Microstrip antenna and method for producing same

Abstract

The present invention addresses the problem of providing a low-cost microstrip antenna which has an effect of gain improvement in the millimeter wave band without being increased in the planar size. The above-described problem is able to be solved by a microstrip antenna which sequentially comprises at least an antenna conductor layer (3), a first polyimide layer (2) and a ground conductor layer (1) in this order, wherein: the first polyimide layer (2) has a thickness of 75 μm to 200 μm; and the dielectric loss tangent at 10 GHz is 0.008 or less.

Description

Microstrip antenna and its manufacturing method

[0001] The present invention relates to a microstrip antenna and a method for manufacturing the same.

[0002] In recent years, advances in information and communication networks have accelerated the development of wireless communication technology, leading to an increase in the number of mobile devices, including smartphones. New system standards also require higher speeds and larger capacities, making it essential for antennas, which are key devices in wireless communication, to have lower loss in higher frequency bands.

[0003] As a method for improving the gain of a microstrip antenna, Patent Document 1 proposes a method of arranging a plurality of antenna conductors in an array.

[0004] As another method for improving the gain of a microstrip antenna, Patent Document 2 proposes a method of using a magnetic material for the substrate.

[0005] Japanese Patent Publication No. 2001-267839 and Japanese Patent Publication No. 2011-528527

[0006] However, the method described in Patent Document 1 has the problem of increasing the planar size due to the use of multiple antenna conductors, while the method described in Patent Document 2 has the problem of not only increasing material costs but also being unable to achieve much gain improvement in the millimeter wave band because the magnetic properties of the magnetic material are significantly reduced in the millimeter wave band.

[0007] An object of one aspect of the present invention is to provide a microstrip antenna that is low-cost and exhibits the effect of improving gain in the millimeter wave band without increasing the planar size, and a method for manufacturing the same.

[0008] As a result of extensive investigation, the present inventors have found that the above-mentioned problems can be solved by the following configuration. That is, one embodiment of the present invention has the following configuration.

[0009] [1] A microstrip antenna having at least an antenna conductor layer / a first polyimide layer / a ground conductor layer in this order, wherein the first polyimide layer has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz.

[0010] [2] A method for manufacturing a microstrip antenna, the microstrip antenna having at least an antenna conductor layer / first polyimide layer / ground conductor layer in this order, and the first polyimide layer is a polyimide film having a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz.

[0011] According to the present invention, transmission loss can be easily prevented, and therefore the present invention can be suitably used for microstrip antenna applications that require high-speed, high-frequency transmission paths.

[0012] FIG. 1 is a schematic cross-sectional view of a microstrip antenna according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a microstrip antenna having a plurality of antenna conductor layers according to one embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of a microstrip antenna having a second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order according to one embodiment of the present invention. FIG. 4 is a schematic cross-sectional view of a microstrip antenna having an adhesive layer 2 according to one embodiment of the present invention. FIG. 5 is a schematic cross-sectional view of a microstrip antenna having a plurality of antenna conductor layers according to one embodiment of the present invention. FIG. 6 is a schematic cross-sectional view of a microstrip antenna in which the insulating layer is a solder resist according to one embodiment of the present invention. FIG. 7 is a schematic cross-sectional view of a microstrip antenna having a plurality of antenna conductor layers according to one embodiment of the present invention.

[0013] A microstrip antenna according to one embodiment of the present invention has at least an antenna conductor layer / a first polyimide layer / a ground conductor layer in this order, and the first polyimide layer has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz.

[0014] Furthermore, a microstrip antenna according to another embodiment of the present invention may be a microstrip antenna that further has a second polyimide layer / adhesive layer, and has at least a second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order, with the second polyimide layer being a polyimide film.

[0015] Alternatively, a microstrip antenna according to another embodiment of the present invention may be a microstrip antenna having an insulating layer instead of the second polyimide layer / adhesive layer, and having at least an insulating layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order, with the insulating layer being a solder resist.

[0016] [1] Polyimide film used for the first polyimide layer

[0017] First, the polyimide film used for the first polyimide layer will be described. The polyimide film used for the first polyimide layer has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz. By using this polyimide film, the reflection loss at the resonant frequency of the microstrip antenna can be reduced to −10 dB or less.

[0018] The polyimide film used for the first polyimide layer is not particularly limited as long as it has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz, and may be a single layer or a multilayer.

[0019] The first polyimide layer preferably includes a thermoplastic polyimide layer and a non-thermoplastic polyimide layer, which is preferable because the first polyimide layer includes a thermoplastic polyimide layer and a non-thermoplastic polyimide layer, making it possible to easily manufacture a microstrip antenna.

[0020] In one embodiment of the present invention, it is preferable to use a polyimide film having a multilayer structure having a three-layer structure in which a thermoplastic polyimide layer is provided on both sides of a non-thermoplastic polyimide layer as the first polyimide layer, which is desirable because it tends to reduce the total production cost of the film, although it requires more steps of laminating or bonding multiple layers compared to using a single-layer polyimide film.

[0021] The polyimide film used for the first polyimide layer is particularly preferably the following film: a film having a thickness of 75 to 200 μm, which is obtained by laminating (compressing) at least two polyimide films each having a three-layer structure with a thermoplastic polyimide layer on both sides of a non-thermoplastic polyimide layer and a thickness of less than 75 μm. This is desirable because it allows the total production cost of the film to be reduced to the lowest possible level.

[0022] (Polyimide adhesive sheet: polyimide film having a three-layer structure with thermoplastic polyimide layers on both sides of a non-thermoplastic polyimide layer) Hereinafter, a polyimide film having a three-layer structure with thermoplastic polyimide layers on both sides of a non-thermoplastic polyimide layer will be described. For convenience, a polyimide film having a three-layer structure with thermoplastic polyimide layers on both sides of a non-thermoplastic polyimide layer will be referred to as a polyimide adhesive sheet. First, the raw material monomer of polyamic acid, which is a precursor of the non-thermoplastic polyimide used in the non-thermoplastic polyimide layer, the method for producing polyamic acid, which is a precursor of the non-thermoplastic polyimide, the method for producing the non-thermoplastic polyimide film, and the thermoplastic polyimide layer will be described in detail in that order.

[0023] (Raw Material Monomers of Polyamic Acid, Precursor of Non-Thermoplastic Polyimide) In one embodiment of the present invention, the raw material monomers of the polyamic acid, which is the precursor of the non-thermoplastic polyimide, are not particularly limited as long as the non-thermoplastic polyimide obtained by imidizing the precursor polyamic acid satisfies the following requirements. That is, the non-thermoplastic polyimide is not particularly limited as long as it has the solder heat resistance, dimensional stability, and flame retardancy required for conventional flexible printed circuit board materials, and the solder heat resistance, dimensional stability, and flame retardancy can be controlled by the primary structure and production method. As the raw material monomers, for example, diamines and acid dianhydrides commonly used in the synthesis of polyamic acids can be used.

[0024] The diamine is not particularly limited as long as it can exhibit the effects of the present invention, and examples thereof include 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-oxydianiline, 3,3'-oxydianiline, 3,4'-oxydianiline, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl N-methylamine, 4,4'-diaminodiphenyl N-phenylamine, 1,4-diaminobenzene(p-phenylenediamine), bis{4-(4-aminophenoxy)phenyl}sulfonate, bis(4-(4-aminophenoxy)phenyl) ... sulfone, bis{4-(3-aminophenoxy)phenyl}sulfone, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 3,3'-diaminobenzophenone, 4,4'-diaminobenzophenone, 2,2-bis(4-aminophenoxyphenyl)propane, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diamino-3,3'-dimethylbiphenyl, 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, bis(4-aminophenyl)terephthalate, 2,2-bis(4-aminophenoxyphenyl)hexafluoropropane, and the like. These may be used alone or in combination.

[0025] Diamines that are advantageous for achieving a low dielectric tangent include aliphatic diamines having 36 carbon atoms, 1,4-diaminobenzene (p-phenylenediamine), 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diamino-3,3'-dimethylbiphenyl, 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl, 4,4'-diaminodiphenyl ether, bis(4-aminophenyl)terephthalate, 2,2-bis(4-aminophenoxyphenyl)propane, 2,2-bis(4-aminophenoxyphenyl)hexafluoropropane, 4,4'-bis(4-aminophenoxy)biphenyl, etc. These may be used alone or in combination. These diamines are preferably contained in an amount of 30 to 100 mol %, more preferably 50 to 100 mol %, and even more preferably 70 to 100 mol %, of the total diamine components.

[0026] Furthermore, the acid dianhydride compound that can be used as a raw material monomer for polyamic acid is not particularly limited as long as it can exhibit the effects of the present invention, and examples thereof include pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 3,4'-oxydiphthalic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propano ... ,4,9,10-perylenetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)propanoic dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)ethane dianhydride, oxydiphthalic dianhydride, bis(3,4-dicarboxyphenyl)sulfonic dianhydride, p-phenylene bis(trimellitic acid monoester acid anhydride), ethylene bis(trimellitic acid monoester acid anhydride), bisphenol A bis(trimellitic acid monoester acid anhydride), and the like. These may be used alone or in combination.

[0027] Examples of acid dianhydrides that are advantageous for achieving a low dielectric dissipation factor include 3,3',4,4'-biphenyltetracarboxylic dianhydride, paraphenylenebis(trimellitate anhydride), 4,4'-oxydiphthalic dianhydride, 2,2'-bis(4-(3,4-dicarboxyphenoxy)phenyl)propanoic dianhydride, and pyromellitic dianhydride. These may be used alone or in combination. These acid dianhydrides are preferably contained in an amount of 30 to 100 mol %, more preferably 50 to 100 mol %, and even more preferably 70 to 100 mol % of the total acid dianhydrides.

[0028] The first polyimide layer can be produced, for example, by the following method: using the diamine and the acid dianhydride as raw materials, a polyamic acid solution is obtained by ring-opening addition polymerization in a solvent, and the polyamic acid is then heated to cause a dehydration cycloreaction (imidization). This allows the dielectric loss tangent of the first polyimide layer at 10 GHz to be controlled to a range of 0.008 or less.

[0029] (Method for Producing Polyamic Acid as a Non-Thermoplastic Polyimide Precursor) Any organic solvent can be used when producing polyamic acid as a non-thermoplastic polyimide precursor, as long as it dissolves non-thermoplastic polyamic acid. For example, amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone are preferred, with N,N-dimethylformamide and N,N-dimethylacetamide being more preferred. The solids concentration of polyamic acid as a non-thermoplastic polyimide precursor is not particularly limited, but if it is within the range of 5% by weight to 35% by weight, a polyamic acid as a non-thermoplastic polyimide precursor having sufficient mechanical strength when formed into a non-thermoplastic polyimide film can be obtained.

[0030] The order of addition of the raw materials, aromatic diamine and aromatic acid dianhydride, is not particularly limited, and the properties of the resulting non-thermoplastic polyimide can be controlled not only by controlling the chemical structures of the raw materials but also by controlling the order of addition.

[0031] A filler can be added to the non-thermoplastic polyamic acid for the purpose of improving various film properties such as sliding properties, thermal conductivity, electrical conductivity, corona resistance, loop stiffness, etc. Any filler may be used, but preferred examples include silica, titanium oxide, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, mica, etc.

[0032] (Method for Producing Non-Thermoplastic Polyimide Film) In one embodiment of the present invention, the non-thermoplastic polyimide film can be preferably obtained by, for example, a method including the following steps i) to iv).

[0033] i) a step of reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent to obtain a polyamic acid solution (hereinafter also referred to as non-thermoplastic polyamic acid), which is a precursor of a non-thermoplastic polyimide; ii) a step of casting a film-forming dope containing the non-thermoplastic polyamic acid solution from a die onto a support to form a resin layer (which may also be referred to as a liquid film); iii) a step of heating the resin layer on the support to form a self-supporting gel film, and then peeling the gel film from the support; iv) a step of further heating to imidize the remaining amic acid and drying to obtain a non-thermoplastic polyimide film.

[0034] In the steps after ii), the imidization method can be roughly divided into a thermal imidization method and a chemical imidization method. The thermal imidization method is a method in which a polyamic acid solution is used as a film-forming dope and cast onto a support, without using a dehydrating ring-closing agent or the like, and the imidization proceeds simply by heating. On the other hand, the chemical imidization method is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization accelerator is used as a film-forming dope to accelerate imidization. Either method can be used, but the chemical imidization method is superior in productivity.

[0035] The dehydration ring-closing agent may preferably be an acid anhydride such as acetic anhydride, and the catalyst may preferably be a tertiary amine such as an aliphatic tertiary amine, an aromatic tertiary amine, or a heterocyclic tertiary amine.

[0036] Suitable supports for casting the film-forming dope include glass plates, aluminum foils, endless stainless steel belts, stainless steel drums, etc. Heating conditions are set depending on the thickness of the final film to be obtained and the production rate, and the film is partially imidized or dried, and then peeled off from the support to obtain a polyamic acid film (hereinafter referred to as a gel film).

[0037] The gel film is dried by fixing the edges to prevent shrinkage during curing, and water, residual solvent, and imidization accelerator are removed from the gel film, and the remaining amic acid is completely imidized to obtain a polyimide-containing film. The heating conditions can be appropriately set depending on the thickness of the final film to be obtained and the production speed.

[0038] (Thermoplastic Polyimide (Layer)) In one embodiment of the present invention, the thermoplastic polyimide contained in the thermoplastic polyimide (layer) is obtained by imidizing its precursor, polyamic acid (hereinafter, the polyamic acid that is the precursor of the thermoplastic polyimide is also referred to as thermoplastic polyamic acid).

[0039] The aromatic diamines and aromatic tetracarboxylic dianhydrides used in the polyamic acid, which is the precursor of the thermoplastic polyimide used in the present invention, include the same as those used in the non-thermoplastic polyimide layer. On the other hand, to obtain a thermoplastic polyimide film, it is preferable to react a flexible diamine with an acid dianhydride. Examples of flexible diamines include 4,4'-diaminodiphenyl ether, 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and 2,2-bis(4-aminophenoxyphenyl)propane. The diamines may be used in combination with 1,4-diaminobenzene and / or 4,4'-diamino-2,2'-dimethylbiphenyl to adjust the glass transition temperature (Tg) of the polyimide film. Examples of acid dianhydrides that can be suitably combined with these diamines include pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and 4,4'-oxydiphthalic dianhydride.

[0040] The thermoplastic polyamic acid of the present invention can be produced by any known method as long as the resulting thermoplastic polyimide satisfies the following requirements: That is, any known method can be used as long as the thermoplastic polyimide obtained by imidizing the resulting polyamic acid has the adhesiveness to metal foil, solder heat resistance, dimensional stability, and flame retardancy required for conventional flexible printed circuit board materials.

[0041] For example, the polymer can be produced by the following steps (A-a) to (A-c): (A-a) reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent with the aromatic diamine in excess to obtain a prepolymer having amino groups at both ends; (A-b) additionally adding an aromatic diamine having a structure different from that used in step (A-a); and (A-c) further adding an aromatic acid dianhydride having a structure different from that used in step (A-a) so that the total amount of the aromatic diamine and the total amount of the aromatic acid dianhydride used in all steps (A-a) to (A-c) are substantially equimolar, and polymerizing the result.

[0042] Alternatively, polyamic acid can be obtained through the following steps (Ba) to (B-c): (Ba) reacting an aromatic diamine with an aromatic acid dianhydride in an organic polar solvent with an excess of the aromatic acid dianhydride to obtain a prepolymer having acid anhydride groups at both ends; (B-b) adding an aromatic acid dianhydride having a structure different from that used in step (Ba); and (B-c) adding an aromatic diamine having a structure different from that used in step (Ba) so that the total amount of the aromatic diamine and the total amount of the aromatic acid dianhydride used in all steps (Ba) to (B-c) are substantially equimolar, followed by polymerization.

[0043] The solids concentration of the polyamic acid, which is the precursor of the thermoplastic polyimide, is not particularly limited, but is usually 5 to 35% by weight, preferably 10 to 30% by weight, in order to obtain an appropriate molecular weight and solution viscosity.

[0044] (Method for Producing Polyimide Adhesive Sheet) A method for producing a laminate having a thermoplastic polyimide layer and a non-thermoplastic polyimide layer in one embodiment of the present invention will be described in detail. The method for producing the laminate may, for example, involve synthesizing a non-thermoplastic polyamic acid in step i), then proceeding through steps ii) to iv) and temporarily recovering the non-thermoplastic polyimide film, coating both sides of the recovered non-thermoplastic polyimide film with the thermoplastic polyamic acid, and then imidization. Alternatively, a polyimide adhesive sheet can also be produced by coating both sides of the non-thermoplastic polyimide film with a thermoplastic polyimide solution capable of forming a thermoplastic polyimide layer and drying the coating.

[0045] Another method is as follows: In step i), a polyamic acid, which is a precursor of a thermoplastic polyimide, is synthesized simultaneously with the synthesis of a non-thermoplastic polyamic acid. In step ii), a dope containing a thermoplastic polyamic acid, a film-forming dope containing a non-thermoplastic polyamic acid solution, and a dope containing a thermoplastic polyamic acid are cast from a die onto a support to form three layers, thereby forming a resin layer (sometimes referred to as a liquid film). Steps iii) and iv) are then similarly carried out to produce the polyimide adhesive sheet of the present invention.

[0046] [2] Microstrip Antenna According to an Embodiment of the Present Invention Hereinafter, a microstrip antenna according to an embodiment of the present invention will be described, namely, a microstrip antenna having at least an antenna conductor layer / first polyimide layer / ground conductor layer in this order, wherein the first polyimide layer has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz.

[0047] A microstrip antenna according to one embodiment of the present invention has an antenna conductor layer, a first polyimide layer, and a ground conductor layer, in this order, as shown in FIG. 1 . In the example shown in FIG. 1 , the microstrip antenna includes a ground conductor layer 1, a first polyimide layer 2 disposed on the ground conductor layer 1, and an antenna conductor layer 3 disposed on the first polyimide layer 2. Furthermore, in the microstrip antenna, a polyimide film having a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz is used for the first polyimide layer. This allows the reflection loss at the resonant frequency to be −10 dB or less. A smaller reflection loss is preferable, more preferably −12 dB or less, even more preferably −15 dB or less, and even more preferably −20 dB or less.

[0048] Furthermore, a microstrip antenna according to one embodiment of the present invention may have a plurality of antenna conductor layers, as shown in Fig. 2. In such a case, the plurality of antenna conductor layers are each disposed on the first polyimide layer. In the example shown in Fig. 2, the microstrip antenna includes a ground conductor layer 1, a first polyimide layer 2 disposed on the ground conductor layer 1, and two antenna conductor layers 3 disposed on the first polyimide layer 2. By providing a plurality of antenna conductor layers, an improvement in gain can be expected, and by controlling the phase of each antenna, it becomes possible to radiate radio waves over a wide range.

[0049] <Method for Manufacturing Microstrip Antenna> Figure 3 shows an example of a method for manufacturing a microstrip antenna according to one embodiment of the present invention. As shown in Figure 3, multiple polyimide adhesive sheets (each polyimide adhesive sheet is a polyimide film having a three-layer structure with a non-thermoplastic polyimide layer 10 and a thermoplastic polyimide layer 11 on both sides) are sandwiched between two sheets of metal foil, such as copper foil, and then bonded together. The microstrip antenna can then be easily formed by etching one of the two copper foils (metal foils) to form the antenna conductor layer 3. Note that while the example in Figure 3 shows an example in which the first polyimide layer includes multiple adhesive sheets, a single adhesive sheet may also be used as the first polyimide layer.

[0050] Alternatively, although multiple polyimide adhesive sheets are used as the first polyimide layer in the example of FIG. 3 , a single-layer thermoplastic polyimide film and a single-layer non-thermoplastic polyimide film may be used instead of the polyimide adhesive sheet, and the following method may be adopted. For example, copper foil (metal foil) / single-layer thermoplastic polyimide / single-layer non-thermoplastic polyimide / single-layer thermoplastic polyimide / single-layer non-thermoplastic polyimide / single-layer thermoplastic polyimide / copper foil (metal foil) may be stacked and bonded together in this order. Then, the antenna conductor layer 3 is formed in the same manner, thereby forming a microstrip antenna. In this case, the total thickness of the thermoplastic polyimide and non-thermoplastic polyimide (the thickness of the first polyimide layer) is preferably 75 μm or more.

[0051] Hereinafter, in this specification, a laminate formed by laminating the metal foil and the first polyimide layer may be referred to as a "metal-clad laminate" or a "flexible metal-clad laminate (FCCL)." Furthermore, a laminate formed by laminating two metal foils with a first polyimide layer sandwiched between them may be referred to as a "double-sided flexible metal-clad laminate," and a laminate formed by laminating one metal foil and the first polyimide layer may be referred to as a "single-sided flexible metal-clad laminate."

[0052] (Thermoplastic polyimide (single layer)) The thermoplastic polyimide (single layer) is a sheet (film) formed by imidizing the same polyamic acid as the polyamic acid that is the precursor of the thermoplastic polyimide described in the section "Thermoplastic polyimide (layer)". The thermoplastic polyimide (single layer) is preferably produced by the same method as the method for producing a non-thermoplastic polyimide film.

[0053] (Non-thermoplastic polyimide (single layer)) The non-thermoplastic polyimide (single layer) is a single-layer sheet (film) obtained by imidizing the same polyamic acid as the polyamic acid that is the precursor of the non-thermoplastic polyimide. The non-thermoplastic polyimide (single layer) is preferably produced by the same method as the method for producing the non-thermoplastic polyimide film.

[0054] As a method for laminating the metal foil and the first polyimide layer (e.g., the polyimide adhesive sheet, or a single-layer thermoplastic polyimide film and a single-layer non-thermoplastic polyimide film) together, various known methods can be applied. However, a thermocompression bonding method using thermocompression bonding is preferred because it can prevent wrinkles and the like from occurring in the metal-clad laminate. Examples of methods for bonding a polyimide adhesive sheet and a metal foil include a batch thermocompression bonding method using a single-plate press, and a continuous thermocompression bonding method using a thermoroll laminator (also called a thermolaminator) or a double-belt press (DBP) device. Among these, a thermocompression bonding method using a thermoroll laminator having one or more pairs of metal rolls is preferred from the standpoint of productivity and equipment costs, including maintenance costs. The term "thermoroll laminator having one or more pairs of metal rolls" used here refers to any device having metal rolls for heating and pressurizing materials, and the specific device configuration is not particularly limited.

[0055] The surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is preferably small in terms of contributing to reducing transmission loss, but it is also necessary to ensure adhesion. Therefore, it is preferably 0.05 μm to 0.5 μm, more preferably 0.08 μm to 0.3 μm, and even more preferably 0.1 μm to 0.2 μm. This surface roughness depends on the surface roughness of the polyimide layer side, and can be controlled by the metal foil used.

[0056] The method of laminating metal foil has been described as a method of forming the ground conductor layer and the antenna conductive layer before etching. However, one or both of the ground conductor layer and the antenna conductive layer before etching may be formed by applying and drying a conductive paste, or by laminating a conductive shielding film.

[0057] In such cases, various known methods can be applied to laminating the ground conductor layer or the antenna conductive layer before etching with the thermoplastic polyimide layer or single layer of thermoplastic polyimide of the polyimide adhesive sheet. However, a thermocompression bonding method, which involves bonding by thermocompression, is preferred because it can prevent the occurrence of poor appearance of the laminate. Examples of bonding methods include a batch thermocompression bonding method using a single-plate press, and a continuous thermocompression bonding method using a hot roll laminator (also called a hot laminator) or a double belt press (DBP) device. From the standpoints of productivity and equipment costs, including maintenance costs, a thermocompression bonding method using a hot roll laminator having one or more pairs of metal rolls is preferred. Here, the term "hot roll laminator having one or more pairs of metal rolls" refers to any device that has metal rolls for heating and pressurizing materials, and the specific device configuration is not particularly limited.

[0058] In the above-mentioned thermocompression bonding method, when two sheets of metal foil are sequentially bonded together, or when a ground conductor layer and an antenna conductive layer before etching (hereinafter, the ground conductor layer and the antenna conductive layer before etching may be collectively referred to as the “conductive layer”) are sequentially formed, the metal foil or conductive layer is subjected to high heat treatment twice, once during the production of the single-sided flexible metal (conductive layer)-clad laminate and once during the production of the double-sided flexible metal (conductive layer)-clad laminate, which can result in a problem of poor appearance due to heat burning and thermal deformation of the metal foil (conductive layer). To improve thermal deformation, it is recommended to use a thermocompression bonding method using a hot roll laminator having one or more pairs of metal rolls in the above-mentioned thermocompression bonding method, and to control the tension between the metal foil or conductive layer and the single-sided flexible metal (conductive layer)-clad laminate, or between the metal foil or conductive layer and the double-sided flexible metal (conductive layer)-clad laminate. Specifically, it is preferable to set the tension before thermocompression bonding high, and the payout tension of a single-sided flexible metal (conductive layer)-clad laminate or a double-sided flexible metal (conductive layer)-clad laminate is preferably 40 kgf / 270 mm or more. In addition, in order to prevent heat burn, it is preferable to use a protective film during hot roll lamination.

[0059] The heating method for the laminated material in the thermocompression bonding method is not particularly limited, and any heating means employing a conventionally known method capable of heating to a predetermined temperature, such as a heat circulation method, a hot air heating method, an induction heating method, etc. Similarly, the pressurizing method for the laminated material in the thermocompression bonding method is not particularly limited, and any pressing means employing a conventionally known method capable of applying a predetermined pressure, such as a hydraulic method, an air pressure method, or a gap pressure method, can be used.

[0060] The heating temperature in the thermocompression bonding step, i.e., the bonding temperature (lamination temperature), during the production of a single-sided flexible metal (conductive layer)-clad laminate should be the lowest temperature at which the polyimide adhesive sheet on the side that adheres to the metal foil (conductive layer) can adhere to the metal foil (conductive layer). The polyimide adhesive sheet on the side that does not adhere to the metal foil (conductive layer) should be at a temperature that does not cause it to stick to other materials, peripheral components, etc. Therefore, the lamination temperature during the production of a single-sided flexible metal (conductive layer)-clad laminate should be the glass transition temperature (Tg) of the polyimide adhesive sheet used + 20°C to + 60°C.

[0061] When the polyimide adhesive sheet is heated at a temperature above Tg, the higher the heating temperature, the softer the polyimide adhesive sheet becomes, making it easier to adhere to surrounding components. In this case, the side of the polyimide adhesive sheet that is not in close contact with the metal foil is preferably less adhesive, since it may come into contact with surrounding components during processing. Therefore, it is preferable to use the lamination temperature.

[0062] On the other hand, when manufacturing a double-sided flexible metal (conductive layer)-clad laminate, it is desirable for all layers to have high adhesion. Therefore, it is preferable to laminate at a higher temperature than for a single-sided flexible metal (conductive layer)-clad laminate. Therefore, the lamination temperature during the manufacture of a double-sided flexible metal (conductive layer)-clad laminate is preferably a temperature of the glass transition temperature (Tg) of the polyimide adhesive sheet used + 20°C to (Tg) + 90°C, and more preferably a temperature of Tg of the adhesive sheet (C) + 50°C to (Tg) + 80°C.

[0063] The lamination speed in the thermocompression bonding step is preferably 0.5 m / min or more, and more preferably 1.0 m / min or more. If it is 0.5 m / min or more, sufficient thermocompression bonding can be achieved, and if it is 1.0 m / min or more, productivity can be further improved.

[0064] The higher the pressure in the thermocompression bonding process, i.e., the lamination pressure, the lower the lamination temperature and the faster the lamination speed. However, too high a lamination pressure generally tends to worsen the dimensional change of the resulting metal (conductive layer)-clad laminate. Conversely, too low a lamination pressure tends to lower the adhesive strength of the metal foil in the resulting metal (conductive layer)-clad laminate. Therefore, the lamination pressure is preferably within the range of 49 N / cm to 490 N / cm (5 kgf / cm to 50 kgf / cm), and more preferably within the range of 98 N / cm to 294 N / cm (10 kgf / cm to 30 kgf / cm). Within these ranges, the three conditions of lamination temperature, lamination speed, and lamination pressure can be optimized, further improving productivity.

[0065] In one embodiment of the present invention, a thermal roll laminator is preferably used to obtain the double-sided flexible metal (conductive layer)-clad laminate, which continuously heats and presses the lamination material. This thermal roll laminator may include a lamination material feeding means for feeding the lamination material upstream of the thermal lamination means, or a lamination material winding means for winding the lamination material downstream of the thermal lamination means. The provision of these means further improves the productivity of the thermal roll laminator. The specific configurations of the lamination material feeding means and lamination material winding means are not particularly limited, and examples include known roll winders capable of winding adhesive sheets, metal foils, or the resulting metal (conductive layer)-clad laminate.

[0066] Furthermore, it is more preferable to provide a winding means and a feeding means for winding and unwinding the protective film. If these winding means and feeding means are provided, the protective film can be reused by winding up a used protective film in the thermocompression bonding process and placing it again on the feeding side. Furthermore, an end position detection means and a winding position correction means may be provided to align both ends of the protective film when it is wound up. This allows the ends to be wound up with high precision, thereby increasing the efficiency of reuse. The specific configurations of these winding means, feeding means, end position detection means, and winding position correction means are not particularly limited, and various conventionally known devices can be used.

[0067] (Metal Foil) The metal foil that can be used in one embodiment of the present invention is not particularly limited. When used in a microstrip antenna according to one embodiment of the present invention for electronic and electrical equipment applications, examples include foils made of copper or copper alloys, stainless steel or its alloys, nickel or nickel alloys (including 42 alloy), or aluminum or aluminum alloys. In general flexible metal-clad laminates, copper foils such as rolled copper foils and electrolytic copper foils are often used as the metal foil, and these can also be preferably used in the present invention. The surface of these metal foils may be coated with an anti-corrosion layer, a heat-resistant layer, or an adhesive layer. The thickness of the metal foil is not particularly limited, and may be any thickness that can exhibit sufficient functionality depending on the application.

[0068] Transmission loss is primarily composed of conductor loss due to conductors, such as metal foils (e.g., copper foil), and dielectric loss due to insulating resin substrates. Conductor loss is affected by the skin effect of metal foils (e.g., copper foil), which becomes more pronounced at higher frequencies. Therefore, to minimize transmission loss in high-frequency applications, metal foils with low roughness (e.g., copper foil) are required. Furthermore, alloys containing magnetic materials (e.g., nickel and cobalt), which are used for corrosion prevention and improved adhesion, are known to exhibit frequency-dependent changes in conductivity, potentially resulting in increased transmission loss. Therefore, caution is required when using them.

[0069] The thickness of the conductor layer such as the metal foil, and therefore the thickness of the antenna conductor layer and the ground conductor layer, is preferably 3 μm to 30 μm, and more preferably 5 μm to 20 μm. The surface roughness (Ra) of the conductor layer such as the metal foil, and therefore the surface roughness (Ra) of the antenna conductor layer and the ground conductor layer on the first polyimide layer side, is preferably 0.05 μm to 0.5 μm, more preferably 0.08 μm to 0.3 μm, and even more preferably 0.1 μm to 0.2 μm, in consideration of adhesion to the polyimide layer. When the surface roughness (Ra) is equal to or greater than the lower limit of this range, adhesion to the polyimide layer is enhanced, and when the Ra is equal to or less than the upper limit of this range, conductor loss is reduced, thereby enabling transmission loss to be suitably reduced.

[0070] (Surface treatment of polyimide adhesive sheet) Because polyimide adhesive sheets have an adhesive layer on the outermost layer, there is no need to perform a general surface treatment to improve adhesion. However, when adhesive sheets are bonded together, the sheets are made of the same material, so the surface conditions are similar, which tends to result in a small anchor effect and poor adhesion. In this case, the adhesion between polyimide adhesive sheets can be improved by performing a surface treatment on the adhesive layer, which is not usually performed, on at least one of the bonding surfaces.

[0071] The method of the surface treatment is not particularly limited, and for example, corona treatment, plasma treatment, sandblasting, etc. can be used.

[0072] [3] Microstrip antenna according to another embodiment of the present invention Hereinafter, a microstrip antenna according to another embodiment of the present invention will be described, namely, a microstrip antenna having, in addition to the antenna conductor layer / first polyimide layer / ground conductor layer, a second polyimide layer / adhesive layer, and having at least the second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order.

[0073] As shown in Fig. 4, the microstrip antenna according to this embodiment includes a second polyimide layer, an adhesive layer (adhesive layer 1), an antenna conductor layer, a first polyimide layer, and a ground conductor layer in this order. In the example shown in Fig. 4, the microstrip antenna includes a ground conductor layer 1, a first polyimide layer 2 disposed on the ground conductor layer 1, an antenna conductor layer 3 disposed on the first polyimide layer 2, an adhesive layer 5 (adhesive layer 1) disposed on the antenna conductor layer 3, and a second polyimide layer 4 disposed on the adhesive layer 5. The adhesive layer 5 (adhesive layer 1) is disposed on the first polyimide layer 2 in a portion where the antenna conductor layer 3 is not disposed. In the microstrip antenna, the first polyimide layer is a polyimide film having a thickness of 75 to 200 µm and a dielectric loss tangent of 0.008 or less at 10 GHz.

[0074] The second polyimide layer may be made of a polyimide film having a dielectric loss tangent of 0.008 or less at 10 GHz, which is used for the first polyimide layer, or a commercially available polyimide film having a dielectric loss tangent of more than 0.008 at 10 GHz.

[0075] The microstrip antenna according to this embodiment has a second polyimide layer on the adhesive layer (adhesive layer 1), but it is also possible to use a film such as a PET film or a liquid crystal film instead of the second polyimide layer as long as it has insulating properties. There are no particular restrictions on the thickness of the second polyimide layer as long as it has insulating properties, but from the viewpoint of the total thickness of the laminate, it is preferably 200 μm or less, and most preferably 25 μm or less.

[0076] Furthermore, the microstrip antenna according to this embodiment may have an adhesive layer (adhesive layer 2) between the antenna conductor layer and the first polyimide layer, as shown in Fig. 5. In the example shown in Fig. 5, the microstrip antenna includes a ground conductor layer 1, a first polyimide layer 2 arranged on the ground conductor layer 1, an adhesive layer 6 (adhesive layer 2) arranged on the first polyimide layer 2, an antenna conductor layer 3 arranged on the adhesive layer 6, an adhesive layer 5 (adhesive layer 1) arranged on the antenna conductor layer 3, and a second polyimide layer 4 arranged on the adhesive layer 5. Note that the adhesive layer 5 (adhesive layer 1) is arranged on the first polyimide layer 2 in a portion of the adhesive layer 6 where the antenna conductor layer 3 is not arranged.

[0077] The adhesive used for the adhesive layer 1 and the adhesive layer 2 is not particularly limited as long as it does not adversely affect the effects of the present invention, but for example, a thermoplastic polyimide resin, an acrylic resin, an epoxy resin, etc. can be used.

[0078] Furthermore, the microstrip antenna according to this embodiment may have a plurality of antenna conductor layers as shown in Fig. 6. In such a case, the plurality of antenna conductor layers are each disposed on the first polyimide layer.

[0079] The microstrip antenna according to this embodiment can be manufactured by bonding together a polyimide film (second polyimide layer) 4, an adhesive layer (bonding sheet) 8, and the microstrip antenna having the antenna conductor layer, first polyimide layer, and ground conductor layer in this order as described in [1] above, as shown in Figure 8. Here, an example is described in which a bonding sheet is used as the adhesive layer. On the other hand, instead of using a bonding sheet, a microstrip antenna can also be manufactured by applying an adhesive to the polyimide film side of a polyimide film (second polyimide layer) or the microstrip antenna having the antenna conductor layer, first polyimide layer, and ground conductor layer in this order as described in [1] above, and then bonding them together.

[0080] Furthermore, a microstrip antenna according to yet another embodiment of the present invention may be a microstrip antenna having at least an insulating layer, an antenna conductor layer, a first polyimide layer, and a ground conductor layer in this order, with the insulating layer being a solder resist, as shown in Fig. 7. In the example shown in Fig. 7, the microstrip antenna includes a ground conductor layer 1, a first polyimide layer 2 disposed on the ground conductor layer 1, an antenna conductor layer 3 disposed on the first polyimide layer 2, and an insulating layer 12, which is a solder resist, disposed on the antenna conductor layer 3. Note that the insulating layer 12 is disposed on the first polyimide layer 2 in a portion of the first polyimide layer 2 where the antenna conductor layer 3 is not disposed. Various commercially available solder resists can be used as the solder resist for the insulating layer, as long as they have insulating properties.

[0081] <Method for manufacturing a microstrip antenna> The microstrip antenna having an antenna conductor layer / first polyimide layer / ground conductor layer in this order as described in the above [1], which is used in manufacturing the microstrip antenna according to this embodiment, can be obtained by laminating a metal foil on the thermoplastic polyimide layer that serves as the adhesive layer of the polyimide adhesive sheet (a polyimide film having a three-layer structure with thermoplastic polyimide layers on both sides of a non-thermoplastic polyimide layer). As shown in Figure 3, metal foil / multiple polyimide adhesive sheets / metal foil may be laminated together to form a double-sided flexible metal-clad laminate (Figure 3b).

[0082] Alternatively, to manufacture a microstrip antenna having the antenna conductor layer / first polyimide layer / ground conductor layer in this order described in [1] above, instead of multiple polyimide adhesive sheets, a polyimide adhesive sheet formed by laminating (stacking) multiple sheets of a single-layer non-thermoplastic polyimide film and a single-layer thermoplastic polyimide film, each having a thickness of 75 μm or less, as shown in FIG. 9 may be used. Next, an antenna conductor layer is formed by etching one side of a double-sided flexible metal-clad laminate (FIGS. 3b and 9b). This antenna conductor layer may be multiple, as shown in FIG. 6. By the above method, a microstrip antenna having the antenna conductor layer / first polyimide layer / ground conductor layer in this order described in [1] above (microstrip antenna: FIG. 3c and FIG. 9c) can be manufactured.

[0083] The surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is preferably small in order to contribute to reducing transmission loss. However, since it is also necessary to ensure adhesion, it is preferably 0.05 μm to 0.5 μm, more preferably 0.08 μm to 0.3 μm, and even more preferably 0.1 μm to 0.2 μm. This surface roughness depends on the surface roughness of the first polyimide layer side of the metal foil (conductor layer) laminated on the double-sided flexible metal (conductor layer)-clad laminate, and can be controlled by the metal foil (conductor layer) used.

[0084] Although a method of laminating metal foil has been described as a method of forming the ground conductor layer and the pre-etching antenna conductive layer, it is also possible to adopt a method similar to the method of forming the ground conductor layer and the pre-etching antenna conductive layer in the manufacturing method of the flexible metal-clad laminate 1. That is, one or both of the ground conductor layer and the pre-etching antenna conductive layer may be formed by applying and drying a conductive paste, or by laminating a conductive shielding film.

[0085] Various known methods can be applied as a method for laminating the metal foil and the thermoplastic polyimide layer or single layer of thermoplastic polyimide of the polyimide adhesive sheet, and the same lamination method as the lamination method in the manufacturing method of the microstrip antenna having the antenna conductor layer / first polyimide layer / ground conductor layer in this order described in [1] above can be adopted.

[0086] (Metal foil) The metal foil that can be used in manufacturing the microstrip antenna according to this embodiment is not particularly limited, and the same metal foil as that used in the microstrip antenna having an antenna conductor layer / first polyimide layer / ground conductor layer in this order as described in [1] above can be used.

[0087] (Surface Treatment of Polyimide Adhesive Sheet) The polyimide adhesive sheet in the microstrip antenna according to this embodiment has an adhesive layer on the outermost layer, similar to the polyimide adhesive sheet in the microstrip antenna described in [1] above, which has an antenna conductor layer, a first polyimide layer, and a ground conductor layer in that order. Therefore, there is no need for a typical surface treatment to improve adhesion. However, when adhesive sheets are bonded together, the sheets are made of the same material, and therefore the surface conditions tend to be similar, resulting in a small anchor effect and poor adhesion. In this case, the adhesion between the polyimide adhesive sheets can be improved by performing a surface treatment on the adhesive layer, which is not normally performed, on at least one of the bonding surfaces.

[0088] The surface treatment method is not particularly limited, and for example, corona treatment, plasma treatment, sandblasting, etc. can be used.

[0089] An embodiment of the present invention may have the following configurations. 1) A microstrip antenna having at least an antenna conductor layer / first polyimide layer / ground conductor layer in this order, wherein the first polyimide layer has a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz. 2) The microstrip antenna according to 1), further having a second polyimide layer / adhesive layer, and having at least the second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order. 3) The microstrip antenna according to 1), further having an insulating layer, and having at least the insulating layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order, wherein the insulating layer is a solder resist. 4) The microstrip antenna according to any one of 1) to 3), characterized in that the return loss at the resonant frequency is −10 dB or less. 5) The microstrip antenna according to any one of 1) to 4), characterized in that the first polyimide layer has a thermoplastic polyimide layer and a non-thermoplastic polyimide layer. 6) The microstrip antenna according to any one of 1) to 5), characterized in that the first polyimide layer has a three-layer structure with a thermoplastic polyimide layer on both sides of a non-thermoplastic polyimide layer. 7) The microstrip antenna according to 6), characterized in that the first polyimide layer is a laminate of two or more polyimide films having a thickness of less than 75 μm and having the three-layer structure. 8) The microstrip antenna according to any one of 1) to 7), characterized in that the antenna conductor layer is a copper layer and further comprises an adhesive layer between the copper layer and the first polyimide layer. 9) The microstrip antenna according to any one of 1) to 8), characterized in that it has two or more antenna conductor layers. 10) The microstrip antenna according to any one of 1) to 9), characterized in that the surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is 0.05 μm to 0.5 μm.11) A method for manufacturing a microstrip antenna, the microstrip antenna having at least an antenna conductor layer / a first polyimide layer / a ground conductor layer in this order, wherein the first polyimide layer is a polyimide film having a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz. 12) A method for manufacturing a microstrip antenna according to 11), the microstrip antenna further having a second polyimide layer / an adhesive layer, the microstrip antenna having at least the second polyimide layer / an adhesive layer / an antenna conductor layer / a first polyimide layer / a ground conductor layer in this order, wherein the second polyimide layer is a polyimide film. 13) A method for manufacturing a microstrip antenna according to 11) or 12), the microstrip antenna having a reflection loss of -10 dB or less at a resonant frequency. 14) A method for manufacturing a microstrip antenna according to any one of 11) to 13), wherein the first polyimide layer has a thermoplastic polyimide layer and a non-thermoplastic polyimide layer. 15) A method for manufacturing a microstrip antenna according to any one of 11) to 14), characterized in that the first polyimide layer is formed by laminating a thermoplastic polyimide film and a non-thermoplastic polyimide film. 16) A method for manufacturing a microstrip antenna according to any one of 11) to 15), characterized in that the first polyimide layer has a three-layer structure with a thermoplastic polyimide layer on both sides of a non-thermoplastic polyimide layer. 17) A method for manufacturing a microstrip antenna according to 16), characterized in that the first polyimide layer is formed by laminating at least two polyimide films having a thickness of less than 75 μm and having the three-layer structure. 18) A method for manufacturing a microstrip antenna according to any one of 11) to 17), characterized in that the antenna conductor layer is a copper layer, and the microstrip antenna further includes an adhesive layer between the copper layer and the first polyimide layer. 19) A method for manufacturing a microstrip antenna according to any one of 11) to 18), characterized in that the microstrip antenna has two or more antenna conductor layers.20) A method for manufacturing a microstrip antenna according to any one of 11) to 19), characterized in that the surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is 0.05 μm to 0.5 μm.

[0090] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0091] The present invention will be described in detail below with reference to examples, but is not limited to these examples. The methods for measuring the dielectric constant and dielectric loss tangent of polyimide films, manufacturing flexible metal-clad laminates (FCCLs), designing and fabricating microstrip antennas, measuring reflection loss, measuring radiation characteristics, measuring peel strength, and evaluating film thickness and copper foil surface roughness (Ra) in the synthesis examples, examples, and comparative examples are as follows:

[0092] (Measurement of Dielectric Constant and Dielectric Loss Tangent) A cavity resonator perturbation method complex dielectric constant evaluation device (manufactured by Kanto Electronics Application Development Co., Ltd.) was used as the measurement device, and the dielectric constant and dielectric loss tangent of the multilayer polyimide film were measured at the following frequencies: Measurement frequency: 10 GHz Measurement conditions: Temperature 22°C to 24°C, humidity 45% to 55% Measurement sample: A sample left to stand for 24 hours under the above measurement conditions was used.

[0093] (Production of Flexible Metal-Clad Laminate (FCCL)) A polyimide laminate (polyimide adhesive sheet) and copper foil were laminated under the following conditions to obtain a double-sided FCCL. Copper foil used: thickness 12 μm, roughness of the surface to be bonded to the polyimide film 0.45 μm or less. Lamination conditions for polyimide and copper foil: lamination temperature 360° C., lamination pressure 0.8 tons, lamination speed 1 m / min.

[0094] (Design and Fabrication of Microstrip Antenna) The microstrip line antenna was designed using electromagnetic field simulation software (ADS, Momentumn, Keysight Technologies) to determine the dimensions of the square patch antenna and the position of the feed point.

[0095] Based on the design, one side of the double-sided FCCL was etched to a patch conductive area (i.e., antenna conductor layer) of approximately 3 mm x 3 mm, and the opposing ground area (i.e., ground conductor layer) was cut to a size of 10 mm x 10 mm. A 0.3 mm diameter drill was used to fabricate a patch antenna FPC. This patch antenna FPC was then bonded to a 30 mm x 30 mm x 1.5 mm thick, gold-plated stainless steel plate via silver paste and fixed by heating at 150°C for 30 minutes. For signal input, a coaxial cable connection terminal was attached via an Anritsu Corporation K103F-R connector and glass beads K100. Conduction between the glass beads K100 terminal and the patch conductive area was achieved by bonding via silver paste and fixing by heating at 150°C for 30 minutes.

[0096] (Measurement of Return Loss of Microstrip Antenna) The obtained microstrip antenna was subjected to the following treatment. Specifically, it was conditioned for 48 hours or more in a test room adjusted to 23°C and 55% RH. Then, the return loss (S11) was measured using a network analyzer E5221B (Keysight Technologies), and the resonant frequency and the return loss amount (dB) at the resonant frequency were measured.

[0097] (Measurement of radiation characteristics of microstrip antenna) Measurements were carried out in an anechoic chamber with radio wave absorbers on six sides, equipped with a turntable and a positioner. The microstrip antenna was placed in the center of the turntable, which rotates 360°, and a receiver for receiving radio waves was placed on a positioner that rotates 180° directly above the antenna.

[0098] Using a network analyzer E5227B (Keysight Technologies), the radio waves emitted from the microstrip antenna when a 28 GHz signal was incident were measured by automatically controlling the turntable and positioner from outside the anechoic chamber. The obtained gain data for each direction was displayed as a relative gain with an omnidirectional antenna as the reference. The measurement data includes directional gain and antenna gain.

[0099] Strictly speaking, millimeter waves refer to frequencies above 30 GHz, but the 28 GHz frequency band used in 5G communications is also referred to as millimeter waves. In this specification, 28 GHz is also included in the millimeter wave band.

[0100] (Method for Measuring Peel Strength) Flexible metal-clad laminates (FCCLs) were analyzed in accordance with "6.5 Peel Strength" of JIS C6471. Specifically, a 1 mm wide metal foil portion was peeled at a peel angle of 90 degrees at a peel rate of 100 mm / min, and the peel load was measured. Peel strengths of 12 N / cm or more were evaluated as "○" (good), and those less than 12 N / cm were evaluated as "×" (poor).

[0101] (Film Thickness) The thickness of the film was measured using a contact type thickness meter, LASER HOLOGAGE manufactured by Mitsutoyo Corporation.

[0102] (Surface Roughness Ra of Copper Foil) The arithmetic mean roughness was measured under the following conditions using an optical interference surface roughness meter (NewView 5030 system manufactured by ZYGO Corporation).

[0103] (Measurement conditions) Objective lens: 50x zoom FDA Res: Normal Analysis conditions: Remove: Cylinder Filter: High Pass Filter Low Wave: 0.002 mm

[0104] (Synthesis Example 1) With the reaction system maintained at 20°C, 11.64 kg of 1,3-bis(4-aminophenoxy)benzene (hereinafter also referred to as TPE-R) and 11.28 kg of 4,4'-diamino-2,2'-dimethylbiphenyl (hereinafter also referred to as m-TB) were added to 328.79 kg of N,N-dimethylformamide (hereinafter also referred to as DMF) and stirred under a nitrogen atmosphere. After visually confirming that the TPE-R and m-TB had dissolved, 14.66 kg of 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter also referred to as BPDA) and 7.39 kg of pyromellitic anhydride (hereinafter also referred to as PMDA) were added and stirred for 30 minutes. Subsequently, 4.31 kg of paraphenylenediamine (hereinafter also referred to as PDA) and 9.85 kg of PMDA were added and stirred for 30 minutes.

[0105] Finally, a solution was prepared by dissolving 0.9 kg of PMDA in DMF to a solids concentration of 7%, and this solution was gradually added to the reaction solution while taking care not to increase the viscosity. The polymerization was terminated when the viscosity reached 3,000 poise.

[0106] An imidization accelerator consisting of acetic anhydride / isoquinoline / DMF (weight ratio 2.0 / 0.7 / 4.0) was added to the polyamic acid solution at a weight ratio of 50%. The mixture was continuously stirred in a mixer, extruded through a T-die, and cast onto a stainless steel endless belt. The resin film was heated at 130°C for 100 seconds, and then the self-supporting gel film was peeled off from the endless belt and fixed to tenter clips. The film was dried and imidized at 250°C for 17 seconds, 350°C for 17 seconds, and 400°C for 120 seconds to obtain a 17 μm-thick polyimide film.

[0107] Synthesis Example 2: With the reaction system maintained at 20°C, 15.76 kg of 4,4'-diaminodiphenyl ether (hereinafter also referred to as ODA) was added to 328.94 kg of DMF and stirred under a nitrogen atmosphere. After visually confirming that the ODA had dissolved, 17.37 kg of BPDA and 2.57 kg of PMDA were added and stirred for 30 minutes. Subsequently, 11.14 kg of m-TB and 12.30 kg of PMDA were added and stirred for 30 minutes.

[0108] Finally, a solution was prepared by dissolving 0.9 kg of PMDA in DMF to a solids concentration of 7%, and this solution was gradually added to the reaction solution while taking care not to increase the viscosity. The polymerization was terminated when the viscosity reached 3,000 poise.

[0109] An imidization accelerator consisting of acetic anhydride / isoquinoline / DMF (weight ratio 2.0 / 0.7 / 4.0) was added to the polyamic acid solution at a weight ratio of 50%. The mixture was continuously stirred in a mixer, extruded through a T-die, and cast onto a stainless steel endless belt. The resin film was heated at 130°C for 100 seconds, and then the self-supporting gel film was peeled off from the endless belt and fixed to tenter clips. The film was dried and imidized at 250°C for 17 seconds, 350°C for 17 seconds, and 400°C for 120 seconds to obtain a 17 μm-thick polyimide film.

[0110] Synthesis Example 3: With the reaction system maintained at 20°C, 10.53 kg of ODA and 32.39 kg of 2,2-bis[4-(4-aminophenoxy)phenyl]propane (hereinafter also referred to as BAPP) were added to 657.82 kg of DMF and stirred under a nitrogen atmosphere. After visually confirming that the ODA and BAPP had dissolved, 16.95 kg of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (hereinafter also referred to as BTDA) and 14.34 kg of PMDA were added and stirred for 30 minutes. Subsequently, 14.22 kg of PDA and 29.83 kg of PMDA were added and stirred for 30 minutes.

[0111] Finally, a solution was prepared by dissolving 1.7 kg of PDA in DMF to a solids concentration of 10%, and this solution was gradually added to the reaction solution while paying attention to the increase in viscosity. The polymerization was terminated when the viscosity reached 3,000 poise.

[0112] An imidization accelerator consisting of acetic anhydride / isoquinoline / DMF (weight ratio 2.0 / 0.7 / 4.0) was added to the polyamic acid solution at a weight ratio of 50%. The mixture was continuously stirred in a mixer, extruded through a T-die, and cast onto a stainless steel endless belt. The resin film was heated at 130°C for 100 seconds, and then the self-supporting gel film was peeled off from the endless belt and fixed to tenter clips. The film was dried and imidized at 250°C for 17 seconds, 350°C for 17 seconds, and 400°C for 120 seconds to obtain a 17 μm-thick polyimide film.

[0113] Synthesis Example 4: With the reaction system maintained at 20°C, 11.2 kg of TPE-R and 33.0 kg of 1,4-diaminobenzene (also referred to as PDA) were added to 850.0 kg of DMF and stirred under a nitrogen atmosphere. After visually confirming that the PDA had dissolved, 63.2 kg of BPDA and 38.0 kg of 4,4'-oxydiphthalic dianhydride (also referred to as ODPA) were added and stirred for 30 minutes. 2.2 kg of PMDA was added to this solution and stirred for 30 minutes.

[0114] Finally, a solution was prepared by dissolving 0.9 kg of PMDA in DMF to a solids concentration of 7%, and this solution was gradually added to the reaction solution while taking care not to increase the viscosity. The polymerization was terminated when the viscosity reached 3,000 poise.

[0115] An imidization accelerator consisting of acetic anhydride / isoquinoline / DMF (weight ratio 2.0 / 0.7 / 4.0) was added to the polyamic acid solution at a weight ratio of 50%. The mixture was continuously stirred in a mixer, extruded through a T-die, and cast onto a stainless steel endless belt. The resin film was heated at 130°C for 100 seconds, and then the self-supporting gel film was peeled off from the endless belt and fixed to tenter clips. The film was dried and imidized at 250°C for 17 seconds, 350°C for 17 seconds, and 400°C for 120 seconds to obtain a 17 μm-thick polyimide film.

[0116] (Synthesis of Thermoplastic Polyimide Precursor (Polyamic Acid)) 29.8 g of BAPP was dissolved in 249 g of DMF cooled to 10°C. 21.4 g of BPDA was added and dissolved, and the mixture was stirred for 30 minutes to form a prepolymer. A separately prepared DMF solution of BAPP (1.57 g of BAPP / 31.4 g of DMF) was carefully added to this solution, and the addition was stopped when the viscosity reached approximately 1,000 poise. After stirring for 1 hour, a polyamic acid solution with a solids concentration of approximately 17 wt% and a rotational viscosity at 23°C of 1,000 poise was obtained.

[0117] <Microstrip Antenna Having Antenna Conductor Layer / First Polyimide Layer / Ground Conductor Layer in That Order> (Example 1) After diluting a thermoplastic polyamic acid solution with DMF to a solids concentration of 10 wt%, polyamic acid was applied to one side of the film obtained in Synthesis Example 1 using a comma coater so that the final thickness on one side was 4 μm, and the film was heated by passing through a drying oven set at 140°C for 1 minute. Polyamic acid was similarly applied to the other side so that the final thickness was 4 μm, and the film was heated by passing through a drying oven set at 140°C for 1 minute. Subsequently, the film was heated for 20 seconds in a far-infrared heater oven at an atmospheric temperature of 360°C to obtain a polyimide laminate having a total thickness of 25.0 μm. Furthermore, copper foil / three polyimide laminates with a total thickness of 25.0 μm / copper foil were stacked in this order, and the stack was thermally laminated using a hot roll laminator under conditions of a lamination temperature of 360°C, a lamination pressure of 0.8 tons, and a lamination speed of 1.0 m / min to produce a double-sided copper-clad board (double-sided FCCL) (copper foil: CF-T49A-HD2, Ra = 0.15 μm, polyimide layer thickness: 75 μm). The three polyimide laminates correspond to the "first polyimide layer."

[0118] Based on a design based on a preliminary electromagnetic field simulation, one side of the double-sided FCCL was etched to make the patch conductive area approximately 3 mm x 3 mm, and the opposing ground area was cut to 10 mm x 10 mm. A feed point was drilled to create a patch antenna FPC. This patch antenna FPC was fixed to a stainless steel plate, and a coaxial cable connection terminal was attached via a connector to create a microstrip antenna.

[0119] (Example 2) A microstrip antenna of a flexible metal-clad laminate was produced in the same manner as in Example 1, except that four polyimide laminates with a total thickness of 25.0 μm obtained in Example 1 were stacked. The four polyimide laminates corresponded to the "first polyimide layer."

[0120] (Example 3) A microstrip antenna of a flexible metal-clad laminate was produced in the same manner as in Example 1, except that six polyimide laminates with a total thickness of 25.0 μm obtained in Example 1 were stacked. The six polyimide laminates corresponded to the "first polyimide layer."

[0121] Example 4 A microstrip antenna of a flexible metal-clad laminate was produced in the same manner as in Example 1, except that eight polyimide laminates with a total thickness of 25.0 μm obtained in Example 1 were stacked. The eight polyimide laminates corresponded to the "first polyimide layer."

[0122] From Examples 1 to 4, it was confirmed that the antenna gain increased as the thickness of the polyimide laminate increased.

[0123] (Example 5) A polyimide laminate was obtained by coating, drying, and heat-treating the thermoplastic polyamic acid solution on the film obtained in Synthesis Example 2 in the same manner as in Example 1. Furthermore, a microstrip antenna of a flexible metal-clad laminate was produced using the same lamination conditions and copper foil as in Example 1.

[0124] (Example 6) A polyimide laminate was obtained by coating, drying, and heat-treating the thermoplastic polyamic acid solution on the film obtained in Synthesis Example 4 in the same manner as in Example 1. Furthermore, a microstrip antenna of a flexible metal-clad laminate was produced using the same lamination conditions and copper foil as in Example 1.

[0125] From Examples 1, 5, and 6, it was confirmed that the antenna gain increased as the dielectric loss tangent of the polyimide laminate decreased.

[0126] Comparative Example 1 A microstrip antenna of a flexible metal-clad laminate was fabricated in the same manner as in Example 1, except that only one polyimide laminate having a total thickness of 25.0 μm obtained in Example 1 was used, and the thickness of the polyimide laminate was set to 25 μm. This one polyimide laminate corresponds to the "first polyimide layer."

[0127] Comparative Example 2 A microstrip antenna of a flexible metal-clad laminate was fabricated in the same manner as in Example 1, except that two polyimide laminates with a total thickness of 25.0 μm obtained in Example 1 were stacked together to make the thickness of the polyimide laminate 50 μm. The two polyimide laminates corresponded to the "first polyimide layer."

[0128] From Example 1 and Comparative Examples 1 and 2, it was confirmed that as the thickness of the polyimide laminate became thinner, the antenna gain decreased and the insertion loss of the flexible metal-clad laminate worsened (the absolute value became larger).

[0129] Comparative Example 3 Four polyimide laminates with a total thickness of 25.0 μm were stacked in the same manner as in Example 2, except that the film obtained in Synthesis Example 3 was used, to prepare a microstrip antenna of a flexible metal-clad laminate.

[0130] It was confirmed from Example 2 and Comparative Example 3 that the use of a polyimide laminate with a large dielectric loss tangent reduces the antenna gain of the flexible metal-clad laminate. Furthermore, it was also found from Example 1 and Comparative Example 3 that the use of a polyimide laminate with a large dielectric loss tangent reduces the antenna gain of the flexible metal-clad laminate, even if the thickness of the laminate is large. These results demonstrate that in order to obtain good antenna gain, it is essential to use a polyimide laminate with a small dielectric loss tangent and to laminate it thickly.

[0131] For Examples 1 to 5 and Comparative Examples 1 to 3, the dielectric constant, dielectric dissipation factor, and thickness of each "first polyimide layer," the peel strength of each double-sided FCCL, and the reflection loss, directional gain, and antenna gain of each microstrip antenna are shown in Table 1. <Microstrip antenna having second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order> (Example 7) After diluting a thermoplastic polyamic acid solution with DMF to a solids concentration of 10 wt%, polyamic acid was applied to one side of the film obtained in Synthesis Example 1 using a comma coater so that the final thickness on one side was 4 μm, and the film was heated in a drying oven set to 140°C for 1 minute. Polyamic acid was similarly applied to the other side so that the final thickness was 4 μm, and the film was heated in a drying oven set to 140°C for 1 minute. Subsequently, the film was heated for 20 seconds in a far-infrared heater oven at an atmospheric temperature of 360°C to obtain a polyimide laminate with a total thickness of 25.0 μm. Further, copper foil / three polyimide laminates with a total thickness of 25.0 μm / copper foil were stacked in this order, and the stack was thermally laminated using a hot roll laminator under conditions of a lamination temperature of 360°C, a lamination pressure of 0.6 tons, and a lamination speed of 1.0 m / min to produce a double-sided copper-clad board (double-sided FCCL) (copper foil: CF-T49A-HD2, Ra=0.15 μm, polyimide laminate thickness: 75 μm). The three polyimide laminates correspond to the "first polyimide layer."

[0132] The polyimide laminate having a total thickness of 25.0 μm, which was used to prepare the "first polyimide layer," was used as the "second polyimide layer."

[0133] One side of the double-sided FCCL including the first polyimide layer was etched to produce a patch antenna conductor layer, which was then bonded to the second polyimide laminate via a bonding sheet SAFY manufactured by Nikkan Industries Co., Ltd., by heating under reduced pressure at 150°C for 30 minutes at 1 to 2 MPa, to obtain the microstrip antenna shown in Figure 4.

[0134] Example 8 A microstrip antenna was fabricated in the same manner as in Example 7, except that four polyimide laminates each having a total thickness of 25.0 μm obtained in Example 7 were stacked. The four polyimide laminates corresponded to the "first polyimide layer."

[0135] Example 9 A microstrip antenna was fabricated in the same manner as in Example 7, except that six polyimide laminates each having a total thickness of 25.0 μm obtained in Example 7 were stacked. The six polyimide laminates corresponded to the "first polyimide layer."

[0136] Example 10 A microstrip antenna was fabricated in the same manner as in Example 7, except that eight polyimide laminates having a total thickness of 25.0 μm obtained in Example 7 were stacked. The eight polyimide laminates corresponded to the "first polyimide layer."

[0137] From Examples 7 to 10, it was confirmed that the antenna gain increased as the thickness of the polyimide laminate increased.

[0138] (Example 11) In the same manner as in Example 7, a thermoplastic polyamic acid solution was applied to the film obtained in Synthesis Example 2, followed by drying and heat treatment to obtain a polyimide laminate. Furthermore, a microstrip antenna was produced using the same lamination conditions and the same copper foil as in Example 7.

[0139] (Comparative Example 4) A microstrip antenna was fabricated in the same manner as in Example 7, except that only one polyimide laminate having a total thickness of 25.0 μm obtained in Example 7 was used, and the thicknesses of the first and second polyimide layers were each 25 μm.

[0140] Comparative Example 5 A microstrip antenna was fabricated in the same manner as in Example 7, except that two polyimide laminates with a total thickness of 25.0 μm obtained in Example 7 were stacked and the thickness of the first polyimide layer was set to 50 μm.

[0141] From Example 7 and Comparative Examples 4 and 5, it was confirmed that the antenna gain decreased as the thickness of the polyimide laminate decreased.

[0142] Comparative Example 6 A polyimide laminate having a total thickness of 25.0 μm was produced using the film obtained in Synthesis Example 3, and a microstrip antenna of a flexible metal-clad laminate was produced in the same manner as in Example 7, except that four polyimide laminates having a total thickness of 25.0 μm were stacked on top of each other. These four polyimide laminates corresponded to the "first polyimide layer."

[0143] It was confirmed from Example 8 and Comparative Example 6 that the use of a polyimide laminate with a large dielectric loss tangent reduces the antenna gain. Furthermore, it was found from Example 7 and Comparative Example 6 that the use of a polyimide laminate with a large dielectric loss tangent reduces the antenna gain even if the thickness of the laminate is large. From the above results, it is clear that in order to obtain a good antenna gain, it is essential to use a polyimide laminate with a small dielectric loss tangent and to laminate it thickly.

[0144] For Examples 7 to 11 and Comparative Examples 4 to 6, the dielectric constants, dielectric loss tangents, and thicknesses of the first polyimide layer, second polyimide layer, and adhesive layer used, the peel strength of each double-sided FCCL, and the return loss, directional gain, and antenna gain of each microstrip antenna are shown in Table 2.

[0145] 1. Ground conductor layer 2. First polyimide layer 3. Antenna conductor layer 4. Second polyimide layer 5. Adhesive layer 1 6. Adhesive layer 2 8. Adhesive layer (bonding sheet) 10. Non-thermoplastic polyimide 11. Thermoplastic polyimide 12. Solder resist (insulating layer)

Claims

1. The antenna conductor layer includes at least an antenna conductor layer, a first polyimide layer, and a ground conductor layer in this order, the first polyimide layer having a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz, The microstrip antenna is characterized in that the first polyimide layer is a laminate of three or more polyimide films having a thickness of less than 75 μm, which has a three-layer structure having a thermoplastic polyimide layer on both sides of a non-thermoplastic polyimide layer.

2. 2. The microstrip antenna according to claim 1, further comprising a second polyimide layer / adhesive layer, and at least a second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order.

3. the antenna conductor layer is formed on the first polyimide layer and on the second polyimide layer, and the first polyimide layer is formed on the second polyimide layer.

2. The microstrip antenna according to claim 1, wherein the insulating layer is a solder resist.

4. 4. The microstrip antenna according to claim 1, wherein a reflection loss at a resonant frequency is −10 dB or less.

5. the antenna conductor layer is a copper layer; The microstrip antenna according to any one of claims 1 to 4, further comprising an adhesive layer between the copper layer and the first polyimide layer.

6. 6. The microstrip antenna according to claim 1, comprising two or more antenna conductor layers.

7. 7. The microstrip antenna according to claim 1, wherein the surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is 0.05 μm to 0.5 μm.

8. A method for manufacturing a microstrip antenna, comprising the steps of: The microstrip antenna has at least an antenna conductor layer / a first polyimide layer / a ground conductor layer in this order, the first polyimide layer having a thickness of 75 to 200 μm and a dielectric loss tangent of 0.008 or less at 10 GHz, A method for manufacturing a microstrip antenna, characterized in that the first polyimide layer is a polyimide film that is a laminate of three or more polyimide films with a thickness of less than 75 μm, having a three-layer structure with a thermoplastic polyimide layer on both sides of a non-thermoplastic polyimide layer.

9. the microstrip antenna further includes a second polyimide layer / adhesive layer, and at least includes a second polyimide layer / adhesive layer / antenna conductor layer / first polyimide layer / ground conductor layer in this order; 9. The method for manufacturing a microstrip antenna according to claim 8, wherein the second polyimide layer is a polyimide film.

10. 10. The method for manufacturing a microstrip antenna according to claim 8, wherein the microstrip antenna has a reflection loss of −10 dB or less at a resonant frequency.

11. The method for manufacturing a microstrip antenna according to any one of claims 8 to 10, characterized in that the first polyimide layer is formed by laminating a thermoplastic polyimide film and a non-thermoplastic polyimide film.

12. the antenna conductor layer is a copper layer; The method for manufacturing a microstrip antenna according to any one of claims 8 to 11, characterized in that the microstrip antenna further comprises an adhesive layer between the copper layer and the first polyimide layer.

13. The method for manufacturing a microstrip antenna according to any one of claims 8 to 12, wherein the microstrip antenna has two or more antenna conductor layers.

14. The method for manufacturing a microstrip antenna according to any one of claims 8 to 13, characterized in that the surface roughness (Ra) of the antenna conductor layer on the first polyimide layer side is 0.05 μm to 0.5 μm.