Thermally conductive sheet precursor, precursor composition, thermally conductive sheet obtained from thermally conductive sheet precursor, and method for producing the same

Abstract

A thermally conductive sheet precursor with excellent thermal conductivity and electrical insulation properties is provided, a precursor composition, a thermally conductive sheet obtained from the thermally conductive sheet precursor, and a method for producing the thermally conductive sheet. One embodiment of the thermally conductive sheet precursor disclosed herein includes: a plurality of first thermally conductive particles, the first thermally conductive particles being an anisotropic thermally conductive particle polymer having void regions between the anisotropic thermally conductive particles constituting the polymer; a plurality of second thermally conductive particles, the second thermally conductive particles being isotropic thermally conductive particles and contained outside the polymer; a plurality of third thermally conductive particles, the third thermally conductive particles being different from both the first and second thermally conductive particles, at least a portion of the third thermally conductive particles being contained in the void regions of the polymer; and an adhesive resin, wherein at least some of the polymer disintegrates when a first pressure of about 0.75 to about 12 MPa is applied to the thermally conductive sheet precursor.

Description

Technical Field

[0001] This disclosure relates to a thermally conductive sheet precursor, a precursor composition, a thermally conductive sheet obtained from the thermally conductive sheet precursor, and a method for producing the thermally conductive sheet. Prior Technology

[0002] Heat-generating components, such as semiconductor elements, may suffer from problems such as reduced performance and damage due to heat generated during use. To eliminate these problems, sheets with thermal conductivity are used, for example, in the power module assembly of electric vehicles (EVs) where semiconductor heat sinks are attached to heat sinks.

[0003] Patent document 1 (JP 5184543 B) discloses a thermally conductive sheet obtained by dispersing an inorganic filler in a thermosetting resin, wherein the inorganic filler contains spherical secondary agglomerated particles formed by isotropic agglomeration and sintering of flake-shaped boron nitride primary particles with an average long diameter of 15 μm or less and / or spherical inorganic powder with an average long diameter of 3 μm to 50 μm, and the inorganic filler contains more than 20 vol% of secondary agglomerated particles with a particle size of 50 μm or greater, and the flake-shaped boron nitride with an average long diameter of 3 μm to 50 μm isotropically oriented in the thermally conductive sheet.

[0004] Patent document 2 (WO2011 / 111684) discloses a thermally conductive composite comprising an insulating layer having at least one filler-containing polyimide resin layer and a metal layer laminated on one or both surfaces of the insulating layer. The filler-containing polyimide resin layer contains a thermally conductive filler in the polyimide resin, wherein the content of the thermally conductive filler in the filler-containing polyimide resin layer is in the range of 35 to 80 vol%, the thermally conductive filler has a maximum particle size of less than 15 μm, the thermally conductive filler comprises plate-shaped filler and spherical filler, the plate-shaped filler has an average long diameter DL in the range of 0.1 to 2.4 μm, and the insulating layer has a thermal conductivity λz of 0.8 W / mK or greater in the thickness direction of the insulating layer.

[0005] Patent document 3 (JP 2019-121708 A) discloses a thermally conductive sheet precursor containing an isotropic thermally conductive polymer, an anisotropic thermally conductive material not constituting the polymer, and an adhesive resin. In the isotropic polymer, the anisotropic thermally conductive primary particles are agglomerated, and at least some of the isotropic thermally conductive polymers disintegrate when a pressure of 3 to 12 MPa is applied to the thermally conductive sheet precursor.

[0006] Patent document 4 (JP 2020-053531 A) discloses a precursor of a heat-conductive sheet containing a polymer, an isotropic heat-conductive material different from the polymers and having an average particle diameter of 20 μm or more, and an adhesive resin, wherein anisotropic heat-conductive primary particles are aggregated in the polymer, and at least some of the polymers disintegrate when a first pressure within the range of 0.75 to 12 MPa is applied to the precursor of the heat-conductive sheet. SUMMARY OF THE INVENTION Problems to be Solved by the Invention

[0007] In recent years, for example, with the miniaturization, increased power, improved performance, and the like of power modules in electric vehicles, there is a demand for heat-conductive sheets with more improved electrical insulating properties in addition to the heat conductivity.

[0008] Therefore, the present disclosure provides a precursor of a heat-conductive sheet, a precursor composition, a heat-conductive sheet obtained from the precursor of the heat-conductive sheet, and a method for producing the heat-conductive sheet, which are excellent in heat conductivity and electrical insulating properties. Means for Solving the Problems

[0009] According to an embodiment of the present disclosure, there is provided a precursor of a heat-conductive sheet, including: a plurality of first heat-conductive particles, which are polymers of anisotropic heat-conductive particles and have void regions between the anisotropic heat-conductive particles constituting the polymers; a plurality of second heat-conductive particles, which are isotropic heat-conductive particles and are included outside the polymers; a plurality of third heat-conductive particles, which are different from both the first heat-conductive particles and the second heat-conductive particles, and at least a part of the third heat-conductive particles are included in the void regions of the polymers; and an adhesive resin, wherein at least some of the polymers disintegrate when a first pressure within the range of about 0.75 to about 12 MPa is applied to the precursor of the heat-conductive sheet.

[0010] According to another embodiment of the present disclosure, there is provided a heat-conductive sheet containing a cured product of the above-mentioned precursor of the heat-conductive sheet

[0011] According to another embodiment of the present disclosure, a precursor composition is provided, which includes: a plurality of first heat-conducting particles, which are aggregates of anisotropic heat-conducting particles, and there are void regions between the anisotropic heat-conducting particles that constitute the aggregate; a plurality of second heat-conducting particles, which are isotropic heat-conducting particles and are included outside the aggregate; a plurality of third heat-conducting particles, which are different from both the first heat-conducting particles and the second heat-conducting particles, and at least a part of the third heat-conducting particles are included in the void regions of the aggregate; and a binder resin, and the plurality of first heat-conducting particles, the plurality of second heat-conducting particles, and the plurality of third heat-conducting particles are dispersed in the binder resin.

[0012] According to yet another embodiment of the present disclosure, a method for producing a heat-conducting sheet is provided, which includes: preparing the precursor composition; forming a heat-conducting sheet precursor by using the composition; and forming a heat-conducting sheet by applying a pressure of at least about 0.75 MPa to the heat-conducting sheet precursor. Advantages of the Invention

[0013] According to the present disclosure, a heat-conducting sheet precursor, a precursor composition, a heat-conducting sheet obtained from the heat-conducting sheet precursor, and a method for producing the heat-conducting sheet with excellent heat conductivity and electrical insulation properties can be provided.

[0014] The above description should not be construed as having disclosed all embodiments of the present disclosure and all advantages of the present disclosure. Brief Description of the Drawings

[0015] FIG. 1 is a cross-sectional SEM image of a heat-conducting sheet according to an embodiment of the present disclosure. FIG. 2 is a graph of the heat conductivity of a heat-conducting sheet, which includes a boron nitride aggregate and isotropic heat-conducting particles of various sizes. FIG. 3 is a graph of the dielectric breakdown voltage associated with the mixing ratio of aluminum nitride particles in a heat-conducting sheet, which contains a boron nitride aggregate and aluminum nitride particles with an average particle diameter of 0.9 μm. FIG. 4 is a graph of the heat conductivity and dielectric breakdown voltage associated with the mixing ratio of the third heat-conducting particles and the second heat-conducting particles in a heat-conducting sheet, which includes a boron nitride aggregate, and small-diameter third heat-conducting particles included inside the aggregate and large-diameter second heat-conducting particles included outside the aggregate, and the boron nitride aggregate is the first heat-conducting particle. Embodiments

[0016] In the following description, for the purpose of illustrating representative embodiments of the invention, the invention will be described in more detail with reference to the necessary drawings, but the invention is not limited to these embodiments.

[0017] In this disclosure, "sheet" encompasses items referred to as "film".

[0018] In this disclosure, "(meth)acrylic acid" means acrylic acid or methacrylic acid.

[0019] In this disclosure, "anisotropic thermal conductivity" or "anisotropy thermal conductivity" means that thermal conductivity varies with direction. For example, it is expected that the thermal conductivity in another direction is reduced by about 50% or more, about 60% or more, or about 70% or more compared to the direction with the highest thermal conductivity. Here, it is expected that the other direction described above may differ from the direction with the highest thermal conductivity by about 10 degrees or more, about 20 degrees or more, or about 30 degrees or more, and about 90 degrees or less. Examples of materials exhibiting such anisotropic thermal conductivity include flake boron nitride. These boron nitrides are known to exhibit anisotropic thermal conductivity, wherein the thermal conductivity is high in the long diameter direction (crystal direction) and low in the short diameter direction (thickness direction, or direction at 90 degrees relative to the long diameter direction).

[0020] In this disclosure, "isotropic thermal conductivity" or "isotropy thermal conductivity" means substantially isotropic, particularly with less anisotropy in terms of thermal conductivity than anisotropic thermally conductive materials. For example, it is known that substantially spherical alumina particles exhibit isotropic thermal conductivity, where the thermal conductivity is substantially equal in all directions. In this disclosure, the term "substantially" means including variations caused by manufacturing errors or the like, and may be intended to mean that a variation of about 5% to about 30%, preferably about 5% to about 20%, is acceptable.

[0021] In this disclosure, the term "disintegration" means the collapse of a secondary structure that aggregates a primary structure, essentially reverting to the form of the primary structure. For example, "disintegration of an aggregate of at least some anisotropically thermally conductive primary particles" means that at least some of the primary particles constituting the aggregate collapse under pressure and essentially revert to the form of primary particles before agglomeration. Here, the term "primary particle" refers to a particle constituting the aggregate, and the phrase "substantially return" can mean, for example, that the shape or size of the primary structure remains, about 70% or more, about 75% or more, or about 80% or more, relative to its shape or size before disintegration.

[0022] In this disclosure, the term "random" refers to a state of disordered direction. For example, the state of the primary particles of the boron nitride polymer shown in Figure 1 corresponds to "random," but in a sheet containing flake-like boron nitride, the state in which multiple boron nitride particles are substantially arranged parallel to the main surface of the sheet does not correspond to "random." Here, whether the primary particles of the polymer are random is determined by examining the state of the entire polymer, not by examining the state of a portion of the polymer. For example, in Figure 1, near the arrow representing "anisotropic thermally conductive BN primary particles constituting the first thermally conductive particle," three particles are arranged in parallel, but when examining the entire polymer, these three particles are not arranged in parallel with the other primary particles, making the state of the primary particles of the boron nitride polymer shown in Figure 1 "random."

[0023] In this disclosure, the term "cured product" includes a product obtained by a chemical reaction such as vulcanization or crosslinking of a rubber-based resin, and a product obtained by curing a thermoplastic resin, other than a product obtained by a chemical reaction such as thermal polymerization of a thermosetting resin.

[0024] In this paper, the components of the thermally conductive sheet precursor (sometimes simply referred to as "precursor") will be further described.

[0025] The polymers of the first thermally conductive particles contained in the thermally conductive sheet precursor disclosed herein are each polymers of anisotropic thermally conductive particles (e.g., secondary polymers, wherein the anisotropic thermally conductive primary particles are aggregated). These polymers have the property that at least some polymers disintegrate when a predetermined pressure is applied to the thermally conductive sheet precursor. Preferably, the polymers comprise randomly aggregated anisotropic thermally conductive primary particles and have a thermal conductivity more isotropic than that of the primary particles. Here, the polymers do not need to disintegrate in the precursor at a predetermined pressure (e.g., all pressures in the range of about 0.75 to about 12 MPa), and at least some polymers only need to disintegrate when any pressure within this range (the first pressure) is applied. The polymers that can disintegrate under this pressure are each polymers with a relatively low density and have void regions between the anisotropic thermally conductive particles constituting the polymer, allowing the third thermally conductive particles described below to enter these void regions. The first thermally conductive particle (polymer) can be used alone or in combination of two or more types.

[0026] In the case of forming sheets from resin materials simply doped with primary particles of anisotropic thermally conductive particles such as flake-like boron nitride, these particles can be aligned in one direction, thus the resulting sheets are less likely to exhibit isotropic thermal conductivity. However, the thermally conductive sheet precursor disclosed herein uses a polymer that can disintegrate under a first pressure, and therefore, the anisotropic thermally conductive primary particles constituting the polymer can be aligned in random directions after disintegration, making the resulting thermally conductive sheet considered to exhibit isotropic thermal conductivity (sometimes simply referred to as "thermal conductivity").

[0027] From the perspective of thermal conductivity, the preferred disintegration rate of the polymer after pressure application is approximately 20% or greater, approximately 30% or greater, or approximately 40% or greater per mm². The upper limit of the disintegration rate is not particularly limited, but can be defined, for example, approximately 100% or less, approximately 95% or less, or approximately 90% or less per mm². Here, "disintegration rate" refers to the rate of change of the area-average diameter obtained from particle distribution analysis of optical micrographs of the polymer recovered from the sheet (ImageJ software (version 1.50i)).

[0028] The polymers disclosed herein are each relatively low-density polymers designed to disintegrate under a first pressure. The density of each polymer can be defined, for example, by its porosity. The porosity is preferably greater than about 50%, and can be about 60% or more, or about 70% or more. These polymers allow the third thermally conductive particles, described below, to easily penetrate the polymer interior, and thus, in addition to further improving thermal conductivity, allow the polymer to disintegrate and randomize under a predetermined pressure, enabling the polymer to exhibit isotropic thermal conductivity relative to a thermally conductive sheet. The upper limit of the porosity is not particularly limited and can be, for example, less than about 100%, about 95% or lower, about 90% or lower, about 85% or lower, or about 80% or lower. For example, this porosity of the polymer can be controlled by adjusting the sintering temperature of the polymer. At high sintering temperatures, the polymer shrinks and densifies, thus increasing its strength but decreasing its porosity. Conversely, at low sintering temperatures, the polymer shrinks less, allowing for an increase in porosity without increasing its strength. Here, at high-temperature sintering, the polymer tends to exhibit a spherical shape, while at low-temperature sintering, it tends to exhibit an imperfect spherical shape, i.e., a non-spherical shape. The porosity of the polymer can be determined, for example, by measuring the pore volume using the mercury intrusion method.

[0029] The size of each polymer is not particularly limited, as long as the polymer size is appropriately selected to achieve the desired properties, such as thermal conductivity, in the final thermally conductive sheet. For example, the average particle size of the polymer can be about 20 μm or larger, about 40 μm or larger, about 60 μm or larger, or about 80 μm or larger. Polymers with this size allow the third thermally conductive particles, as described below, to easily enter the polymer interior, and thus, in addition to further improving thermal conductivity, the anisotropic thermally conductive primary particles constituting the polymer are easily randomized after disintegration, allowing the polymer to exhibit isotropic thermal conductivity relative to the thermally conductive sheet. The upper limit of the average particle size is not particularly limited, and from the viewpoint of reducing the resistance of the thermally conductive sheet precursor and the like, it can be, for example, about 300 μm or smaller, about 250 μm or smaller, or about 200 μm or smaller.

[0030] The size of the polymer can also be defined by D50 (particle size at 50% cumulative frequency), which is calculated from particle size distribution data. The D50 of the polymer can be about 20 μm or larger, about 40 μm or larger, or about 60 μm or larger, and can be about 300 μm or smaller, about 250 μm or smaller, or about 200 μm or smaller.

[0031] The size of the polymer can also be defined by D90 (the particle size at 90% cumulative frequency), which is calculated from particle size distribution data. The D90 of the polymer can be, for example, about 30 μm or larger, about 50 μm or larger, or about 70 μm or larger, and can be about 350 μm or smaller, about 300 μm or smaller, or about 250 μm or smaller.

[0032] Polymers of this size can be randomized after disintegration, and thus exhibit isotropic thermal conductivity relative to the thermally conductive sheet, allowing the third thermally conductive particles described below to easily enter the interior.

[0033] Here, the average particle sizes D50 and D90 of the polymer can be determined using, for example, laser diffraction / scattering, or various microscopes (such as optical microscopes, scanning electron microscopes (SEM), and transmission electron microscopes (TEM)). When determining the average particle size using a microscope, the area circle-equivalent particle diameter of the polymer can be defined as the average particle size. For example, it is conceivable to obtain the particle size by converting it into circular particles having the same area as the projected area of ​​the polymer observed by an electron microscope. This area circle-equivalent particle diameter can be defined as the average of 10 polymers or more (e.g., 50 polymers). Preferably, the volume average diameter is obtained by measuring the particle size distribution using laser diffraction (wet measurement, LS 13 320, available from Beckman Coulter, Inc.), and in this disclosure, the average particle size of these polymers generally refers to the volume average size obtained by this measurement method.

[0034] The amount of binder in the thermally conductive sheet precursor is not particularly limited, provided that the mixing ratio is appropriately adjusted to achieve the desired properties, such as thermal conductivity, in the final thermally conductive sheet. For example, when the binder of the first, second, and third thermally conductive particles (described below) is considered in terms of thermal conductivity, mechanical strength, and the like, it is collectively defined as a "thermally conductive filler component." This thermally conductive filler component can be mixed in the thermally conductive sheet precursor at about 45 vol% or more, about 50 vol% or more, or about 55 vol% or more, and can be mixed at about 80 vol% or less, about 75 vol% or less, or 70 vol% or less. In the thermally conductive sheet disclosed herein, a specific binder (first thermally conductive particle), a third thermally conductive particle contained within the binder, and a second thermally conductive particle contained outside the binder are used. Therefore, even if the thermally conductive filler component is not filled to about 90 vol%, isotropic thermal conductivity can still be adequately exhibited. Here, voids are included in the thermally conductive sheet precursor, pre-disintegration polymer, and the like, but voids are not included in the above volume percentage values ​​because the actual density of each material is used when calculating the volume percentage.

[0035] In the thermally conductive filler component, the content of the binder can be about 20 vol% or more, about 25 vol% or more, or about 30 vol% or more, and the content can be about 90 vol% or less, about 85 vol% or less, or about 80 vol% or less. A thermally conductive sheet precursor containing this content of binder can further improve properties such as the isotropic thermal conductivity of the final thermally conductive sheet. Here, the binder content can be calculated from the amount of binder (vol%) relative to the total amount (vol%) of the thermally conductive filler component.

[0036] The anisotropic thermally conductive particles (anisotropic thermally conductive primary particles) constituting the polymer are not particularly limited, as long as they are primary particles exhibiting anisotropic thermal conductivity. For example, inorganic primary particles having needle-like, flat, or flake-like shapes can be used alone or in combination of two or more types. Examples of materials constituting inorganic primary particles include at least one selected from aluminum nitride, silicon nitride, and boron nitride. Among these, boron nitride is preferred, and flake-shaped hexagonal boron nitride (h-BN) is even more preferred because, in addition to good thermal conductivity, it also imparts excellent electrical insulation properties and the like after the polymer disintegrates. Boron nitride is excellent in both thermal conductivity and electrical insulation properties, therefore, the use of such particles can improve both properties of the thermally conductive sheet. In this disclosure, the anisotropic thermally conductive particles constituting the polymer may also be referred to as "first anisotropic thermally conductive particles".

[0037] The size of the anisotropic thermally conductive particles constituting the polymer is not particularly limited, as long as the size is appropriately selected to obtain the desired properties, such as thermal conductivity, in the final thermally conductive sheet. For example, the average long diameter of the anisotropic thermally conductive particles can be about 1.5 μm or larger, about 2.0 μm or larger, or about 2.5 μm or larger, and can be about 25 μm or smaller, about 20 μm or smaller, or about 15 μm or smaller.

[0038] Anisotropic thermally conductive particles of this size can be randomized after the polymer breaks down, and thus can exhibit isotropic thermal conductivity relative to the thermally conductive sheet.

[0039] Here, the average major diameter of the anisotropic thermally conductive particles can be determined using various microscopes, such as laser microscopes, scanning electron microscopes (SEM), or transmission electron microscopes (TEM). Among these, SEM is preferred. The SEM method can also be used to appropriately measure the average major diameter of primary particles in a polymeric state. Specifically, for example, a sheet of polymer embedded in an adhesive resin is prepared using a substrate film as needed. Then, the cross-section of the sheet in the thickness direction is observed by SEM, and the major diameter of 10 or more (e.g., 50) anisotropic thermally conductive particles is measured from the obtained SEM cross-sectional image, so that the average major diameter of the anisotropic thermally conductive particles can be determined from the average value.

[0040] The second thermally conductive particles contained in the thermally conductive sheet precursor disclosed herein are isotropic thermally conductive particles that differ from the aforementioned polymer, and are not particularly limited, as long as they are contained outside the polymer before disintegration. For example, an isotropic thermally conductive material that does not disintegrate under a first pressure applied to the thermally conductive sheet precursor can be used. By using such a material, the thermally conductive sheet obtained according to the method disclosed herein can exhibit even higher isotropic thermal conductivity. Here, "different from the aforementioned polymer" means that at least one of the following groups differs from the aforementioned polymer in terms of material, average particle size, void fraction, and structure (e.g., non-polymeric structure, a structure that disintegrates under the first pressure).

[0041] As a second thermally conductive particle, particularly, substantially spherical inorganic primary particles or polymers can be used alone or in combination of two or more types. Examples of materials constituting inorganic primary particles or polymers include those selected from aluminum nitride, alumina (bauxite), silicon carbide, and boron nitride. Using this material can further improve properties such as the isotropic thermal conductivity of the resulting thermally conductive sheet. Among these, aluminum nitride, alumina, or boron nitride systems are preferred from the viewpoints of thermal conductivity, electrical insulation properties, production cost, and the like, aluminum nitride or alumina systems are even better, and aluminum nitride systems are particularly good.

[0042] Here, the substantially spherical shape disclosed herein can be defined by, for example, roundness (4π × area / (square of circumference)), and a roundness in the range of about 0.7 or greater, about 0.8 or greater, or about 0.9 or greater, and about 1.0 or less can be defined as substantially spherical.

[0043] A substantially spherical inorganic polymer that does not disintegrate under a first pressure applied to a thermally conductive sheet precursor can be appropriately prepared, for example by sintering the polymer at a high temperature, in which the aforementioned anisotropic thermally conductive particles are agglomerated.

[0044] Regarding thermal conductivity, electrical insulation properties, and the like, the amount of second thermally conductive particles contained in the exterior of the polymer before disintegration may be about 80 vol% or more, about 85 vol% or more, about 90 vol% or more, about 95 vol% or more, or about 98 vol% or more, and preferably about 100 vol% or more, that is, the second thermally conductive particles are not contained within the polymer before disintegration. Here, the amount of second thermally conductive particles contained in the exterior of the polymer before disintegration can be calculated from the amount (vol%) of the second thermally conductive particles contained in the exterior of the polymer before disintegration relative to the total amount (vol%) of the second thermally conductive particles.

[0045] From the viewpoint of impermeability, thermal conductivity, electrical insulation properties, and the like of the pre-disintegration binder, the second thermally conductive particles preferably have an average particle size of about 4.5 μm or larger, about 5.0 μm or larger, about 8.0 μm or larger, about 10 μm or larger, about 15 μm or larger, about 20 μm or larger, about 30 μm or larger, or about 40 μm or larger. When second thermally conductive particles of this size are included, the interfacial ratio between the second thermally conductive particles and the binder resin is reduced, and isotropic thermal conduction pathways are easily obtained, making it easier for the thermally conductive sheet to exhibit isotropic thermal conductivity. The upper limit of the average particle size is not particularly limited, and from the viewpoint of reducing the resistance of the thermally conductive sheet precursor and the like, it can be, for example, about 200 μm or smaller, about 150 μm or smaller, or about 100 μm or smaller.

[0046] The size of the second thermally conductive particle can also be defined by D50, which is calculated from particle size distribution data. The D50 of the second thermally conductive particle can be about 4.5 μm or larger, about 5.0 μm or larger, about 8.0 μm or larger, about 10 μm or larger, about 15 μm or larger, about 20 μm or larger, about 30 μm or larger, about 40 μm or larger, or about 50 μm or larger, and can also be about 200 μm or smaller, about 150 μm or smaller, or about 100 μm or smaller.

[0047] The size of the second thermally conductive particle can also be defined by D90, which is calculated from particle size distribution data. The D90 of the second thermally conductive particle can be, for example, about 9.0 μm or larger, about 10 μm or larger, about 16 μm or larger, about 20 μm or larger, about 30 μm or larger, about 40 μm or larger, or about 50 μm or larger, and can be about 400 μm or smaller, about 300 μm or smaller, about 200 μm or smaller, or about 150 μm or smaller.

[0048] The second thermally conductive particle of this size has a smaller interface-to-adhesive resin ratio, in which isotropic thermal conductive paths are easily obtained, thus exhibiting isotropic thermal conductivity to the thermally conductive sheet and improving electrical insulation properties.

[0049] Here, similar to the aforementioned polymer, the average particle sizes D50 and D90 of the second thermally conductive particles can be determined using, for example, laser diffraction / scattering, or various microscopes (such as optical microscopes, scanning electron microscopes (SEM), and transmission electron microscopes (TEM)). When determining the average particle size using a microscope, the area circle equivalent particle size of the second thermally conductive particles can be defined as the average particle size. For example, it is conceivable to obtain the particle size by converting it into a circular particle having the same area as the projected area of ​​the second thermally conductive particle observed by the microscope. This area circle equivalent particle size can be defined as the average value of 10 or more (e.g., 50) second thermally conductive particles. Preferably, the volume average diameter is obtained by particle size distribution measurement using laser diffraction (wet measurement, LS 13 320, available from Beckman Coulter, Inc.), and in this disclosure, the average particle size of these second thermally conductive particles generally refers to the volume average size obtained by such measurement methods.

[0050] The amount of the second thermally conductive particles in the thermally conductive sheet precursor is not particularly limited, provided that the mixing ratio is appropriately adjusted to achieve the desired properties, such as thermal conductivity, electrical insulation properties, or similar properties, in the final thermally conductive sheet. For example, in the thermally conductive filler component, the second thermally conductive particles can be mixed at about 5 vol% or more, about 10 vol% or more, about 15 vol% or more, about 20 vol% or more, about 25 vol% or more, about 30 vol% or more, about 35 vol% or more, or about 40 vol% or more, and can be mixed at about 75 vol% or less, about 70 vol% or less, about 65 vol% or less, about 60 vol% or less, about 55 vol% or less, about 50 vol% or less, about 45 vol% or less, about 40 vol% or less, about 35 vol% or less, or about 30 vol% or less. This mixing ratio of the thermally conductive sheet precursor, including the second thermally conductive particles, can further improve properties such as the isotropic thermal conductivity of the final thermally conductive sheet. Here, the mixing ratio of the second thermally conductive particles can be calculated from the amount (vol%) of the second thermally conductive particles relative to the total amount (vol%) of the thermally conductive filler components.

[0051] The thermally conductive sheet precursor disclosed herein contains a plurality of third thermally conductive particles, which differ from either the first or second thermally conductive particles described above. These third thermally conductive particles are in the form of substantially spherical particles or similar, and at least some of these third thermally conductive particles are contained within the voids of the polymer, such as aluminum nitride particles with an average particle size of about 0.9 μm in Figure 1. Here, "different from either the first or second thermally conductive particles" means that at least one of the third thermally conductive particles is selected from the group consisting of: material, average particle size, void fraction, and structure (e.g., non-polymeric structure, structure not separated under the first pressure) that differs from either the first or second thermally conductive particles described above.

[0052] When the size of the third thermally conductive particle decreases, the thermal conductivity of the thermally conductive sheet generally tends to decrease, as shown in Comparative Examples 5 and 9 in Figure 2. However, the inventors have discovered that when the size of the third thermally conductive particle is reduced to a size that allows the third thermally conductive particle to penetrate into the interior of the polymer, the thermal conductivity of the thermally conductive sheet actually improves, as shown in Comparative Example 1 in Figure 2.

[0053] On the other hand, as shown in Figure 3, a third thermally conductive particle of this size can cause a decrease in the electrical insulation properties of the thermally conductive sheet. However, the inventors have discovered that this problem regarding electrical insulation properties can be solved by simultaneously incorporating the aforementioned second thermally conductive particle into the thermally conductive sheet precursor.

[0054] For example, various thermally conductive particles other than the first thermally conductive particle (polymer) and the second thermally conductive particle mentioned above (e.g., anisotropic thermally conductive particles, isotropic thermally conductive particles) can be used as the third thermally conductive particle. The third thermally conductive particle can be used alone or in combination of two or more types. Here, when the third thermally conductive particle is anisotropic, such anisotropic thermally conductive particle can be called a "second anisotropic thermally conductive particle", and when the third thermally conductive particle is isotropic, such isotropic thermally conductive particle can be called a "second isotropic thermally conductive particle".

[0055] In the third thermally conductive particles, some or more of the third thermally conductive particles need to be contained only within the polymer (in the void regions between the anisotropic thermally conductive particles constituting the polymer), or all of the third thermally conductive particles may be contained within the polymer. In the case where some third thermally conductive particles are contained within the polymer and the remainder are contained outside the polymer, the third thermally conductive particles may at least partially fill the low-density portions (such as the voids between equal polymers before the application of the first pressure) after the application of the first pressure to reduce electron intrusion and thus improve the properties of the thermally conductive sheet, such as electrical insulation properties.

[0056] As the third thermally conductive particle disclosed herein, for example, at least one of inorganic primary particles (such as aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, and boron nitride) having substantially spherical, substantially needle-shaped, substantially flat, or substantially flake-shaped forms, and secondary particles in which inorganic primary particles are agglomerated, can be used. From the viewpoint of thermal conductivity intrusion into the aggregate, the thermally conductive sheet, and the like, substantially spherical inorganic primary particles or secondary particles in which such inorganic primary particles are agglomerated are preferred; substantially spherical aluminum nitride and / or aluminum oxide or secondary particles in which such aluminum nitride and / or aluminum oxide are agglomerated are even more preferred; and substantially spherical aluminum nitride or secondary particles in which such aluminum nitride are agglomerated are particularly preferred.

[0057] From the viewpoint of intrusive polymers, thermally conductive sheets, and the like, the average long diameter or average particle size of the third thermally conductive particles disclosed herein is preferably about 3.0 μm or less, about 2.0 μm or less, about 1.0 μm or less, less than about 1.0 μm, or about 0.9 μm or less. The lower limit of the average long diameter or average particle size of the third thermally conductive particles may be, for example, about 0.05 μm or more, about 0.07 μm or more, or about 0.1 μm or more.

[0058] The size of the third thermally conductive particle can also be defined by D50, which is calculated from particle size distribution data. The D50 of the third thermally conductive particle can be about 3.0 μm or smaller, about 2.0 μm or smaller, about 1.0 μm or smaller, less than about 1.0 μm, or about 0.9 μm or smaller, and can be about 0.05 μm or larger, about 0.07 μm or larger, or about 0.1 μm or larger.

[0059] The size of the third thermally conductive particle can also be defined by D90, which is calculated from particle size distribution data. The D90 of the third thermally conductive particle can be about 6.0 μm or smaller, about 4.0 μm or smaller, about 2.5 μm or smaller, less than about 2.5 μm, about 2.4 μm or smaller, or about 2.0 μm or smaller, and can also be about 0.1 μm or larger, about 0.2 μm or larger, or about 0.3 μm or larger.

[0060] The average major diameter of the third thermally conductive particles can be determined using various microscopes, such as laser microscopes, scanning electron microscopes (SEM), or transmission electron microscopes (TEM). SEM is preferred. The average major diameter of primary particles in sheet form can also be appropriately measured using SEM. Specifically, for example, a sheet in which the third thermally conductive particles are embedded in an adhesive resin is prepared using a substrate film as needed. The thickness-direction cross-section of the sheet is then observed using SEM, and the major diameters of 10 or more (e.g., 50) third thermally conductive particles are measured from the obtained SEM cross-sectional images, allowing the average major diameter of the third thermally conductive particles to be determined from an average value. The average particle size D50 and D90 of the third thermally conductive particles can be determined using, for example, laser diffraction / scattering methods. In this disclosure, generally speaking, when the third thermally conductive particle system is substantially spherical, it is preferred to use an average particle size or similar by laser diffraction / scattering method; when the third thermally conductive particle system has particles of other shapes, it is preferred to use an average long diameter by scanning electron microscopy (SEM).

[0061] The amount of the third thermally conductive particles in the thermally conductive sheet precursor is not particularly limited, provided that the mixing ratio is appropriately adjusted to achieve the desired properties, such as thermal conductivity, electrical insulation properties, or similar properties, in the final thermally conductive sheet. For example, in the thermally conductive filler component, the third thermally conductive particles can be mixed at about 5 vol% or more, about 10 vol% or more, about 15 vol% or more, about 20 vol% or more, about 25 vol% or more, about 30 vol% or more, about 35 vol% or more, or about 40 vol% or more, and can be mixed at about 75 vol% or less, about 70 vol% or less, about 65 vol% or less, about 60 vol% or less, about 55 vol% or less, about 50 vol% or less, about 45 vol% or less, about 40 vol% or less, about 35 vol% or less, or about 30 vol% or less. This mixing ratio of the thermally conductive sheet precursor, including the third thermally conductive particles, can further improve properties such as the isotropic thermal conductivity of the final thermally conductive sheet. Here, the mixing ratio of the third thermally conductive particles can be calculated from the amount (vol%) of the third thermally conductive particles relative to the total amount (vol%) of the thermally conductive filler components.

[0062] The adhesive resin included in the thermally conductive sheet precursor disclosed herein may be appropriately selected, and is not particularly limited, depending on the intended use of the final thermally conductive sheet. Examples of adhesive resins include thermoplastic resins, thermosetting resins, and rubber resins. Adhesive resins may be used alone or in combination of two or more types.

[0063] Examples of thermoplastic resins include polyolefin resins (such as polyethylene and polypropylene), polyester resins (such as polyethylene terephthalate and polyethylene naphthalate), polycarbonate resins, polyamide resins, and polyphenylene sulfide resins.

[0064] Examples of thermosetting resins may include epoxy resins, (meth)acrylic resins, carbamate resins, polysiloxane resins, unsaturated polyester resins, phenolic resins, melamine resins, and polyimide resins. Among these, epoxy resins are preferred from the viewpoints of formability of thermally conductive sheets, adhesion to other objects, electrical insulation properties, and the like. Examples of epoxy resins include bisphenol-A epoxy resins, bisphenol-F epoxy resins, o-cresol phenolic epoxy resins, phenolic phenolic epoxy resins, alicyclic epoxy resins, aliphatic epoxy resins, alicyclic aliphatic epoxy resins, and glycidylaminophenol epoxy resins.

[0065] Examples of rubber resins may include polysiloxane rubber, isopropylene rubber, butadiene rubber, styrene-butadiene rubber, chloroprene rubber, ethylene propylene rubber, ethylene-propylene-diene rubber, nitrile rubber, acrylonitrile-butadiene rubber (NBR), hydrogenated NBR, acrylic rubber, carbamate rubber, fluororubber, and natural rubber.

[0066] The amount of binder resin in the thermally conductive sheet precursor is not particularly limited, provided that it is appropriately adjusted to achieve the desired properties (such as thermal conductivity, electrical insulation properties, adhesion, or similar) in the final thermally conductive sheet. For example, the binder resin can be blended in about 20 vol% or more, about 25 vol% or more, or about 30 vol% or more, and can be blended in about 80 vol% or less, about 75 vol% or less, about 70 vol% or less, about 65 vol% or less, about 60 vol% or less, about 55 vol% or less, about 50 vol% or less, or about 45 vol% or less. Including the binder resin in this blending amount in the thermally conductive sheet precursor can further improve properties such as the thermal conductivity, electrical insulation properties, and mechanical strength of the final thermally conductive sheet. Here, voids are included in the thermally conductive sheet precursor, pre-disintegration polymer, and the like, but voids are not included in the above volume percentage values ​​because the actual density of each material is used when calculating the volume percentage.

[0067] In addition to the first thermally conductive particles (polymer), second thermally conductive particles, and third thermally conductive particles described above, the thermally conductive sheet precursor disclosed herein may further contain, as optional components, such as flame retardants, pigments, dyes, fillers (e.g., fuming silica), reinforcing materials, leveling agents, coupling agents, defoamers, dispersants, heat stabilizers, light stabilizers, crosslinking agents, thermosetting agents, photocuring agents, curing accelerators, tackifiers, plasticizers, reactive diluents, or solvents, provided that they do not adversely affect the effectiveness of this disclosure. These optional components may be used alone or in combination of two or more types. Here, the thermal conductivity of the powders (e.g., pigments, fillers, reinforcing materials) among the optional components is typically about 10 W / mK or less, and these powders do not have thermal conductivity, but for example, the thermal conductivity is about 15 W / mK or greater, or about 20 W / mK or greater, and the powder contained within the polymer can be considered as the aforementioned third thermally conductive particle.

[0068] The thickness of the thermally conductive sheet precursor disclosed herein is not particularly limited, as long as the thickness can be appropriately adjusted according to the intended use of the final thermally conductive sheet or similar applications. For example, the thickness of the thermally conductive sheet precursor can be greater than the maximum value of the minor axis length (length of the smallest side) of the aforementioned polymer. This thickness can reduce defects such as polymer shedding. Specifically, this thickness can be, for example, about 80 μm or greater, about 100 μm or greater, about 150 μm or greater, about 170 μm or greater, about 200 μm or greater, or about 250 μm or greater, and can be about 650 μm or less, about 600 μm or less, about 500 μm or less, about 450 μm or less, about 400 μm or less, about 350 μm or less, or about 300 μm or less. The thickness of the thermally conductive sheet precursor can be defined as the average value calculated by measuring the thickness of a freely selected portion of the thermally conductive sheet precursor at least five times using a high-accuracy digital micrometer (MDH-25MB, available from Mitutoyo Corporation).

[0069] Here, the image data of the polymer can be obtained by, for example, by capturing an image of the polymer using an optical microscope. Then, the minor axis length of the polymer is determined using the particle analysis function of ImageJ software (version 1.50i) for the captured image data, where the minor axis length is determined as the minor axis diameter obtained by approximation of an ellipse. The maximum value of the minor axis length of these polymers can be defined as the maximum value among the minor axis lengths of 100 polymers determined.

[0070] The thermally conductive sheet obtained from the thermally conductive sheet precursor disclosed herein exhibits excellent isotropic thermal conductivity and electrical insulation properties.

[0071] The thermally conductive sheets obtained from the thermally conductive sheet precursors disclosed herein may have thermal conductivity of, for example, about 4.5 W / m·K or more, and about 5.0 W / m·K or more, about 5.5 W / m·K or more, about 6.0 W / m·K or more, about 6.5 W / m·K or more, about 7.0 W / m·K or more, about 7.5 W / m·K or more, or about 8.0 W / m·K or more, although varying depending on the amount of thermally conductive filler components or the like. There is no particular upper limit to the thermal conductivity, and it may be defined, for example, as about 20.0 W / m·K or less, about 18.0 W / m·K or less, or about 15.0 W / m·K or less. Thermally conductive sheets with this thermal conductivity are sufficient for use, for example, in electric vehicle (EV) power modules and the like. Here, the thermal conductivity measurement can be determined by, for example, the thermal conductivity evaluation test described below. Since these tests examine the thermal conductivity from the bottom surface to the top surface of the thermally conductive sheet, the obtained thermal conductivity is an isotropic indicator.

[0072] The electrical insulation properties of the thermally conductive sheet obtained from the thermally conductive sheet precursor disclosed herein can be evaluated by, for example, dielectric breakdown voltage. The thermally conductive sheet disclosed herein, when fabricated into a thermally conductive sheet with a thickness of 150 μm, can exhibit dielectric breakdown voltages of approximately 6.5 kV or greater, approximately 7.0 kV or greater, approximately 7.5 kV or greater, or approximately 8.0 kV or greater, and preferably, in the field of power modules, exhibits a dielectric breakdown voltage of, for example, approximately 7.0 kV or greater. The upper limit of the dielectric breakdown voltage is not particularly limited and can be, for example, approximately 30 kV or less, approximately 25 kV or less, approximately 20 kV or less, or approximately 15 kV or less. Here, the dielectric breakdown voltage of the thermally conductive sheet can be measured, for example, by means of the dielectric breakdown voltage evaluation test described below.

[0073] The thickness of the thermally conductive sheet disclosed herein is not particularly limited, provided that the thickness can be appropriately adjusted according to the intended use of the final thermally conductive sheet or similar applications. For example, it can be approximately 80 μm or greater, approximately 100 μm or greater, or approximately 150 μm or greater, and can be approximately 400 μm or less, approximately 350 μm or less, and approximately 300 μm or less. The thickness of the thermally conductive sheet can be defined as the average value calculated by measuring the thickness of a freely selected portion of the thermally conductive sheet at least five times using a high-accuracy digital micrometer (MDH-25MB, available from Mitutoyo Corporation).

[0074] The thermally conductive sheet disclosed herein can be manufactured by, for example, the following method. The materials described above can be used similarly as various materials used in this method.

[0075] In a given container, an adhesive resin, a solvent, and optionally a curing agent, and the like, are blended and stirred and mixed using a high-speed mixer or similar at about 1000 to 3000 rpm for about 10 seconds to 5 minutes to prepare a mixture. The mixture is then further blended with a first thermally conductive particle (polymer), a second thermally conductive particle, a third thermally conductive particle, and optionally a solvent, and stirred and mixed using a high-speed mixer or similar at about 1,000 to about 3,000 rpm for about 10 seconds to 5 minutes to prepare a precursor composition. For the precursor composition, at least a plurality of first thermally conductive particles, each of which is an anisotropic thermally conductive particle aggregate, and the aggregate has void regions between the anisotropic thermally conductive particles constituting the aggregate; a plurality of second thermally conductive particles, each of which is an isotropic thermally conductive particle and contained outside the aggregate; and a plurality of third thermally conductive particles, wherein the third thermally conductive particles are different from both the first and second thermally conductive particles, and at least a portion of the third thermally conductive particles are contained in the void regions of the aggregate, and the third thermally conductive particles are dispersed in the adhesive resin. As used herein, the term "dispersion" also includes a state in which the aggregates are dispersed in the adhesive resin with the third thermally conductive particles contained in the void regions of each of the aggregates. Next, using known coating methods, such as rod coaters and blade coaters, the precursor composition is applied to the release liner and dried if necessary, to obtain a thermally conductive sheet precursor. Through this mixing and coating step, the third thermally conductive particles disclosed herein can penetrate into the interior of the polymer.

[0076] Drying can be a single step, or it can be two or more steps, such as drying at about 50°C to 120°C for about 1 to 10 minutes, followed by drying at about 50°C to 120°C for about 1 to 10 minutes. Through such multi-step drying, a porous thermally conductive sheet precursor can be obtained.

[0077] Next, a predetermined pressure (first pressure) is applied to the obtained thermally conductive sheet precursor at approximately 50°C to 200°C for approximately 1 to 60 minutes to produce a thermally conductive sheet containing a cured product of the thermally conductive sheet precursor. This pressure can be appropriately set considering the disintegration properties of the polymer, and can be approximately 0.75 MPa or greater, approximately 1.0 MPa or greater, or approximately 3.0 MPa or greater, and can be approximately 12 MPa or less, approximately 10 MPa or less, or approximately 8.0 MPa or less. Here, "cured product of thermally conductive sheet precursor" means a cured product of thermally conductive sheet precursor in which at least a portion of the polymer is contained in a state of disintegration by the first pressure.

[0078] Here, when using a thermosetting agent, the heat from the drying step described above can be used to perform curing, and it can be performed separately in other steps (e.g., pressure application step and additional heating step).

[0079] The thermally conductive sheet disclosed herein can be suitably used as a heat dissipation article for applications such as electric vehicles (EVs), consumer electronics, and computer devices, and as a heat dissipation assembly configured to fill the space between a heat-generating component (such as an IC chip) and a heat dissipation component (such as a heat sink or heat pipe), thereby effectively transferring heat generated from the heat-generating component to the heat dissipation assembly. It can be suitably used as a heat dissipation assembly for power modules.

[0080] The thermally conductive sheet disclosed herein can also be made adhesive by appropriately selecting the adhesive resin. For example, when using epoxy resin as the adhesive resin, the thermally conductive sheet disclosed herein can be used as a thermally adhesive type thermally conductive adhesive sheet. Example

[0081] Specific embodiments of this disclosure will be illustrated in the following examples; however, the invention is not limited to these embodiments. Unless otherwise stated, all parts and percentages are by weight. Values ​​substantially include errors due to the measuring principle and measuring device. Values ​​are indicated by significant figures that have been properly rounded.

[0082] The products and similar products used in these examples are shown in Table 1 below. Test Example 1

[0083] In Test Example 1, the thermal conductivity and electrical insulation properties of a thermally conductive sheet containing boron nitride polymer (first thermally conductive particles) (wherein the anisotropic thermally conductive boron nitride particles are agglomerated), and a second or third thermally conductive particle were examined. Preparation of thermally conductive sheet precursors

[0084] EOCN104S, HP-7250, and PKHH were placed in 20ml glass vials and mixed at 2,000 rpm for 2 minutes using a high-speed mixer. Subsequently, DISPERBYK-2013 and V-09GB were added and mixed, followed by Amicure (trade name) CG1400 and TS-720, and mixed to prepare mixed solution A. A second or third thermally conductive particle was added to mixed solution A and mixed for 2 minutes or until all solids were dispersed. Next, a coating liquid (precursor composition) for producing thermally conductive sheet precursors was prepared by adding and mixing Rheobyk-430. Then, the boron nitride polymer was mixed with the solvent and mixed until the solids were dispersed. Here, the mixing amounts shown in Table 2 refer to the mixing amounts of each component.

[0085] The coating liquid was applied to a PET release liner (available from DU PONT-TORAY CO.,LTD., A55, 38 μm thick), then coated using a blade coater (280 μm gap), and dried in an oven at 110°C for 2 minutes to obtain an adhesive film. The two layers of the adhesive film were then laminated at 80°C and a rate of 1 m / min to obtain a thermally conductive sheet precursor with adhesive properties.

[0086] The properties of the thermally conductive sheets prepared using the thermally conductive sheet precursors were evaluated using the following methods. The results are shown in Table 2 and Figures 2 and 3. Here, Figure 3 is a graph based on the results of Comparative Examples 1 to 3. Thermal conductivity evaluation test

[0087] Thermal diffusivity was measured using a flash analysis method in a Hyperflash (trademark name) LFA 467 purchased from Netzsch. Three to four thermally conductive sheet precursors were applied between two release liner pads and placed in a hot press (heated platen press N5042-00, available from NPA System Co., Ltd.). The precursors were cured by applying a predetermined pressure (3 to 7 MPa) at 180°C for 30 minutes to produce a sample A of thermally conductive sheet with a thickness of approximately 400 to 500 μm. Next, sample A was cut into small pieces of 10 mm × 10 mm each using a knife cutter to create sample B, which was then attached to a sample holder. Before measurement, both surfaces (top and bottom) of sample B were coated with a thin layer of graphite (GRAPHIT 33, available from Kontakt Chemie) to prepare sample C. In the measurements, the temperature of the top surface of sample C was measured using an InSb IR detector after the bottom surface was irradiated with a light pulse (by a xenon flash lamp, 230V, 300μs duration). Next, the thermal diffusivity was calculated by fitting a thermal analysis plot using the cowon method. Three measurements were performed on sample C at 23°C. For each coating formulation, four samples were prepared and measured. Based on the thermal diffusivity, density, and specific heat capacity of each sample, the thermal conductivity was calculated using Proteus software (purchased from Netzsch). Evaluation test of dielectric breakdown voltage

[0088] Sample A of the thermally conductive sheet was prepared using the same procedure as described above, except that the thickness was changed to the thickness listed in the table. The dielectric breakdown voltage of sample A was measured in air at a speed of 0.5 kV / s using a puncture tester (TP-5120A) purchased from Asao Electrons Co., Ltd. Three measurements were performed at different points on sample A, and the average value was taken as the dielectric breakdown voltage value. Adhesion assessment test

[0089] Peel strength was measured using a Tensilon tester (available from RTC-1250A, ORIENTEC CO.,LTD.), which is an adhesive guide. A thermally conductive sheet precursor with adhesive properties (prepared using the same procedure as described above) was cut into 50mm × 10mm pieces, sandwiched between two unfinished copper foils, and subjected to a predetermined pressure (3 to 7 MPa) for 30 minutes at 180°C in a hot press (heated plate stamping machine N5042-00, available from NPA System Co.,Ltd.). After returning to room temperature, peel strength (T-peel strength) was measured at a rate of 20 mm / min. Test Example 2

[0090] In Test Example 2, the thermal conductivity and electrical insulation properties of the thermally conductive sheet were examined when the boron nitride polymer (first thermally conductive particle) (in which the anisotropic thermally conductive boron nitride particles are agglomerated) was mixed with the second and third thermally conductive particles. Preparation of thermally conductive sheet precursors

[0091] The thermally conductive sheet precursor for Test Example 2 was prepared in the same manner as in Test Example 1, except that the mixing amounts were changed to those shown in Table 3. Here, when the second and third thermally conductive particles are used in combination, they are added to the mixed solution A simultaneously.

[0092] The properties of the thermally conductive sheets prepared using the thermally conductive sheet precursor were evaluated using the methods described above. The results are shown in Table 3 and Figure 4. Here, Figure 4 is a graph based on Comparative Example 2, Example 1, Example 2, and Comparative Example 13.

[0093] Please note that the SEM image in Figure 1 was obtained as follows. An IM4000 Plus ion milling machine from Hitachi High-Technologies Corporation was used to fabricate a cross-sectional sample of the thermally conductive sheet, and a 2nm Pt / Pd layer was coated onto the cross-sectional sample using a sputtering device. The cross-section of the sample was then observed using an S3400N from Hitachi High Technologies Corporation, and an SEM image was obtained.

[0094] It will be apparent to those skilled in the art that various modifications can be made to the above embodiments and examples without departing from the basic principles of the invention. Furthermore, it will be apparent to those skilled in the art that various improvements and modifications can be made to the invention without departing from its spirit and scope.

Claims

1. A thermally conductive sheet precursor comprising: a plurality of first thermally conductive particles, the first thermally conductive particles being an anisotropic thermally conductive particle polymer having void regions between the anisotropic thermally conductive particles constituting the polymer, wherein the first thermally conductive particles comprise a boron nitride polymer having an average particle size of 20 μm or greater; and a plurality of second thermally conductive particles, the second thermally conductive particles being isotropic thermally conductive particles and contained outside the polymer, wherein the second thermally conductive particles are selected from at least one of aluminum nitride, alumina, silicon carbide, and boron nitride. The polymer has an average particle size of 4.5 μm or greater; a plurality of third thermally conductive particles, which are different from both the first and second thermally conductive particles, at least a portion of which are contained in the void region of the polymer, wherein the third thermally conductive particles are selected from at least one of aluminum nitride, alumina, silicon carbide, and boron nitride and have an average long diameter or average particle size of 3.0 μm or less; and an adhesive resin, wherein at least some of the polymers disintegrate when a first pressure in the range of 0.75 to 12 MPa is applied to the thermally conductive sheet precursor.

2. The precursor of claim 1, wherein the second thermally conductive particles do not disintegrate when the first pressure is applied.

3. The precursor of claim 1, wherein the polymer has a void fraction greater than 50%.

4. The precursor of claim 1, wherein the third thermally conductive particles are second isotropic thermally conductive particles.

5. As claimed in claim 1, wherein 45 vol% to 80 vol% of the thermally conductive filler component is included in the thermally conductive sheet precursor, and 20 vol% to 90 vol% of the first thermally conductive particles, 5 vol% to 75 vol% of the second thermally conductive particles, and 5 vol% to 75 vol% of the third thermally conductive particles are included in the thermally conductive filler component.

6. The precursor of claim 1 has a thickness greater than the maximum value of the minor axis length of the polymer.

7. A thermally conductive sheet comprising a cured product of the thermally conductive sheet precursor as described in claim 1.

8. A precursor composition comprising: a plurality of first thermally conductive particles, the first thermally conductive particles being an anisotropic thermally conductive particle aggregate having void regions between the anisotropic thermally conductive particles constituting the aggregate, wherein the first thermally conductive particles comprise a boron nitride aggregate having an average particle size of 20 μm or greater; and a plurality of second thermally conductive particles, the second thermally conductive particles being isotropic thermally conductive particles and contained outside the aggregate, wherein the second thermally conductive particles are selected from at least one of aluminum nitride, alumina, silicon carbide, and boron nitride. It has an average particle size of 4.5 μm or greater; a plurality of third thermally conductive particles, which are different from the first and second thermally conductive particles, at least a portion of which are contained in the void region of the polymer, wherein the third thermally conductive particles are selected from at least one of aluminum nitride, alumina, silicon carbide, and boron nitride and have an average long diameter or average particle size of 3.0 μm or less; and an adhesive resin in which the plurality of first, second, and third thermally conductive particles are dispersed.

9. The precursor composition as claimed in claim 8, wherein the polymer has a void fraction greater than 50%.

10. A method for producing a thermally conductive sheet, the method comprising: preparing a precursor composition as described in claim 8; forming a thermally conductive sheet precursor by using the composition; and forming a thermally conductive sheet by applying a pressure of at least 0.75 MPa to the thermally conductive sheet precursor.

Images