) boron nitride particles constituting the (C1) aggregates) is preferably 0.1 μm or more, more preferably 0.5 μm or more, and is preferably 10 μm or less, more preferably 5 μm or less.

(C) The average primary particle size of boron nitride particles means the average particle size of the primary particles on a volume basis. (C) The average primary particle size of boron nitride particles can be measured using a "laser diffraction particle size distribution measuring device" manufactured by Horiba, Ltd.

From the viewpoint of packing the (C) boron nitride particles at a high density, effectively increasing thermal conductivity, and effectively suppressing the occurrence of voids, the average aspect ratio of the (C1) boron nitride particle aggregates is preferably 5 or less, more preferably 3 or less, even more preferably 2 or less, even more preferably 1.8 or less, particularly preferably 1.6 or less, and most preferably 1.5 or less.

The aspect ratio is the major axis/minor axis. The average aspect ratio is determined by averaging the aspect ratios of multiple (C1) aggregates. It is preferable to average the aspect ratios of 50 arbitrarily selected (C1) aggregates.

From the viewpoint of further improving dispersibility in the resin, stability over time, and thermal conductivity, the average particle size of the aggregates of boron nitride particles (C1) is preferably 1 μm or more, more preferably 5 μm or more, and is preferably 100 μm or less, more preferably 80 μm or less.

The average particle size of the aggregates of (C1) boron nitride particles is determined by measuring the particle size on a volume basis using particle size distribution measurement. The average particle size of the aggregates of (C1) boron nitride particles can be measured using a "laser diffraction particle size distribution measuring device" manufactured by Horiba, Ltd. It is preferable to average the particle sizes of 50 arbitrarily selected (C1) aggregates.

From the viewpoint of effectively increasing thermal conductivity, the average sphericity of the aggregates of boron nitride particles (C1) is preferably 0.8 or more, more preferably 0.85 or more, and even more preferably 0.9 or more.

(C1) Examples of methods for producing aggregates of boron nitride particles include spray drying and fluidized bed granulation. The spray drying method (also called spray drying) is preferred. Depending on the spray method, the spray drying method can be classified into a two-fluid nozzle method, a disk method (also called a rotary method), and an ultrasonic nozzle method, and any of these methods can be used.

From the viewpoint of effectively increasing the compressive strength and voltage resistance, it is preferable that the thermosetting sheet according to the present invention contains (A) a thermosetting compound, (B) a thermosetting agent, (C) boron nitride particles (which may contain (C1) aggregates), and (D) an insulating filler that is not boron nitride particles (sometimes simply referred to as (D) insulating filler).

By adopting a composition containing components (A) through (D), the heat dissipation properties of the cured product can be significantly improved. For example, by using a combination of (A) a thermosetting compound, (B) a thermosetting agent, (C) boron nitride particles (which may contain (C1) aggregates), and (D) an insulating filler, the heat dissipation properties, compressive strength, and voltage resistance are effectively improved compared to when these are not used in combination. With the present invention, it is possible to achieve high levels of the effects of high heat dissipation, high compressive strength, and high voltage resistance, which were previously difficult to achieve.

Further details of the present invention are described below.

((A) Thermosetting Compound)
Examples of the (A) thermosetting compound include a styrene compound, a phenoxy compound, an oxetane compound, an epoxy compound, an episulfide compound, a (meth)acrylic compound, a phenol compound, an amino compound, an unsaturated polyester compound, a polyurethane compound, a silicone compound, and a polyimide compound. Only one type of (A) thermosetting compound may be used, or two or more types may be used in combination.

From the viewpoint of effectively increasing both thermal conductivity and voltage resistance, it is preferable that the (A) thermosetting compound contains an epoxy compound.

As the (A) thermosetting compound, (A1) a thermosetting compound having a molecular weight of less than 10,000 (sometimes simply referred to as (A1) thermosetting compound) may be used. As the (A) thermosetting compound, (A2) a thermosetting compound having a molecular weight of 10,000 or more (sometimes simply referred to as (A2) thermosetting compound) may be used. As the (A) thermosetting compound, both (A1) and (A2) thermosetting compounds may be used.

Of the components contained in the thermosetting sheet, the content of the thermosetting compound (A) is preferably 10% by weight or more, more preferably 20% by weight or more, out of 100% by weight of the components excluding the boron nitride particles (C) and the insulating filler (D). Of the components contained in the thermosetting sheet, the content of the thermosetting compound (A) is preferably 90% by weight or less, more preferably 80% by weight or less, even more preferably 70% by weight or less, particularly preferably 60% by weight or less, and most preferably 50% by weight or less, out of 100% by weight of the components excluding the boron nitride particles (C) and the insulating filler (D). If the content of the thermosetting compound (A) is equal to or greater than the lower limit, the adhesiveness and heat resistance of the cured product are further improved. If the content of the thermosetting compound (A) is equal to or less than the upper limit, the coating property during the preparation of the thermosetting sheet is improved.

The components contained in the thermosetting sheet, excluding (C) boron nitride particles and (D) insulating filler, are components excluding (C) boron nitride particles when the thermosetting sheet does not contain (D) insulating filler, and are components excluding (C) boron nitride particles and (D) insulating filler when the thermosetting sheet contains (D) insulating filler.

(A1) Thermosetting compound having a molecular weight of less than 10,000:
The thermosetting compound (A1) may be a thermosetting compound having a cyclic ether group. The cyclic ether group may be an epoxy group or an oxetanyl group. The thermosetting compound having a cyclic ether group is preferably a thermosetting compound having an epoxy group or an oxetanyl group. The thermosetting compound (A1) may be used alone or in combination of two or more kinds.

The (A1) thermosetting compound may contain (A1a) a thermosetting compound having an epoxy group (sometimes simply referred to as (A1a) thermosetting compound) or (A1b) a thermosetting compound having an oxetanyl group (sometimes simply referred to as (A1b) thermosetting compound).

From the viewpoint of further improving the heat resistance and moisture resistance of the cured product, it is preferable that the thermosetting compound (A1) has an aromatic skeleton.

The aromatic skeleton is not particularly limited, and examples thereof include a naphthalene skeleton, a fluorene skeleton, a biphenyl skeleton, an anthracene skeleton, a pyrene skeleton, a xanthene skeleton, an adamantane skeleton, and a bisphenol A skeleton. From the viewpoint of further improving the thermal cycle resistance and heat resistance of the cured product, a biphenyl skeleton or a fluorene skeleton is preferred.

(A1a) Thermosetting compounds include epoxy monomers having a bisphenol skeleton, epoxy monomers having a dicyclopentadiene skeleton, epoxy monomers having a naphthalene skeleton, epoxy monomers having an adamantane skeleton, epoxy monomers having a fluorene skeleton, epoxy monomers having a biphenyl skeleton, epoxy monomers having a bi(glycidyloxyphenyl)methane skeleton, epoxy monomers having a xanthene skeleton, epoxy monomers having an anthracene skeleton, and epoxy monomers having a pyrene skeleton. Hydrogenated products or modified products of these may also be used. (A1a) Thermosetting compounds may be used alone or in combination of two or more.

Examples of the epoxy monomer having a bisphenol skeleton include bisphenol A type, bisphenol F type, and bisphenol S type epoxy monomers having a bisphenol skeleton.

Examples of the epoxy monomer having a dicyclopentadiene skeleton include dicyclopentadiene dioxide and phenol novolac epoxy monomer having a dicyclopentadiene skeleton.

Examples of the epoxy monomer having a naphthalene skeleton include 1-glycidylnaphthalene, 2-glycidylnaphthalene, 1,2-diglycidylnaphthalene, 1,5-diglycidylnaphthalene, 1,6-diglycidylnaphthalene, 1,7-diglycidylnaphthalene, 2,7-diglycidylnaphthalene, triglycidylnaphthalene, and 1,2,5,6-tetraglycidylnaphthalene.

Examples of the epoxy monomer having an adamantane skeleton include 1,3-bis(4-glycidyloxyphenyl)adamantane and 2,2-bis(4-glycidyloxyphenyl)adamantane.

Examples of the epoxy monomer having the fluorene skeleton include 9,9-bis(4-glycidyloxyphenyl)fluorene, 9,9-bis(4-glycidyloxy-3-methylphenyl)fluorene, 9,9-bis(4-glycidyloxy-3-chlorophenyl)fluorene, 9,9-bis(4-glycidyloxy-3-bromophenyl)fluorene, 9,9-bis(4-glycidyloxy-3-fluorophenyl)fluorene, 9,9-bis(4-glycidyloxy-3-methoxyphenyl)fluorene, 9,9-bis(4-glycidyloxy-3,5-dimethylphenyl)fluorene, 9,9-bis(4-glycidyloxy-3,5-dichlorophenyl)fluorene, and 9,9-bis(4-glycidyloxy-3,5-dibromophenyl)fluorene.

Examples of the epoxy monomer having a biphenyl skeleton include 4,4'-diglycidylbiphenyl and 4,4'-diglycidyl-3,3',5,5'-tetramethylbiphenyl.

Examples of the epoxy monomer having the bi(glycidyloxyphenyl)methane skeleton include 1,1'-bi(2,7-glycidyloxynaphthyl)methane, 1,8'-bi(2,7-glycidyloxynaphthyl)methane, 1,1'-bi(3,7-glycidyloxynaphthyl)methane, 1,8'-bi(3,7-glycidyloxynaphthyl)methane, 1,1'-bi(3,5-glycidyloxynaphthyl)methane, 1,8'-bi(3,5-glycidyloxynaphthyl)methane, 1,2'-bi(2,7-glycidyloxynaphthyl)methane, 1,2'-bi(3,7-glycidyloxynaphthyl)methane, and 1,2'-bi(3,5-glycidyloxynaphthyl)methane.

Examples of the epoxy monomer having a xanthene skeleton include 1,3,4,5,6,8-hexamethyl-2,7-bis-oxiranylmethoxy-9-phenyl-9H-xanthene.

Specific examples of the (A1b) thermosetting compound include 4,4'-bis[(3-ethyl-3-oxetanyl)methoxymethyl]biphenyl, 1,4-benzenedicarboxylic acid bis[(3-ethyl-3-oxetanyl)methyl]ester, 1,4-bis[(3-ethyl-3-oxetanyl)methoxymethyl]benzene, and oxetane-modified phenol novolak. Only one type of (A1b) thermosetting compound may be used, or two or more types may be used in combination.

From the viewpoint of further improving the heat resistance of the cured product, it is preferable that the (A1) thermosetting compound contains a thermosetting compound having two or more cyclic ether groups.

From the viewpoint of further improving the heat resistance of the cured product, the content of the thermosetting compound having two or more cyclic ether groups in 100% by weight of the (A1) thermosetting compound is preferably 70% by weight or more, more preferably 80% by weight or more, and preferably 100% by weight or less. The content of the thermosetting compound having two or more cyclic ether groups in 100% by weight of the (A1) thermosetting compound may be 10% by weight or more and 100% by weight or less. In addition, the entire (A1) thermosetting compound may be a thermosetting compound having two or more cyclic ether groups.

The molecular weight of the (A1) thermosetting compound is less than 10,000. The molecular weight of the (A1) thermosetting compound is preferably 200 or more, preferably 1,200 or less, more preferably 600 or less, and even more preferably 550 or less. When the molecular weight of the (A1) thermosetting compound is equal to or greater than the above lower limit, the tackiness of the surface of the cured product is reduced, and the handleability of the curable composition is further improved. When the molecular weight of the (A1) thermosetting compound is equal to or less than the above upper limit, the adhesiveness of the cured product is further improved. Furthermore, the cured product is less likely to become hard and brittle, and the adhesiveness of the cured product is further improved.

In this specification, the molecular weight of the (A1) thermosetting compound means the molecular weight that can be calculated from the structural formula when the (A1) thermosetting compound is not a polymer and when the structural formula of the (A1) thermosetting compound can be specified, and means the weight average molecular weight when the (A1) thermosetting compound is a polymer.

Of the components contained in the thermosetting sheet, the content of the thermosetting compound (A1) is preferably 10% by weight or more, more preferably 20% by weight or more, based on 100% by weight of the components excluding the boron nitride particles (C) and the insulating filler (D). Of the components contained in the thermosetting sheet, the content of the thermosetting compound (A1) is preferably 90% by weight or less, more preferably 80% by weight or less, even more preferably 70% by weight or less, particularly preferably 60% by weight or less, and most preferably 50% by weight or less, based on 100% by weight of the components excluding the boron nitride particles (C) and the insulating filler (D). If the content of the thermosetting compound (A1) is equal to or greater than the lower limit, the adhesiveness and heat resistance of the cured product are further improved. If the content of the thermosetting compound (A1) is equal to or less than the upper limit, the coating property during the preparation of the thermosetting sheet is improved.

(A2) Thermosetting compound having a molecular weight of 10,000 or more:
The (A2) thermosetting compound is a thermosetting compound having a molecular weight of 10,000 or more. Since the (A2) thermosetting compound has a molecular weight of 10,000 or more, the (A2) thermosetting compound is generally a polymer, and the above molecular weight generally means a weight average molecular weight.

From the viewpoint of further increasing the heat resistance and moisture resistance of the cured product, it is preferable that the (A2) thermosetting compound has an aromatic skeleton. When the (A2) thermosetting compound is a polymer and has an aromatic skeleton, the (A2) thermosetting compound may have an aromatic skeleton in any part of the entire polymer, and may have the aromatic skeleton in the main chain skeleton or in a side chain. From the viewpoint of further increasing the heat resistance of the cured product and further increasing the moisture resistance of the cured product, it is preferable that the (A2) thermosetting compound has an aromatic skeleton in the main chain skeleton. Only one type of (A2) thermosetting compound may be used, or two or more types may be used in combination.

The aromatic skeleton is not particularly limited, and examples thereof include a naphthalene skeleton, a fluorene skeleton, a biphenyl skeleton, an anthracene skeleton, a pyrene skeleton, a xanthene skeleton, an adamantane skeleton, and a bisphenol A skeleton. A biphenyl skeleton or a fluorene skeleton is preferred. In this case, the cured product has higher cold-heat cycle resistance and heat resistance.

(A2) Thermosetting compounds are not particularly limited, and examples include styrene resins, phenoxy resins, oxetane resins, epoxy resins, episulfide compounds, (meth)acrylic resins, phenolic resins, amino resins, unsaturated polyester resins, polyurethane resins, silicone resins, and polyimide resins.

From the viewpoint of suppressing oxidation deterioration of the cured product, further improving the cold-heat cycle resistance and heat resistance of the cured product, and further reducing the water absorption rate of the cured product, the thermosetting compound (A2) is preferably a styrene resin, a phenoxy resin, or an epoxy resin, more preferably a phenoxy resin or an epoxy resin, and even more preferably a phenoxy resin. In particular, the use of a phenoxy resin or an epoxy resin further increases the heat resistance of the cured product. In addition, the use of a phenoxy resin further reduces the elastic modulus of the cured product, and further increases the cold-heat cycle resistance of the cured product. The thermosetting compound (A2) does not have to have a cyclic ether group such as an epoxy group.

Specific examples of the styrene resin that can be used include homopolymers of styrene monomers and copolymers of styrene monomers and acrylic monomers. Styrene polymers having a styrene-glycidyl methacrylate structure are preferred.

Examples of the styrene monomer include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-methoxystyrene, p-phenylstyrene, p-chlorostyrene, p-ethylstyrene, p-n-butylstyrene, p-tert-butylstyrene, p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene, p-n-dodecylstyrene, 2,4-dimethylstyrene, and 3,4-dichlorostyrene.

The above-mentioned phenoxy resin is specifically a resin obtained by reacting, for example, epihalohydrin with a divalent phenol compound, or a resin obtained by reacting a divalent epoxy compound with a divalent phenol compound.

The phenoxy resin preferably has a bisphenol A type skeleton, a bisphenol F type skeleton, a bisphenol A/F mixed skeleton, a naphthalene skeleton, a fluorene skeleton, a biphenyl skeleton, an anthracene skeleton, a pyrene skeleton, a xanthene skeleton, an adamantane skeleton, or a dicyclopentadiene skeleton. The phenoxy resin more preferably has a bisphenol A type skeleton, a bisphenol F type skeleton, a bisphenol A/F mixed skeleton, a naphthalene skeleton, a fluorene skeleton, or a biphenyl skeleton, and even more preferably has at least one skeleton selected from the fluorene skeleton and the biphenyl skeleton. By using a phenoxy resin having these preferred skeletons, the heat resistance of the cured product is further improved.

The epoxy resin is an epoxy resin other than the phenoxy resin. Examples of the epoxy resin include styrene skeleton-containing epoxy resin, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, phenol novolac type epoxy resin, biphenol type epoxy resin, naphthalene type epoxy resin, fluorene type epoxy resin, phenol aralkyl type epoxy resin, naphthol aralkyl type epoxy resin, dicyclopentadiene type epoxy resin, anthracene type epoxy resin, epoxy resin having an adamantane skeleton, epoxy resin having a tricyclodecane skeleton, and epoxy resin having a triazine nucleus in the skeleton.

The molecular weight of the (A2) thermosetting compound is 10,000 or more. The molecular weight of the (A2) thermosetting compound is preferably 30,000 or more, more preferably 40,000 or more, and preferably 1,000,000 or less, more preferably 250,000 or less. When the molecular weight of the (A2) thermosetting compound is equal to or greater than the above lower limit, the cured product is less susceptible to thermal deterioration. When the molecular weight of the (A2) thermosetting compound is equal to or less than the above upper limit, the compatibility of the (A2) thermosetting compound with other components is increased. As a result, the heat resistance of the cured product is further increased.

Of the components contained in the thermosetting sheet, the content of the (A2) thermosetting compound is preferably 20% by weight or more, more preferably 30% by weight or more, and preferably 60% by weight or less, more preferably 50% by weight or less, out of 100% by weight of the components excluding the (C) boron nitride particles and the (D) insulating filler. When the content of the (A2) thermosetting compound is equal to or more than the above lower limit, the thermosetting sheet becomes easy to handle. When the content of the (A2) thermosetting compound is equal to or less than the above upper limit, the (C) boron nitride particles and the (D) insulating filler are easily dispersed.

((B) Heat Curing Agent)
The (B) heat curing agent is not particularly limited. As the (B) heat curing agent, an appropriate heat curing agent capable of curing the (A) heat curable compound can be used. In this specification, the (B) heat curing agent includes a curing catalyst. As the (B) heat curing agent, only one type may be used, or two or more types may be used in combination.

From the viewpoint of further improving the heat resistance of the cured product, it is preferable that the (B) heat curing agent has an aromatic skeleton or an alicyclic skeleton. It is preferable that the (B) heat curing agent contains an amine curing agent (amine compound), an imidazole curing agent, a phenolic curing agent (phenolic compound) or an acid anhydride curing agent (acid anhydride), and it is more preferable that the (B) heat curing agent contains an amine curing agent. It is preferable that the above-mentioned acid anhydride curing agent contains an acid anhydride having an aromatic skeleton, a water additive of the acid anhydride, or a modified product of the acid anhydride, or contains an acid anhydride having an alicyclic skeleton, a water additive of the acid anhydride, or a modified product of the acid anhydride.

The above-mentioned amine curing agent includes dicyandiamide, an imidazole compound, diaminodiphenylmethane, and diaminodiphenylsulfone. From the viewpoint of further enhancing the adhesion between the cured product and the thermal conductor and the conductive layer, it is more preferable that the above-mentioned amine curing agent is dicyandiamide or an imidazole compound. From the viewpoint of further enhancing the storage stability of the curable composition, it is preferable that the (B) heat curing agent contains a curing agent having a melting point of 180°C or higher, and it is more preferable that the (B) heat curing agent contains an amine curing agent having a melting point of 180°C or higher.

The above imidazole curing agents include 2-undecylimidazole, 2-heptadecylimidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 1-benzyl-2-phenylimidazole, 1,2-dimethylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazolium trimellitate, and 1-cyanoethyl-2-phenylimidazolium trimethylolate. Examples include melitate, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid adduct, 2-phenylimidazole isocyanuric acid adduct, 2-methylimidazole isocyanuric acid adduct, 2-phenyl-4,5-dihydroxymethylimidazole, and 2-phenyl-4-methyl-5-dihydroxymethylimidazole.

Examples of the phenol curing agent include phenol novolac, o-cresol novolac, p-cresol novolac, t-butylphenol novolac, dicyclopentadiene cresol, polyparavinylphenol, bisphenol A type novolac, xylylene modified novolac, decalin modified novolac, poly(di-o-hydroxyphenyl)methane, poly(di-m-hydroxyphenyl)methane, and poly(di-p-hydroxyphenyl)methane. From the viewpoint of further increasing the flexibility and flame retardancy of the cured product, phenolic resins having a melamine skeleton, phenolic resins having a triazine skeleton, and phenolic resins having an allyl group are preferred.

Commercially available phenolic hardeners include MEH-8005, MEH-8010, and MEH-8015 (all manufactured by Meiwa Kasei Co., Ltd.), YLH903 (manufactured by Mitsubishi Chemical Corporation), LA-7052, LA-7054, LA-7751, LA-1356, and LA-3018-50P (all manufactured by DIC Corporation), and PS6313 and PS6492 (all manufactured by Gun-ei Chemical Co., Ltd.).

Examples of the above-mentioned acid anhydrides having an aromatic skeleton, water additives of the acid anhydrides, or modified products of the acid anhydrides include styrene/maleic anhydride copolymer, benzophenone tetracarboxylic anhydride, pyromellitic anhydride, trimellitic anhydride, 4,4'-oxydiphthalic anhydride, phenylethynylphthalic anhydride, glycerol bis(anhydrotrimellitate) monoacetate, ethylene glycol bis(anhydrotrimellitate), methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and trialkyltetrahydrophthalic anhydride.

Commercially available products of the above-mentioned acid anhydrides having an aromatic skeleton, water additives of the acid anhydrides, or modified products of the acid anhydrides include SMA Resin EF30, SMA Resin EF40, SMA Resin EF60, and SMA Resin EF80 (all manufactured by Sartomer Japan), ODPA-M, and PEPA (all manufactured by Manac), RIKACID MTA-10, RIKACID MTA-15, and RIKACID TMT. A, RIKACID TMEG-100, RIKACID TMEG-200, RIKACID TMEG-300, RIKACID TMEG-500, RIKACID TMEG-S, RIKACID TH, RIKACID HT-1A, RIKACID HH, RIKACID MH-700, RIKACID MT-500, RIKACID DSDA and RIKACID TDA-100 (all manufactured by New Japan Chemical Co., Ltd.), as well as EPICLON B4400, EPICLON B650 and EPICLON B570 (all manufactured by DIC Corporation).

The acid anhydride having an alicyclic skeleton, the water additive of the acid anhydride, or the modified product of the acid anhydride is preferably an acid anhydride having a polyalicyclic skeleton, the water additive of the acid anhydride, or the modified product of the acid anhydride, or an acid anhydride having an alicyclic skeleton obtained by an addition reaction of a terpene compound with maleic anhydride, the water additive of the acid anhydride, or the modified product of the acid anhydride. The use of these curing agents further improves the flexibility of the cured product as well as the moisture resistance and adhesion of the cured product.

Examples of the acid anhydrides having an alicyclic skeleton, water-added products of the acid anhydrides, or modified products of the acid anhydrides include methylnadic acid anhydride, acid anhydrides having a dicyclopentadiene skeleton, or modified products of the acid anhydrides.

Commercially available products of the acid anhydrides having the above alicyclic skeleton, water additives of the acid anhydrides, or modified products of the acid anhydrides include RIKACID HNA and RIKACID HNA-100 (both manufactured by New Japan Chemical Co., Ltd.), as well as EPICURE YH306, EPICURE YH307, EPICURE YH308H, and EPICURE YH309 (all manufactured by Mitsubishi Chemical Corporation).

(B) It is also preferable that the heat curing agent is methylnadic anhydride or trialkyltetrahydrophthalic anhydride. The use of methylnadic anhydride or trialkyltetrahydrophthalic anhydride increases the water resistance of the cured product.

Of the components contained in the thermosetting sheet, the content of the (B) thermosetting agent is preferably 0.1% by weight or more, more preferably 1% by weight or more, and preferably 40% by weight or less, more preferably 25% by weight or less, out of 100% by weight of the components excluding the (C) boron nitride particles and the (D) insulating filler. When the content of the (B) thermosetting agent is equal to or more than the above lower limit, it is easy to sufficiently cure the thermosetting sheet. When the content of the (B) thermosetting agent is equal to or less than the above upper limit, it is difficult to generate excess (B) thermosetting agent that is not involved in curing. This further increases the heat resistance and adhesion of the cured product.

((C) Boron nitride particles)
From the viewpoint of effectively enhancing heat dissipation and compressive strength, the ratio of the average particle size of the (C) boron nitride particles to the average particle size of the (D) insulating filler is preferably 0.01 or more, more preferably 0.2 or more, and is preferably 200 or less, more preferably 10 or less.

In 100% by volume of the thermosetting sheet and in 100% by volume of the cured product, the content of (C) boron nitride particles is preferably 10% by volume or more, more preferably 30% by volume or more, and preferably 80% by volume or less, more preferably 75% by volume or less. When the content of (C) boron nitride particles is equal to or greater than the above lower limit, the heat dissipation and compressive strength are effectively improved. When the content of (C) boron nitride particles is equal to or less than the above upper limit, it is easy to sufficiently cure the thermosetting sheet. When the content of (C) boron nitride particles is equal to or less than the above upper limit, the thermal conductivity and adhesiveness of the cured product are further improved.

In the thermosetting sheet, the content of the aggregates of the (C1) boron nitride particles is preferably 20 vol% or more, more preferably 50 vol% or more, even more preferably 60 vol% or more, particularly preferably 70 vol% or more, and most preferably 80 vol% or more, and is preferably 100 vol% or less, out of a total of 100 vol% of the (C1) boron nitride particles. When the content of the (C1) aggregates is equal to or more than the lower limit, the heat dissipation and compressive strength are effectively increased.

In 100% by volume of the thermosetting sheet and in 100% by volume of the cured product, the total content of the (C) boron nitride particles and the (D) insulating filler is preferably 10% by volume or more, more preferably 20% by volume or more, even more preferably 30% by volume or more, and preferably 80% by volume or less, more preferably 75% by volume or less. When the total content of the (C) boron nitride particles and the (D) insulating filler is equal to or greater than the above lower limit, the heat dissipation and compressive strength are effectively improved. When the total content of the (C) boron nitride particles and the (D) insulating filler is equal to or less than the above upper limit, the thermosetting sheet is easily cured sufficiently. When the total content of the (C) boron nitride particles and the (D) insulating filler is equal to or less than the above upper limit, the thermal conductivity and adhesiveness of the cured product are further increased.

From the viewpoint of effectively increasing heat dissipation and compressive strength, the content of (C) boron nitride particles in the thermosetting sheet is preferably 10 vol% or more, more preferably 20 vol% or more, even more preferably 50 vol% or more, and preferably 100 vol% or less, out of a total of 100 vol% of (C) boron nitride particles and (D) insulating filler in the thermosetting sheet. The content of (C) boron nitride particles in the thermosetting sheet may be less than 100 vol% or may be 99 vol% or less, out of a total of 100 vol% of (C) boron nitride particles and (D) insulating filler in the thermosetting sheet.

(D) Insulating filler other than boron nitride particles)
The (D) insulating filler has insulating properties. The (D) insulating filler may be an organic filler or an inorganic filler. From the viewpoint of effectively increasing heat dissipation, the (D) insulating filler is preferably an inorganic filler. From the viewpoint of effectively increasing heat dissipation, the (D) insulating filler preferably has a thermal conductivity of 10 W/m·K or more. The (D) insulating filler is preferably not a particle aggregate. Only one type of (D) insulating filler may be used, or two or more types may be used in combination. Note that insulating means that the volume resistivity of the filler is 10 ⁶ Ω·cm or more.

From the viewpoint of further improving the heat dissipation properties of the cured product, the thermal conductivity of the insulating filler (D) is preferably 10 W/m·K or more, more preferably 15 W/m·K or more, and even more preferably 20 W/m·K or more. There is no particular upper limit to the thermal conductivity of the insulating filler (D). Inorganic fillers with a thermal conductivity of about 300 W/m·K are widely known, and inorganic fillers with a thermal conductivity of about 200 W/m·K are easily available.

The material of the (D) insulating filler is not particularly limited. The material of the (D) insulating filler is preferably alumina, synthetic magnesite, aluminum nitride, silicon nitride, silicon carbide, zinc oxide, magnesium oxide or diamond, and more preferably alumina, aluminum nitride, silicon nitride, silicon carbide, zinc oxide or magnesium oxide. By using these preferred insulating fillers, the heat dissipation properties of the cured product are further improved.

(D) The insulating filler is preferably spherical particles or non-agglomerated particles with an aspect ratio of more than 2. The use of these insulating fillers further improves the heat dissipation properties of the cured product. The aspect ratio of the spherical particles is 2 or less.

(D) The new Mohs hardness of the insulating filler material is preferably 12 or less, and more preferably 9 or less. (D) If the new Mohs hardness of the insulating filler material is 9 or less, the processability of the cured product will be even higher.

From the viewpoint of further improving the processability of the cured product, it is preferable that the material of the insulating filler (D) is synthetic magnesite, crystalline silica, zinc oxide, or magnesium oxide. The new Mohs hardness of these inorganic filler materials is 9 or less.

From the viewpoint of effectively improving heat dissipation, the average particle size of the insulating filler (D) is preferably 1 μm or more, and preferably 100 μm or less. When the average particle size is equal to or more than the above lower limit, the insulating filler (D) can be easily packed at a high density. When the average particle size is equal to or less than the above upper limit, the voltage resistance of the cured product is further improved.

The above "average particle size" refers to the average particle size determined from the volume-average particle size distribution measurement results measured using a laser diffraction particle size distribution analyzer.

In 100% by volume of the thermosetting sheet and in 100% by volume of the cured product, the content of the insulating filler (D) is preferably 5% by volume or more, more preferably 10% by volume or more, and preferably 75% by volume or less, more preferably 65% by volume or less. When the content of the insulating filler (D) is equal to or more than the above lower limit and equal to or less than the above upper limit, the heat dissipation and compressive strength of the cured product are effectively increased.

(Other ingredients)
The thermosetting sheet may contain, in addition to the above-mentioned components, other components that are generally used in thermosetting compositions and thermosetting sheets, such as a dispersant, a chelating agent, and an antioxidant.

(Other details of the thermosetting sheet and the cured product)
A cured product can be obtained by curing the thermosetting sheet. This cured product is a cured product of the thermosetting sheet and is formed from the thermosetting sheet.

Figure 1 is a cross-sectional view showing a schematic diagram of a cured thermosetting sheet according to one embodiment of the present invention. Note that for convenience of illustration, the actual size and thickness are different in Figure 1.

The cured material 1 shown in FIG. 1 includes a cured material portion 11, an aggregate 12 of boron nitride particles, and an insulating filler 13. The insulating filler 13 is not an aggregate of boron nitride particles. In FIG. 1, the aggregate 12 of boron nitride particles is marked with a diagonal line extending from the upper left to the lower right. In FIG. 1, the insulating filler 13 is marked with a diagonal line extending from the upper right to the lower left. In FIG. 1, the aggregate 12 of boron nitride particles is shown in a simplified diagram.

The cured portion 11 is a portion in which the thermosetting component containing the thermosetting compound and the thermosetting agent has hardened, and is obtained by hardening the thermosetting component.

The thermosetting sheet and the cured product can be used in various applications requiring high heat dissipation and compressive strength. For example, the cured product is used in electronic devices by being placed between heat-generating and heat-dissipating components.

The present invention will be clarified by providing specific examples of the present invention below. The present invention is not limited to the following examples.

( Reference Example 1)
Preparation of thermosetting sheet:
An aggregate of boron nitride particles having an average particle size of 25 μm ("PTX25" manufactured by Momentive Corp.), an epoxy resin ("E1256" manufactured by Japan Epoxy Resins Co., Ltd.), a curing agent ("MH-700" manufactured by New Japan Chemical Co., Ltd.), and a dispersing agent ("Disperbyk-2070" manufactured by BYK Japan KK) were prepared.

50 parts by weight of the boron nitride particle aggregates, 47 parts by weight of the epoxy resin, 1.25 parts by weight of the curing agent, and 1.75 parts by weight of the dispersant were mixed to obtain a resin composition.

Next, the resin composition was applied to a thickness of 100 μm on a release PET sheet (thickness 50 μm) and dried in an oven at 90°C for 10 minutes to obtain a thermosetting sheet.

The thermosetting sheet was pre-cured by heating at 110°C for 30 minutes to obtain a first cured sheet. The degree of orientation of the boron nitride particles in the first cured sheet measured by X-ray diffraction analysis was 35.1.

Next, both sides of the pre-cured thermosetting sheet were sandwiched between copper foil and an aluminum plate, respectively, and vacuum pressed for 60 minutes at a temperature of 195°C and a pressure of 12 MPa to produce a second cured sheet. The degree of orientation of the boron nitride particles in the second cured sheet measured by X-ray diffraction analysis was 85.

( Reference Example 2)
A second cured sheet was produced in the same manner as in Reference Example 1, except that the pressure during vacuum pressing was changed to 3 MPa. The degree of orientation of the boron nitride particles in the second cured sheet was 38 as measured by X-ray diffraction analysis.

( Reference Example 3)
Preparation of boron nitride particle agglomerates:
100 g of boron nitride particles (MBN-010T, manufactured by Mitsui Chemicals, Inc.), 5 g of polyvinyl alcohol, and 390 g of ethanol were mixed and dispersed with ultrasonic waves for 30 minutes to prepare a slurry to be sprayed.

Next, the resulting spray slurry was spray-dried at 90°C using a spray dryer (B-290, manufactured by Buchi) equipped with an ultrasonic nozzle to obtain spherical aggregates with an average particle size of 30 μm.

The resulting agglomerates were sintered in a nitrogen atmosphere at 1000°C for 2 hours, and then sintered further at 1500°C for 2 hours. This sintering process resulted in spherical agglomerates of boron nitride particles from which the PVB had been removed.

Preparation of thermosetting sheet:
A thermosetting sheet, a first cured sheet, and a second cured sheet were prepared in the same manner as in Reference Example 1, except that the obtained agglomerates of boron nitride particles were used instead of agglomerates of boron nitride particles having an average particle size of 25 μm (PTX25, manufactured by Momentive Corp.).

The degree of orientation of the boron nitride particles in the first hardened sheet measured by X-ray diffraction analysis was 6.5. The degree of orientation of the boron nitride particles in the second hardened sheet measured by X-ray diffraction analysis was 8.9.

Example 4
A thermosetting sheet, a first cured sheet, and a second cured sheet were prepared in the same manner as in Reference Example 1, except that 45 parts by weight of an aggregate of boron nitride particles ("PTX25" manufactured by Momentive Corp.) and 5 parts by weight of aluminum oxide ("AX10-75" manufactured by Micron Corp.) were used instead of the 50 parts by weight of the aggregate of boron nitride particles.

The degree of orientation of the boron nitride particles in the first hardened sheet measured by X-ray diffraction analysis was 35.1. The degree of orientation of the boron nitride particles in the second hardened sheet measured by X-ray diffraction analysis was 72.

(Comparative Example 1)
A second cured sheet was produced in the same manner as in Reference Example 1, except that the pressure during vacuum pressing was changed to 1 MPa. The degree of orientation of the boron nitride particles in the second cured sheet was 36 as measured by X-ray diffraction analysis.

(evaluation)
<Measurement of Orientation Degree S1 and Orientation Degree S2>
The X-ray diffraction patterns of the first and second cured sheets were measured using an X-ray diffractometer (Rigaku Corporation "SmartLab Multipurpose"). From the obtained diffraction patterns, the diffraction peak intensity I (002) corresponding to the (002) plane of the boron nitride particles and the diffraction peak intensity I (100) corresponding to the (100) plane of the boron nitride particles were calculated. The ratio of the diffraction peak intensity I (002) calculated using the first cured sheet to the diffraction peak intensity I (100) was defined as the degree of orientation S1, and the ratio of the diffraction peak intensity I (002) calculated using the second cured sheet to the diffraction peak intensity I (100) was defined as the degree of orientation S2.

<Measurement of thermal conductivity>
The thermal conductivity was measured by a laser flash method using a measurement sample obtained by cutting the obtained thermosetting sheet into a 1 cm square and spraying carbon black on both sides. The thermal conductivity in Table 1 is a relative value with the value of Comparative Example 1 set to 1.00.

<Dielectric breakdown voltage (voltage resistance)>
The obtained thermosetting sheet was cut into a size of 100 mm x 100 mm to obtain a test sample. The obtained test sample was cured in an oven at 120 ° C for 1 hour and then in an oven at 200 ° C for 1 hour to obtain a cured sheet. Using a voltage resistance tester (EXTECH Electronics "MODEL 7473"), an AC voltage was applied between the cured sheets so that the voltage increased at a rate of 1 kV/sec. The voltage at which the cured sheet broke down was taken as the dielectric breakdown voltage.

The results are shown in Table 1 below.

1...cured product 11...cured product part (cured part of thermosetting component)
12...Agglomerates of boron nitride particles 13...Insulating filler

Claims (8)

pre-curing a thermosetting sheet comprising a thermosetting compound, a thermosetting agent, a plurality of boron nitride particles, and an insulating filler that is not a boron nitride particle to obtain a first cured sheet;
and post-curing the first cured sheet to obtain a cured sheet.
the thermosetting compound comprises an epoxy compound,
the boron nitride particles contained in the thermosetting sheet include aggregates in which boron nitride particles are aggregated,
The insulating filler has a thermal conductivity of 10 W/m K or more;
a degree of orientation S1 of boron nitride particles in the first cured sheet and a degree of orientation S2 of boron nitride particles in the resulting cured sheet satisfy the relationship: 1.05×S1≦S2≦3.0×S1. The method for producing a cured sheet according to claim 1 , wherein the content of the boron nitride particles in the thermosetting sheet is 10% by volume or more and 80% by volume or less. 3. The method for producing a cured sheet according to claim 1 , wherein a total content of the boron nitride particles and the insulating filler in the thermosetting sheet is 20% by volume or more and 80% by volume or less. The method for producing a cured sheet according to any one of claims 1 to 3 , wherein the content of the boron nitride particles is 50% by volume or more based on a total of 100% by volume of the boron nitride particles and the insulating filler in the thermosetting sheet. The method for producing a cured sheet according to claim 1 , wherein the degree of orientation S1 is 5 or more and 60 or less. The method for producing a cured sheet according to any one of claims 1 to 5 , wherein the average primary particle size of the boron nitride particles contained in the thermosetting sheet is 0.1 µm or more and 10 µm or less. The method for producing a cured sheet according to any one of claims 1 to 6 , wherein a ratio of an average particle size of the boron nitride particles contained in the thermosetting sheet to an average particle size of the insulating filler is 0.01 or more and 200 or less. The method for producing a cured sheet according to any one of claims 1 to 7 , wherein the insulating filler has an average particle size of 1 µm or more and 100 µm or less.

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