JP6875854B2 - Hexagonal Boron Nitride Primary Particle Aggregates and Their Applications - Google Patents

Description

The present invention is contained in a hexagonal boron nitride primary particle agglomerate, specifically, a thermal interface material that transfers heat of a heat-generating electronic component such as a power device or a power module to a heat-dissipating member, and an insulating layer of a printed wiring board. The present invention relates to a crystallized boron nitride primary particle agglomerate, a method for producing the same, and its use.

In heat-generating electronic components such as power devices, transistors, thyristors, and CPUs, and power modules in which these are combined, how to efficiently dissipate heat generated during use has become an important issue. Conventionally, as a measure for improving heat dissipation efficiency in this field, for example, the insulating layer of a printed wiring board on which a heat-generating electronic component is mounted is made highly thermally conductive, and a heat-generating electronic component or a printed wiring board on which a heat-generating electronic component is mounted is mounted. It has been generally practiced to attach a plate to a heat radiating member such as a heat sink via an electrically insulating thermal interface material. As a main material such as an insulating layer of a printed wiring board and a thermal interface material, for example, a resin composition in which a silicone resin or an epoxy resin is filled with ceramic powder having high thermal conductivity is preferably used.

In recent years, as electronic devices have become lighter, thinner, shorter, and smaller, the mounting density of heat-generating electronic components has increased, while the heat-generating density inside electronic devices has also increased year by year. Has been sought. Further, the demand for reliability of equipment, particularly the demand for electrical reliability is increasing, and at the same time, ceramic powder having high electrical insulation is also required.

Due to the above background, hexagonal boron nitride, which is basically excellent as an electrically insulating material having high thermal conductivity, is attracting attention. However, due to its crystal structure, the primary particles of hexagonal boron nitride have a thermal conductivity of 400 W / (m · K) in the in-plane direction (also referred to as the a-axis direction) and a thickness direction (also referred to as the c-axis direction). ) Is 2 W / (m · K), and the anisotropy of the thermal conductivity is extremely large. When such hexagonal boron nitride primary particles are filled in the resin as they are, the primary particles are aligned in the same direction and oriented. Therefore, for example, when manufacturing an insulating layer for a printed wiring board and a thermal interface material, hexagonal boron nitride primary particles are used. The in-plane direction of the particles (that is, the direction having high thermal conductivity) and the thickness direction of the insulating layer of the printed wiring board and the thermal interface material (that is, the direction in which heat should be transferred) are perpendicular to each other, and the hexagonal boron nitride particles The high thermal conductivity could not be fully utilized.

In order to solve the problem caused by the anisotropy of thermal conductivity, the scaly hexagonal boron nitride primary particles are intended to have more isotropic thermal conductivity. Primary particle agglomerates have been developed.

For example, Patent Documents 1 and 2 propose the use of hexagonal boron nitride primary particle aggregates in which hexagonal boron nitride primary particles are aggregated so as not to be oriented in the same direction, and the anisotropy of thermal conductivity is increased. It has been shown to be suppressed.

Japanese Unexamined Patent Publication No. 2014-40341 Japanese Unexamined Patent Publication No. 2013-241321

The shape of the hexagonal boron nitride primary particle agglomerates proposed in Patent Document 1 and Patent Document 2 is substantially spherical, and is effective in suppressing anisotropy related to thermal conductivity. However, among the hexagonal boron nitride primary particles, which is a conventional technique, an alkali metal or alkaline soil derived from a binder agent (also referred to as a sintering aid) used when the primary particles are agglomerated by sintering is used. It is not particularly taken into consideration that metallic impurities such as similar metals remain, and the insulating layer and heat of the printed wiring board produced by filling the resin with the hexagonal boron nitride primary particle agglomerates. Further improvements were required with respect to the reliability of the insulation of the interface material. That is, as power devices and power modules are installed in hybrid vehicles, electric vehicles, fuel cell vehicles, which have been energetically developed in recent years, and railway vehicles, which are increasingly being electrically controlled. Hexagonal nitride as an insulating layer and thermal interface material for printed wiring boards used in power devices and the like, which has high thermal conductivity and long-term insulation (also referred to as moisture absorption insulation) in a high temperature and high humidity environment. Development of boron primary particle aggregates is required. However, simply reducing the binder material that hinders the moisture absorption and insulation properties reduces the mechanical strength of the hexagonal boron nitride primary particle agglomerates, and the hexagonal boron nitride primary particle agglomerates and the resin are kneaded to form a resin composition. When the primary particle agglomerates are destroyed, the thermal conductivity is lowered. Hexagonal boron nitride primary particle coagulation that has strength enough to withstand kneading with resin during composition preparation, has few metallic impurities, and can withstand long-term use in a high temperature and high humidity environment. There was a demand for the development of aggregates and methods for producing them. That is, a hexagonal boron nitride primary particle agglomerate that satisfies all of high mechanical strength, isotropic thermal conductivity, and hygroscopic insulation has not been developed so far.

In view of the above prior art, the present invention provides a hexagonal boron nitride primary particle agglomerate and a method for producing the same, which contributes to high mechanical strength, thermal conductivity and long-term insulation in a high temperature and high humidity environment of the resin composition. One of the issues is to do. The present invention also provides a resin composition containing the hexagonal boron nitride primary particle aggregate, which is applicable to the present invention, and further provides an insulating layer and a thermal interface material for a printed wiring board using the resin composition. Make it an issue.

In order to solve the above problems, the following means are adopted in the present invention. That is, the present invention
[1] Hexagonal boron nitride primary particles satisfying the following (1) and (2) are aggregated, the average particle diameter is 10 μm or more and 200 μm or less, the crushing strength of the aggregate is 1 MPa or more and 4 MPa or less, and the content of alkali metal is contained. Hexagonal boron nitride primary particle agglomerates having a ratio of 10 ppm or less.
(1) The graphitization index calculated by the powder X-ray diffraction method is 1.8 or more and 2.2 or less.
(2) The proportion of particles having a long side length of less than 1 μm is 25 to 50%, the proportion of particles having a long side length of 1 μm or more and 5 μm or less is 50 to 75%, and the proportion of particles exceeding 5 μm is 5% or less.
[2]
The hexagonal boron nitride primary particle aggregate of [1], wherein the alkali metal is sodium or lithium.
[3] After primary firing of melamine borate in a nitrogen atmosphere of 400 ° C. or higher and 1200 ° C. or lower for 1 hour or longer and 5 hours or lower to obtain a primary fired product, alkali is applied to 100 parts by mass of the primary fired product. The hexagonal boron nitride primary particle aggregate of [1] or [2], which is obtained by adding 5 parts by mass or more and 15 parts by mass or less of metal borate and further firing in a nitrogen atmosphere of 1900 ° C. or higher and 2200 ° C. or lower. Production method.
[4] A resin composition containing the hexagonal boron nitride primary particle aggregates of [1] or [2].
[5] An insulating layer of a printed wiring board containing the resin composition of [4].
[6] A thermal interface material containing the resin composition of [4].

The hexagonal boron nitride primary particle agglomerates of the present invention provide a resin composition having high thermal conductivity and high insulating properties in a high temperature and high humidity environment without breaking during kneading with a resin to form a resin composition. It becomes possible to obtain. Therefore, the resin composition containing the hexagonal boron nitride primary particle aggregate of the present invention can be suitably used as an insulating layer and a thermal interface material for a printed wiring board.

6 is an electron micrograph of a cross section of a hexagonal boron nitride primary particle aggregate produced in Example 1.

In the present invention, scaly hexagonal boron nitride primary particles are referred to as "hexagonal boron nitride primary particles", and a state in which two or more of the primary particles are bonded to each other by sintering is referred to as "hexagonal boron nitride primary particle agglomerates". Is defined as. For bonding by sintering, for example, a scanning electron microscope (for example, JSM-6010LA, manufactured by JEOL Ltd.) can be used to confirm the bonded portion of the primary particles in the cross section of the hexagonal boron nitride primary particle aggregate. As a pretreatment for easier observation, after embedding the hexagonal boron nitride primary particle agglomerate with a resin, the cross section is processed by a CP (cross section polisher) method and fixed to a sample table. Further osmium coating is preferably performed.

<Manufacturing method of hexagonal boron nitride primary particle aggregate>
As a method for producing a hexagonal boron nitride primary particle aggregate according to the present invention, a powder of a compound containing boron and a powder of a compound containing nitrogen (starting raw material) are used as a starting material for hexagonal boron nitride at the time of firing. A mixed powder containing a powder of a firing aid that promotes conversion (hereinafter, these boron-containing compounds, a nitrogen-containing compound, and a mixed powder containing a sintering aid are collectively referred to as a mixed raw material powder) is referred to as nitrogen. , A method of firing in an atmosphere of helium, argon, ammonia or the like is preferably applied.

Here, as the compound containing boron, boric acid, boron oxide, borax and the like are preferable, and boric acid can be particularly preferably used. Further, as the nitrogen-containing compound, cyandiamide, melamine, urea and the like are preferable, and melamine can be particularly preferably selected. Further, the molar ratio of boron atom to nitrogen atom contained in the starting material does not necessarily have to be fixed at 5: 5, and the molar ratio of boron atom to nitrogen atom may be changed to 2: 5 depending on the reactivity and yield. It can be appropriately changed in the range of 8 to 8: 2, preferably in the range of 3: 7 to 7: 3.

The sintering aid used in the present invention is preferably inexpensive and does not easily volatilize and remain in the firing process. Specific examples include boric acid, an alkyl metal borate, and an alkali metal compound capable of reacting with boric acid to form an alkali metal borate. More specifically, examples of alkali metal compounds such as sodium borate and lithium borate that can react with boric acid to form an alkali metal borate include sodium carbonate and lithium carbonate. Alkali metal borates other than sodium and lithium have a high boiling point, and the metal remains after firing, and the moisture absorption and insulation property when the hexagonal boron nitride primary particle aggregate is filled in the resin is lowered.

The firing is performed by primary firing a mixture of boric acid and melamine in a non-oxidizing atmosphere of 400 ° C. or higher and 1200 ° C. or lower for 1 hour or longer and 5 hours or lower to obtain an amorphous BN, and then for 100 parts by mass of the primary fired product. Alkali metal borate is added in an amount of 5 parts by mass or more and 15 parts by mass or less, and secondary firing is performed in a non-oxidizing atmosphere of 1900 ° C. or higher and 2200 ° C. or lower. If the temperature of the primary firing is less than 400 ° C., carbon and oxygen remain in the primary firing product, and the crystallinity of the hexagonal boron nitride primary particles becomes low. If the temperature exceeds 1200 ° C., the amount of carbon and oxygen remaining in the primary fired product is small, and the crystal growth of the hexagonal boron nitride primary particles progresses too much, resulting in loosening of aggregation. Further, if the amount of the alkali metal borate added is less than 5 parts by mass, the crystallization of the hexagonal boron nitride primary particles does not proceed, and if it exceeds 15 parts by mass, the crystal growth of the hexagonal boron nitride primary particles proceeds too much. Therefore, the agglomeration is loosened. If the temperature of the secondary firing is less than 1900 ° C., alkali metal borate remains, and if it exceeds 2200 ° C., the crystal growth of the hexagonal boron nitride primary particles progresses too much, resulting in loosening of agglomeration. The firing temperature may be kept constant or may be changed continuously or discontinuously, and the firing time and the rate of heating and cooling are not particularly limited. Further, the firing device is not particularly limited, but a container made of hexagonal boron nitride can be used as the container for storing the raw material, and a firing furnace using an electric heater, for example, can be used as the heating device. Can be done. Further, it is also possible to further add heating, cooling, humidifying, drying, and washing operations within a range that does not deviate from the object of the present invention between the time when the starting materials are mixed to form a powder mixture and the time when firing is completed. Is.

The hexagonal boron nitride primary particle agglomerates of the present invention include the average particle size, crush strength, moisture absorption insulation, alkali metal content of a specific agglomerate, and the ratio of the long side length of the specific hexagonal boron nitride primary particles. By having a graphitization index, it is possible to obtain a hexagonal boron nitride primary particle agglomerate having excellent thermal conductivity and moisture absorption and insulation properties, which could not be achieved by conventional techniques.

The hexagonal boron nitride primary particle agglomerates of the present invention have a graphitization index of 1.8 to 2.2, a ratio of long side lengths of less than 1 μm is 25 to 50%, and a ratio of 1 μm or more and 5 μm or less is 50. Hexagonal boron nitride primary particles are aggregated, characterized in that the proportion of particles exceeding ~ 75% and 5 μm is 5% or less, the average particle size is 10 to 200 μm, the alkali metal content is 10 ppm or less, and the particle is crushed. It is a hexagonal boron nitride primary particle agglomerate having a strength of 1 MPa or more and 4 MPa or less. Boron nitride powder designed in this way has never existed, and has the crushing strength that can withstand the shear stress of kneading when filling the resin, and the high thermal conductivity and high moisture absorption insulation of the resin composition filled with these. It is a very important factor in terms of securing.

<Graphitization index>
The graphitization index (also referred to as GI, Grafitation Index, etc.) is originally an index value indicating the degree of crystallinity of a graphite sample (J. Thomas, et. Al, J. Am. Chem. Soc. 84, 4619 (J. Am. Chem. Soc. 84, 4619). 1962)), in the present invention, this is a value applied to hexagonal boron nitride. That is, the integrated intensity of each diffraction peak by the (100) plane, (101) plane, and (102) plane in the spectrum diagram of the hexagonal boron nitride primary particles measured by the powder X-ray diffraction method, that is, each diffraction peak and its base. The area value (unit is arbitrary) surrounded by the line is _{a value calculated by obtaining S 100} , S ₁₀₁ , and S ₁₀₂ , respectively, and substituting them into the equation of GI = (S ₁₀₀ + S ₁₀₁ ) / S _102.
It is said that the GI value of a completely crystallized powder is 1.60, but in the case of a scaly hexagonal boron nitride powder with high crystallinity and sufficiently grown particles, the particles are easily oriented. The fully crystallized GI becomes even smaller. That is, GI is an index of crystallinity of scaly hexagonal boron nitride powder, and the smaller this value is, the higher the crystallinity is.
In the present invention, the GI of the hexagonal boron nitride primary particles to be aggregated is preferably 1.8 to 2.2. If the GI is larger than 2.2, the crystallinity of the hexagonal boron nitride primary particles is low, so that high thermal conductivity may not be obtained. On the other hand, if the GI is smaller than 1.8, the scale shape is overdeveloped, so that it may be difficult to maintain the aggregated structure, and the particle strength may decrease.

<Long side length of primary particles>
Further, in the hexagonal boron nitride primary particle aggregate of the present invention, the proportion of particles having a long side length of less than 1 μm is 25 to 50%, the proportion of particles having a long side length of 1 μm or more and 5 μm or less is 50 to 75%, and the proportion of particles exceeding 5 μm is 5%. It is an agglomerate in which hexagonal boron nitride primary particles are agglomerated, which is characterized by the following. In addition, this ratio is the number ratio. The long side length and ratio of the hexagonal boron nitride primary particles are determined by embedding the hexagonal boron nitride primary particle aggregate with a resin, processing the cross section by the CP (cross section polisher) method, and scanning the cross section with scanning electrons. Photographed with a microscope, for example, "JSM-6010LA" (manufactured by JEOL Ltd.) at a magnification of 2000, and the obtained particle image was taken into image analysis software, for example, "MacView" (Thermermo Electron SpA) and measured from the photograph. To do. If the ratio of the long side length deviates from the above ratio, for example, if the ratio of hexagonal boron nitride primary particles having a long side length of less than 1 μm exceeds 50%, the shearing of kneading received when filling the resin It is not possible to obtain particle strength that can withstand stress. If the proportion of hexagonal boron nitride primary particles having a long side length of 1 μm or more and 5 μm or less exceeds 75%, or the proportion of hexagonal boron nitride primary particles having a long side length of more than 5 μm exceeds 5%, aggregation occurs. There is a problem that it becomes difficult to maintain the structure.

<Alkali metal content>
Further, in the hexagonal boron nitride primary particle aggregate of the present invention, the content of the alkali metal is 10 ppm or less. If the content of the alkali metal exceeds 10 ppm, the hygroscopic insulation property when the resin is filled is lowered. The alkali metal content of the hexagonal boron nitride primary particle aggregate is obtained by adding 1 g of the hexagonal boron nitride primary particle aggregate to 40 ml of distilled water and heating at 120 ° C. and 0.2 MPa for 24 hours using a pressure vessel. The supernatant was measured using a high frequency inductively coupled plasma emission spectrometer (for example, "ES-71" (HORIBA)).

<Crushing strength>
Further, the crushing strength of the hexagonal boron nitride primary particle aggregate of the present invention is 1 to 4 MPa. The crushing strength of the boron nitride primary particle agglomerates referred to in the present invention is a value based on the actual measurement value of the crushing strength of the individual agglomerated particles. The particle crushing strength can be measured in accordance with JIS R1639-5 using a commercially available compression tester capable of measuring the crushing strength of fine particles. At this time, assuming that the particle diameter of the hexagonal boron nitride primary particle aggregate is d (unit is mm) and the fracture test force is P (unit is N), the crushing strength Cs (unit is MPa) is Cs = 2.48P /. It is calculated from the formula of πd ^2. In the present invention, the average value of the crushing intensities of 10 agglomerated particles was defined as the particle crushing intensities. If the crushing strength is less than 1 MPa, for example, when the composition is kneaded with a resin, the aggregate particles are destroyed, which is not preferable. On the other hand, if it is 4 MPa or more, when kneaded with the resin, the shape of the agglomerated particles is maintained and the flexibility of the resin composition is lost, the adhesion between the resin composition and the heat radiating member is lost, and the heat radiating property is lowered. When the crushing strength is in the range of 1 to 4 MPa, the hexagonal boron nitride primary particle aggregate partially disintegrates when mixed with the resin composition, and exhibits high thermal conductivity while maintaining the flexibility of the resin composition.

<Average particle size>
In the hexagonal boron nitride primary particle agglomerates of the present invention, the average particle size of the agglomerates is 10 μm or more and 200 μm or less. Generally, as the average particle size becomes smaller, the thermal conductivity tends to decrease due to an increase in contact thermal resistance as the total number of aggregate-resin interfaces increases. However, as the average particle size increases, the mechanical strength of the agglomerates tends to decrease, and a part of the structure of the agglomerates is destroyed by the shear stress received during kneading with the resin, and the particles return to the primary particles. Since the scaly hexagonal boron nitride is oriented in the same direction, high thermal conductivity may not be exhibited. Therefore, the average particle size of the hexagonal boron nitride primary particle aggregate is 200 μm or less. The average particle size referred to in the present invention is a particle size of 50% of the cumulative value of the cumulative particle size distribution based on the volume in the particle size distribution measurement by the laser diffraction light scattering method. When measuring the particle size distribution, water can be used as the solvent for dispersing the aggregates, and hexametaphosphate can be used as the dispersant. At this time, 1.33 can be used for the refractive index of water, and 1.80 can be used for the refractive index of the boron nitride powder. The measurement time per measurement is not particularly limited, but is usually 5 seconds or more and 120 seconds or less, and is generally set to about 15 seconds or more and 60 seconds or less.

<Resin composition containing hexagonal boron nitride primary particle aggregates>
Next, a resin composition containing hexagonal boron nitride primary particle aggregates, which is the second embodiment of the present invention, will be described. The proportion of the agglomerates contained in the resin composition is preferably 20% by volume or more and 80% by volume or less. At this time, various ceramic mix powders (hereinafter referred to as various ceramic powders) having an average particle diameter smaller than that of the hexagonal boron nitride primary particle aggregate of the present invention, such as aluminum nitride, hexagonal boron nitride, boron nitride, and silicon nitride, are used. , Aluminum oxide, zinc oxide, magnesium oxide, magnesium hydroxide, silicon dioxide, and silicon carbide may be appropriately added in an amount of one or more as long as the object of the present invention is not impaired.

<Resin>
The type of resin that can be used in the resin composition containing the hexagonal boron nitride primary particle aggregate according to the second embodiment of the present invention is not particularly limited, but for example, epoxy resin, silicone resin, and silicone rubber. , Acrylic resin, phenol resin, melamine resin, urea resin, unsaturated polyester, fluororesin, polyimide, polyamideimide, polyetherimide and other polyimides, polybutylene terephthalate, polyethylene terephthalate and other polyesters, polyphenylene ether, polyphenylene sulfide, total fragrance Group polyester, polysulfone, liquid crystal polymer, polyether sulfone, polycarbonate, maleimide modified resin, ABS resin, AAS (acrylonitrile-acrylic rubber / styrene) resin, AES (acrylonitrile / ethylene / propylene / diene rubber-styrene) resin, polyglycolic acid resin , Polyphthalamide, polyacetal, nylon resin and the like can be preferably mentioned. These resins, especially thermosetting resins, are appropriately cured with a curing agent, an inorganic filler, a silane coupling agent, and further improve wettability and leveling property and reduce viscosity to reduce the occurrence of defects during heat and pressure molding. Additives can be included. Examples of this additive include a defoaming agent, a surface conditioner, a wet dispersant and the like. Further, the epoxy resin is suitable as an insulating layer of a printed wiring board because it has excellent heat resistance and adhesive strength to a copper foil circuit. Further, silicone resin and silicone rubber are suitable as thermal interface materials because they are excellent in heat resistance, flexibility, and adhesion to heat sinks and the like.

When the hexagonal boron nitride primary particle aggregate of the present invention and the resin are mixed to form a resin composition, an organic solvent may be added if necessary in order to facilitate mixing of the two. Examples of the organic solvent include alcohols such as ethanol and isopropanol, 2-methoxyethanol, 1-methoxyethanol, 2-ethoxyethanol, 1-ethoxy-2-propanol, 2-butoxyethanol, 2- (2-methoxyethoxy). ) Ether alcohols such as ethanol, 2- (2-ethoxyethoxy) ethanol and 2- (2-butoxyethoxy) ethanol, glycol ethers such as ethylene glycol monomethyl ether and ethylene glycol monobutyl ether, acetone, methyl ethyl ketone, methyl isobutyl ketone. And ketones such as diisobutylketone, and aromatic hydrocarbons such as toluene and xylene. These organic solvents may be used alone or in combination of two or more.

The insulating layer and thermal interface material of the printed wiring board according to the third embodiment of the present invention are further required by molding a resin composition containing the hexagonal boron nitride primary particle agglomerates according to the second embodiment. It is an insulating layer and a thermal interface material of a printed wiring board, which are made in combination with other materials according to the situation.

<Examples 1 to 10, Comparative Examples 1 to 9: Preparation of hexagonal boron nitride primary particle agglomerates>
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
Example 1 was prepared as follows. 260 kg of boric acid and 240 kg of melamine were humidified and mixed, and fired in a nitrogen atmosphere in an electrically heated rotary kiln at 1000 ° C. for 2 hours as primary firing. 10 kg of sodium borate was added to 100 kg of the obtained primary fired product, mixed, and fired in a nitrogen atmosphere in a nitrogen atmosphere at 2100 ° C. for 4 hours to obtain a secondary fired product. Then, it was crushed to obtain a hexagonal boron nitride primary particle agglomerate. Further, in the same operation, 19 kinds of powders A to S were produced according to the conditions shown in Table 1.

<Filling in resin>
Epoxy resin (manufactured by Mitsubishi Chemical Corporation, Epicoat 807) and curing agent (manufactured by Nippon Synthetic Chem Industry Co., Ltd., Acmex) to evaluate the practical characteristics of hexagonal boron nitride primary particle aggregates as an insulating layer and thermal interface material for printed wiring boards. Hexagonal boron nitride primary particle aggregates are mixed with H-84B) so as to be 60% by volume, coated on a PET sheet so as to have a thickness of 1.0 mm, and then defoamed under reduced pressure at 500 Pa. It was done for 10 minutes. ^{Then, press heating and pressurization was performed for 60 minutes under two} conditions of a temperature of 150 ° C. and a pressure of 160 kg / cm to obtain a sheet having a thickness of 0.5 mm. Evaluation was carried out using the sheet.

<Thermal conductivity>
Thermal conductivity (H (W / (m · K)) is thermal diffusivity (A (m ² / sec)), density (B (kg / m ³ )) and specific heat capacity (C). Calculated from (J / (kg · K)) as H = A × B × C. The thermal diffusivity was obtained by processing a sheet as a measurement sample into a width of 10 mm × 10 mm × thickness of 0.5 mm and using a laser flash. The measuring device was a xenon flash analyzer (LFA447NanoFlash, manufactured by NETZSCH). The density was determined by the Archimedes method. The specific heat capacity was determined by using a DSC measuring device (ThermoPlus Evo DSC8230, manufactured by Rigaku). I asked for it.

<Hygroscopic insulation>
For hygroscopic insulation, after measuring the initial insulation resistance of the sheet, the sheet is moistened with a constant temperature and humidity controller (manufactured by Yamato Scientific Co., Ltd., IX110) at 85 ° C. and 85% RH, and the insulation resistance is 85, which is the initial insulation resistance. Shown as the elapsed time until it becomes less than%. The insulation resistance is an insulation resistance tester (manufactured by Kikusui Electronics Co., Ltd.) in which the sheet is sandwiched between an aluminum plate and a copper nickel-plated plate 55 × 70 mm and attached with a torque of 7 kgf · cm using M5 screws. , TOS7200) was used for measurement at an applied voltage of 500 V.

From the results in Table 2, the sheet using the resin composition containing the hexagonal boron nitride primary particle aggregate of the present invention has a thermal conductivity of 8 (W / m · K) or more and a moisture absorption of 100 (hours) or more. It can be seen that the insulation is compatible. Further, the sheet using the resin composition containing the hexagonal boron nitride primary particle aggregate of the comparative example has both a thermal conductivity of 8 (W / m · K) or more and a moisture absorption insulation property of 100 (hours) or more. It turns out that it cannot be done. The cause is as explained below.

In Comparative Example 1, the primary firing temperature was too low, and the carbon and oxygen of the primary fired product remained, so that the crystallinity of the hexagonal boron nitride primary particles became low. Therefore, high thermal conductivity cannot be obtained.
In Comparative Example 2, the primary firing temperature was too high, and the residual amount of carbon and oxygen in the primary firing product was small, so that the crystallinity of the hexagonal boron nitride primary particles was high. The scaly shape develops too much, making it difficult to maintain the aggregated structure and making it impossible to obtain high thermal conductivity.
In Comparative Example 3, the primary firing time was too short, and carbon and oxygen remained, so that the crystallinity of the hexagonal boron nitride primary particles became low. Therefore, high thermal conductivity cannot be obtained.
In Comparative Example 4, the primary firing time was too long, and the residual amount of carbon and oxygen was small, so that the crystallinity of the hexagonal boron nitride primary particles was high. The scaly shape develops too much, making it difficult to maintain the aggregated structure and making it impossible to obtain high thermal conductivity.
In Comparative Example 5, since the amount of the alkali metal borate added is too small and the amount of the sintering aid is small, the average particle size of the hexagonal boron nitride primary particle aggregate is small, and high thermal conductivity cannot be obtained.
In Comparative Example 6, the amount of the alkali metal borate added was too large, and by acting as a sintering aid, the average particle size of the hexagonal boron nitride primary particle aggregate became large, and high thermal conductivity could not be obtained.
In Comparative Example 7, the boiling point of calcium borate was high, and calcium remained after the secondary firing, resulting in a decrease in hygroscopic insulation.
In Comparative Example 8, the secondary firing temperature was too low, and the alkali metal borate did not completely volatilize. Therefore, the alkali metal borate promotes the crystallization of hexagonal boron nitride primary particles too much, it becomes difficult to maintain the aggregated structure, the particle crushing strength is lowered, and high thermal conductivity cannot be obtained. In addition, the hygroscopic insulation was lowered because the alkali metal borate remained.
In Comparative Example 9, the secondary firing temperature was too high, and the crystallinity of the square boron nitride primary particles became high. The scaly shape develops too much, it becomes difficult to maintain the aggregated structure, the particle crushing strength decreases, and high thermal conductivity cannot be obtained.

The hexagonal boron nitride primary particle agglomerate of the present invention has both high thermal conductivity and moisture absorption and insulation properties, and an electrically insulating member using a resin composition containing the same can be used as an insulating layer and a thermal interface material for a printed wiring board. It can be preferably used.

Claims (6)

Hexagonal boron nitride primary particles satisfying the following (1) and (2) are aggregated, the average particle diameter is 10 μm or more and 200 μm or less, the crushing strength of the aggregate is 1 MPa or more and 4 MPa or less, and the alkali metal content is 10 ppm. The following hexagonal boron nitride primary particle aggregates.
(1) The graphitization index calculated by the powder X-ray diffraction method is 1.8 or more and 2.2 or less.
(2) The proportion of particles having a long side length of less than 1 μm is 25 to 50%, the proportion of particles having a long side length of 1 μm or more and 5 μm or less is 50 to 75%, and the proportion of particles exceeding 5 μm is 5% or less. The hexagonal boron nitride primary particle agglomerate according to claim 1, wherein the alkali metal is sodium or lithium. After primary firing of melamine borate at 400 ° C. or higher and 1200 ° C. or lower in a nitrogen atmosphere of 1 hour or longer and 5 hours or lower to obtain a primary fired product, alkali metal boric acid is added to 100 parts by mass of the primary fired product. The method for producing a hexagonal boron nitride primary particle aggregate according to claim 1 or 2, wherein 5 parts by mass or more and 15 parts by mass or less of salt is added, and further secondary firing is performed in a nitrogen atmosphere of 1900 ° C. or higher and 2200 ° C. or lower. A resin composition containing the hexagonal boron nitride primary particle aggregate according to claim 1 or 2. An insulating layer of a printed wiring board containing the resin composition of claim 4. A thermal interface material containing the resin composition of claim 4.

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