Electromagnetic wave shield member

Abstract

To provide an electromagnetic wave shield member exhibiting a wide absorption band and excellent absorption characteristics and shield characteristics in high frequency bands of 60 to 90 GHz.SOLUTION: An electromagnetic wave shield member comprises an electromagnetic wave absorption layer containing electromagnetic wave absorption fillers and a binder resin, and a reflection layer installed on an opposite face side of an electromagnetic wave incident face of the electromagnetic wave absorption layer. In the electromagnetic wave shield member, a dielectric constant of the electromagnetic wave absorption layer is 4 to 30, and a dielectric tangent thereof is 0.1 to 2.0. A content of the electromagnetic wave absorption fillers is 1 to 20 mass% with respect to a total mass of the electromagnetic wave absorption layer, and a volume resistivity of the reflection layer is 1×10-4 to 1×10-1 Ω cm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave shielding member suitable for use in devices compatible with high-speed, large-capacity communication and semiconductor devices used in vehicles. [Background technology]

[0002] High-speed, large-capacity communications using radio waves in the millimeter wave band, with frequencies between 30 and 300 GHz, are expected to be used in a wide range of fields, including 5G-compatible communications devices, autonomous driving systems, and remote medical systems. Among these, electromagnetic wave absorbing materials are known as components that absorb unwanted leakage and reflection of radio waves, and play a role in maintaining a stable communications environment and preventing malfunctions.

[0003] In the field of development of millimeter-wave electromagnetic wave absorbing materials, there have been many reports of the development of λ / 4 type electromagnetic wave absorbing sheets, which consist of a dielectric layer made of resin filled with carbon-based filler and a reflective layer made of conductive material attached to this. However, due to the absorption principle behind these electromagnetic wave absorbing sheets, they were only able to obtain narrow-band absorption characteristics, with performance limited to an absorption band of about 10 GHz at an attenuation of 10 dB and an absorption band of about 5 GHz at an attenuation of 20 dB (Patent Documents 1 and 2).

[0004] On the other hand, electromagnetic wave absorbers designed with two or more dielectric layers to obtain broadband absorption characteristics have been reported (Patent Document 3). However, such multilayered electromagnetic wave absorbers require control of many parameters, such as the dielectric properties of each layer, film thickness, and lamination method, which creates a high barrier to practical application and also leaves open the problem of a significant increase in manufacturing costs.

[0005] Therefore, when considering practicality and convenience, it is thought that a millimeter-wave electromagnetic wave absorbing material with high absorption performance, with an attenuation of 10 dB or more, and a wide absorption band of 20 GHz or more, would be more effective; however, there have been no reports to date that clearly describe the above-mentioned performance of λ / 4 type electromagnetic wave absorbing materials. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2018 / 168859 [Patent Document 2] Japanese Patent Application Publication No. 2023-129145 [Patent Document 3] Japanese Patent Application Publication No. 2020-145278 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above problems, and has an object to provide an electromagnetic wave shielding member that exhibits a wide absorption band and excellent absorption and shielding properties in the high frequency band of 60 to 90 GHz. [Means for solving the problem]

[0008] As a result of intensive research conducted by the present inventors to achieve the above object, they have discovered the following: an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing filler and a binder resin, and having a dielectric constant of 4 to 30 and a dielectric loss tangent of 0.1 to 2.0; and a dielectric film disposed on the opposite side of the electromagnetic wave absorbing layer to the electromagnetic wave incident surface, and having a volume resistivity of 1×10 -4 ~1×10 -1 The inventors have found that an electromagnetic wave shielding member having a reflective layer with a resistivity of Ω·cm and an electromagnetic wave absorbing filler content of 1 to 20 mass % relative to the total mass of the electromagnetic wave absorbing layer exhibits a wide absorption band and excellent absorption and shielding properties in the high frequency band of 60 to 90 GHz, which has led to the completion of the present invention.

[0009] Therefore, the present invention provides the following electromagnetic wave shielding member. 1. An electromagnetic wave shielding member having an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing filler and a binder resin, and a reflective layer placed on the side of the electromagnetic wave absorbing layer opposite to the electromagnetic wave incident surface, wherein the electromagnetic wave absorbing layer has a dielectric constant of 4 to 30 and a dielectric loss tangent of 0.1 to 2.0, the content of the electromagnetic wave absorbing filler is 1 to 20 mass% with respect to the total mass of the electromagnetic wave absorbing layer, and the volume resistivity of the reflective layer is 1×10 -4 ~1×10 -1 An electromagnetic wave shielding member characterized by a resistivity of Ω·cm. 2. The electromagnetic wave shielding member according to the above item 1, wherein the electromagnetic wave absorbing filler is one or more selected from the group consisting of carbon black, carbon nanotubes and carbon fibers. 3. The electromagnetic wave shielding member according to the above item 1 or 2, wherein the binder resin has a dielectric constant of 5 or less. 4. The electromagnetic wave shielding member according to the above item 1 or 2, wherein the reflective layer is made of a carbon material. 5. The electromagnetic wave shielding member according to 4 above, wherein the carbon material is at least one selected from the group consisting of carbon nanotubes, carbon fibers, graphite and graphene. 3. The electromagnetic wave shielding member according to 1 or 2 above, which has a transmission attenuation of 30 dB or more over the entire frequency band of 6.60 to 90 GHz. 3. The electromagnetic shielding member according to 1 or 2 above, wherein the frequency bandwidth in which the return loss is 10 dB or more in the frequency band of 7.60 to 90 GHz is 20 GHz or more. 3. The electromagnetic wave shielding member according to 1 or 2 above, which has a return loss of 20 dB or more in at least a part of the frequency band of 8.60 to 90 GHz. [Effects of the Invention]

[0010] The electromagnetic wave shielding material of the present invention can exhibit a wide absorption range and excellent absorption and shielding properties in the high frequency band of 60 GHz to 90 GHz. Because it can be made thinner than conventional electromagnetic wave shielding materials, it can be installed in narrow spaces within devices and is therefore extremely useful as an electromagnetic wave shielding material. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view of an electromagnetic wave shielding member (electromagnetic wave shielding sheet) of the present invention.

[0012] The present invention will be described in detail below, but the present invention is not limited thereto.

[0013] The electromagnetic wave shielding member of the present invention is characterized by having an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing filler and a binder resin, and a reflective layer. The content of the electromagnetic wave absorbing filler is 1 to 20% by mass, preferably 1 to 10% by mass, based on the total mass of the electromagnetic wave absorbing layer. The volume resistivity of the reflective layer is 1×10 -4 ~1×10 -1 Ω cm, preferably 1×10 -3 ~1×10 -1 Ω·cm. In the present invention, the volume resistivity is a value measured by the four-probe method described in JIS K7194:1994.

[0014] The method for bonding the electromagnetic wave absorbing layer and the reflective layer is not particularly limited, and an adhesive or the like may be used as an adhesive layer between the electromagnetic wave absorbing layer and the reflective layer. Alternatively, a simple lamination method by pressure bonding may be used without chemical bonding such as an adhesive.

[0015] Furthermore, the electromagnetic wave shielding member of the present invention may have a functional film provided on the outer peripheral surface thereof for the purposes of imparting insulating properties or adhesiveness, or physical protection. Each layer will be described in detail below.

[0016] <Electromagnetic wave absorbing layer> The electromagnetic wave absorbing layer of the present invention is characterized by containing an electromagnetic wave absorbing filler and a binder resin. The electromagnetic wave absorbing layer is used for the purposes of converting incident electromagnetic waves into heat and absorbing them, and for the purposes of controlling the frequency of the electromagnetic waves incident on the electromagnetic wave absorbing layer.

[0017] The dielectric constant of the electromagnetic wave absorbing layer is 4 to 30, and preferably 4 to 15. If the dielectric constant is 30 or less, the reflected component from the surface of the electromagnetic wave absorbing layer is small and the transmitted component is large, which is preferable because the absorption efficiency of the electromagnetic wave absorbing layer is high.

[0018] The dielectric loss tangent of the electromagnetic wave absorbing layer is a coefficient for absorbing electromagnetic waves and converting them into heat, and therefore must be 0.1 to 2.0, and preferably 0.3 to 1.6. The dielectric loss tangent is preferably within a range of ±20% around a value that satisfies the following formula (1). If the dielectric loss tangent and dielectric constant satisfy the following formula (1), the matching conditions for the electromagnetic wave reflected from the dielectric layer and the electromagnetic wave reflected from the reflective layer can be matched.

[0019] tanδ=1.27ε r -0.5 (1) (In the above formula, tanδ is the dielectric tangent of the electromagnetic wave absorbing layer, ε r is the dielectric constant of the electromagnetic wave absorbing layer.)

[0020] In the present invention, the dielectric constant and dielectric loss tangent were measured by the free space method.

[0021] The thickness of the electromagnetic wave absorbing layer in the present invention is not particularly limited, but is preferably within a range of ±20% around a value that satisfies the following formula (2): If the thickness of the electromagnetic wave absorbing layer is a value that satisfies the following formula (2), the matching conditions for the electromagnetic wave reflected from the dielectric layer and the electromagnetic wave reflected from the reflective layer can be made to match in any frequency band, and an attenuation of 20 dB or more can theoretically be obtained.

[0022] d=0.25cf -1 ε r -0.5 +0.5Ncf -1 ε r -0.5 (2) (In the above formula, d is the thickness (m) of the electromagnetic wave absorbing layer, c is the luminous flux (m / s), f is the frequency (Hz), ε r is the dielectric constant of the electromagnetic wave absorbing layer, and N is an integer equal to or greater than 0.)

[0023] The method for producing the composition constituting the electromagnetic wave absorbing layer in the present invention is not particularly limited, but mixing can be performed using a known kneading machine, such as a kneader, Banbury mixer, two-roll mill, three-roll mill, planetary mixer, gate mixer, or rotation-revolution mixer. Three-roll mills and planetary mixers are preferred because they are easy to operate and can achieve uniform mixing. Furthermore, when mixing, the binder resin, electromagnetic wave absorbing filler, and optional components may be kneaded simultaneously, or they may be added one by one. The resulting composition can be molded using known methods such as vacuum molding, press molding, and injection molding.

[0024] (binder resin) The binder resin used in the electromagnetic wave absorbing layer is used as a binder for the electromagnetic wave absorbing filler. The binder resin is not particularly limited, but examples thereof include polycarbonate resin, polyethylene resin, polypropylene resin, polystyrene resin, polyimide resin, thermosetting polyimide resin, polyethylene terephthalate resin, polyamide resin, polyester resin, polyvinyl chloride resin, polyurethane resin, maleimide resin, polyether ketone resin, silicone resin, phenolic resin, epoxy resin, and unsaturated polyester resin. These resins may be used alone or in combination of two or more. Among these, resins with a dielectric constant of 5 or less are preferred, such as silicone resin, maleimide resin, bismaleimide resin, and polyether ketone resin. A dielectric constant of 5 or less is preferred because the reflected component from the surface of the electromagnetic wave absorbing layer is small and the transmitted component is large, thereby increasing the absorption efficiency of the electromagnetic wave absorbing layer.

[0025] (electromagnetic wave absorbing filler) The electromagnetic wave absorbing filler used in the electromagnetic wave absorbing layer is used for the purposes of controlling the dielectric constant of the electromagnetic wave absorbing layer and absorbing electromagnetic waves and converting them into heat. The electromagnetic wave absorbing filler is not particularly limited, and may be used alone or in combination of two or more. Examples of the electromagnetic wave absorbing filler include graphite, carbon fiber, graphene, graphene oxide, graphite, carbon nanotubes, fullerene, silicon carbide, iron oxide, nano-iron oxide, and ferrite. Among these, carbon black, carbon nanotubes, carbon fiber, and silicon carbide are preferred, as they have good absorption performance in the high frequency range. Examples of carbon black include furnace black, channel black, acetylene black, and thermal black. Examples of carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes. Examples of carbon fibers include PAN-based carbon fiber and pitch-based carbon fiber. The amount of the electromagnetic wave absorbing filler added is typically 1 to 20% by mass, preferably 1 to 10% by mass, of the total mass of the electromagnetic wave absorbing layer.

[0026] (optional ingredient) In addition to the above-mentioned components, optional components may be appropriately blended into the electromagnetic wave absorbing layer to the extent that the objectives and effects of the present invention are not impaired. For example, various optional components may be added for the purpose of appropriately adjusting the shrinkage rate during curing and the thermal expansion coefficient, mechanical strength, heat resistance, chemical resistance, flame retardancy, gas permeability, thermal conductivity, oxidation resistance, etc. of the resulting cured product. Such optional components include inorganic fillers such as silica, fumed silica, quartz powder, glass fiber, titanium dioxide, calcium carbonate, magnesium carbonate, aluminum hydroxide, alumina (aluminum oxide), aluminum nitride, magnesium oxide, and boron nitride, and antioxidants such as hydroquinone and 2,6-tert-butyl-p-cresol.

[0027] <Reflection layer> The reflective layer is used for the purpose of converting the electromagnetic waves transmitted through the electromagnetic wave absorbing layer into heat, and for the purpose of preventing unnecessary electromagnetic waves from leaking to the semiconductor device on the backside as a shielding material. The reflective layer of the electromagnetic wave shielding member according to this embodiment is a member disposed on the side opposite to the electromagnetic wave incident surface of the electromagnetic wave absorbing layer.

[0028] The thickness of the reflective layer is not particularly limited as long as the transmission attenuation of the reflective layer is 30 dB or more. If the thickness is too thin, the transmission attenuation may decrease, and electromagnetic waves incident on the reflective layer may be transmitted.

[0029] The volume resistivity of the reflective layer is 1×10 -4 ~1×10 -1 Ω cm, preferably 1×10 -3 ~1×10 -1 Ω·cm. Volume resistivity is 1×10 -4 If the volume resistivity is less than Ω·cm, the efficiency of converting the electromagnetic waves transmitted through the electromagnetic wave absorbing layer into heat may decrease. -1 If the resistivity exceeds Ω·cm, a sufficient shielding effect may not be obtained. The material constituting the reflective layer is not particularly limited as long as it is within the above-mentioned volume resistivity range, but examples include carbon materials such as carbon fiber, carbon nanotubes, graphene, and graphite, and metal powders such as silver, copper, and aluminum. Of these, carbon materials are preferred. A reflective layer made of such a material has a high efficiency of converting electromagnetic waves transmitted through the electromagnetic wave absorbing layer into heat, further improving both high absorption and broadband absorption.

[0030] [Frequency characteristics of electromagnetic wave absorbing materials] The transmission attenuation is preferably 30 dB or more across the entire frequency band from 60 to 90 GHz. If it is 30 dB or more, electromagnetic waves incident on the electromagnetic wave absorbing layer can be sufficiently reflected without passing through the reflective layer. Furthermore, it is preferable that the return loss is 20 dB or more in at least a portion of the frequency range from 60 to 90 GHz, and that the frequency range in which the return loss is 10 dB or more is 20 GHz or higher. If the frequency range in which the electromagnetic wave attenuation is 10 dB or more is 20 GHz or higher, it becomes possible to flexibly accommodate electromagnetic wave devices with different frequencies, and further, a material with an attenuation of 20 dB or more will provide an even better electromagnetic wave shielding member. [Example]

[0031] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0032] <Creating an electromagnetic wave shielding sheet> The binder resin, electromagnetic wave absorbing filler, and optional ingredients were blended in the amounts shown in Tables 1 and 2 relative to the total mass of the electromagnetic wave absorbing layer, and the blend was mixed for 1 hour using a planetary mixer to obtain an electromagnetic wave absorber composition. The obtained electromagnetic wave absorber composition was press-molded into a 120 mm x 120 mm square sheet with the electromagnetic wave absorbing layer thickness shown in Tables 1 and 2, and then heat-cured at 150°C for 2 hours to obtain a sheet-like electromagnetic wave absorbing layer. The obtained electromagnetic wave absorbing layer and reflective layer were bonded together and pressure-bonded under conditions of 1 MPa and 10 minutes to produce an electromagnetic wave shielding sheet (electromagnetic wave shielding member). FIG. 1 shows an electromagnetic wave shielding sheet 1 having an electromagnetic wave absorbing layer 1a and a reflective layer 1b.

[0033] The materials used in the electromagnetic wave shielding sheets of the examples are as follows: Silicone resin: KE-106 (Shin-Etsu Chemical Co., Ltd.) Bismaleimide resin: SLK-3000 (Shin-Etsu Chemical Co., Ltd.) Carbon black: Ketjenblack (Lion Specialty Chemicals) Carbon fiber: ZY-300-25M (Nippon Graphite Fiber Co., Ltd.) Alumina: AC-5500SI (Admatechs) Carbon nanotube sheet (CNT sheet): S-T01AVB-12 (Huntsman Corporation) Carbon fiber sheet: ASA-60S-A4 manufactured by Sakai Ovex Graphite sheet: WW-T68A-140140, manufactured by Wide Work Co., Ltd.

[0034] <Measurement of dielectric properties and electromagnetic wave shielding properties> The above-mentioned electromagnetic wave shielding sheet was used as the measurement object, and the permittivity, dielectric loss tangent, and electromagnetic wave shielding properties were measured by the free space method. Specifically, using a network analyzer (manufactured by Keycom Co., Ltd.), a horn antenna (manufactured by Keycom Co., Ltd.), and a dielectric lens (manufactured by Keycom Co., Ltd.), as shown in Figure 1, electromagnetic waves were irradiated perpendicularly onto the electromagnetic wave shielding sheet 1 from the side opposite to the reflective layer 1b, and measurements were made from the incident wave and reflected wave.

[0035] Minimum return loss: Minimum return loss in the 60 to 90 GHz frequency band Maximum return loss: Maximum return loss in the 60 to 90 GHz frequency band Minimum transmission attenuation: Minimum transmission attenuation in the 60 to 90 GHz frequency band Maximum transmission attenuation: Maximum transmission attenuation in the 60 to 90 GHz frequency band 10 dB or greater return loss region width: A frequency range that shows an attenuation of 10 dB or greater in the 60 to 90 GHz frequency band.

[0036] [Table 1]

[0037] [Table 2]

[0038] Examples 1 to 4 show examples in which carbon black was used as an electromagnetic wave absorbing filler, including a system in which no optional components were added (Example 1), a system in which alumina was used as an optional component (Example 2), a system in which the dielectric loss tangent in the above formula (1) was 20% or more higher (Example 3), and a system in which the film thickness in the above formula (2) was 20% or more thicker (Example 4). Table 1 shows that in all examples, the return loss and transmission loss were large in the frequency range of 60 GHz to 90 GHz, and the absorption region showing attenuation of 10 dB or more was also wide. In particular, it can be seen that high maximum return loss was obtained in Examples 1 and 2, in which the dielectric loss tangent and film thickness were controlled to within 20% of the values ​​in the above formulas (1) and (2).

[0039] On the other hand, in the case of Comparative Example 1 in which no electromagnetic wave absorbing filler was used, in the case of Comparative Example 2 in which only a reflective layer was used, in the case of Comparative Example 3 in which no reflective layer was provided, and in the case where the volume resistivity was 1×10 -4 In Comparative Example 4, in which a copper foil having a dielectric constant of Ω·cm or less was used for the reflective layer, and Comparative Example 5, in which the electromagnetic wave absorbing layer had a dielectric constant of more than 30, it was found that neither of them achieved high return loss, high transmission loss, or a wide attenuation range of 10 dB or more, and therefore did not meet the desired performance.

[0040] Furthermore, within the scope of the present invention, Example 5, in which carbon fiber was used as the electromagnetic wave absorbing filler, Examples 6 and 7, in which a carbon fiber sheet or a graphite sheet was used in addition to a CNT sheet in the reflective layer, and a system in which a bismaleimide resin was used as the binder resin can also satisfy the performance objective of the present invention.

[0041] As described above, the electromagnetic wave shielding member of the present invention has a wide absorption band and exhibits excellent absorption and shielding properties in the high frequency band of 60 to 90 GHz. [Industrial Applicability]

[0042] The electromagnetic wave shielding member of the present invention has excellent electromagnetic wave shielding properties and can therefore be suitably used in devices compatible with high-speed, large-capacity communication and in-vehicle semiconductor devices.

[0043] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention.

Claims

1. An electromagnetic wave shielding member having an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing filler and a binder resin, and a reflective layer disposed on the side of the electromagnetic wave absorbing layer opposite to the electromagnetic wave incident surface, wherein the electromagnetic wave absorbing layer has a dielectric constant of 4 to 30 and a dielectric loss tangent of 0.1 to 2.0, the content of the electromagnetic wave absorbing filler is 1 to 20 mass % with respect to the total mass of the electromagnetic wave absorbing layer, and the volume resistivity of the reflective layer is 1×10 -4 ~1 x 10 -1 Electromagnetic wave shielding member characterized by having a resistivity of Ω·cm.

2. 2. The electromagnetic wave shielding member according to claim 1, wherein the electromagnetic wave absorbing filler is at least one selected from the group consisting of carbon black, carbon nanotubes, and carbon fibers.

3. 3. The electromagnetic wave shielding member according to claim 1, wherein the binder resin has a dielectric constant of 5 or less.

4. 3. The electromagnetic wave shielding member according to claim 1, wherein the reflective layer is made of a carbon material.

5. 5. The electromagnetic wave shielding member according to claim 4, wherein the carbon material is at least one selected from the group consisting of carbon nanotubes, carbon fibers, graphite, and graphene.

6. 3. The electromagnetic wave shielding member according to claim 1, wherein the transmission attenuation is 30 dB or more over the entire frequency band of 60 to 90 GHz.

7. 3. The electromagnetic wave shielding member according to claim 1, wherein the frequency bandwidth in which the return loss is 10 dB or more in the frequency band of 60 to 90 GHz is 20 GHz or more.

8. 3. The electromagnetic wave shielding member according to claim 1, wherein the return loss is 20 dB or more in at least a part of the frequency band of 60 to 90 GHz.

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