Fluororesin sheet, copper-clad laminate, circuit substrate, and antenna

Abstract

Provided are: a fluororesin sheet having both a low linear expansion coefficient (CTE) and improved adhesion to a copper foil having a smooth surface; and a copper-clad laminate using the fluororesin sheet. A fluororesin sheet containing a fluororesin and silica particles, the fluororesin sheet having an oxygen element ratio of 3.0 at% or more and a linear expansion coefficient (CTE) of 100 ppm / DEG C or less when measured by X-ray photoelectron spectroscopy (XPS) on one or both surfaces thereof.

Description

Technical Field

[0001] The present disclosure relates to a fluororesin sheet, a copper-clad laminate, a circuit substrate, and an antenna. Background Art

[0002] High-frequency printed wiring boards are required to have low transmission loss. It is known to use a fluororesin film in such high-frequency printed wiring boards (Patent Document 1, etc.).

[0003] In such a printed wiring board, the adhesion to the copper foil is improved by subjecting the fluororesin film to a surface treatment.

[0004] Patent Document 2 proposes that the adhesion to copper foil be improved by subjecting a fluororesin film to surface treatment and annealing.

[0005] Patent Document 3 proposes that the surface of a fluororesin substrate be hydrophilized to have amino groups and hydroxyl groups, thereby improving adhesion to a metal foil.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-8260

[0009] Patent Document 2: Japanese Patent Application Publication No. 2022-112028

[0010] Patent Document 3: Japanese Patent Application Publication No. 2022-114351 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] An object of the present disclosure is to obtain a fluororesin sheet that achieves both a low coefficient of linear expansion (CTE) and improved adhesion to a copper foil having a smooth surface, and a copper-clad laminate using the same.

[0013] Means for solving problems

[0014] The present disclosure relates to a fluororesin sheet comprising a fluororesin and silica particles, wherein on one or both sides of the fluororesin sheet, the oxygen element ratio is 3.0 atomic % or more and the coefficient of linear expansion (CTE) is 100 ppm / °C or less as measured by X-ray photoelectron spectroscopy (XPS).

[0015] The fluororesin is preferably polytetrafluoroethylene.

[0016] Furthermore, it is preferable that the fluororesin sheet has a nitrogen element ratio of 1.35 atomic % or more when measured by XPS on the same surface.

[0017] Furthermore, it is preferable that the fluororesin sheet has a silicon element ratio of 0.5 atomic % or more when measured by XPS on the same surface.

[0018] Furthermore, it is preferable that the fluororesin sheet be one in which a 2 μL droplet is placed on the same surface and a static contact angle of water measured 1 second after the droplet lands is 105° or less.

[0019] The silicon dioxide particles are preferably spherical silicon dioxide.

[0020] As the silica particles, silica particles treated with a silane coupling agent are preferably used.

[0021] The average particle size of the silica particles is preferably 10 μm or less.

[0022] The fluororesin sheet of the present disclosure preferably does not contain glass fibers.

[0023] The content of the silica particles relative to the total amount of the fluororesin sheet is preferably 30% by mass or more.

[0024] The content of the silica particles relative to the total amount of the fluororesin sheet is preferably 50% by mass or more.

[0025] The content of the silica particles is preferably 50% by mass or more and 65% by mass or less based on the total amount of the fluororesin sheet.

[0026] The fluororesin sheet preferably has a dielectric loss tangent value of 0.0015 or less at 10 GHz.

[0027] The thickness of the fluororesin sheet is preferably 5 μm to 250 μm.

[0028] The present invention also relates to a method for producing the fluororesin sheet, characterized in that fluororesin particles and silica particles are mixed to form a film, and then surface treated.

[0029] Furthermore, a method for producing a fluororesin sheet is preferred, in which a film is formed using a composition consisting essentially of fluororesin particles and filler particles containing at least silica particles, and the film is then surface-treated.

[0030] The present disclosure also relates to a copper-clad laminate having copper foil and the fluororesin sheet as essential layers.

[0031] The surface roughness (Rz) of the copper foil is preferably 2.0 μm or less.

[0032] The surface roughness (Rq) of the copper foil is preferably 0.01 to 0.15 μm.

[0033] Preferably, the copper foil and the fluororesin sheet are directly laminated, and the peel strength at the interface between the copper foil having a surface roughness (Rz) of 2.0 μm or less and the fluororesin sheet is preferably 0.5 kN / m or more.

[0034] The present disclosure also relates to a method for manufacturing the copper-clad laminate, characterized in that the fluororesin sheet and copper foil are laminated, heated at 180° C. to 390° C., and press-molded at a pressure of 0.5 MPa to 5 MPa under vacuum or in an inert gas atmosphere.

[0035] The present disclosure also relates to a circuit substrate having the copper-clad laminate.

[0036] The present disclosure also relates to an antenna formed using the circuit substrate.

[0037] The antenna is preferably a millimeter wave antenna facing the moving object.

[0038] Effects of the Invention

[0039] The fluororesin sheet disclosed herein has both a low coefficient of linear expansion (CTE) and improved adhesion to a copper foil having a smooth surface. DETAILED DESCRIPTION

[0040] Hereinafter, the present disclosure will be described in detail.

[0041] In high-frequency substrates of 5G and above, a smooth interface between the fluororesin sheet and the copper foil reduces transmission loss and improves the characteristics of the high-frequency substrate. Therefore, good adhesion between the fluororesin sheet and the smooth surface of the copper foil is required.

[0042] However, sheets composed solely of fluororesin have a high coefficient of linear expansion, which can sometimes lead to substrate warping and circuit defects. Furthermore, even with surface treatment, it is difficult to form oxygen-containing functional groups in sheets composed solely of polytetrafluoroethylene (PTFE) resin. Consequently, sufficient adhesion to smooth copper foil cannot be achieved, and further improvements are required. This problem arises because it is difficult to generate functional groups on the surface of polytetrafluoroethylene resin, and the generated functional groups are prone to molecular motion, so the functional groups migrate from the surface to the interior of the resin, making it difficult to expose them on the surface. This makes it difficult to achieve the effects of surface treatment.

[0043] The present disclosure has found that by subjecting the surface of a fluororesin sheet containing fluororesin and silica particles to a surface treatment such as plasma treatment, the oxygen element ratio on the surface of the fluororesin sheet is increased, thereby improving adhesion between the fluororesin sheet and a copper foil having a smooth surface and an Rz of 2.0 μm or less.

[0044] The present disclosure incorporates silica particles, allowing the surface treatment of the fluororesin sheet to impart functional groups derived from the surface treatment to the silica surface, thereby increasing the oxygen atomic ratio on the fluororesin sheet surface. This improves the adhesion between the fluororesin sheet and the copper foil, further enhancing the peel strength of these bonded surfaces. Furthermore, the present disclosure further reduces the linear expansion coefficient of the fluororesin sheet by using a composite of fluororesin and silica particles.

[0045] As described above, the fluororesin sheet of the present disclosure can achieve both a low linear expansion coefficient and good peel strength.

[0046] The present invention discloses a fluororesin sheet comprising a fluororesin and silica particles, wherein the sheet has an oxygen ratio of 3.0 atomic percent or greater and a coefficient of linear expansion (CTE) of 100 ppm / °C or less, as measured by X-ray photoelectron spectroscopy (XPS). As described above, by forming a fluororesin sheet comprising a fluororesin and silica particles, the oxygen ratio on the surface of the fluororesin sheet can be increased, thereby enabling good adhesion to smooth copper foil. Furthermore, the low coefficient of linear expansion (CTE) effectively suppresses substrate warping and circuit defects.

[0047] The fluororesin sheet disclosed herein has an oxygen ratio of 3.0 atomic % or greater on one or both sides, as measured by X-ray photoelectron spectroscopy (XPS). In particular, the oxygen ratio on the surface of the sheet bonded to copper foil, as measured by X-ray photoelectron spectroscopy (XPS), should be 3.0 atomic % or greater. By achieving an oxygen ratio of 3.0 atomic % or greater, the sheet adheres to the surface of the copper foil, improving the peel strength relative to the copper foil.

[0048] The oxygen atomic ratio is preferably 3.0 atomic % or more, more preferably 5.0 atomic % or more, and even more preferably 10.0 atomic % or more. There is no particular upper limit, but in view of the effects on productivity and other physical properties, it is preferably 25.0 atomic % or less.

[0049] Specifically, the above-mentioned measurement by X-ray photoelectron spectroscopy (XPS) was performed using a scanning X-ray photoelectron spectrometer (XPS / ESCA) PHI5000 VersaProbeII (manufactured by ULVAC-PHI Co., Ltd.).

[0050] In the fluororesin sheet of the present disclosure, the surface having an oxygen element ratio of 3.0 atomic % or greater as measured by X-ray photoelectron spectroscopy (XPS) preferably has a nitrogen element ratio of 1.35 atomic % or greater as measured by X-ray photoelectron spectroscopy (XPS). The nitrogen atomic ratio is more preferably 1.35 atomic % or greater, further preferably 2.5 atomic % or greater, and most preferably 3.0 atomic % or greater.

[0051] By increasing the nitrogen element ratio on the surface of the fluororesin sheet, which contributes to adhesion, sufficient peel strength with the copper foil can be obtained without impairing the dielectric properties.

[0052] The upper limit is not particularly specified, but is preferably 25.0 atomic % or less in view of the effects on productivity and other physical properties.

[0053] In the fluororesin sheet of the present invention, the surface having an oxygen element ratio of 3.0 atomic % or greater as measured by X-ray photoelectron spectroscopy (XPS) preferably has a silicon element ratio of 0.5 atomic % or greater as measured by X-ray photoelectron spectroscopy (XPS). The silicon atomic ratio is more preferably 1.0 atomic % or greater, further preferably 1.5 atomic % or greater, and most preferably 2.0 atomic % or greater.

[0054] Since the silicon element ratio of the surface of the fluororesin sheet contributing to adhesion is high in this manner, the silica particles that can be subjected to surface treatment are exposed, and the peeling strength with the copper foil can be improved.

[0055] The upper limit is not particularly specified, but is preferably 10.0 atomic % or less in view of the influence on productivity, strength, and other physical properties.

[0056] For the surface of the fluororesin sheet of the present disclosure having an oxygen element ratio of 3.0 atomic % or more as measured by X-ray photoelectron spectroscopy (XPS), the static contact angle of water measured 1 second after a 2 μL droplet has landed is preferably 105° or less, more preferably 103° or less, and even more preferably 100° or less.

[0057] By satisfying such a range, new functional groups are generated on the fluororesin sheet, and the peeling strength with the copper foil is increased.

[0058] The static contact angle of water was measured using a contact angle meter ("DropMaster" manufactured by Kyowa Interface Science Co., Ltd.) at 23°C with a droplet volume of 2 μL for 1 second after landing.

[0059] The coefficient of linear expansion (CTE) of the fluororesin sheet disclosed herein is 100 ppm / ° C. or less. By being within the above range, it is preferable in that the fluororesin sheet has low shrinkage and excellent dimensional stability.

[0060] The upper limit is more preferably 70 ppm / ° C., further preferably 50 ppm / ° C., and even more preferably 40 ppm / ° C. The lower limit is preferably 10 ppm / ° C., and more preferably 18 ppm / ° C.

[0061] The linear expansion coefficient in this specification is determined as follows: TMA measurement is performed in the tensile mode using a TMA-7100 (manufactured by Hitachi High-Technologies Corporation). A fluororesin sheet cut to a length of 20 mm, a width of 5 mm, and a thickness of 150 μm is used as a sample sheet. The distance between the chucks is set to 10 mm. While applying a load of 49 mN, the linear expansion coefficient is determined from the displacement of the sample at a temperature increase rate of 2°C / min at a temperature between 0 and 150°C.

[0062] (Fluororesin)

[0063] Fluororesin has low dielectric properties and can therefore be preferably used in the present invention.

[0064] Fluororesins that can be used in the present disclosure are not particularly limited, and examples thereof include polytetrafluoroethylene (PTFE), tetrafluoroethylene [TFE] / hexafluoropropylene [HFP] copolymers [FEP], TFE / alkyl vinyl ether copolymers [PFA], TFE / HFP / alkyl vinyl ether copolymers [EPA], TFE / chlorotrifluoroethylene [CTFE] copolymers, TFE / ethylene copolymers [ETFE], polyvinylidene fluoride [PVdF], and tetrafluoroethylene having a molecular weight of 300,000 or less [LMW-PTFE]. These fluororesins can be used alone or as a mixture of two or more.

[0065] Wherein, from the viewpoint of low dielectric property, low linear expansion coefficient, particularly preferably polytetrafluoroethylene (PTFE).PTFE preferably has fibrillar property.PTFE with fibrillar property refers to the PTFE that can carry out paste extrusion of unfired polymer powder.

[0066] Modified PTFE comprises TFE units based on TFE and modified monomer units based on the modified monomer. The modified monomer units are part of the molecular structure of the modified PTFE and are derived from the modified monomer. The modified PTFE preferably comprises 0.001 to 0.500% by mass of the modified monomer units, and more preferably 0.01 to 0.30% by mass of the modified monomer units, based on the total monomer units. The total monomer units are derived from all monomers in the molecular structure of the modified PTFE.

[0067] The modifying monomer is not particularly limited as long as it can copolymerize with TFE. Examples thereof include perfluoroolefins such as hexafluoropropylene (HFP); chlorofluoroolefins such as chlorotrifluoroethylene (CTFE); hydrogen-containing fluoroolefins such as trifluoroethylene and vinylidene fluoride (VDF); perfluorovinyl ether; perfluoroalkylethylene (PFAE); ethylene. The modifying monomers used may be one or more.

[0068] The perfluorovinyl ether is not particularly limited, and examples thereof include perfluorounsaturated compounds represented by the following general formula (1).

[0069] CF2=CF-ORf…(1)

[0070] (Wherein, Rf represents a perfluoroorganic group.)

[0071] In this specification, a perfluoroorganic group is an organic group in which all hydrogen atoms bonded to carbon atoms are replaced by fluorine atoms. The perfluoroorganic group may have an ether oxygen.

[0072] Examples of perfluorovinyl ethers include perfluoro(alkyl vinyl ether) (PAVE) wherein Rf in the above general formula (1) is a perfluoroalkyl group having 1 to 10 carbon atoms. The perfluoroalkyl group preferably has 1 to 5 carbon atoms.

[0073] Examples of the perfluoroalkyl group in PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, and perfluorohexyl. Preferred PAVEs include perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE).

[0074] The perfluoroalkylethylene (PFAE) is not particularly limited, and examples thereof include perfluorobutylethylene (PFBE) and perfluorohexylethylene (PFHE).

[0075] The modifying monomer in the modified PTFE is preferably at least one selected from the group consisting of HFP, CTFE, VDF, PAVE, PFAE, and ethylene.

[0076] The fluororesin is preferably non-melt-processable. Non-melt-processable means that even when heated above its melting point, the resin lacks sufficient fluidity and cannot be molded using conventional melt molding methods. PTFE meets this requirement.

[0077] In the present invention, it is preferred to use such non-melt-processable fluororesin pellets and produce a fluororesin sheet by a molding method of fibrillating the pellets. This molding method will be described later.

[0078] The preferred standard specific gravity (SSG) of the above-mentioned PTFE is 2.0 to 2.3. If such PTFE is used, it is easy to obtain a PTFE membrane with high strength (cohesive force and puncture strength per unit thickness). PTFE with a large molecular weight has long molecular chains, and is therefore difficult to form a structure with regular arrangement of molecular chains. In this case, the length of the amorphous portion increases, and the degree of entanglement between molecules increases. It is believed that: when the degree of entanglement between molecules is high, the PTFE membrane is difficult to deform for the applied load and demonstrates excellent mechanical strength. In addition, if PTFE with a large molecular weight is used, it is easy to obtain a PTFE membrane with a small average pore size.

[0079] The lower limit of the SSG is more preferably 2.05, and even more preferably 2.1. The upper limit of the SSG is more preferably 2.25, and even more preferably 2.2.

[0080] Standard specific gravity [SSG] is obtained by measuring the specific gravity of a sample prepared in accordance with ASTM D-4895-89 by the water displacement method.

[0081] The refractive index of the PTFE is preferably within the range of 1.2 to 1.6. This refractive index is preferred from the perspective of low dielectric constant. The refractive index can be adjusted to fall within the above range by adjusting the polarizability and the flexibility of the main chain. The lower limit of the refractive index is more preferably 1.25, more preferably 1.30, and most preferably 1.32. The upper limit of the refractive index is more preferably 1.55, more preferably 1.50, and most preferably 1.45.

[0082] The above-mentioned refractive index is a value measured using a refractometer (Abbemat 300).

[0083] Furthermore, the maximum endothermic peak temperature (crystalline melting point) of the PTFE is preferably 340±7°C.

[0084] The PTFE may be low-melting-point PTFE having a maximum peak temperature of 338°C or lower in the endothermic curve on the crystalline melting curve measured by a differential scanning calorimeter, or high-melting-point PTFE having a maximum peak temperature of 342°C or higher in the endothermic curve on the crystalline melting curve measured by a differential scanning calorimeter.

[0085] Low melting point PTFE powder is a powder particle produced by emulsion polymerization, having the above-mentioned maximum endothermic peak temperature (crystalline melting point), a dielectric constant (ε) of 2.08 to 2.2, and a dielectric loss tangent (tanδ) of 1.9×10 -4 ~4.0×10 -4 Examples of commercially available products include Polyflon Fine Powder F201, F203, F205, F301, and F302 manufactured by Daikin Industries, Ltd.; CD090 and CD076 manufactured by Asahi Glass Industries, Ltd.; and TF6C, TF62, and TF40 manufactured by DuPont.

[0086] High melting point PTFE powder is also produced by emulsion polymerization. It has the above-mentioned maximum endothermic peak temperature (crystalline melting point), a dielectric constant (ε) of 2.0 to 2.1, and a dielectric loss tangent (tanδ) of 1.6×10 -4 ~2.2×10 -4Commercially available products include, for example, POLYFLON FINE POWDER F104 and F106 manufactured by Daikin Industries, Ltd.; CD1, CD141, and CD123 manufactured by Asahi Glass Industries, Ltd.; and TF6 and TF65 manufactured by DuPont.

[0087] The average particle size of the powder obtained by secondary agglomeration of the two PTFE polymer particles is preferably 250 μm to 2000 μm. In particular, granulated powder obtained by granulation using a solvent is preferred from the perspective of improving fluidity during mold filling during premolding.

[0088] (Silicon dioxide particles)

[0089] The fluororesin sheet of the present disclosure necessarily contains silica particles.

[0090] The shape of the silica particles is not particularly limited, but is particularly preferably spherical. Spherical silica particles are preferred because they are easy to process uniformly during drilling, have a small specific surface area, and have low transmission loss. In the present disclosure, spherical silica is most preferably used.

[0091] The above-mentioned spherical silica refers to particles whose particle shape is close to a true sphere. Specifically, the sphericity is preferably 0.80 or more, more preferably 0.85 or more, further preferably 0.90 or more, and most preferably 0.95 or more. Regarding sphericity, a photograph is taken using an SEM, and the area and perimeter of the observed particles are calculated as a value calculated by (sphericity) = {4π × (area) ÷ (perimeter) 2}. The closer to 1, the closer to a true sphere. Specifically, the average value of 100 particles measured using an image processing device (Spectris Co., Ltd.: FPIA-3000) is used.

[0092] The parameters of the spherical silica can also be measured by heating a fluororesin sheet at 600° C. for 30 minutes in an air atmosphere to burn off the fluororesin, taking out the spherical silica particles, and then using the above-mentioned methods.

[0093] The silica particles are preferably surface-treated silica particles. Surface treatment allows interaction between the silica particles, thereby reducing the linear expansion coefficient of the fluororesin sheet.

[0094] The surface treatment is not particularly limited, and any known surface treatment can be used. Specifically, for example, treatment with a silane coupling agent such as epoxy silane, amino silane, isocyanate silane, vinyl silane, acrylic silane, hydrophobic alkyl silane, phenyl silane, or fluorinated alkyl silane having a reactive functional group, plasma treatment, or fluorination treatment can be mentioned. Among these, treatment with a silane coupling agent is preferred.

[0095] In the present disclosure, silica particles treated with a silane coupling agent are preferably used. Silane coupling agent treatment of silica particles can reduce the linear expansion coefficient and improve adhesion to copper foil, which is advantageous in these respects. Furthermore, silane coupling agent treatment reacts with polar functional groups on the silica particle surface, reducing the amount of polar functional groups and resulting in excellent electrical properties. Furthermore, the functional groups contained in the silane coupling agent react with the copper foil surface, further enhancing the peel strength from the copper foil.

[0096] Examples of the silane coupling agent include epoxy silanes such as γ-glycidoxypropyltriethoxysilane and β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, amino silanes such as aminopropyltriethoxysilane and N-phenylaminopropyltrimethoxysilane, isocyanate silanes such as 3-isocyanatepropyltrimethoxysilane, vinyl silanes such as vinyltrimethoxysilane, and acryl silanes such as acryloxytrimethoxysilane.

[0097] The average particle size of the silica particles is preferably 10 μm or less. When the average particle size of the silica particles is 10 μm or less, the surface roughness of the sheet is reduced, which is preferred.

[0098] The upper limit of the average particle size of the silica particles is more preferably 8 μm or less, further preferably 5 μm or less.

[0099] The lower limit of the average particle size of the silica particles is not particularly limited, but is preferably 0.5 μm or greater. If the average particle size is less than 0.5 μm, aggregation of the filler occurs, and sufficient effects tend to be unattainable.

[0100] In addition, the average particle size here is the value of D50 measured by a laser analysis type particle size distribution analyzer.

[0101] The silica particles can be commercially available silica particles that satisfy the above-mentioned properties. Examples of commercially available silica particles include Denka fused silica FB grade (manufactured by Denka Co., Ltd.), Denka fused silica SFP grade (manufactured by Denka Co., Ltd.), EXCELICA (manufactured by Tokuyama Co., Ltd.), high-purity synthetic spherical silica ADMAFINE (manufactured by Admatechs Co., Ltd.), ADMANANO (manufactured by Admatechs Co., Ltd.), and ADMAFUSE (manufactured by Admatechs Co., Ltd.).

[0102] Above-mentioned silicon dioxide particles are preferably more than 30 quality % with respect to the content of fluororesin sheet total amount.By being set to such coordination amount, it is preferred that aspect reducing linear expansion coefficient while being able to maintain low dielectric constant, low loss.In addition, when using PTFE as fluororesin, even if PTFE carries out plasma treatment and is also difficult for forming the functional group of oxygen, nitrogen, but by having silicon dioxide particles, can make silicon dioxide surface also have the functional group that is derived from surface treatment.As a result, can improve the oxygen atomic ratio on fluororesin sheet surface, the adhesion of fluororesin sheet and copper foil becomes good, can make the peeling strength of bonding surface better.

[0103] The above-mentioned compounding amount is more preferably 35 mass% or more, further preferably 50 mass% or more, and even more preferably 55 mass% or more. The upper limit of the compounding amount of silica particles is not particularly limited, but is preferably 70 mass% or less, more preferably 68 mass% or less, and even more preferably 65 mass% or less.

[0104] (piece)

[0105] The fluororesin sheet disclosed herein preferably has a dielectric loss tangent of 0.0015 or less at 10 GHz. Setting the dielectric loss tangent within this range is preferred for minimizing electrical signal loss in circuits. The dielectric loss tangent is more preferably 0.0012 or less, and even more preferably 0.0011 or less. Meanwhile, the lower limit of the dielectric loss tangent is preferably 0.00001.

[0106] The fluororesin sheet of the present disclosure preferably has a relative dielectric constant of 3.5 or less at 10 GHz. Setting the relative dielectric constant within this range is preferred in terms of low dielectric loss.

[0107] The upper limit of the relative dielectric constant is more preferably 3.2, and even more preferably 3.1. On the other hand, the lower limit of the relative dielectric constant is preferably 2.0, and even more preferably 2.5.

[0108] The relative dielectric constant (Dk) and dielectric loss tangent (Df) at 10 GHz in this specification were determined by measuring Dk and Df at 25° C. and 10 GHz using a split-cylinder dielectric constant and dielectric loss tangent measuring apparatus (manufactured by EM lab).

[0109] The thickness of the fluororesin sheet disclosed herein is preferably 5 to 250 μm. The lower limit of the thickness is more preferably 15 μm or greater, and even more preferably 30 μm or greater. The upper limit of the thickness is more preferably 230 μm or less, and even more preferably 200 μm or less. This thickness can be selected based on the balance between the electrical properties and the linear expansion coefficient of the laminate.

[0110] The fluororesin sheet of the present disclosure may contain components other than the fluororesin and silica particles. The components contained are not particularly limited, and examples thereof include fluorine-free thermosetting resins and thermoplastic resins.

[0111] The fluororesin sheet disclosed herein preferably does not contain glass fibers. By not containing glass fibers or using a cloth composed of glass fibers, the fluororesin sheet can be made thinner and more flexible, making it easier to use in applications where the sheet is bent.

[0112] In the present disclosure, fillers other than silica particles may be used within a range not hindering the purpose of the present disclosure.

[0113] (Method for producing fluororesin sheet)

[0114] The fluororesin sheet disclosed herein can be obtained, for example, by mixing the fluororesin particles and silica particles, forming a film, and then surface treating the film. The film forming method is not limited and can be performed by paste extrusion molding, powder calendering molding, or the like.

[0115] As described above, the fluororesin particles used in the fluororesin sheet of the present disclosure are preferably non-melt-processable fluororesin particles. When such fluororesin particles are used, they are preferably formed into a sheet by fibrillating the powdered PTFE as a raw material.

[0116] The powdered PTFE preferably has a primary particle size of 0.05 to 10 μm. This provides excellent moldability and dispersibility. The primary particle size here is measured according to ASTM D 4895.

[0117] The powdered PTFE preferably contains 50% by mass or more, more preferably 80% by mass or more, of polytetrafluoroethylene resin having a secondary particle size of 500 μm or more. The PTFE having a secondary particle size of 500 μm or more within this range has the advantage of enabling the production of a high-strength fluororesin sheet.

[0118] By using PTFE having a secondary particle size of 500 μm or more, a sheet having lower electrical resistance and excellent toughness can be obtained.

[0119] The lower limit of the secondary particle size is more preferably 300 μm, and even more preferably 350 μm. The upper limit of the secondary particle size is more preferably 700 μm or less, and even more preferably 600 μm or less. The secondary particle size can be determined, for example, by sieving.

[0120] From the perspective of obtaining a sheet with higher strength and excellent homogeneity, the average primary particle size of the powdered PTFE is preferably 50 nm or greater, more preferably 100 nm or greater, further preferably 150 nm or greater, and particularly preferably 200 nm or greater.

[0121] The average primary particle size of PTFE is larger, and when using this powder to carry out paste extrusion molding, the rising of paste extrusion pressure can be suppressed more, and formability is also more excellent. The upper limit is not particularly limited and can be 500nm. From the aspect of the productivity in the polymerization process, it is preferably 350nm.

[0122] The average primary particle size can be determined by preparing a calibration curve using an aqueous dispersion of PTFE obtained by polymerization, comparing the transmittance of projected light at 550 nm per unit length of the aqueous dispersion adjusted to a polymer concentration of 0.22% by mass, and the average primary particle size determined by measuring the orientation diameter in a transmission electron micrograph. The transmittance can be determined based on the calibration curve by measuring the transmittance of the aqueous dispersion to be measured.

[0123] The PTFE particles used in the present disclosure can have a core-shell structure. As PTFE particles with a core-shell structure, for example, modified polytetrafluoroethylene (PTFE) comprising a core of a high molecular weight polytetrafluoroethylene and a shell of a lower molecular weight polytetrafluoroethylene or modified polytetrafluoroethylene (PTFE) in the particle can be enumerated. As such modified polytetrafluoroethylene, for example, the polytetrafluoroethylene described in writing in the Japanese Patent Application Laid-Open No. 2005-527652 communique can be enumerated.

[0124] The specific methods of the paste extrusion molding and powder calendering molding are not particularly limited, and general methods are described below.

[0125] (Paste extrusion molding)

[0126] The method for producing the fluororesin sheet may include: step (1a), mixing powdered fluororesin particles obtained using a hydrocarbon-based surfactant, silica particles, and an extrusion aid; step (1b), subjecting the obtained mixture to paste extrusion molding; step (1c), calendering the extrudate obtained by extrusion molding; step (1d), drying the calendered sheet; and step (1e), firing the dried sheet to obtain a molded body.

[0127] The paste extrusion molding may be performed by adding conventionally known additives such as pigments and fillers to the fluororesin particles and silica particles.

[0128] The extrusion aid is not particularly limited, and a generally known extrusion aid can be used, for example, hydrocarbon oil.

[0129] (Powder calendering molding)

[0130] The fluororesin sheet can also be formed by powder calendering. Powder calendering is a method of applying shear force to powdered fluororesin particles to fibrillate them, thereby forming a sheet. This process may then include a calendering step to obtain a molded body.

[0131] More specifically, it can be obtained by the following manufacturing method, etc., which has the following characteristics:

[0132] Step (1) of applying shear force while mixing a raw material composition containing fluororesin particles and filler particles;

[0133] Step (2), forming the mixture obtained in the above step (1) into a block; and

[0134] In step (3), the block-shaped mixture obtained in step (2) is rolled into a sheet.

[0135] Furthermore, the process may include a step (4) of calcining the sheet obtained above at 200 to 400° C. for 1 to 60 minutes.

[0136] Alternatively, step (2) may be omitted.

[0137] When producing a fluororesin sheet by the aforementioned powder calendering method, it is preferred to form the film using a composition that contains no liquid component and consists essentially of fluororesin particles and filler particles containing at least silica particles. Furthermore, the phrase "consisting essentially of fluororesin particles and filler particles containing at least silica particles" means that the content of components other than the fluororesin particles and the filler particles is 3% by mass or less relative to the total weight of the composition.

[0138] The filler particles include at least silica particles. Other filler particles are preferably aluminum oxide, zinc oxide, titanium oxide, calcium carbonate, magnesium oxide, or the like. Filler particles other than silica particles may be omitted, used alone, or mixed. The proportion of filler particles other than silica particles in the filler particles is preferably 0.1 to 80% by mass.

[0139] It is particularly preferred to mix only the fluororesin particles and the silica particles and then perform molding.

[0140] The fluororesin sheet obtained by the above-described method for producing a fluororesin sheet before surface treatment preferably has an oxygen ratio of 1.5 atomic % or greater, as measured by X-ray photoelectron spectroscopy (XPS), on at least the surface treated surface. The presence of oxygen on the surface of the fluororesin sheet before surface treatment enables good adhesion to copper foil having a smooth surface. Furthermore, in order to ensure that the oxygen ratio of the fluororesin sheet after surface treatment, as measured by X-ray photoelectron spectroscopy (XPS), is within the above-specified range, it is also preferred to do so.

[0141] The oxygen atomic ratio is preferably 1.65 atomic % or more, more preferably 1.7 atomic % or more, and even more preferably 1.8 atomic % or more. There is no particular upper limit, but in view of the effects on productivity and other physical properties, it is preferably 4.9 atomic % or less.

[0142] The method for adjusting the oxygen element ratio of the fluororesin sheet before surface treatment to the above range is not particularly limited, and can be controlled by, for example, adjusting the blending amount of silica particles, adjusting the molding method, and the like.

[0143] The fluororesin sheet before surface treatment preferably has a silicon ratio of 0.2 atomic % or greater, as measured by X-ray photoelectron spectroscopy (XPS), on at least the surface treated. The presence of silicon on the surface of the fluororesin sheet before surface treatment enables good adhesion to copper foil having a smooth surface. This range is also preferred in order to achieve a silicon ratio of 0.5 atomic % or greater, as measured by X-ray photoelectron spectroscopy (XPS), on the surface treated fluororesin sheet after surface treatment.

[0144] The silicon element ratio is preferably 0.3 atomic % or more, more preferably 0.5 atomic % or more, and even more preferably 0.7 atomic % or more. There is no particular upper limit, but in view of the effects on productivity and other physical properties, it is preferably 2.0 atomic % or less.

[0145] The silicon element ratio of the fluororesin sheet before surface treatment may be adjusted to fall within the above range by adjusting the amount of silica particles added.

[0146] (Surface Treatment)

[0147] The fluororesin sheet obtained by the above-mentioned method can be made into a fluororesin sheet satisfying the above-mentioned requirements by subjecting one or both surfaces of the sheet to surface treatment under appropriate conditions.

[0148] Fluororesin is a raw material that is generally difficult to bond to copper foil. Therefore, in the present disclosure, a fluororesin sheet is surface treated to improve lamination properties. This surface treatment is generally known to increase the oxygen atomic weight of the resin surface.

[0149] The specific method of the surface treatment is not particularly limited, and the surface treatment can be performed by any known method.

[0150] The surface treatment of the fluororesin sheet may be carried out by conventional discharge treatments such as plasma discharge treatment, corona discharge treatment, glow discharge treatment, and sputtering treatment. Among these, plasma treatment is preferred.

[0151] The plasma treatment is a treatment in which plasma is brought into contact with a fluororesin sheet to etch the fluororesin on the outer surface of the fluororesin sheet, thereby adding oxygen atoms, nitrogen atoms, etc. to the outer surface of the fluororesin sheet.

[0152] For example, the surface free energy can be controlled by introducing oxygen, nitrogen, hydrogen, helium, argon, or the like into the discharge atmosphere.

[0153] Alternatively, the surface to be modified may be exposed to an inert gas containing an organic compound, i.e., an atmosphere containing an inert gas containing an organic compound, and a high-frequency voltage may be applied between the electrodes to induce discharge, thereby generating active species on the surface, followed by the introduction of functional groups of the organic compound or the graft polymerization of the polymerizable organic compound, thereby performing surface treatment.

[0154] Examples of the organic compound in the inert gas containing organic compounds include polymerizable or non-polymerizable organic compounds containing oxygen atoms, such as vinyl esters such as vinyl acetate and vinyl formate; acrylic esters such as glycidyl methacrylate; ethers such as vinyl ethyl ether, vinyl methyl ether, and glycidyl methyl ether; carboxylic acids such as acetic acid and formic acid; alcohols such as methanol, ethanol, phenol, and ethylene glycol; ketones such as acetone and methyl ethyl ketone; carboxylic acid esters such as ethyl acetate and ethyl formate; and acrylic acids such as acrylic acid and methacrylic acid. Among these, vinyl esters, acrylic esters, and ketones are preferred, with vinyl acetate and glycidyl methacrylate being particularly preferred, due to their resistance to surface deactivation after modification, i.e., their long lifespan, safety, and ease of handling.

[0155] The concentration of the organic compound in the inert gas containing the organic compound varies depending on the type of the organic compound, the type of fluororesin to be surface treated, etc., and is generally 0.1 to 3.0% by volume, preferably 0.1 to 1.0% by volume, more preferably 0.15 to 1.0% by volume, and further preferably 0.30 to 1.0% by volume. The discharge conditions can be appropriately selected according to the degree of target surface treatment, the type of fluororesin, the type and concentration of the organic compound, etc. Usually, at a discharge rate of 50 W·min / m 2 Above 1500W·min / m 2 Below, preferably 70W·min / m 2 Above 1400W·min / m 2The discharge treatment is performed within the following range. The treatment temperature may be any temperature within the range of 0°C to 100°C. Due to concerns about elongation and wrinkles of the fluororesin sheet, the temperature is preferably 80°C or lower.

[0156] The fluororesin sheet of the present invention surface-treated as described above can be used as a sheet for a circuit board by laminating it with other base materials.

[0157] A laminated body can be prepared by bonding copper foil to one or both surfaces of the fluororesin sheet.

[0158] The present disclosure also relates to a copper-clad laminate comprising copper foil and the aforementioned fluororesin sheet as essential layers. As described above, the fluororesin sheet comprising fluororesin and silica particles of the present disclosure exhibits excellent adhesive properties. Therefore, a laminate in which the surface-treated surface of the fluororesin sheet of the present disclosure is directly laminated with copper foil is preferred.

[0159] The surface roughness (Rz) of the copper foil is preferably 2.0 μm or less. A surface roughness (Rz) of 2.0 μm or less is preferred because the interface between the copper foil and the fluororesin sheet becomes smooth, increasing the metal interface conductivity and reducing transmission loss. The Rz of the copper foil on at least the surface bonded to the fluororesin sheet is not particularly limited, provided that the Rz of the other surface is 2.0 μm or less.

[0160] The fluororesin sheet of the present disclosure has excellent adhesiveness to a copper foil having high smoothness such as Rz2.0 μm or less, further Rz1.0 μm or less, and further Rz0.5 μm or less.

[0161] Rz is the sum of the highest peak height (maximum peak height: Rp) and the deepest valley depth (maximum valley depth: Rv). Rz is the ten-point average roughness specified in JIS-B0601. In this specification, Rz is measured using a surface roughness meter (trade name: SURFCOM 470A, manufactured by Tokyo Seiki Co., Ltd.) with a measurement length of 4 mm.

[0162] The surface roughness (Rq) of the copper foil is preferably 0.01 to 0.15 μm.

[0163] When Rq is within the above range, the metal interface conductivity can be improved and the transmission loss can be reduced favorably, which is preferable.

[0164] The copper foil may have an Rq of at least 0.01 to 0.15 μm on the surface bonded to the fluororesin sheet, and the Rq of the other surface is not particularly limited. The Rq of the surface bonded to the fluororesin sheet is more preferably 0.015 to 0.1 μm, and even more preferably 0.03 to 0.08 μm.

[0165] The Rq is a root mean square height. In this specification, the Rq is a value measured using a stylus surface roughness measuring instrument (trade name: SE600A, manufactured by Kosaka Laboratory Co., Ltd.) with a measurement length of 4 mm.

[0166] The thickness of the copper foil is not particularly limited, but is preferably 1 to 100 μm, more preferably 5 to 50 μm, and even more preferably 9 to 35 μm.

[0167] The copper foil is not particularly limited, and specific examples thereof include rolled copper foil and electrolytic copper foil.

[0168] The copper foil having an Rz of 2.0 μm or less and / or an Rq of 0.01 to 0.15 μm is not particularly limited, and commercially available copper foil can be used. Examples of such commercially available copper foil include electrolytic copper foil CF-T9DA-SV-18 (thickness 18 μm / Rz 0.85 μm / Rq 0.05 μm) (manufactured by Fukuda Metal Foil Co., Ltd.).

[0169] The copper foil may be surface-treated in order to increase the peeling strength with the fluororesin sheet of the present disclosure.

[0170] In the copper-clad laminate of the present disclosure, when the surface-treated surface of the fluororesin sheet and the copper foil are directly laminated, the peel strength of the interface between the copper foil and the fluororesin sheet is preferably 0.5 kN / m or more.

[0171] By using the fluororesin sheet of the present disclosure, the peel strength described above can be achieved.

[0172] When the peel strength is 0.5 kN / m or more, the copper-clad laminate can be suitably used as a copper-clad laminate or a circuit board. The peel strength is more preferably 0.7 kN / m or more.

[0173] The upper limit of the peel strength is not particularly limited, but may be 3.0 kN / m or less.

[0174] It should be noted that the peel strength herein refers to the peel strength measured under the conditions described in the Examples.

[0175] In the case of a laminate having a copper foil bonded to the surface treated surface of a fluororesin sheet having only one surface treated, the untreated fluororesin sheet surface may be additionally surface treated to improve adhesion of the laminate to other materials.

[0176] The copper-clad laminate of the present disclosure may further include layers other than the copper foil and the fluororesin sheet.

[0177] In the copper-clad laminate of the present disclosure, the copper foil layer may be formed on one or both sides of the roll sheet. Examples of methods for forming the copper foil layer include laminating (bonding) copper foil on the surface of the roll sheet, vapor deposition, and plating.

[0178] Examples of the method for laminating the copper foil include hot pressing, etc. The hot pressing temperature is from the melting point of the sheet - 150°C to the melting point of the sheet + 40°C. The hot pressing time is, for example, 1 minute to 30 minutes.

[0179] For example, a method is preferably used in which a fluororesin sheet and copper foil are laminated, heated at 180 to 390° C., and press-molded at a pressure of 0.5 to 5 MPa in a vacuum or in an inert gas atmosphere to produce a copper-clad laminate.

[0180] By performing press molding under such conditions, the fluororesin sheet can be bonded to the copper foil without being degraded.

[0181] To obtain the above-mentioned laminate structure, the fluororesin sheet of the present disclosure is used by bonding copper foil to one or both sides. As described above, the fluororesin sheet of the present disclosure has excellent adhesion. Therefore, it also has excellent adhesion to copper foil with a smoothness of Rz 2.0 μm or less.

[0182] Copper foil used in circuit substrates has traditionally been given a certain degree of surface roughness to ensure adhesion to fluororesin sheets. However, surface roughness on the copper foil is not preferred for high-frequency applications because it can lead to electrical signal loss. The above-described laminate exhibits suitable adhesion even to highly smooth copper foil, making it suitable for use as a circuit substrate.

[0183] The copper-clad laminate of the present disclosure is not particularly limited in its use, and is used as a circuit substrate. The present disclosure also relates to a circuit substrate having the copper-clad laminate.

[0184] The copper-clad laminate of the present invention is also used as a circuit substrate, so the metal interface conductivity is preferably high. It is preferably 1.00 (1e7 S / m) or higher, more preferably 3.00 (1e7 S / m) or higher, and even more preferably 5.00 (1e7 S / m) or higher. The upper limit is not particularly limited, but is preferably 6.00 (1e7 S / m) or lower.

[0185] The copper-clad laminate disclosed herein is also used as a circuit substrate, so the transmission loss is preferably close to 0. The transmission loss at a frequency of 67 GHz is preferably -4.20 dB / 100 mm or more, more preferably -4.00 dB / 100 mm or more, and even more preferably -3.30 dB / 100 mm or more.

[0186] The present disclosure also relates to an antenna formed from the above-mentioned circuit substrate, and is particularly preferably a millimeter wave antenna for use with mobile objects such as automobiles and airplanes.

[0187] The laminate for a circuit board may be obtained by laminating a copper foil layer, the above-mentioned fluororesin sheet, and a base material layer.

[0188] In the laminate, the order of laminating the copper foil layer, the substrate, and the fluororesin sheet or the method of producing the laminate is not particularly limited, and the laminate can be configured to suit the intended purpose.

[0189] Specific examples of the above-mentioned stacking order include a stacking order consisting of base material layer / fluororesin sheet / copper foil layer, a stacking order consisting of copper foil layer / fluororesin sheet / base material layer / fluororesin sheet / copper foil layer, and a stacking order consisting of copper foil layer / base material layer / fluororesin sheet / base material layer / copper foil layer.

[0190] In addition, other layers may be included as needed.

[0191] Example

[0192] Hereinafter, the present disclosure will be described in detail based on examples. However, the present invention is not limited to the following examples. In the following examples, "parts" and "%" respectively represent "parts by mass" and "% by mass" unless otherwise specified.

[0193] The obtained samples were evaluated based on the following criteria.

[0194] [Thickness of fluororesin sheet]

[0195] The measurement was performed using a micrometer.

[0196] [XPS measurement of fluororesin sheet surface]

[0197] The measurement was performed using a scanning X-ray photoelectron spectroscopy analyzer (XPS / ESCA) PHI5000 VersaProbeII (manufactured by ULVAC-PHI Co., Ltd.).

[0198] [Coefficient of linear expansion (CTE)]

[0199] TMA measurement was performed using a TMA-7100 (manufactured by Hitachi High-Technologies Corporation) in tensile mode. A fluororesin sheet cut to a length of 20 mm, a width of 5 mm, and a thickness of 150 μm was used as a sample sheet. The distance between the chucks was set to 10 mm. The linear expansion coefficient was determined from the displacement of the sample at a temperature increase rate of 2°C / min at a temperature of 0°C to 150°C while applying a load of 49 mN.

[0200] [Dielectric Constant and Dielectric Loss Tangent of Fluororesin Sheet]

[0201] The dielectric constant and dielectric loss tangent were measured at 25° C. and 10 GHz using a split-cylinder dielectric constant and dielectric loss tangent measuring apparatus (manufactured by EM lab).

[0202] [Static contact angle of water]

[0203] The static contact angle of water on the produced fluororesin sheet was measured using a contact angle meter ("DropMaster" manufactured by Kyowa Interface Science Co., Ltd.) at 23°C with a droplet volume of 2 μL and 1 second after landing.

[0204] [Peel strength]

[0205] The surface-treated fluororesin sheet was placed on top and bottom with copper foil (CF-T9DA-SV-18, Rz = 0.85 μm, Rq = 0.05 μm) placed so that the treated surface was in close contact with the fluororesin sheet. The sample was then heated and pressurized using a vacuum hot press (360°C, 2.5 MPa, 300 s).

[0206] The obtained sample was cut into strips 10 mm wide and the peel strength was measured by using a Tensilon universal testing machine (manufactured by Shimadzu Corporation) while clamping the unbonded portion of the strip sample between the upper and lower chucks of the Tensilon and stretching it at a speed of 50 mm per minute. The obtained value was used as the peel strength.

[0207] <Method for preparing slices A to G>

[0208] (Paste extrusion molding)

[0209] PTFE powder (average particle size: 500 μm, apparent density: 460 g / L, standard specific gravity: 2.17) and the silica particles shown in Table 1 were weighed to give the mass ratios shown in Table 1 and mixed in a mixer in the presence of dry ice. The temperature during mixing was -10°C or lower.

[0210] 18 to 23% of oil (Isopar H) was added to the obtained mixed powder, mixed, and aged for about 5 hours.

[0211] The aged composition was preformed under a pressure of 3 MPa, and the preformed body was extruded at 40° C. and 50 mm / min to obtain an extruded sample.

[0212] The extruded sample was rolled with a twin roll to obtain a sample having a film thickness of 125 μm, and the sample was dried by passing through a drying roll at 200° C.

[0213] Furthermore, the gap and pressure between the two rolls were adjusted to confirm that a sample with a film thickness of 30 μm could be produced.

[0214] <Method for producing sheet H>

[0215] (Powder calendering molding)

[0216] PTFE powder (average particle size: 500 μm, apparent density: 460 g / L, standard specific gravity: 2.17) and silica particles shown in Table 1 were weighed to give the mass ratio shown in Table 1, and stirred twice for 30 seconds at room temperature using a Wonder crusher at memory 6.

[0217] The obtained mixture was rolled using a twin-roller (roll gap: set to 100 μm, roll temperature: 100° C.) to obtain a sample having a film thickness of 130 μm.

[0218] It should be noted that the silica used in each example is as shown in Table 1. As spherical silica, SC6500-SQ (average particle size 2.1 μm) manufactured by Admatechs and SC6500-SQ (average particle size 2.1 μm) manufactured by Admatechs were surface-treated with 3-aminopropyltriethoxysilane (at a treatment amount of 0.5% by mass or 1% by mass relative to the mass of the silica particles).

[0219] [Table 1]

[0220] piece Fluororesin Silicon dioxide Silica surface treatment Fluororesin:Silicon dioxide (mass ratio) A PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 40:60 B PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 0.5% 40:60 C PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 50:50 D PTFE SC6500-SQ none 40:60 E PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 80:20 F PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 90:10 G PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 95:5 H PTFE SC6500-SQ 3-Aminopropyltriethoxysilane 1% 40:60

[0221] (Examples 1 to 10)

[0222] (Surface treatment of fluororesin sheets)

[0223] A fluororesin sheet was placed between upper and lower electrodes in a treatment chamber (direct plasma surface treatment apparatus, manufactured by Air Water) having upper and lower electrodes. After the treatment chamber was filled with the following mixed gas atmosphere, discharge plasma treatment was performed on the surface of the fluororesin sheet for the treatment time shown in Table 2.

[0224] (Processing atmosphere)

[0225] A: A mixture of argon, helium and oxygen

[0226] B: A mixture of argon, helium, nitrogen and oxygen

[0227] The elemental composition of the surface-treated fluororesin sheet was measured by XPS.

[0228] (Comparative Example 1)

[0229] Sheet A was not subjected to surface treatment, and the elemental composition was measured by XPS.

[0230] (Comparative Examples 2 to 4)

[0231] The discharge plasma treatment of the fluororesin sheet surface was carried out in the same manner as in Example 5 except that sheets E to G were used.

[0232] The elemental composition of the surface-treated fluororesin sheet was measured by XPS.

[0233] Table 2 shows the results of Examples 1 to 10 and Comparative Examples 1 to 4.

[0234]

[0235] [Measurement of Metal Interface Conductivity]

[0236] Copper foil (CF-T9DA-SV-18, manufactured by Fukuda Metal Foil Powder) was placed on top and bottom of the sheet produced in Example 5, with the treated surface of the foil in close contact with the sheet. The sheet was then placed in a vacuum hot press, heated from room temperature to 360°C under vacuum, and pressed at 3 MPa at 360°C to produce a double-sided copper-clad laminate. Circular dielectric samples (no copper on both sides) and circular dielectric samples (patterned copper foil on one side) were prepared from the produced double-sided copper-clad laminate. The metal layer interface conductivity (1e7 S / m) was measured using the balanced circular plate resonator method over a frequency range of 14.5 GHz to 111.5 GHz. The average metal layer interface conductivity of the double-sided copper-clad laminate produced in Example 5 was 5.66 (1e7 S / m), which is equivalent to that of the reference copper circular plate.

[0237] Furthermore, the average metal layer interface conductivity was similarly measured using commercially available RO3003 (manufactured by Rogers Corporation), which is a double-sided copper-clad laminate of a composite sheet of a fluororesin and a filler. The result was 0.56 (1e7 S / m).

[0238] This shows that the double-sided copper-clad laminate using the sheet produced in Example 5 has higher metal interface conductivity than commercially available double-sided copper-clad laminates.

[0239] [Transmission loss measurement]

[0240] Copper foil (CF-T9DA-SV-18, manufactured by Fukuda Metal Foil Powder, Rz = 0.85 μm) was overlapped on the upper and lower sides of the fluororesin sheet produced in Example 5 so that the treated surface was in close contact with the fluororesin sheet. The sheet was then inserted into a vacuum hot press and heated from room temperature to 360°C under vacuum. The sheet was then pressed at a pressure of 3 MPa at 360°C to produce a double-sided copper-clad laminate.

[0241] Microstrip lines were used to form transmission lines on the fabricated double-sided copper-clad laminate. 50 and 68 GHz signals on the printed circuit board were processed using a network analyzer ("E8361A" manufactured by Keysight Technologies, Inc.), and the S21 parameter, which represents transmission loss, was measured using a universal test fixture as a probe. The transmission loss was measured with a characteristic impedance of 50Ω and a transmission line length of 100 mm on the printed circuit board.

[0242] As a measure of transmission loss, the S21 parameter, one of the circuit network parameters used to represent the characteristics of high-frequency electronic circuits and high-frequency electronic components, is used as the transmission loss value. The closer this value is to 0, the smaller the transmission loss.

[0243] The transmission loss is -2.42dB / 100mm at a frequency of 50GHz and -2.96dB / 100mm at a frequency of 67GHz.

[0244] In addition, the transmission loss was similarly measured using commercially available RO3003G2 (manufactured by Rogers Corporation), which is a double-sided copper-clad laminate of a composite sheet of fluororesin and filler. The results showed that the transmission loss was -3.47 dB / 100 mm at a frequency of 50 GHz and -4.53 dB / 100 mm at a frequency of 67 GHz.

[0245] This shows that the double-sided copper-clad laminate using the fluororesin sheet produced in Example 5 has lower transmission loss than commercially available double-sided copper-clad laminates.

[0246] The above experimental results show that the fluororesin sheets of the examples have low CTE and can be well bonded to highly smooth copper foil. The laminated bodies with highly smooth copper foil have high metal interface conductivity and low transmission loss, making them suitable for use as circuit board materials.

[0247] Industrial Applicability

[0248] The fluororesin sheet disclosed herein can be suitably used in a copper-clad laminate for a circuit board.

Claims

1. A fluororesin sheet comprising a fluororesin and silica particles, wherein: On one or both sides thereof, the oxygen element ratio measured by X-ray photoelectron spectroscopy (XPS) is 3.0 atomic % or more, and the coefficient of linear expansion (CTE) is 100 ppm / ° C. or less.

2. The fluororesin sheet according to claim 1, wherein The above-mentioned fluororesin is polytetrafluoroethylene.

3. The fluororesin sheet according to claim 1 or 2, wherein Furthermore, on the same surface, the nitrogen element ratio measured by X-ray photoelectron spectroscopy (XPS) is 1.35 atomic % or more.

4. The fluororesin sheet according to any one of claims 1 to 3, wherein Furthermore, on the same surface, the silicon element ratio measured by X-ray photoelectron spectroscopy (XPS) is 0.5 atomic % or more.

5. The fluororesin sheet according to any one of claims 1 to 4, wherein Furthermore, a droplet volume of 2 μL was placed on the same surface, and the static contact angle of water measured 1 second after the droplet landed was 105° or less.

6. The fluororesin sheet according to any one of claims 1 to 5, wherein The silicon dioxide particles are spherical silicon dioxide.

7. The fluororesin sheet according to any one of claims 1 to 6, wherein As the silica particles, silica particles treated with a silane coupling agent are used.

8. The fluororesin sheet according to any one of claims 1 to 7, wherein The average particle size of the silica particles is 10 μm or less. 9 . The fluororesin sheet according to claim 1 , which contains no glass fiber.

10. The fluororesin sheet according to any one of claims 1 to 9, wherein The content of the silica particles is 30% by mass or more based on the total amount of the fluororesin sheet.

11. The sheet according to any one of claims 1 to 9, wherein The content of the silica particles is 50% by mass or more based on the total amount of the fluororesin sheet.

12. The sheet according to any one of claims 1 to 9, wherein The content of the silica particles is 50% by mass or more and 65% by mass or less based on the total amount of the fluororesin sheet.

13. The fluororesin sheet according to any one of claims 1 to 12, wherein The dielectric loss tangent value at 10 GHz is 0.0015 or less. 14 . The fluororesin sheet according to claim 1 , which has a thickness of 5 μm to 250 μm.

15. A method for producing a fluororesin sheet according to any one of claims 1 to 14, characterized in that: Fluororesin particles and silica particles are mixed to form a film, and then surface treated.

16. The method for producing a fluororesin sheet according to claim 15, wherein A film is formed using a composition consisting essentially of fluororesin particles and filler particles containing at least silica particles, and then surface-treated.

17. A copper-clad laminate comprising a copper foil and the fluororesin sheet according to any one of claims 1 to 14 as essential layers.

18. The copper-clad laminate according to claim 17, wherein The copper foil has a surface roughness Rz of 2.0 μm or less.

19. The copper-clad laminate according to claim 17 or 18, wherein The surface roughness Rq of the copper foil is 0.01 μm to 0.15 μm. 20 . The copper-clad laminate according to claim 17 , wherein the copper foil and the fluororesin sheet are directly laminated, and the peel strength at the interface between the copper foil and the fluororesin sheet is 0.5 kN / m or more.

21. A method for producing a copper-clad laminate, the method for producing a copper-clad laminate according to any one of claims 17 to 20, characterized in that: The fluororesin sheet according to claim 1 is laminated with copper foil, heated at 180° C. to 390° C., and press-molded at a pressure of 0.5 MPa to 5 MPa in a vacuum or in an inert gas atmosphere. 22 . A circuit board comprising the copper-clad laminate according to claim 17 .

23. An antenna formed from the circuit substrate according to claim 22. The antenna according to claim 23 , which is a millimeter wave antenna for a moving object.

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