High-frequency substrate and method for manufacturing high-frequency substrate

Abstract

An embodiment of the present invention relates to a high-frequency substrate or a method for manufacturing a high-frequency substrate, wherein the high-frequency substrate includes a PTFE film (a) satisfying (I)-(II) and an LCP film (b), and satisfies (X).　(I): The porosity calculated from a cross-sectional SEM image of the film in a range including a surface and a depth of 5 μm from the surface is 0.1% or lower. (II): Rz is 1 μm or less. (X): The adhesive strength between the PTFE film (a) and the LCP film (b) is 3.0 N / cm or greater.

Description

High frequency substrate and method for manufacturing the same

[0001] An embodiment of the present invention relates to a high frequency substrate or a method for manufacturing a high frequency substrate.

[0002] In recent years, in fields such as electronic devices, communications devices, and computers, there has been a demand for more diverse and faster information processing, and the frequencies used are shifting to higher frequencies such as the gigahertz range. High-frequency substrates (substrates used to transmit high-frequency signals) used in such high-frequency ranges are required to have low transmission loss, and therefore are required to have low dielectric constants and low dielectric loss tangents.

[0003] Polyimide (PI) has been mainly used as a material for forming the high-frequency substrate, but there has been a growing demand for materials with lower dielectric loss. In recent years, high-frequency substrates using low-dielectric-constant polyimide (MPI), liquid crystal polymer (LCP), etc. have been put to practical use, but these high-frequency substrates have room for improvement in terms of dielectric properties in order to be used in frequency bands exceeding so-called sub-6 (less than 6 GHz).

[0004] Fluorine-based resins, particularly polytetrafluoroethylene (PTFE), are materials with particularly excellent dielectric properties, and research has been conducted into the use of PTFE in high-frequency substrates (see, for example, Patent Document 1).

[0005] Patent No. 6925814

[0006] High-frequency substrates are typically used as laminates containing a metal layer such as a copper layer. Because PTFE has poor mechanical properties, such as a high linear expansion coefficient, it is sometimes used as a laminate with a resin film such as polyimide, polyester, polyolefin, or epoxy resin. Such laminates require excellent adhesion between the PTFE film and the metal layer and / or resin film. However, PTFE films are poorly bonded, and Patent Document 1 discloses that the adhesive strength between the PTFE film and the metal layer peaks at 3.0 N / cm. Since PTFE films are used, the adhesive strength with resin films is thought to be at most about 3.0 N / cm, leaving room for improvement in this regard.

[0007] Furthermore, since PTFE is a poorly adhesive material, in order to improve the adhesiveness of the laminate, the PTFE film has conventionally been subjected to surface treatment to roughen the surface during lamination. However, it has been found that when a PTFE film with a roughened surface is used, the dielectric properties are reduced and noise may occur when used at high frequencies.

[0008] One embodiment of the present invention relates to a high-frequency substrate having a PTFE film with a smooth surface, in which a PTFE film and an LCP film are laminated together, and the high-frequency substrate has excellent adhesive strength between the films and a low dielectric constant and a low dielectric loss tangent.

[0009] An example of the configuration of the present invention is as follows.

[0010] [1] A high-frequency substrate comprising: a polytetrafluoroethylene (PTFE) film a satisfying the following requirements (I) and (II); and a liquid crystal polymer (LCP) film b, and satisfying the following requirement (X): Requirement (I): the porosity calculated from a cross-sectional SEM image of the film in the range from the surface to a depth of 5 μm from the surface is 0.1% or less; Requirement (II): the maximum height roughness Rz is 1 μm or less; and Requirement (X): the adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more.

[0011] [2] The high-frequency substrate according to [1], wherein no adhesive layer is provided between the PTFE film a and the LCP film b.

[0012] [3] The high-frequency substrate according to [1] or [2], wherein the PTFE film a has a thickness of 6 to 200 μm. [4] The high-frequency substrate according to any one of [1] to [3], wherein the LCP film b has a thickness of 5 to 200 μm.

[0013] [5] The high-frequency substrate according to any one of [1] to [4], wherein the PTFE film a is a modified PTFE film.

[0014] [6] The high-frequency substrate according to any one of [1] to [5], which has a metal layer on the side of the PTFE film a opposite to the LCP film b. [7] The high-frequency substrate according to [6], in which the adhesive strength between the PTFE film a and the metal layer is 3.0 N / cm or more.

[0015] [8] A method for producing a high-frequency substrate, comprising: Step 1 of forming a PTFE film (a) by heating a PTFE-cut film or a modified PTFE film at a temperature equal to or higher than the melting point of PTFE or modified PTFE and applying pressure of 0.1 MPa or more; Step 2 of performing a reduced-pressure plasma treatment, which is any one of Steps 2-1 to 2-3 below; and Step 3 of laminating the PTFE film (a) and an LCP film (b) using the film obtained in Step 2. Step 2-1: performing a reduced-pressure plasma treatment on at least the surface of the PTFE film (a) obtained in Step 1 that faces the LCP film (b); Step 2-2: performing a reduced-pressure plasma treatment on at least the surface of the LCP film (b) that faces the PTFE film (a); and Step 2-3: performing a reduced-pressure plasma treatment on at least the surface of the PTFE film (a) obtained in Step 1 that faces the LCP film (b), and performing a reduced-pressure plasma treatment on at least the surface of the LCP film (b) that faces the PTFE film (a).

[0016] [9] The method for producing a high-frequency substrate according to [8], wherein the PTFE film a formed in step 1 satisfies the following requirements (I) and (II): Requirement (I): The porosity calculated from a cross-sectional SEM image of the film in the range from the surface to a depth of 5 μm from the surface is 0.1% or less, and Requirement (II): The maximum height roughness Rz is 1 μm or less.

[0017]

[10] The method for producing a high-frequency substrate according to [8] or [9], wherein the high-frequency substrate satisfies the following requirement (X): the adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more.

[0018]

[11] The method for manufacturing a high-frequency substrate according to any one of [8] to

[10] , wherein no adhesive is used during lamination in step 3.

[0019]

[12] The method for producing a high-frequency substrate according to any one of [8] to

[11] , wherein the low-pressure plasma treatment carried out in step 2 is a low-pressure plasma treatment using plasma generated under a reduced pressure of 1 to 100,000 Pa in the presence of a material gas containing an amino group.

[0020]

[13] The method for producing a high-frequency substrate according to any one of [8] to

[12] , further comprising step 4 of laminating a metal layer on the side of the PTFE film a opposite to the LCP film b in the high-frequency substrate obtained in step 3.

[0021] According to one embodiment of the present invention, a high-frequency substrate having a smooth surface and laminated with an LCP film can be provided, which has excellent adhesive strength between the films and a low dielectric constant and dielectric loss tangent. While PTFE films have poor mechanical properties, such as a high linear expansion coefficient, the high-frequency substrate according to one embodiment of the present invention is a laminate of a PTFE film and an LCP film. Therefore, the substrate as a whole is a high-frequency substrate with a relatively low linear expansion coefficient that is resistant to deformation due to heat, etc. Even when laminated with a metal layer, a high-frequency substrate that is resistant to peeling due to deformation can be easily obtained. Furthermore, according to one embodiment of the present invention, a flexible high-frequency substrate can be easily obtained.

[0022] The left side of Fig. 1 is a photograph of the appearance of the PTFE-cut film used in Production Example 1 below, and the right side of Fig. 1 is a photograph of the appearance of PTFE film a-1 obtained in Production Example 1 below. Fig. 2 is an SEM image of the cross section of the PTFE-cut film used in Production Example 1 below. Fig. 3 is an SEM image of the cross section of PTFE film a-1 obtained in Production Example 1 below. Fig. 4 is an SEM image of the cross section of the high-frequency substrate obtained in Example 2, which was hot-pressed under conditions of a temperature of 250°C and a pressure of 20 MPa. Fig. 5 is an SEM image of the cross section of the high-frequency substrate produced in Example 8 below.

[0023] In the present invention, polytetrafluoroethylene (PTFE) includes modified PTFE unless otherwise specified. In the present invention, there is no particular distinction between films and sheets. In this specification, a numerical range described using "to" means that the numerical values ​​described before and after "to" are included as the lower limit or upper limit, and when a numerical range is described in stages, the upper and / or lower limits of each numerical range can be combined arbitrarily.

[0024] <High-Frequency Substrate> A high-frequency substrate according to one embodiment of the present invention (hereinafter also referred to as "the substrate") includes a PTFE film a and an LCP film b that satisfy the following requirements (I) and (II), and also satisfies the following requirement (X): Requirement (I): The porosity calculated from a cross-sectional SEM image of the film in the range from the surface to a depth of 5 μm from the surface is 0.1% or less; Requirement (II): The maximum height roughness Rz is 1 μm or less; and Requirement (X): The adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more.

[0025] The adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more, preferably 4.0 N / cm or more, more preferably 5.0 N / cm or more, and the upper limit is not particularly limited, but is, for example, 6.0 N / cm. When the adhesive strength is within this range, a high-frequency substrate that is resistant to deformation due to heat, etc., and has a low dielectric constant and a low dielectric loss tangent can be easily obtained. The adhesive strength is specifically measured by the method described in the examples below. This substrate having an adhesive strength within this range can be specifically produced by the present manufacturing method described below.

[0026] The dielectric constant (ε) of the present substrate at a frequency of 10 GHz is preferably 3.2 or less, more preferably 2.6 or less. The lower limit of the dielectric constant is not particularly limited, but is, for example, 2.2. A substrate having a dielectric constant within the above range can be said to have a low dielectric constant and can be suitably used as a high-frequency substrate.

[0027] The dielectric loss tangent (tanδ) of this substrate at a frequency of 10 GHz is preferably 0.008 or less, more preferably 0.001 or less. The lower limit of the dielectric loss tangent is not particularly limited, but is, for example, 0.0001. A substrate having a dielectric loss tangent within the above range can be said to have a low dielectric loss tangent and can be suitably used as a high-frequency substrate. The relative permittivity and dielectric loss tangent of the substrate can be measured specifically by the method described in the Examples below.

[0028] The substrate is preferably a flexible substrate (e.g., a flexible printed circuit board). A suitable example of the substrate is an electronic circuit board, and more preferably, it can be used as a wiring board for modules of mobile phones, computers, antennas, wearable devices, etc.

[0029] <PTFE film a> The PTFE film a satisfies the following requirements (I) and (II). At least one surface of the PTFE film a satisfies the following requirements (I) and (II), and usually, at least one of its main surfaces (the surface with the largest area) satisfies the following requirements (I) and (II), and it is preferable that both of the main surfaces satisfy the following requirements (I) and (II). Note that when only one of the main surfaces satisfies the following requirement (I), the surface that satisfies requirement (I) is the surface facing the LCP film b, preferably the surface that contacts the LCP film b.

[0030] Requirement (I): The porosity calculated from a cross-sectional SEM image of the film between the surface and a depth of 5 μm from the surface is 0.1% or less. The porosity is preferably 0.08% or less, more preferably 0.02% or less. Since PTFE film a preferably has no voids, the lower limit of the porosity is preferably 0%. Conventionally, the maximum adhesive strength between a PTFE film and a resin film has been considered to be approximately 3.0 N / cm. However, because the porosity of PTFE film a is within the above range, a substrate having an adhesive strength with LCP film b within the above range (3.0 N / cm or more) can be obtained. Furthermore, because the porosity of PTFE film a is within the above range, a substrate having an adhesive strength with a metal layer within the above range (3.0 N / cm or more) can also be obtained. When the porosity is within the above range, a high-frequency substrate that is resistant to deformation due to heat, etc., and has a low dielectric constant and low dielectric loss tangent can be easily obtained. Specifically, the porosity is measured by the method described in the examples below.

[0031] Requirement (II): The maximum height roughness Rz of the surface of the PTFE film a is 1 μm or less. The Rz is preferably 0.9 μm or less, more preferably 0.7 μm or less. The lower limit of the Rz is not particularly limited, but is, for example, 0.2 μm. A PTFE film a having an Rz within the above range can be said to have a smooth surface. Because of this smooth surface, it is easy to obtain a high-frequency substrate with a lower dielectric constant and a lower dielectric dissipation factor. It has traditionally been difficult to bond a PTFE film a having a smooth surface to other layers, such as an LCP film b, with high adhesive strength (3.0 N / cm or more). However, according to one embodiment of the present invention, a substrate having high adhesive strength (3.0 N / cm or more) can be obtained while including such a PTFE film a having a smooth surface. In this specification, the arithmetic mean roughness Ra and maximum height roughness Rz are values ​​measured using a laser microscope, specifically the laser microscope described in the examples.

[0032] Specifically, the PTFE film a satisfying the above requirements (I) and (II) can be obtained by the following step 1.

[0033] The relative dielectric constant of the PTFE film a at a frequency of 10 GHz is preferably 2.8 or less, more preferably 2.3 or less. The lower limit of the relative dielectric constant is not particularly limited, but is, for example, 2.00. The PTFE film a having a relative dielectric constant in the above range can be suitably used for high-frequency substrates.

[0034] The dielectric loss tangent (tanδ) of the PTFE film a at a frequency of 10 GHz is preferably 0.008 or less, more preferably 0.001 or less. The lower limit of the dielectric loss tangent is not particularly limited, but is, for example, 0.00001. PTFE film a having a dielectric loss tangent in the above range can be suitably used for high-frequency substrates. The relative dielectric constant and dielectric loss tangent of the PTFE film a can be measured specifically by the method described in the following examples.

[0035] The PTFE film a has an arithmetic mean roughness Ra of preferably 0.2 μm or less, more preferably 0.1 μm or less. The lower limit of Ra is not particularly limited, but is, for example, 0.03 μm.

[0036] The thickness of the PTFE film a is preferably 6 to 200 μm, more preferably 10 to 100 μm, and even more preferably 10 to 50 μm. In this substrate, the ratio of the thickness of the PTFE film a to the thickness of the LCP film b (thickness of PTFE film a / thickness of LCP film b) is preferably 0.1 to 20, and even more preferably 0.4 to 8. When the thickness or thickness ratio of the PTFE film a is within the above range, a high-frequency substrate with a low dielectric constant and a low dielectric loss tangent can be easily obtained.

[0037] The PTFE film a may be an unmodified PTFE film or a modified PTFE film, but is preferably a modified PTFE film, since this makes it easier to obtain a substrate having excellent adhesive strength with other layers (e.g., LCP film b, metal layer).

[0038] As the modified PTFE, can be listed the conventionally known modified PTFE described in Japanese Patent Laid-Open No. Hei 9-52955, Japanese Patent Laid-Open No. Hei 11-71574, Japanese Patent Laid-Open No. Hei 11-286501, Japanese Patent Laid-Open No. 2008-137327 etc., and specific example can be listed the copolymer obtained by using tetrafluoroethylene (TFE) and one or two or more kinds of monomer other than TFE.

[0039] Examples of the monomer include fluoroolefins such as hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE), perfluoro(alkyl vinyl ethers) such as perfluoro(alkyl vinyl ethers) having a perfluoroalkyl group having 1 to 6 carbon atoms, fluorodioxole, perfluoroalkylethylene, and ω-hydroperfluoroolefin. Among these, perfluoro(alkyl vinyl ethers) are preferred.

[0040] In the modified PTFE, the content of the constituent units derived from the monomer is preferably 0.001 to 10% by mass, more preferably 0.01 to 5% by mass, and even more preferably 0.01 to 2% by mass.

[0041] The modified PTFE film may contain additives such as plasticizers, other resins such as epoxy resins, curing agents, curing accelerators, organic fillers, inorganic fillers, coupling agents, and flame retardants.

[0042] <LCP Film b> The LCP film b is not particularly limited, and any conventionally known LCP film can be used.

[0043] The dielectric constant of the LCP film b at a frequency of 10 GHz is preferably 4.0 or less, more preferably 3.6 or less. The lower limit of the dielectric constant is not particularly limited, but is, for example, 2.8. The LCP film b having a dielectric constant in the above range can be suitably used for high-frequency substrates.

[0044] The dielectric loss tangent (tanδ) of the LCP film b at a frequency of 10 GHz is preferably 0.008 or less, more preferably 0.002 or less. The lower limit of the dielectric loss tangent is not particularly limited, but is, for example, 0.0003. LCP film b having a dielectric loss tangent in the above range can be suitably used for high-frequency substrates. The relative dielectric constant and dielectric loss tangent of the LCP film b can be measured specifically by the method described in the examples below.

[0045] The linear expansion coefficient of the LCP film b is preferably −10 to 60 ppm / ° C., more preferably 10 to 30 ppm / ° C., from the viewpoint of easily suppressing warping when laminated with a metal layer, etc. The linear expansion coefficient can be measured by thermomechanical analysis (TMA).

[0046] The thickness of the LCP film b is preferably 5 to 200 μm, more preferably 10 to 100 μm, and even more preferably 10 to 50 μm. When the thickness of the LCP film b is within this range, it is difficult to deform due to heat or the like, and a high-frequency substrate with a relatively low coefficient of linear expansion can be easily obtained.

[0047] The LCP is not particularly limited, and examples thereof include wholly aromatic polyesters obtained by homopolymerizing or copolymerizing monomers such as aromatic dicarboxylic acids, aromatic diols, aromatic hydroxycarboxylic acids, aromatic hydroxyamines, aromatic diamines, and aromatic aminocarboxylic acids.

[0048] Specific examples of the LCP include (co)polymers containing structural units represented by the following formulas (a) to (n): Among these, (co)polymers containing either the structural unit represented by formula (a) or (e) are preferred because they are expected to have the effect of improving dielectric properties and flame retardancy.

[0049]

[0050] The LCP film b may contain additives such as a plasticizer, other resins such as epoxy resin, a curing agent, a curing accelerator, an organic filler, an inorganic filler, a coupling agent, and a flame retardant.

[0051] <Configuration of the present substrate> The present substrate is not particularly limited as long as it includes the PTFE film a and the LCP film b, but it is preferable that the PTFE film a and the LCP film b are in contact with each other in terms of better demonstrating the effects of the present invention. A preferred embodiment of the present substrate is a substrate including the PTFE film a, the LCP film b, and the PTFE film a in this order. In this case, the two PTFE films a may be the same film or different films. It is preferable that they are the same film in terms of further reducing deformation of the present substrate, such as twisting and warping.

[0052] It is preferable that the present substrate does not have an adhesive layer between the PTFE film a and the LCP film b, since it is possible to easily obtain a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent. PTFE films are poorly adhesive, so adhesive layers (including bonding sheets) have traditionally been used in laminates using such PTFE films. However, these adhesive layers generally have poor dielectric properties, so it is preferable to not have an adhesive layer in order to obtain a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent.

[0053] In this substrate, the laminate containing the PTFE film a and the LCP film b is usually used as an insulating layer in a conventionally known high-frequency substrate. That is, this substrate may have the same configuration as a conventionally known high-frequency substrate, except that a laminate containing the PTFE film a and the LCP film b is used as the insulating layer of the conventionally known high-frequency substrate.

[0054] This substrate usually has a metal layer on the side of the PTFE film a opposite to the LCP film b. In this case, it is preferable not to have an adhesive layer between the PTFE film a and the metal layer, as this makes it easier to obtain a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent. In this case, it is also preferable that the PTFE film a and the metal layer are in contact with each other, as this will more effectively achieve the effects of the present invention.

[0055] Examples of metals constituting the metal layer include conductive materials such as copper, aluminum, iron, silver, palladium, nickel, chromium, molybdenum, tungsten, and alloys thereof, and among these, copper is preferred.

[0056] The thickness of the metal layer is not particularly limited, and may be the same as that of metal layers in conventionally known high-frequency substrates, for example, 5 to 30 μm.

[0057] The metal layer may be a pre-formed or commercially available metal film (metal foil), or may be a metal layer formed on the PTFE film a by plating or vapor deposition.

[0058] The adhesive strength between the PTFE film a and the metal layer is preferably 3.0 N / cm or more, more preferably 4 N / cm or more, and even more preferably 5 N / cm or more. The upper limit is not particularly limited, but is, for example, 8 N / cm. When the adhesive strength is within this range, a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent can be easily obtained. The adhesive strength is specifically measured by the method described in the examples below. This substrate having an adhesive strength within the above range can be specifically produced by the present manufacturing method described below.

[0059] <<Method for Manufacturing High-Frequency Substrate>> A method for manufacturing a high-frequency substrate according to one embodiment of the present invention (also referred to as "this manufacturing method") includes: Step 1 of forming PTFE film a by heating a PTFE-cut film or a modified PTFE film at a temperature equal to or higher than the melting point of PTFE or modified PTFE and pressing it at a pressure of 0.1 MPa or more; Step 2 of a reduced-pressure plasma treatment which is any one of Steps 2-1 to 2-3 below; and Step 3 of laminating PTFE film a and LCP film b using the film obtained in Step 2. According to this manufacturing method, this substrate can be easily manufactured, and a suitable example of this manufacturing method is the method for manufacturing this substrate.

[0060] In the high-frequency substrate manufactured by this method, the adhesive strength between the PTFE film a and the LCP film b is preferably within the adhesive strength range described in the column for the substrate.Furthermore, the high-frequency substrate manufactured by this method preferably has the relative dielectric constant and dielectric loss tangent described in the column for the substrate.

[0061] <Step 1> Step 1 is a step of forming PTFE film a by heating PTFE-cut film or modified PTFE film at a temperature above the melting point of PTFE or modified PTFE, and pressurizing it at a pressure of 0.1 MPa or more.This step 1 can reduce the voids present in PTFE-cut film and modified PTFE film, specifically, the porosity of PTFE film a obtained in step 1 can be made to be the porosity described in the section of PTFE film a in this substrate, and the surface of PTFE-cut film and modified PTFE film can be made smooth, specifically, the Rz of PTFE film a obtained in step 1 can be made to be the Rz described in the section of PTFE film a in this substrate.

[0062] Examples of the PTFE cut film and modified PTFE film used in step 1 include commercially available PTFE cut films and modified PTFE films.

[0063] The temperature during heating in step 1 may be any temperature at which PTFE or modified PTFE can melt and the surface of the PTFE cutting film and the modified PTFE film can be deformed, and is equal to or higher than the melting point of PTFE or modified PTFE, preferably equal to or higher than the melting point of PTFE or modified PTFE and lower than the decomposition temperature of PTFE or modified PTFE, more preferably 330 to 390°C, and even more preferably 340 to 360°C.

[0064] The pressure applied in step 1 may be any pressure that can deform the surfaces of the PTFE cut film and modified PTFE film used in step 1, and is 0.1 MPa or more, preferably 0.5 to 20 MPa, and more preferably 1 to 10 MPa.

[0065] The time for heating and pressing in step 1 is not particularly limited as long as it is possible to deform the surfaces of the PTFE cut film and modified PTFE film used in step 1. Although it depends on the pressure and the like, the time is preferably 10 seconds to 20 minutes, more preferably 30 seconds to 10 minutes.

[0066] The heating and pressing in the step 1 may be carried out using a conventionally known heating and pressing machine. By using a roll press (including a belt press) or the like as the heating and pressing machine, the desired PTFE film a can be continuously mass-produced.

[0067] The PTFE film a formed in step 1 preferably has the porosity, Rz, relative dielectric constant, dielectric loss tangent, and thickness described in the section for the PTFE film a in the present substrate.

[0068] <Step 2> The reduced pressure plasma treatment step 2 is any one of the following steps 2-1 to 2-3. Step 2-1: performing reduced pressure plasma treatment on at least the surface of the PTFE film a obtained in step 1 facing the LCP film b. Step 2-2: performing reduced pressure plasma treatment on at least the surface of the LCP film b facing the PTFE film a. Step 2-3: performing reduced pressure plasma treatment on at least the surface of the PTFE film a obtained in step 1 facing the LCP film b, and performing reduced pressure plasma treatment on at least the surface of the LCP film b facing the PTFE film a.

[0069] The step 2-1 is preferably a step of performing a reduced pressure plasma treatment on at least both main surfaces of the PTFE film a obtained in the step 1, the step 2-2 is preferably a step of performing a reduced pressure plasma treatment on at least both main surfaces of the LCP film b, and the step 2-3 is preferably a step of performing a reduced pressure plasma treatment on at least both main surfaces of the PTFE film a obtained in the step 1 and performing a reduced pressure plasma treatment on at least both main surfaces of the LCP film b. Of these, the step 2 is preferably the step 2-3 from the viewpoint that a high-frequency substrate having excellent adhesive strength between layers can be easily obtained, and more preferably a step of performing a reduced pressure plasma treatment on both main surfaces of the PTFE film a obtained in the step 1 and performing a reduced pressure plasma treatment on both main surfaces of the LCP film b.

[0070] The step 2 is preferably a dry process that does not produce waste liquid, which is simpler and less costly. Furthermore, by passing through the step 2, a high-frequency substrate having excellent adhesive strength between films, a low dielectric constant, and a low dielectric loss tangent can be produced through the step 3 while maintaining the porosity and Rz of the PTFE film a obtained in the step 1.

[0071] The LCP film b used in the step 2 may be the same as the LCP film b in the main substrate.

[0072] The low-pressure plasma treatment is preferably carried out in the presence of a source gas containing an amino group, since this allows for the production of a high-frequency substrate with superior adhesive strength between layers. When the low-pressure plasma treatment is carried out in the presence of a source gas containing an amino group, a portion of the surface of the treated film (the PTFE film a and / or the LCP film b obtained in step 1) is substituted with an amino group, which is thought to introduce hydrophilic functional groups into the treated film surface. This is thought to improve the adhesiveness of the treated film to other layers (e.g., the PTFE film a, the LCP film b, and the metal layer).

[0073] Examples of the source gas containing amino groups include ammonia and gaseous hydrazine. These source gases are preferred because they can be ionized in plasma discharge to supply highly reactive amino groups to the surface of the film to be treated. Among these source gases, ammonia is preferred from the standpoint of ease of handling. The source gas containing amino groups may also be a mixed gas of an amino group-containing gas with a rare gas or an inert gas (e.g., He, Ne, Ar, Kr, Xe, N2). The source gas containing amino groups is preferably used at a flow rate of 1 sccm (standard cm 3 It is preferable that the gas is introduced into the reduced pressure plasma treatment apparatus at a flow rate of at least 1000 kJ / min.

[0074] The reduced pressure in the reduced-pressure plasma treatment can be, for example, 1 to 100,000 Pa, and preferably 1 to 1,000 Pa. By performing the plasma treatment under the reduced pressure, the surface treatment of the film to be treated can be carried out more sufficiently.

[0075] The low-pressure plasma treatment is preferably a high-frequency low-pressure plasma treatment. The conditions for the high-frequency low-pressure plasma treatment include a high-frequency wave of 1 to 100 MHz and an intensity of 0.01 to 3 W / cm. 2 By performing treatment under such conditions, the surface of the film to be treated can be treated more sufficiently.

[0076] The low-pressure plasma treatment can be carried out using a conventionally known apparatus, for example, the apparatus described in Patent Document 1.

[0077] In the step 2, when the PTFE film a obtained in the step 1 is subjected to reduced pressure plasma treatment, the PTFE film a obtained in the step 2 preferably has the porosity, Rz, relative dielectric constant, dielectric loss tangent and thickness described in the column for the PTFE film a in the present substrate.

[0078] <Step 3> Step 3 is a step of laminating a PTFE film a and an LCP film b using the film obtained in step 2.

[0079] A method of overlapping the PTFE film a and the LCP film b and applying heat and pressure is preferred as the lamination method in step 3. An adhesive may be used during lamination in step 3, but it is preferable not to use an adhesive, since a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent can be easily obtained.

[0080] The heating temperature in step 3 is preferably a temperature lower than the melting points of PTFE, modified PTFE, and LCP, more preferably 100 to 300°C, and even more preferably 150 to 280°C, from the viewpoint that a high-frequency substrate having a lower dielectric constant and a lower dielectric dissipation factor can be easily obtained.

[0081] The pressure applied during pressurization in step 3 is preferably 1 to 50 MPa, more preferably 5 to 40 MPa. From the viewpoint that high frequency substrates with excellent adhesive strength can be mass-produced using a conventionally known device that does not require a special heating and pressurizing device, the pressure applied during pressurization in step 3 is preferably 1 to 25 MPa, more preferably 2 to 15 MPa. According to this manufacturing method, since step 2 has been performed, high frequency substrates with excellent adhesive strength can be manufactured at the above heating temperature and pressure.

[0082] The time for heating and pressurizing in step 3 is not particularly limited, and although it depends on the pressure and other factors, it is preferably 10 seconds to 30 minutes, more preferably 1 to 20 minutes.

[0083] The heating and pressing in step 3 may be performed using a conventionally known heating and pressing machine. By using a roll press (including a belt press) or the like as the heating and pressing machine, high frequency substrates can be continuously mass-produced.

[0084] <Other Steps> The present manufacturing method may further include other steps that have been performed in conventional methods for manufacturing high-frequency substrates, in addition to the steps 1 to 3. Examples of such other steps include step 4 of laminating a metal layer on the side of the PTFE film a opposite to the LCP film b in the high-frequency substrate obtained in step 3.

[0085] Examples of the lamination method in step 4 include (i) a method in which a pre-formed or commercially available metal film (metal foil) is superimposed on the PTFE film a in the high-frequency substrate obtained in step 3, and then heated and pressed, and (ii) a method in which a metal layer is directly formed by plating or vapor deposition on the PTFE film a in the high-frequency substrate obtained in step 3. An adhesive may be used in the lamination in step 4, but it is preferable not to use an adhesive because a high-frequency substrate with a lower dielectric constant and a lower dielectric loss tangent can be easily obtained.

[0086] In both methods (i) and (ii), it is preferable that the surface of the PTFE film a on which the metal layer is formed is the surface that has undergone the reduced-pressure plasma treatment step, from the viewpoint of easily obtaining a high-frequency substrate having excellent adhesive strength between the PTFE film a and the metal layer. In steps 2-1 and 2-3, when reduced-pressure plasma treatment is performed on both main surfaces of the PTFE film a obtained in step 1, method (i) or (ii) may be performed directly on the high-frequency substrate obtained in step 3. When step 2-2 is performed as step 2, or when reduced-pressure plasma treatment is performed on one main surface of the PTFE film a obtained in step 1 in steps 2-1 and 2-3, it is preferable to perform method (i) or (ii) on the PTFE film a surface of the high-frequency substrate obtained in step 3 after performing a reduced-pressure plasma treatment similar to step 2. In the steps 2-1 and 2-3, even when both main surfaces of the PTFE film a obtained in the step 1 are subjected to reduced pressure plasma treatment, the PTFE film a surface of the high-frequency substrate obtained in the step 3 may be subjected to a reduced pressure plasma treatment step similar to the step 2, and then the method (i) or (ii) may be carried out.

[0087] The conditions for heating and pressing in the method (i) include the same conditions for heating and pressing as in the step 3. The plating or vapor deposition in the method (ii) may be carried out by a conventionally known method.

[0088] In the case of the method (i), it is preferable to surface-treat at least the surface of the metal layer facing the PTFE film a, from the viewpoint of easily obtaining a high-frequency substrate having a higher adhesive strength between the PTFE film a and the metal layer, etc. The surface treatment is not particularly limited, but a treatment similar to the low-pressure plasma treatment in the above-mentioned step 2 is preferable, from the viewpoint of easily obtaining a high-frequency substrate having an excellent adhesive strength between the PTFE film a and the metal layer, a low dielectric constant, and a low dielectric loss tangent, etc.

[0089] In the high-frequency substrate manufactured through step 4, the adhesive strength between the PTFE film a and the metal layer is preferably in the same range as the adhesive strength between the PTFE film a and the metal layer described in the section on the configuration of the present substrate, etc. The type and thickness of the metal layer in the high-frequency substrate manufactured through step 4 are the same as the type and thickness of the metal layer described in the section on the configuration of the present substrate, etc.

[0090] Next, one embodiment of the present invention will be described in more detail by showing examples, but the present invention is not limited to these.

[0091] [Preparation Example 1] Preparation of PTFE film a-1 (100 μm) A PTFE cutting film (Valqua Corporation, VALFLON cutting tape, length: 30 cm, width: 25 cm, thickness: 100 μm) was heated and pressed using a belt press machine manufactured by SGIC Corporation under conditions of a press temperature of 350 ° C, a press pressure of 1 MPa, and a conveying speed of 0.5 m / min, to produce a PTFE film a-1 (100 μm) having a thickness of 100 μm. The appearances of the PTFE cutting film used and the produced PTFE film a-1 (100 μm) were photographed using a digital camera. The photograph obtained is shown in FIG. 1.

[0092] [Preparation example 2] Preparation of PTFE film a-1 (10 μm, 25 μm, 50 μm) Except that the PTFE cutting film used is 10 μm, 25 μm or 50 μm thick film, prepare PTFE film a-1 (10 μm), PTFE film a-1 (25 μm) and PTFE film a-1 (50 μm) respectively in the same manner as preparation example 1.

[0093] [Preparation example 3] Preparation of (modified) PTFE film a-2 (100 μm) In preparation example 1, except that in preparation example 1, use modified PTFE film (manufactured by Valqua Corporation, New Valflon cutting tape, length: 30 cm, width: 25 cm, thickness: 100 μm), prepare (modified) PTFE film a-2 (100 μm) in the same manner as preparation example 1.

[0094] [Preparation example 4] Preparation of (modified) PTFE film a-2 (25 μm, 50 μm) Except that the modified PTFE film used is the film with thickness of 25 μm or 50 μm, in the same manner as preparation example 3, prepare (modified) PTFE film a-2 (25 μm) and (modified) PTFE film a-2 (50 μm), respectively.

[0095] [Preparation Example 5] Preparation of LCP film b-1 LCP film b-1 (film obtained by removing the PI film from the laminate, thickness: 50 μm) was prepared in the same manner as the preparation method of LCP film A3 described in the examples of WO 2022 / 131045.

[0096] <Arithmetic mean roughness Ra and maximum height roughness Rz> The arithmetic mean roughness Ra and maximum height roughness Rz of the PTFE-cut film used in Preparation Example 1, the PTFE film a-1 (100 μm) produced in Preparation Example 1, the modified PTFE film used in Preparation Example 3, and the (modified) PTFE film a-2 (100 μm) produced in Preparation Example 3 were measured using a laser microscope (manufactured by Keyence Corporation). The results are shown in Table 1. The PTFE-cut film has a rake face (the surface that the blade contacts when cutting the PTFE burette) and a flank face (the surface that the blade does not contact when cutting the PTFE burette). However, since the Ra and Rz were almost the same between the rake face and the flank face, Table 1 shows the Ra and Rz of the rake face. For the PTFE film a-1 (100 μm), the Ra and Rz of the surface corresponding to the rake face of the PTFE cutting film are also shown in Table 1.

[0097] The Ra and Rz of each PTFE film a-1 produced in Preparation Example 2 are considered to be similar to the Ra and Rz of the PTFE film a-1 produced in Preparation Example 1, and the Ra and Rz of each (modified) PTFE film a-2 produced in Preparation Example 4 are considered to be similar to the Ra and Rz of the (modified) PTFE film a-2 produced in Preparation Example 3.

[0098] <Porosity> Using a cryo-cross-section polisher manufactured by JEOL Ltd., cross-sectional samples for cross-sectional SEM image observation were prepared from the PTFE cut film used in Preparation Example 1, the PTFE film a-1 (100 μm) prepared in Preparation Example 1, the modified PTFE film used in Preparation Example 3, and the (modified) PTFE film a-2 (100 μm) prepared in Preparation Example 3. FE-SEM (manufactured by JEOL Ltd.) was used to obtain SEM images of each prepared cross-sectional sample. The SEM image of the PTFE cut film used in Preparation Example 1 is shown in FIG. 2, and the SEM image of the PTFE film a-1 (100 μm) prepared in Preparation Example 1 is shown in FIG. 3. In addition, Figure 2 is an SEM image of the surface and the area 5 μm deep from the surface when using the PTFE cutting film used in Preparation Example 1, and Figure 3 is an SEM image of the surface and the area 5 μm deep from the surface when using the PTFE film a-1 (100 μm) produced in Preparation Example 1.

[0099] Each of the obtained SEM images was binarized using image processing software Image J, and the void area in the range from the surface to a depth of 5 μm from the surface was determined to be 0.001 μm. 2 The area content of the above voids (porosity) was calculated. The results are shown in Table 1. When SEM images were checked, when the PTFE cutting film and PTFE film a-1 (100 μm) were used, there was a difference in the SEM images between the surface and a depth of 5 μm from the surface, and there was no significant difference in the SEM images at depths of more than 5 μm from the surface, so the porosity was calculated between the surface and a depth of 5 μm from the surface.

[0100] The porosity of each PTFE film a-1 produced in Preparation Example 2 is considered to be the same as the porosity of the PTFE film a-1 produced in Preparation Example 1, and the porosity of each (modified) PTFE film a-2 produced in Preparation Example 4 is considered to be the same as the porosity of the (modified) PTFE film a-2 produced in Preparation Example 3.

[0101]

[0102] On the PTFE cutting film, marks due to cutting were observed, and voids thought to be due to cutting were observed on the surface and within a range of 5 μm deep from the surface. On the other hand, as shown in Figures 1 to 3 and Table 1, the PTFE film a-1 was a film with a smooth surface, with no marks or voids. Similarly, the (modified) PTFE film a-2 was a film with a smooth surface and reduced voids within a range of the surface and 5 μm deep from the surface compared to the modified PTFE film.

[0103] <Dielectric breakdown strength> Using an AC withstand voltage tester (manufactured by Shintoyo Kiki Co., Ltd.), a voltage was applied at a rate of 0 V to 1000 V / sec to each of the PTFE cut film used in Preparation Example 1 and the PTFE film a-1 (100 μm) prepared in Preparation Example 1 at room temperature (25 ° C.), and the dielectric breakdown voltage was measured. The measured dielectric breakdown voltage was divided by the thickness (100 μm) of each film to obtain the dielectric breakdown strength (kV / mm). The dielectric breakdown strength was calculated for 10 sheets of each film, and the average value and standard deviation are shown in Table 2.

[0104]

[0105] The PTFE film a-1 had a higher dielectric breakdown strength and a reduced variation in dielectric breakdown strength compared to the PTFE cut film. Since dielectric breakdown is thought to occur from the processing marks and voids, the improvement in dielectric breakdown strength and the reduction in variation are thought to be due to the reduction in the processing marks and voids.

[0106] <Tensile Strength and Elongation> Test specimens were prepared based on ASTM D1708 from the PTFE cut film used in Preparation Example 1 and the PTFE film a-1 (100 μm) prepared in Preparation Example 1. Test specimens were prepared in the length direction and width direction for each film. Using a tabletop tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the prepared test specimens were pulled at a pulling speed of 50 mm / min, and the tensile strength and elongation were measured. The results are shown in Table 3.

[0107]

[0108] The PTFE film a-1 had higher tensile strength and elongation values ​​than the PTFE cut film. Note that the PTFE cut film had lower tensile strength and elongation values ​​due to the presence of cutting marks in the width direction, but the PTFE film a-1 had fewer cutting marks and voids, reducing the anisotropy in the length and width directions of the film.

[0109] <Dielectric constant and dielectric loss tangent> The dielectric constant and dielectric loss tangent of the PTFE cutting film (50 μm) used in Preparation Example 2, the PTFE film a-1 (50 μm) produced in Preparation Example 2, the modified PTFE film (50 μm) used in Preparation Example 4, the (modified) PTFE film a-2 (50 μm) produced in Preparation Example 4, and the LCP film b-1 produced in Preparation Example 5 were measured as follows. Using a PNA-X network analyzer N5245B (manufactured by keysight technologies), a split cylinder resonator 85072A (manufactured by keysight technologies), and material measurement software N1500A (manufactured by keysight technologies), the relative permittivity and dielectric loss tangent were measured in accordance with IPC-TM-650 2.5.5.13 under the conditions of a frequency of 10 GHz, a temperature of 23°C ± 2°C, and a humidity of 50 ± 10% RH. The results are shown in Table 4.

[0110]

[0111] [Example 1] The PTFE film a-1 (50 μm) produced in Production Example 2 and the LCP film b-1 produced in Production Example 5 were placed in the chamber of a high-frequency low-pressure plasma device (a parallel-plate low-pressure plasma device manufactured by Denki Giken Co., Ltd.), and low-pressure plasma treatment was carried out under the following conditions: (Conditions) Treatment gas type: NH3 Gas flow rate: 150 sccm Treatment pressure: 50 Pa Frequency: 13.56 MHz Treatment power: 750 W (0.19 W / cm 2 ) Processing time: 5 minutes Distance between electrodes: 80 mm

[0112] The PTFE film a-1 after the reduced pressure plasma treatment and the LCP film b-1 after the reduced pressure plasma treatment were placed in a heat press machine and heat pressed for 10 minutes under conditions of a temperature of 200°C or 250°C and a pressure of 10 MPa, 20 MPa or 35 MPa, to produce a high-frequency substrate.

[0113] [Example 2] In Example 1, instead of using the PTFE film a-1 (50 μm) produced in Preparation Example 2, use the (modified) PTFE film a-2 (50 μm) produced in Preparation Example 4, except that in the same manner as in Example 1, a high-frequency substrate was produced.

[0114] [Reference Example 1] In Example 1, instead of using the PTFE film a-1 (50 μm) that is produced in Preparation Example 2, use the PTFE cutting film that is used in Preparation Example 2 and has a thickness of 50 μm, and so on, in the same manner as in Example 1, to produce a high-frequency substrate.

[0115] <Adhesive Strength> Using a tabletop tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the adhesive surfaces between the PTFE film and LCP film of the high-frequency substrates produced in Examples 1 and 2 and Reference Example 1 were torn in the adhesive direction (a tensile load was applied in the direction perpendicular to the adhesive surface) at a speed of 50 mm / sec, and the adhesive strength (N / cm) was measured. The results are shown in Table 5. Note that "-" in Table 5 indicates that no measurement was performed under the conditions in that column.

[0116]

[0117] By using PTFE film a-1, the adhesive strength with the LCP film was increased compared to when a PTFE-cut film was used. Because the voids exist in the PTFE-cut film, the PTFE-cut film itself is torn at the voids when measuring the adhesive strength, so it is thought that when a PTFE-cut film is used, the adhesive strength will not be 3.0 N / cm or more. On the other hand, it is thought that the voids in the PTFE film a-1 are reduced, resulting in a high-frequency substrate with high adhesive strength between the PTFE film and the LCP film (3.0 N / cm or more). Similarly, when (modified) PTFE film a-2 is used, the adhesive strength with the LCP film is thought to be higher compared to when a modified PTFE film (commercially available) is used. By using the reduced-pressure plasma treatment, it was possible to bond a PTFE film and an LCP film with high adhesive strength without using an adhesive, and even when the heat-pressing temperature during bonding was lower than their melting points and the heat-pressing pressure during bonding was 10 to 20 MPa, the PTFE film and the LCP film could still be bonded with high adhesive strength. Therefore, there is no need to use a machine that can apply a high pressure of 35 MPa when bonding the PTFE film and the LCP film, and a high-frequency substrate can be manufactured using a heat-pressing machine that can be mass-produced, such as a roll.

[0118] The high-frequency substrate obtained in Example 2 was heat-pressed using a cryo-cross-section polisher manufactured by JEOL Ltd. under conditions of a temperature of 250°C and a pressure of 20 MPa, and then cut in the thickness direction. An SEM image (5000x magnification) of the cross section was obtained using an FE-SEM (manufactured by JEOL Ltd.). The SEM image is shown in Figure 4. The obtained SEM image was binarized using Image J, an image processing software, and it was found that the void area in the range from the surface of the (modified) PTFE film a-2 to a depth of 5 µm from the surface was 0.001 µm. 2The area content of the voids (porosity) was calculated to be 0.018%. Furthermore, as shown in Figure 4, the maximum height roughness Rz of the surface of the (modified) PTFE film a-2 was 1 µm or less. In other words, it was confirmed that the porosity and Rz of the (modified) PTFE film a-2 before lamination could be maintained even after lamination of the (modified) PTFE film a-2 and the LCP film b-1.

[0119] Example 3 In the same manner as in Example 1, the PTFE film a-1 (50 μm) produced in Preparation Example 2 and the LCP film b-1 produced in Preparation Example 5 were subjected to a reduced pressure plasma treatment.

[0120] Furthermore, rolled copper foil (manufactured by Hitachi Metals Neomaterial, Ltd., thickness: 18 μm) was placed in the chamber of a high-frequency low-pressure plasma device (manufactured by Denshi Giken Co., Ltd., parallel-plate low-pressure plasma device) and subjected to low-pressure plasma treatment under the following conditions: (Conditions) Treatment gas type: NH3 Gas flow rate: 150 sccm Treatment pressure: 50 Pa Frequency: 13.56 MHz Treatment power: 500 W (0.13 W / cm 2 ) Processing time: 5 minutes

[0121] The rolled copper foil after reduced pressure plasma treatment, the PTFE film a-1 after reduced pressure plasma treatment, and the LCP film b-1 after reduced pressure plasma treatment were placed in this order in a heat press machine, and heat pressed for 10 minutes under conditions of a temperature of 200°C and 35 MPa or 250°C and 10 MPa to produce a high-frequency substrate.

[0122] [Example 4] In Example 3, instead of using the PTFE film a-1 (50 μm) produced in Preparation Example 2, use the (modified) PTFE film a-2 (50 μm) produced in Preparation Example 4, except that in Example 3, a high-frequency substrate was produced in the same manner as in Example 3.

[0123] <Adhesive Strength> Using a tabletop tensile tester (EZ-SX, manufactured by Shimadzu Corporation), the adhesive surfaces between the PTFE film and copper foil of the high-frequency substrates produced in Examples 3 and 4 were torn in the adhesive direction (a tensile load was applied in the direction perpendicular to the adhesive surface) at a speed of 50 mm / sec, and the adhesive strength (N / cm) was measured. The results are shown in Table 6. Note that "-" in Table 6 indicates that measurements were not made under the conditions in that column.

[0124]

[0125] [Example 5] In Example 1, instead of using the PTFE film a-1 (50 μm) produced in Preparation Example 2, use the PTFE film a-1 (25 μm) produced in Preparation Example 2, except that in Example 1, a high-frequency substrate was produced in the same manner as in Example 1.

[0126] [Example 6] In Example 1, instead of using the PTFE film a-1 (50 μm) produced in Preparation Example 2, use the PTFE film a-1 (10 μm) produced in Preparation Example 2, except that in Example 1, a high-frequency substrate was produced in the same manner as in Example 1.

[0127] [Example 7] In Example 1, instead of using the PTFE film a-1 (50 μm) produced in Preparation Example 2, use the (modified) PTFE film a-1 (25 μm) produced in Preparation Example 4, in the same manner as in Example 1, to produce a high-frequency substrate.

[0128] Reference Example 2 A high-frequency substrate was produced in the same manner as in Example 7, except that, instead of using the LCP film b-1 (50 μm) produced in Production Example 5, a polyimide (PI) film, Upilex-50S (manufactured by UBE Corporation, thickness: 50 μm), was used.

[0129] <Relative permittivity and dielectric dissipation factor> The relative permittivity and dielectric dissipation factor of the high-frequency substrates produced in Examples 1 and 2, and Examples 5 to 7, and the high-frequency substrate produced in Reference Example 2 were measured as follows. Using a PNA-X network analyzer N5245B (manufactured by keysight technologies), a split cylinder resonator 85072A (manufactured by keysight technologies), and material measurement software N1500A (manufactured by keysight technologies), the relative permittivity and dielectric dissipation factor were measured in accordance with IPC-TM-650 2.5.5.13 under conditions of a frequency of 10 GHz, a temperature of 23°C ± 2°C, and a humidity of 50 ± 10% RH. The results are shown in Table 7.

[0130]

[0131] The high-frequency substrate according to one embodiment of the present invention (Example) was a flexible substrate with a low dielectric constant and a low dielectric loss tangent. For example, the high-frequency substrate obtained in Example 1 had low dielectric constant and low dielectric loss tangent values ​​that are not achieved by currently commercialized high-frequency substrates that meet UL standards. Furthermore, since the high-frequency substrate according to one embodiment of the present invention (Example) includes a PTFE film and an LCP film, it is believed to have a lower linear expansion coefficient than a substrate made of a PTFE film. Furthermore, the dielectric properties can be adjusted by adjusting the thickness of each film that makes up the high-frequency substrate.

[0132] Example 8 In the same manner as in Example 1, two PTFE films a-1 (10 μm) prepared in Preparation Example 2 and one LCP film b-1 prepared in Preparation Example 5 were subjected to reduced pressure plasma treatment. Similarly to Example 3, two rolled copper foils (manufactured by Hitachi Metals Neomaterial, Ltd., thickness: 18 μm) were subjected to reduced pressure plasma treatment. The rolled copper foil after reduced pressure plasma treatment, the PTFE film a-1 after reduced pressure plasma treatment, the LCP film b-1 after reduced pressure plasma treatment, the PTFE film a-1 after reduced pressure plasma treatment, and the rolled copper foil after reduced pressure plasma treatment were placed in a heat press in this order, and heat pressed at a temperature of 250°C and a pressure of 20 MPa for 10 minutes to produce a high-frequency substrate. The produced high-frequency substrate was cut in the thickness direction using a cryo-cross-section polisher manufactured by JEOL Ltd., and a cross-sectional SEM image (700x magnification) was obtained using an FE-SEM (manufactured by JEOL Ltd.). The SEM image is shown in FIG.

[0133] As shown in Figure 5, a high-frequency substrate was obtained in which there was no lifting or peeling at the interface of each film and the interface of each film was smooth. It is believed that the smooth interface of each film allows the high-frequency substrate to have a lower dielectric constant and a lower dielectric loss tangent.

Claims

1. A high-frequency substrate comprising: a PTFE film a, which is a polytetrafluoroethylene film that satisfies the following requirements (I) and (II); and an LCP film b, which is a liquid crystal polymer film, and which satisfies the following requirement (X): Requirement (I): the porosity calculated from a cross-sectional SEM image of the film in the range from the surface to a depth of 5 μm from the surface is 0.1% or less; Requirement (II): the maximum height roughness Rz is 1 μm or less; and Requirement (X): the adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more.

2. The high frequency board according to claim 1, which has no adhesive layer between the PTFE film a and the LCP film b.

3. The high frequency board according to claim 1, wherein the thickness of said PTFE film a is 6 to 200 μm.

4. The high frequency board according to claim 1, wherein the thickness of said LCP film b is 5 to 200 μm.

5. The high frequency board according to claim 1, wherein the PTFE film a is a modified PTFE film.

6. The high-frequency board according to any one of claims 1 to 5, further comprising a metal layer on the side of said PTFE film a opposite to said LCP film b.

7. The high frequency board according to claim 6, wherein the adhesive strength between said PTFE film a and said metal layer is 3.0 N / cm or more.

8. A method for producing a high frequency substrate, comprising: step 1 of forming a PTFE film a by heating a PTFE-cut film or a modified PTFE film at a temperature equal to or higher than the melting point of PTFE or modified PTFE and applying a pressure of 0.1 MPa or more; step 2 of performing a reduced pressure plasma treatment which is any one of steps 2-1 to 2-3 below; and step 3 of laminating the PTFE film a and an LCP film b using the film obtained in step 2. Step 2-1: performing a reduced pressure plasma treatment on at least the surface of the PTFE film a obtained in step 1 facing the LCP film b; step 2-2: performing a reduced pressure plasma treatment on at least the surface of the LCP film b facing the PTFE film a; and step 2-3: performing a reduced pressure plasma treatment on at least the surface of the PTFE film a obtained in step 1 facing the LCP film b, and performing a reduced pressure plasma treatment on at least the surface of the LCP film b facing the PTFE film a.

9. The method for producing a high-frequency board according to claim 8, wherein the PTFE film a formed in step 1 satisfies the following requirements (I) and (II): Requirement (I): the porosity calculated from a cross-sectional SEM image of the film in the range from the surface to a depth of 5 μm from the surface is 0.1% or less; Requirement (II): the maximum height roughness Rz is 1 μm or less.

10. The method for producing a high-frequency substrate according to claim 8, wherein the high-frequency substrate satisfies the following requirement (X): the adhesive strength between the PTFE film a and the LCP film b is 3.0 N / cm or more.

11. The method for producing a high frequency board according to claim 8, wherein no adhesive is used during lamination in step 3.

12. The method for producing a high-frequency substrate according to claim 8, wherein the low-pressure plasma treatment carried out in step 2 is a low-pressure plasma treatment using plasma generated under a reduced pressure of 1 to 100,000 Pa in the presence of a material gas containing an amino group.

13. A method for producing a high-frequency board according to any one of claims 8 to 12, further comprising step 4 of laminating a metal layer on the side of the PTFE film a opposite to the LCP film b in the high-frequency board obtained in step 3.

Images