Heat-conducting sheet and production method therefor

Abstract

A heat-conducting sheet (10) contains a binder (11) and a filler (12) dispersed in the binder (11). The filler (12) comprises a plurality of particles (20) having a multilayer structure, and each of the particles (20) includes a core (21) formed of a non-magnetic material and a coating layer (22) formed of a magnetic material covering the surface of the core (21). Since the particles (20) are aligned in the thickness direction of the heat-conducting sheet (10), the heat-conducting sheet (10) has anisotropy such that the heat conductivity in the thickness direction is greater than the heat conductivity in the in-plane direction of the heat-conducting sheet (10).

Description

Thermally Conductive Sheet and Method for Producing the Same

[0001] The present invention relates to a thermally conductive sheet and a method for producing the same.

[0002] Anisotropic thermally conductive sheets can efficiently dissipate heat in a desired direction. For example, Patent Document 1 discloses a thermally conductive sheet and a method for producing the same, in which a thermally conductive composition, prepared by dispersing a first filler that is magnetic powder having a thin plate shape and second and third fillers that are thermally conductive in a binder, is formed into a sheet, and then a magnetic field is applied. When the first filler, which is magnetic powder, is oriented along the thickness direction of the sheet by the magnetic field, the first filler having a thin plate shape presses the second filler, which also has a thin plate shape, to orient the second filler parallel to the first filler.

[0003] Japanese Unexamined Patent Application Publication No. 2022-126535

[0004] However, the second filler itself, which is a non-magnetic material, does not orient even when a magnetic field is applied. If the content of the first filler around the second filler is low, the orientation may be insufficient, so the content of the first filler that is magnetic powder is limited. Since the first filler oriented by the magnetic field mechanically orients the second filler having the same shape, the third filler such as spherical alumina cannot be oriented.

[0005] Accordingly, an object of the present invention is to provide a thermally conductive sheet having anisotropy in thermal conductivity without blending a magnetic filler and a non-magnetic filler, and a method for producing the same.

[0006] A thermally conductive sheet according to one aspect of the present invention is a thermally conductive sheet comprising a binder and a filler dispersed in the binder, wherein the filler consists of a plurality of multilayer-structured particles, each of the particles comprises a core formed of a non-magnetic material and a coating layer formed of a magnetic material covering the surface of the core, and due to the particles being aligned in the thickness direction of the thermally conductive sheet, the thermally conductive sheet has anisotropy in which the thermal conductivity in the thickness direction is higher than the thermal conductivity in the in-plane direction of the thermally conductive sheet.

[0007] According to this embodiment, since a multilayer particle structure including a non-magnetic core and a magnetic coating layer is used, the particles can be aligned in a chain-like fashion along the thickness direction of the sheet. A thermal conductive sheet having anisotropic thermal conductivity and a method for manufacturing the same can be provided without blending magnetic and non-magnetic fillers.

[0008] In the above embodiment, the core may have a D90 particle diameter of 1.0 to 10 μm.

[0009] According to this embodiment, since the D90 particle diameter of the core is 10 μm or less, the particles disperse easily in the binder. Since the D90 particle diameter of the core is 1.0 μm or more, the influence of the coating layer on the entire particle is not excessive.

[0010] In the above embodiment, the coating layer may have a thickness of 0.1 to 1.0 μm.

[0011] According to this embodiment, since the thickness of the coating layer is 0.1 μm or more, it is easy to align the particles when a magnetic field is applied. Since the thickness of the coating layer is 1.0 μm or less, it is less likely to hinder heat conduction by the core.

[0012] In the above embodiment, the core may be formed from diamond, boron nitride, or aluminum nitride.

[0013] According to this embodiment, since the core is formed of a material with high thermal conductivity, a thermal conductive sheet with excellent heat dissipation in the thickness direction can be obtained.

[0014] In the above embodiment, the coating layer may be formed from nickel, cobalt, or iron, or an alloy thereof.

[0015] In this embodiment, since the core is covered with a ferromagnetic material, it is easier to align the particles when a magnetic field is applied.

[0016] In the above embodiment, the particles may further include an insulating layer covering the surface of the coating layer.

[0017] According to this embodiment, since the surface of the particles is insulating, short circuits are less likely to occur when attached to electronic components, and there is less risk of overcurrent flowing through the thermal conductive sheet.

[0018] In the above embodiment, the thickness of the insulating layer may be 0.05 to 0.50 μm.

[0019] According to this embodiment, since the thickness of the insulating layer is 0.05 μm or more, insulation failure is less likely to occur. Since the thickness of the insulating layer is 0.50 μm or less, heat conduction by the core is less likely to be inhibited.

[0020] In the above embodiment, the binder may be an ultraviolet-curing resin.

[0021] According to this embodiment, the binder can be hardened by irradiating it with ultraviolet light at a desired timing, thereby fixing the aligned particles in place.

[0022] In the above embodiment, the binder may be an epoxy resin, a silicone resin, a polyurethane resin, or an acrylic resin, or a mixture thereof.

[0023] According to this embodiment, readily available and inexpensive UV-curing resin binders can be prepared.

[0024] A method for manufacturing a thermal conductive sheet according to one aspect of the present invention includes: covering the surface of a core made of a non-magnetic material with a coating layer made of a magnetic material to form multilayer particles; forming a sheet in which a filler consisting of a plurality of particles is dispersed in an uncured binder; applying a magnetic field parallel to the thickness direction of the sheet to align the particles in the thickness direction; and curing the binder to fix the particles.

[0025] According to this embodiment, since a multilayer particle structure including a non-magnetic core and a magnetic coating layer is used, the particles can be aligned in a chain-like fashion along the thickness direction of the sheet by applying a magnetic field and then curing the binder. A thermal conductive sheet having anisotropic thermal conductivity and a method for manufacturing the same can be provided without blending magnetic and non-magnetic fillers.

[0026] In the above embodiment, aligning the particles may include arranging the sheets so that the horizontal direction perpendicular to the vertical direction and the thickness direction are parallel.

[0027] In this configuration, since the thickness direction of the sheet to which the magnetic field is applied is perpendicular to the vertical direction, which is susceptible to the effects of gravity, aligned particles are less likely to be unevenly distributed in the thickness direction of the sheet.

[0028] In the above embodiment, the particles may be aligned by applying a magnetic field using a pulsed magnetic field, a DC magnetic field, an AC magnetic field, a gradient magnetic field, or a combination thereof.

[0029] In the above embodiment, aligning the particles may include applying acoustic or vibrational stimulation to the uncured binder.

[0030] According to these embodiments, particle aggregation can be mitigated, allowing particles to be aligned in a more uniform distribution state.

[0031] In the above embodiment, aligning the particles may include preparing multiple uncured sheets under different conditions, and fixing the particles may include laminating multiple sheets and then curing them together.

[0032] According to this embodiment, a thermal conductive sheet having a characteristic gradient in the thickness direction of the sheet can be obtained.

[0033] In the above embodiment, forming multilayer particles may include further covering the surface of the coating layer with an insulating layer.

[0034] According to this embodiment, since the surface of the conductive coating layer is further covered with an insulating layer, short circuits are less likely to occur when attached to electronic components, and the risk of overcurrent flowing through the thermal conductive sheet is reduced.

[0035] According to the present invention, it is possible to provide a thermal conductive sheet having anisotropic thermal conductivity without blending magnetic fillers and non-magnetic fillers, as well as a method for manufacturing the same.

[0036] Figure 1 is a cross-sectional view showing an example of the internal structure of a thermal conductive sheet according to one embodiment of the present invention. Figure 2 is a cross-sectional view showing an example of the internal structure of the multilayer particles shown in Figure 1. Figure 3 is a flowchart illustrating an example of a manufacturing method for producing the thermal conductive sheet shown in Figure 1. Figure 4 is a block diagram showing an example of a manufacturing line for continuously producing the thermal conductive sheet shown in Figure 1.

[0037] Preferred embodiments of the present invention will be described with reference to the attached drawings. In each drawing, components denoted by the same reference numerals have the same or similar configurations. The present invention will be described in detail below with reference to the drawings.

[0038] Figure 1 is a cross-sectional view showing an example of the internal structure of a thermal conductive sheet 10 according to one embodiment of the present invention. The thermal conductive sheet 10 includes a binder 11 and a filler 12 dispersed in the binder 11. In the following description, the thermal conductive sheet 10 may be simply referred to as "sheet 10".

[0039] The binder 11 is, for example, an epoxy resin, silicone resin, polyurethane resin, or acrylic resin, or a UV-curable resin made by mixing these, which can be cured at a desired timing by irradiation with ultraviolet light. The binder 11 is not limited to UV-curable resins, but may also be a thermosetting resin.

[0040] As shown in Figure 1, the multiple particles 20 constituting the filler 12 are aligned in a chain-like manner in the thickness direction of the thermal conductive sheet 10. Because the particles 20, which have a higher thermal conductivity than the binder 11, are aligned in the thickness direction, the thermal conductive sheet 10 has anisotropy in which the thermal conductivity in the thickness direction is greater than the thermal conductivity in the in-plane direction of the thermal conductive sheet 10.

[0041] The in-plane direction of the thermal conductive sheet 10, as used here, is the direction parallel to the first surface (front) and / or second surface (back) of the thermal conductive sheet 10, which are the main surfaces of the thermal conductive sheet 10. The thickness direction of the thermal conductive sheet 10 is the direction in which the first and second surfaces of the thermal conductive sheet 10 face each other, and is perpendicular to the in-plane direction. Increasing the filling rate of the filler 12 can increase the thermal conductivity of the thermal conductive sheet 10, but it also increases the elastic modulus, causing the thermal conductive sheet 10 to no longer adhere closely to the irregularities of the electronic components.

[0042] In the case of anisotropic thermal conduction, rows of the particles 20 are arranged at intervals in the in-plane direction of the heat conductive sheet 10. Accordingly, the flexible heat conductive sheet 10 excellent in heat dissipation can be obtained. It is preferable that the interconnected particles 20 in a rosary-like arrangement are in surface contact with each other to achieve efficient heat conduction. Note that the particles 20 are not limited to surface contact with each other, and may be in any type of contact, or may have a slight gap therebetween.

[0043] FIG. 2 is a cross-sectional view showing an example of the internal structure of the multilayer-structured particles 20 illustrated in FIG. 1. Each of the particles 20 has a multilayer structure, and includes at least a core 21 formed of a non-magnetic material and a coating layer 22 formed of a magnetic material that covers the surface of the core 21.

[0044] The core 21 is formed of, for example, a material having high thermal conductivity such as diamond, boron nitride (BN) or aluminum nitride (AlN). Although these materials are insulators, they can be oriented by applying a magnetic field after being covered with the coating layer described below.

[0045] The D90 particle diameter of the core 21 is, for example, 1.0 to 10 μm. The D90 particle diameter can be measured by a laser diffraction / scattering method based on ISO 13320:2020. If the core 21 is smaller than 10 μm, it is easy to disperse. If the core 21 is larger than 1.0 μm, it is not too small, so the influence of the coating layer does not become excessive.

[0046] The coating layer 22 is formed of a ferromagnetic material such as, for example, nickel, cobalt, iron, or alloys thereof. The film thickness of the coating layer 22 is, for example, 0.1 to 1.0 μm. If the film thickness of the coating layer 22 is too large, the influence of the coating layer 22 becomes excessive and the thermal conductivity decreases. If the film thickness of the coating layer 22 is too small, the influence of the coating layer 22 becomes insignificant, and it is difficult to align the particles even when a magnetic field is applied. The film thickness of the coating layer 22 is preferably about 10% of the D90 particle diameter of the core.

[0047] In the illustrated example, each of the particles 20 further includes an insulating layer 23 that covers the surface of the coating layer 22. The structure of the particles 20 is not limited to the illustrated example, and may be a two-layer structure including only the core 21 and the coating layer 22, or may be a multilayer structure of four or more layers further including an outer layer. The insulating layer 23 is formed of, for example, an inorganic insulating material such as an oxide or a nitride. When the insulating layer 23 is provided, no electric leakage occurs even when the particles are attached to an electronic component. The film thickness of the insulating layer 23 is, for example, 0.05 to 0.50 µm.

[0048] FIG. 3 is a flowchart illustrating an example of a manufacturing method for manufacturing the thermally conductive sheet 10 shown in FIG. 1. As shown in FIG. 3, first, the surface of a core 21 formed of a non-magnetic material is covered with a coating layer 22 formed of a magnetic material to form the multilayer-structured particles 20 shown in FIG. 2 (step S1).

[0049] The coating method may be electroplating, electroless plating, or a CVD method, and can be appropriately selected from known methods. Step S1 may further include a step of further covering the surface of the coating layer 22 with the insulating layer 23.

[0050] Next, a filler 12 composed of a plurality of the particles 20 is dispersed in an uncured binder 11 (step S2). The blending amount of the filler 12 is 30 to 60% by volume relative to the total amount of the binder 11 and the filler 12. The method for dispersing the filler 12 in the binder 11 may be a bead mill, a roll mill, or an ultrasonic stirrer, and can be appropriately selected from known methods.

[0051] The binder 11 is formed into a sheet shape (step S3). After an uncured sheet is formed, a magnetic field parallel to the thickness direction of the sheet is applied to align the particles 20 so as to form a beads-like chain in the thickness direction (step S5). In step S5, the magnetic field may be applied using a pulsed magnetic field, a DC magnetic field, an AC magnetic field, a gradient magnetic field, or a combination thereof. An acoustic stimulus or a vibration stimulus may be applied to the uncured binder 11. In the illustrated example, prior to step S5, the sheet is disposed such that the thickness direction thereof is horizontal (step S4). Step S4 is not an essential step and may be omitted.

[0052] In step S5, the binder 11 of the sheet, in which the particles 20 are arranged in a chain-like fashion, is cured to fix the aligned particles 20 (step S6). This makes it possible to obtain the heat-conductive sheet 10 shown in Figure 1. The method for curing the binder 11 may be ultraviolet curing or heat curing.

[0053] In step S5, multiple uncured sheets can be prepared by changing the material of the filler 12, the film thickness of the coating layer 22 and the insulating layer 23, the magnetic field strength, and the presence or absence of acoustic stimulation, etc., and then in step S6, these sheets can be laminated and cured together to obtain a thermal conductive sheet 10 having a characteristic gradient in the thickness direction of the sheet.

[0054] By laminating multiple sheets, for example, a thermal conductive sheet 10 may be manufactured comprising an upper layer made of a sheet in which particles 20 are arranged with high positional accuracy, an intermediate layer made of a sheet manufactured at a reduced cost, and a lower layer made of a sheet that prioritizes electrical insulation, etc. The characteristics of each sheet constituting the laminated sheet can be optimized according to the application and required characteristics.

[0055] Figure 4 is a block diagram showing an example of a manufacturing line for continuously producing the thermal conductive sheet 10 shown in Figure 1. The binder storage tank 31 stores, for example, an epoxy-based ultraviolet curing resin binder 11. The filler storage tank 32 stores, for example, a filler 12 consisting of particles 20, where the core 21 is diamond with a D90 particle diameter of 5 μm, the coating layer 22 is nickel with a film thickness of 0.3 μm, and the insulating layer 23 is silicon dioxide (silica) with a film thickness of 0.1 m.

[0056] A binder 11 and filler 12 are supplied in fixed amounts from a binder storage tank 31 and a filler storage tank 32 to a grinder 33 such as a wet bead mill, and the mixture is kneaded in the binder 11 to prevent the filler 12 from agglomerating, thereby preparing a suspension. The suspension is discharged onto a substrate such as a film unwound from a winding machine using a twin-screw extruder 34, and coated to a thickness of 50 to 100 μm using a coating machine 35 such as a knife coater to form an uncured sheet. A magnetic field of 0.1 to 1.0 T perpendicular to the vertical direction is applied to the formed sheet using a magnetic field generator 37 equipped with a Helmholtz coil or the like to arrange the particles 20.

[0057] In the illustrated example, a 20 kHz ultrasonic vibration is applied by an exciter 36 such as an ultrasonic oscillator immediately before applying the magnetic field. The sheet may also be vibrated while the magnetic field is being applied. The binder 11 is cured by ultraviolet light from the ultraviolet irradiation device 38 to fix the arrangement of the particles 20, and the sheet is wound up by the winding machine 39, allowing for the continuous production of the thermal conductive sheet 10.

[0058] The illustrated manufacturing line can be easily incorporated into a roll-to-roll system or conveyor transport process for the thermal conductive sheet 10 from an unwinding machine to a winding machine 39. By installing a magnetic field generator 37 and a vibration exciter 36 along the sheet transport direction and controlling them in synchronization with the transport speed, it is possible to manufacture large-area and high-throughput thermal conductive sheets 10.

[0059] According to the heat conductive sheet 10 and its manufacturing method of one embodiment of the present invention configured as described above, since the multilayer particles 20 including a non-magnetic core 21 and a magnetic coating layer 22 are used, the particles 20 can be aligned in a chain-like fashion along the thickness direction of the heat conductive sheet 10 by applying a magnetic field and then curing the binder 11. A heat conductive sheet 10 having anisotropic heat conduction can be provided without blending magnetic fillers and non-magnetic fillers.

[0060] The embodiments described above are provided to facilitate understanding of the present invention and are not intended to limit its interpretation. The elements, arrangement, materials, conditions, shapes, and sizes of the embodiments are not limited to those exemplified and can be modified as appropriate. Furthermore, it is possible to partially substitute or combine the configurations shown in different embodiments.

[0061] 10...Heat conductive sheet, 11...Binder, 12...Filler, 20...Particles, 21...Core, 22...Coating layer, 23...Insulating layer, 31...Binder storage tank, 32...Filler storage tank, 33...Crusher, 34...Twin-screw extruder, 35...Coating machine, 36...Vibrator, 37...Magnetic field generator, 38...Ultraviolet irradiation device, 39...Winding machine.

Claims

1. A thermal conductive sheet comprising a binder and a filler dispersed in the binder, wherein the filler consists of a plurality of multilayer particles, each of which comprises a core made of a non-magnetic material and a coating layer made of a magnetic material covering the surface of the core, and the particles are aligned in the thickness direction of the thermal conductive sheet, resulting in an anisotropy in which the thermal conductivity in the thickness direction is greater than the thermal conductivity in the in-plane direction of the thermal conductive sheet.

2. The thermal conductive sheet according to claim 1, wherein the core has a D90 particle diameter of 1.0 to 10 μm.

3. The thermal conductive sheet according to claim 1, wherein the coating layer has a film thickness of 0.1 to 1.0 μm.

4. The thermal conductive sheet according to claim 1, wherein the core is formed from diamond, boron nitride, or aluminum nitride.

5. The thermal conductive sheet according to claim 1, wherein the coating layer is formed from nickel, cobalt, iron, or an alloy thereof.

6. The thermal conductive sheet according to claim 1, wherein the particles further comprise an insulating layer covering the surface of the coating layer.

7. The thermal conductive sheet according to claim 6, wherein the thickness of the insulating layer is 0.05 to 0.50 μm.

8. The thermal conductive sheet according to claim 1, wherein the binder is an ultraviolet curing resin.

9. The thermal conductive sheet according to claim 8, wherein the binder is an epoxy resin, a silicone resin, a polyurethane resin, or an acrylic resin, or a mixture thereof.

10. A method for manufacturing a thermal conductive sheet, comprising: covering the surface of a core made of a non-magnetic material with a coating layer made of a magnetic material to form multilayer particles; forming a sheet in which a filler consisting of a plurality of the particles is dispersed in an uncured binder; applying a magnetic field parallel to the thickness direction of the sheet to align the particles in the thickness direction; and curing the binder to fix the particles.

11. The method for manufacturing a thermal conductive sheet according to claim 10, wherein aligning the particles includes arranging the sheet such that the horizontal direction perpendicular to the vertical direction is parallel to the thickness direction.

12. The method for manufacturing a thermal conductive sheet according to claim 10, wherein the alignment of the particles is performed by applying a magnetic field using a pulsed magnetic field, a DC magnetic field, an AC magnetic field, or a gradient magnetic field, or a combination thereof.

13. The method for manufacturing a thermal conductive sheet according to claim 10, wherein aligning the particles includes applying acoustic or vibrational stimulation to the uncured binder.

14. The method for manufacturing a thermal conductive sheet according to claim 10, wherein aligning the particles includes preparing a plurality of uncured sheets under different conditions, and fixing the particles includes laminating the plurality of sheets and then curing them together.

15. The method for manufacturing a thermal conductive sheet according to claim 10, wherein forming the multilayer particles comprises further covering the surface of the coating layer with an insulating layer.

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