WO2025187577A1 - Sheet, copper-clad laminate, circuit board, and production method for sheet - Google Patents

Abstract

Provided is a sheet that exhibits little change in dielectric loss tangent during water absorption. The sheet contains a fluororesin and a filler, and has a specific gravity of 2.05-2.19.

Description

Sheet, copper clad laminate, circuit board, and method for manufacturing the sheet

This disclosure relates to a sheet, a copper clad laminate, a circuit board, and a method for manufacturing the sheet.

High-frequency printed wiring boards with low transmission loss are in demand. The use of fluororesin film in such high-frequency printed wiring boards is known (see, for example, Patent Document 1). Furthermore, Patent Documents 2 and 3 describe the use of fluororesin containing filler as a wiring board material.

Furthermore, Patent Document 4 discloses the use of a fluororesin composition in which spherical silica particles are blended with a fluororesin for circuit boards.

Patent Publication No. 2015-8260 Japanese Patent Application Publication No. 63-259907 Special Publication No. 2022-510017 International Publication No. 2020/145133

The objective of this disclosure is to provide a sheet that exhibits a small rate of change in dielectric tangent upon water absorption.

The present disclosure provides a sheet containing a fluororesin and a filler, and having a specific gravity of 2.05 or more and 2.19 or less.
The fluororesin is preferably a polytetrafluoroethylene resin.
The filler is preferably silica.
The silica is preferably spherical silica.
The silica preferably has a particle size of 0.1 to 3 μm.
The silica content is preferably 20 to 80% by mass based on the sheet weight.
The surface of the silica is preferably treated with a surface treatment agent.
It is preferred that the fluororesin is a polytetrafluoroethylene resin having a standard specific gravity of 2.0 to 2.3, the filler is spherical silica, the particle size of the spherical silica is 0.5 to 2.1 μm, the content of the polytetrafluoroethylene resin is 40 to 60 mass % relative to the sheet weight, and the content of the spherical silica is 40 to 60 mass % relative to the sheet weight.

The present disclosure also relates to a sheet containing 20 to 80% by mass of polytetrafluoroethylene resin relative to the sheet weight and surface-treated spherical silica with a particle size of 0.1 to 3 μm, the silica content being 20 to 80% by mass relative to the sheet weight, and having a specific gravity of 2.05 or more and 2.19 or less.

The sheet preferably has a water absorption rate of 0.10% or less.
Preferably, the sheet is obtained by a manufacturing process that includes a densification step that increases the specific gravity of the sheet by 1% or more.
When the rate of change in the dielectric loss tangent of the sheet before and after water absorption is compared, it is preferable that the rate of change in the dielectric loss tangent after densification is lower than that before densification.

The sheet preferably has a linear expansion coefficient of 120 ppm/K or less.
The sheet preferably has a thickness of 0.1 to 2 mm.
The sheet is preferably an insulating material for a circuit board.

The present disclosure also relates to a metal clad laminate having a metal layer and the above-mentioned sheet as essential layers.
The metal layer is preferably a copper foil.
The present disclosure also provides a circuit board comprising the sheet and a metal layer.
The metal constituting the metal layer is preferably copper.
The copper is preferably rolled copper or electrolytic copper.
The circuit board is preferably a printed circuit board, a laminated circuit board, or a high-frequency board.

The present disclosure also provides a method for producing the above-mentioned sheet, comprising: a step (1) of mixing the above-mentioned fluororesin and the filler and rolling the mixture into a sheet; and a step (2) of densifying the rolled sheet obtained by the step (1).
In the densification step (2), the rolled sheet is pressurized so that the specific gravity of the rolled sheet increases by 1% or more, the fluororesin is polytetrafluoroethylene resin having a standard specific gravity of 2.0 to 2.3, the filler is spherical silica having a particle size of 0.5 to 2.1 μm, the polytetrafluoroethylene resin content is 40 to 60 mass% relative to the sheet weight, and the spherical silica content is 40 to 60 mass% relative to the sheet weight.

The sheet disclosed herein has a small rate of change in dielectric tangent when absorbing water.

The present disclosure will be described in detail below.
Many studies have been conducted on compositions containing fluororesins and fillers. Meanwhile, in the field of high-frequency printed wiring boards, increasingly high levels of performance, such as low dielectric constant, low loss, and low expansion, are being demanded in recent years.

In such applications, the electrochemical properties of the sheet must be stable and unaffected by the surrounding environment. However, sheets made of filler and fluororesin are prone to change in dielectric tangent when they absorb water. Since sheets containing fluororesin are required to have low dielectric constant and low loss, in order to achieve stable low loss, it is preferable that the sheet containing fluororesin has low water absorption. Low water absorption makes it possible to reduce the rate of change in dielectric tangent when water is absorbed.

In this disclosure, the density is increased to reduce the rate of change in the dielectric tangent upon water absorption. In other words, by reducing the voids in the resin sheet, water is prevented from entering the sheet upon water absorption, thereby achieving the above-mentioned objective.

Resin compositions containing large amounts of filler are prone to gaps at the interface between the resin and filler, and when formed into a sheet using a conventional molding method, voids are likely to form. Furthermore, voids are generated when the molding aid is removed from the fluororesin. It is believed that this allows water to enter the voids, causing the above-mentioned problems.

Therefore, this problem can be solved by reducing these voids and creating a high-density sheet, for example by performing a densification process after molding.

In view of the above, the sheet of the present disclosure contains a fluororesin and a filler, and has a specific gravity of 2.05 or more and 2.19 or less.
The specific gravity in this disclosure is a value measured by the density and specific gravity measurement method according to JIS Z 8807 (Method for measuring density and specific gravity of solids) 8, submerged weighing method.

If the specific gravity is less than 2.05, the aforementioned problem of a low rate of change in dielectric tangent upon water absorption cannot be fully achieved. Furthermore, if the specific gravity exceeds 2.19, the relative dielectric constant becomes too high, which is undesirable.

The lower limit of the specific gravity is more preferably 2.05. The upper limit of the specific gravity is preferably 2.18, more preferably 2.17, and even more preferably 2.16.
A sheet having such a specific gravity has a low porosity, which provides the above-mentioned effects.

The sheet of the present disclosure preferably has a water absorption rate of 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. By setting the specific gravity within the above range, the sheet of the present disclosure has a low porosity. It is preferable to reduce the porosity and achieve the water absorption rate described above. A water absorption rate of the above value is preferable in that it can effectively achieve the goal of reducing the rate of change in dielectric tangent upon water absorption.

In this disclosure, the water absorption rate is determined by drying a 50 x 50 (mm) sheet at 110°C for 1 hour, cooling it to 23°C in a desiccator, and then immersing it in water at 23°C for 24 hours. The water absorption rate is calculated from the weight before and after immersion in water.

The water absorption rate is more preferably 0.08% or less, and even more preferably 0.07% or less. Since a low water absorption rate does not pose any particular problems, the lower limit is not particularly limited and may be 0%. The lower limit is preferably 0.001%.

The sheet of the present disclosure preferably has a linear expansion coefficient of 120 ppm/K or less. Having such a linear expansion coefficient is preferable in that it results in a dielectric sheet with low shrinkage and excellent dimensional stability. The linear expansion coefficient is more preferably 70 ppm/K or less, and even more preferably 50 ppm/K or less.

There is no particular lower limit for the linear expansion coefficient, but it is more preferably 5 ppm/K, and even more preferably 10 ppm/K.

The linear expansion coefficient in this specification was determined by performing TMA measurements in tension mode using a TMA-7100 (Hitachi High-Tech Science Corporation). A sample piece was cut out of a sheet measuring 20 mm in length, 5 mm in width, and 150 μm in thickness. The distance between the chucks was set to 10 mm, and a load of 49 mN was applied at a heating rate of 2°C/min. The change in sample length was determined from the amount of change in sample length from -10 to 160°C.

The sheet of the present disclosure preferably has a film thickness of 0.03 to 2 mm. By achieving a film thickness within this range, the sheet becomes suitable for applications such as those described in detail below in this disclosure. The lower limit is more preferably 0.05 mm, and even more preferably 0.1 mm. The upper limit is more preferably 1 mm, and even more preferably 0.5 mm.

The film thicknesses in this disclosure are values measured using a film thickness gauge.

The sheet of the present disclosure preferably has a rate of change in relative dielectric constant in the temperature range of -50 to 150°C of 0.025 or less, more preferably 0.023 or less, and even more preferably 0.021 or less. A sheet within this range is preferred in that it minimizes temperature-related changes in electrical properties and provides stable performance when used in high-frequency printed circuit boards.

As described above, the sheet of the present disclosure contains a fluororesin and a filler. A sheet containing these has a low dielectric constant and therefore low transmission loss, making it particularly suitable for use as a printed wiring board.
These components that make up the sheets of the present disclosure are described in detail below.

(Fluorine resin)
The composition of the present disclosure contains a fluororesin, which has low dielectric properties and can therefore be suitably used for the purposes of the present disclosure.

Fluororesins that can be used in the present disclosure are not particularly limited, but examples include polytetrafluoroethylene (PTFE), tetrafluoroethylene [TFE]/hexafluoropropylene [HFP] copolymer [FEP], TFE/alkyl vinyl ether copolymer [PFA], TFE/HFP/alkyl vinyl ether copolymer [EPA], TFE/chlorotrifluoroethylene [CTFE] copolymer, TFE/ethylene copolymer [ETFE], polyvinylidene fluoride [PVdF], and tetrafluoroethylene with a molecular weight of 300,000 or less [LMW-PTFE]. They may be used alone or in combination of two or more. From the perspective of low dielectric constant, polytetrafluoroethylene resin (PTFE) is particularly preferred. PTFE that has fibrillar properties is preferred. Fibrillar PTFE refers to PTFE that can be paste-extruded as unsintered polymer powder.

The PTFE may be modified polytetrafluoroethylene (hereinafter referred to as modified PTFE), homopolytetrafluoroethylene (hereinafter referred to as homoPTFE), or a mixture of modified PTFE and homoPTFE. From the perspective of maintaining good moldability of polytetrafluoroethylene, the content of modified PTFE in the polymeric PTFE is preferably 10% by mass or more and 98% by mass or less, and more preferably 50% by mass or more and 95% by mass or less. The homo-PTFE is not particularly limited, and homo-PTFE disclosed in JP-A-53-60979, JP-A-57-135, JP-A-61-16907, JP-A-62-104816, JP-A-62-190206, JP-A-63-137906, JP-A-2000-143727, JP-A-2002-201217, WO 2007/046345 pamphlet, WO 2007/119829 pamphlet, WO 2009/001894 pamphlet, WO 2010/113950 pamphlet, WO 2013/027850 pamphlet, etc. can be suitably used. Among these, homo-PTFE, which has high stretchability and is disclosed in JP 57-135 A, JP 63-137906 A, JP 2000-143727 A, JP 2002-201217 A, WO 2007/046345, WO 2007/119829, WO 2010/113950, etc., is preferred.

Modified PTFE is composed of TFE and a monomer other than TFE (hereinafter referred to as the modified monomer). Modified PTFE includes, but is not limited to, PTFE that is uniformly modified with the modified monomer, PTFE that is modified at the beginning of the polymerization reaction, and PTFE that is modified at the end of the polymerization reaction. The modified PTFE is preferably a TFE copolymer obtained by polymerizing a small amount of a monomer other than TFE together with TFE, within a range that does not significantly impair the properties of the TFE homopolymer. The modified PTFE can be suitably used, for example, those disclosed in JP-A-60-42446, JP-A-61-16907, JP-A-62-104816, JP-A-62-190206, JP-A-64-1711, JP-A-2-261810, JP-A-11-240917, JP-A-11-240918, WO 2003/033555 pamphlet, WO 2005/061567 pamphlet, WO 2007/005361 pamphlet, WO 2011/055824 pamphlet, WO 2013/027850 pamphlet, etc. Among these, modified PTFE with high stretchability disclosed in JP 61-16907 A, JP 62-104816 A, JP 64-1711 A, JP 11-240917 A, WO 2003/033555 A, WO 2005/061567 A, WO 2007/005361 A, WO 2011/055824 A, etc. are preferred.

Modified PTFE contains TFE units based on TFE and modified monomer units based on modified monomers. The modified monomer units are part of the molecular structure of the modified PTFE and are derived from the modified monomers. The modified PTFE preferably contains modified monomer units in an amount of 0.001 to 0.500 mass% of the total monomer units, and more preferably 0.01 to 0.30 mass%. The total monomer units are the portions derived from all monomers in the molecular structure of the modified PTFE.

The modifying monomer is not particularly limited as long as it is copolymerizable with TFE, and examples 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, etc. One or more types of modifying monomers may be used.

The perfluorovinyl ether is not particularly limited, and examples thereof include perfluorounsaturated compounds represented by the following general formula (1).
CF ₂ =CF-ORf...(1)

In the formula, Rf represents a perfluoroorganic group.

In this specification, a perfluoroorganic group is an organic group in which all hydrogen atoms bonded to carbon atoms are substituted with fluorine atoms. The perfluoroorganic group may also have an ether oxygen.

An example of a perfluorovinyl ether is perfluoro(alkyl vinyl ether) (PAVE), where Rf in the above general formula (1) is a perfluoroalkyl group having 1 to 10 carbon atoms. The number of carbon atoms in the perfluoroalkyl group is preferably 1 to 5. Examples of perfluoroalkyl groups in PAVE include perfluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, and perfluorohexyl groups. Preferred PAVEs are perfluoropropyl vinyl ether (PPVE) and perfluoromethyl vinyl ether (PMVE).

The perfluoroalkylethylene (PFAE) is not particularly limited, and examples include perfluorobutylethylene (PFBE), perfluorohexylethylene (PFHE), etc.

The modifying monomer in the modified PTFE is preferably at least one (e.g., 1 to 6) selected from the group consisting of HFP, CTFE, VDF, PAVE, PFAE, and ethylene.

The above-mentioned fluororesin is preferably non-melt-moldable. "Non-melt-moldable" means that the resin does not have sufficient fluidity even when heated above its melting point, and cannot be molded using melt-molding techniques commonly used for resins. PTFE falls into this category.

In the present disclosure, it is preferable to use such fluororesin that cannot be melt-molded and turn it into a fluororesin sheet using a molding method that fibrillates it. This molding method will be described later.

The PTFE preferably has an SSG of 2.0 to 2.3. Using such PTFE makes it easier to obtain a PTFE membrane with high strength (cohesion and puncture strength per unit thickness). PTFE with a large molecular weight has long molecular chains, making it difficult to form a structure with regularly arranged molecular chains. In this case, the length of the amorphous portion increases, and the degree of entanglement between molecules increases. When the degree of entanglement between molecules is high, the PTFE membrane is less likely to deform under an applied load and is thought to exhibit excellent mechanical strength. Furthermore, using PTFE with a large molecular weight makes it easier to obtain a PTFE membrane with a small average pore size.

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.

Standard specific gravity (SSG) is measured by preparing a sample in accordance with ASTM D-4895-89 and measuring the specific gravity of the resulting sample using the water displacement method.

In this embodiment, the molecular weight (number average molecular weight) of the PTFE constituting the PTFE powder is, for example, in the range of 2 million to 12 million. The lower limit of the molecular weight of PTFE may be 3 million or 4 million. The upper limit of the molecular weight of PTFE may be 10 million.

There are two methods for measuring the number average molecular weight of PTFE: one is to determine it from the standard specific gravity, and the other is to measure it using dynamic viscoelasticity in the melt. The method for determining it from the standard specific gravity can be performed using a sample molded in accordance with ASTM D-4895 98 and the water displacement method in accordance with ASTM D-792. The dynamic viscoelasticity measurement method is explained, for example, by S. Wu in Polymer Engineering & Science, 1988, Vol. 28, 538, and the same publication, 1989, Vol. 29, 273.

The refractive index of the PTFE is preferably in the range of 1.2 to 1.6. Having such a refractive index is preferable in that it has low dielectric constant. The refractive index can be adjusted to fall within the above range by adjusting the polarizability or 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.

The above refractive index was measured using a refractometer (Abbemat 300).

Furthermore, it is preferable that the maximum endothermic peak temperature (crystalline melting point) of the above-mentioned PTFE is 340±7°C.

The PTFE may be low-melting-point PTFE, with a maximum peak temperature of 338°C or less on the endothermic curve of the crystalline melting curve measured with a differential scanning calorimeter, or high-melting-point PTFE, with a maximum peak temperature of 342°C or more on the endothermic curve of the crystalline melting curve measured with a differential scanning calorimeter.

The low-melting-point PTFE is a powder produced by emulsion polymerization, and preferably has the above-mentioned maximum endothermic peak temperature (crystalline melting point), a dielectric constant (ε) of 2.08 to 2.2, and a dielectric dissipation factor (tan δ) of 1.9 × 10 ^-4 to 4.0 × 10 ^-4 . 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 Industry Co., Ltd.; and TF6C, TF62, and TF40 manufactured by DuPont.

High-melting-point PTFE powder is also a powder produced by emulsion polymerization, has the above-mentioned maximum endothermic peak temperature (crystalline melting point), and has a generally low dielectric constant (ε) of 2.0 to 2.1 and a dielectric dissipation factor (tan δ) of 1.6 × 10 ^-4 to 2.2 × 10 ^-4 . Commercially available products include Polyflon Fine Powder F104 and F106 manufactured by Daikin Industries, Ltd.; CD1, CD141, and CD123 manufactured by Asahi Glass Co., Ltd.; and TF6 and TF65 manufactured by DuPont.

Powdered PTFE that meets the above-mentioned parameters can be obtained using conventional manufacturing methods. For example, it can be produced by following the manufacturing methods described in International Publication Nos. 2015-080291 and 2012-086710.

(Filler)
The filler that can be used in the present disclosure is not particularly limited, and examples thereof include organic fillers that are one or more selected from aramid fibers, polyphenyl esters, polyphenylene sulfide, polyimides, polyether ether ketones, polyphenylenes, polyamides, and wholly aromatic polyester resins, and inorganic fillers that are one or more selected from ceramics, talc, mica, aluminum oxide, zinc oxide, tin oxide, titanium oxide, silica, calcium carbonate, calcium oxide, magnesium oxide, potassium titanate, glass fibers, glass chips, glass beads, silicon carbide, calcium fluoride, boron nitride, barium sulfate, molybdenum disulfide, and potassium carbonate whiskers. Two or more of these may also be used in combination.

The shape of the filler is not particularly limited, but spherical fillers are particularly preferred. Spherical shapes are preferred because they facilitate uniform processing during drilling, have a small specific surface area, and result in low transmission loss.

Among these, it is particularly preferable to use silica, and it is most preferable to use spherical silica particles.

The spherical silica particles mentioned above refer to particles whose shape is close to a perfect sphere. Specifically, the sphericity is preferably 0.80 or higher, more preferably 0.85 or higher, even more preferably 0.90 or higher, and most preferably 0.95 or higher. Sphericity is calculated by taking a photograph with an SEM and using the area and perimeter of the observed particles, as follows: (sphericity) = {4π x (area) ÷ (perimeter)2}. The closer the value is to 1, the closer the particle is to a perfect sphere. Specifically, the average value measured for 100 particles using an image processing device (Spectris Inc.: FPIA-3000) is used.

The spherical silica particles used in this disclosure preferably have a D90/D10 of 2 or more (preferably 2.3 or more, or 2.5 or more) and a D50 of 10 μm or less, when integrating the volume from the smallest particle size. Furthermore, it is preferable that D90/D50 is 1.5 or more (even more preferably 1.6 or more). It is preferable that D50/D10 is 1.5 or more (even more preferably 1.6 or more). Furthermore, it is even more preferable that D50 is 5 μm or less. Since small-sized spherical silica particles can enter the gaps between larger-sized spherical silica particles, excellent filling properties and high fluidity can be achieved. In particular, it is preferable that the particle size distribution has a higher frequency on the small particle size side compared to a Gaussian curve. The particle size can be measured using a laser diffraction/scattering particle size distribution analyzer. Furthermore, because coarse particles make it difficult to form a thin film sheet, it is preferable that coarse particles above a certain particle size have been removed using a filter or the like.

The above-mentioned spherical silica particles preferably have a water absorption of 1.0% or less, and more preferably 0.5% or less. Water absorption is based on the mass of the silica particles when dry. Water absorption is measured by leaving a dry sample at 40°C and 80% RH for 1 hour, and then measuring and calculating the amount of water generated by heating to 200°C using a Karl Fischer moisture analyzer.

Furthermore, the spherical silica particles can be measured using the above-mentioned methods after the fluororesin sheet is heated in an air atmosphere at 600°C for 30 minutes to burn off the fluororesin and the spherical silica particles are removed.

It is preferable that the silica particles have been surface-treated. Pre-surface treatment is preferable because it can prevent the silica particles from aggregating and allow the silica particles to be well dispersed in the resin composition.

The surface treatment is not particularly limited, and any known treatment can be used. Specific examples include treatment with a silane coupling agent such as an epoxy silane, amino silane, isocyanate silane, vinyl silane, acrylic silane, hydrophobic alkyl silane, phenyl silane, or fluorinated alkyl silane, which has a reactive functional group; plasma treatment; and fluorination treatment.

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 acrylic silanes such as acryloxytrimethoxysilane.

The spherical silica particles may 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 Company Limited), Denka Fused Silica SFP Grade (manufactured by Denka Company Limited), Excelica (manufactured by Tokuyama Corporation), high-purity synthetic spherical silica particles Admafine (manufactured by Admatechs Co., Ltd.), Admanano (manufactured by Admatechs Co., Ltd.), and Admafuse (manufactured by Admatechs Co., Ltd.).

The filler used in the present disclosure preferably has a ratio of (dielectric tangent of the filler measured at 10 GHz)/(surface area of the filler (m ² /g)) of 0.00001 to 0.00035. Use of a filler that satisfies this relationship is preferred in that it allows for the production of a sheet with particularly excellent low dielectric constant, low loss, and low expansion.

The value of (dielectric loss tangent of filler measured at 10 GHz)/(surface area of filler (m ² /g)) can be adjusted by the shape and size of the filler, whether or not surface treatment is performed, and the like. More specifically, it is preferable to use spherical silica particles of a predetermined size as the above-mentioned spherical silica particles, which are further surface-treated. It is preferable that the surface of the silica is treated with a surface treatment agent. When surface treatment is performed, the type of surface treatment agent also affects the above parameter. More specifically, it is particularly preferable that the silica is surface-treated with aminopropyltriethoxysilane, aminosilane, vinylsilane, hydrophobic alkylsilane, phenylsilane, 3-mercaptopropylsilane, 3-acryloxypropylsilane, 3-methacryloxypropylsilane, p-styrylsilane, silylpropylsuccinic anhydride, 3-isocyanatopropylsilane, 2-(3,4-epoxycyclohexyl)ethylsilane, or the like. By carrying out surface treatment with these silane coupling agents, the polar functional groups present on the filler surface react, reducing the amount of polar functional groups, resulting in excellent electrical properties.

The filler is preferably contained in an amount of 20 to 80% by mass relative to the sheet weight. This amount is preferable in that it achieves low thermal expansion while maintaining a low dielectric constant and low loss. The amount is more preferably 40% by mass or more, even more preferably 53% by mass or more, and even more preferably 56% by mass or more. There is no particular upper limit to the filler amount, but it is preferably 75% by mass or less, even more preferably 70% by mass or less, even more preferably 65% by mass or less, and even more preferably 60% by mass or less.

The upper limit of the above (dielectric loss tangent of the filler measured at 10 GHz)/(surface area of the filler (m ² /g)) is more preferably 0.00030, and even more preferably 0.00025.

In this disclosure, the dielectric loss tangent of the filler measured at 10 GHz was measured using a cylindrical cavity resonator and network analyzer. A filler powder sample was filled into a quartz tube and loaded into the resonator. The resonator's characteristics (resonance frequency and Q value) were obtained before and after inserting the sample, and the dielectric loss tangent was calculated from the results. This measurement method complies with Japanese Industrial Standard JIS 2565, Microwave Ferrite Core Test Method, and measurements were performed in an environment with a room temperature of 25°C and humidity of 40%.

In the present disclosure, the dielectric loss tangent of the filler measured at 10 GHz is not particularly limited, but is preferably 0.0015 or less. This value is preferable in that it results in low loss for the fluororesin sheet. The upper limit is more preferably 0.0025, and even more preferably 0.002.

In the present disclosure, the surface area (m ² /g) of the filler is not particularly limited, but is preferably 1 to 10. Setting it within this range is preferable in that it achieves a good balance between low loss and low linear expansion of the fluororesin sheet. The lower limit is more preferably 1.2, and even more preferably 1.5. The upper limit is more preferably 9, and even more preferably 7.

In the present disclosure, the surface area (m ² /g) of the filler is a value based on the BET method, and can be measured using a specific surface area measuring device "Macsorb HM model-1208" (manufactured by MACSORB Co., Ltd.). When the fluororesin sheet of the present disclosure contains two or more types of fillers, the surface area measured for all the blended fillers falls within the above-mentioned range.

In the present disclosure, the average particle size of the filler is preferably 0.1 μm or more, and more preferably 0.5 μm or more. In the present disclosure, the average particle size of the filler is preferably 250 μm or less, more preferably 100 μm or less, even more preferably 10 μm or less, even more preferably 3 μm or less, even more preferably 2.5 μm or less, and particularly preferably 2.1 μm or less.

In the present disclosure, the filler may have an average particle size of 0.5 to 250 μm. Note that the average particle size here is the D50 value measured using a laser analytical particle size distribution analyzer. If the average particle size is 0.5 μm or greater, the filler is less likely to aggregate, and sufficient effects tend to be obtained.

(About the composition)
The sheet of the present disclosure contains the above-described filler and fluororesin. If necessary, it may contain components other than the filler and fluororesin, or it may consist only of the filler and fluororesin. The content of components other than the filler and fluororesin is preferably 10% by mass or less (0% by mass, i.e., not contained, or greater than 0% by mass and 10% by mass or less). In other words, the total content of the fluororesin and filler is preferably, for example, 90% by mass or more and 100% by mass or less relative to the sheet weight. The ratio Wf/Wr of the mass of the filler Wf to the mass Wr of the fluororesin may be, for example, 0.6 to 1.5.

The sheet of the present disclosure preferably has a fluororesin content of 20 to 80% by mass relative to the total amount of the sheet. Incorporating a filler in this range is preferable in that it reduces the linear expansion coefficient and makes the sheet easier to mold. The lower limit of the filler content is not particularly limited, but from the perspective of reducing the linear expansion coefficient, it is preferably 30% by mass, and more preferably 40% by mass. The upper limit is more preferably 75% by mass, even more preferably 70% by mass, even more preferably 65% by mass, and even more preferably 60% by mass.

(Sheet manufacturing method)
The method for manufacturing a sheet according to the present disclosure is not particularly limited, but an example thereof will be described. The method for manufacturing a sheet according to the present disclosure includes, for example, step (1) of mixing the fluororesin and the filler and rolling them into a sheet; and step (2) of densifying the rolled sheet obtained by step (1). More specifically, the sheet according to the present disclosure can be obtained by mixing fluororesin particles and a filler to form a film in step (1), and then rolling the sheet obtained by the film formation in step (2). By rolling, the amount of voids in the sheet is reduced, thereby achieving high density. The method for forming the film on the sheet in step (1) is not limited, but can be performed by paste extrusion molding, powder rolling molding, or the like.

As mentioned above, it is preferable to use a fluororesin that cannot be melt-molded as the fluororesin used in the sheet of the present disclosure. When using such a fluororesin and molding it into a sheet, it is preferable to do so by fibrillating powdered PTFE as the raw material.

It is preferable to use powdered PTFE with a primary particle diameter of 0.05 to 10 μm. Using such a material has the advantage of excellent moldability and dispersibility. The primary particle diameter here is a value measured in accordance with ASTM D 4895.

The above-mentioned powdered PTFE preferably contains 50% by mass or more, and more preferably 80% by mass or more, of polytetrafluoroethylene resin with a secondary particle diameter of 500 μm or more. Having PTFE with a secondary particle diameter of 500 μm or more within this range has the advantage of allowing the production of a high-strength sheet. Using PTFE with a secondary particle diameter of 500 μm or more makes it possible to obtain a sheet with lower resistance and greater toughness.

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

The powdered PTFE preferably has an average primary particle diameter of 50 nm or more, as this allows for the production of a sheet with higher strength and excellent homogeneity. It is more preferably 100 nm or more, even more preferably 150 nm or more, and particularly preferably 200 nm or more. The larger the average primary particle diameter of PTFE, the more effectively it can suppress an increase in paste extrusion pressure when paste extrusion molding is performed using the powder, and the better the moldability. There is no particular upper limit, but it may be 500 nm. From the viewpoint of productivity in the polymerization process, a value of 350 nm is preferred.

The average primary particle diameter can be determined by creating a calibration curve between the transmittance of 550 nm incident light per unit length of an aqueous dispersion of PTFE obtained by polymerization, with the polymer concentration adjusted to 0.22% by mass, and the average primary particle diameter determined by measuring the unidirectional diameter in a transmission electron microscope photograph, and then measuring the transmittance for the aqueous dispersion to be measured, and then using the calibration curve.

The PTFE used in this disclosure may have a core-shell structure. Examples of PTFE with a core-shell structure include modified polytetrafluoroethylene particles containing a core of high-molecular-weight polytetrafluoroethylene and a shell of lower-molecular-weight polytetrafluoroethylene or modified polytetrafluoroethylene. Examples of such modified polytetrafluoroethylene include the polytetrafluoroethylene described in JP-A-2005-527652.

The specific methods for paste extrusion molding and powder rolling molding in step (1) are not particularly limited, but general methods are described below.

(Paste extrusion molding)
The method for producing the sheet may include the steps of: (1a) mixing the PTFE powder obtained using a hydrocarbon surfactant with an extrusion aid; (1b) paste-extrusion molding the resulting mixture; (1c) rolling the extrudate obtained by extrusion; (1d) drying the rolled sheet; and (1e) firing the dried sheet to obtain a molded product. The paste extrusion molding may also be carried out by adding conventionally known additives such as pigments and fillers to the PTFE powder.

The extrusion aid is not particularly limited, and commonly known agents can be used. Examples include hydrocarbon oils.

(Powder rolling molding)
The sheet can also be formed by powder rolling. Powder rolling is a method of applying shear force to resin powder to fibrillate it and form it into a sheet. This method may include a subsequent step of firing the powder to obtain a molded product. More specifically, the method includes a step (1-1) of applying shear force while mixing a raw material composition containing a fluororesin and a filler.
a step (1-2) of forming the mixture obtained in the step (1-1) into a bulk form, and a step (1-3) of rolling the bulk mixture obtained in the step (1-2) into a sheet form.
When a sheet is formed by such powder rolling molding, it is preferable to mix only the fluororesin particles and the inorganic filler and then mold the mixture.

(Densification process)
In order to obtain the sheet of the present disclosure, it is preferable to perform the above-mentioned densification treatment in step (2). The densification treatment is not particularly limited, and specific examples include pressing with a pressure roll or a pressure press.

The densification treatment preferably increases the specific gravity of the sheet by 1% or more. More specifically, in step (2), the rolled sheet obtained in step (1) is preferably pressurized so that the specific gravity of the rolled sheet increases by 1% or more. Increasing the specific gravity in this manner allows the objectives of the present disclosure to be successfully achieved. To achieve such an increase rate, it is preferable to appropriately adjust the pressure, temperature, pressurization time, etc. The upper limit of the increase rate of specific gravity is not particularly limited, but is, for example, 10% or less.

Furthermore, when comparing the rate of change in dielectric tangent before and after water absorption, the densification treatment preferably results in a lower rate of change in the dielectric tangent of the densified sheet compared to the sheet before densification. That is, the dielectric tangent of the sheet before densification treatment and the sheet after densification treatment are measured before and after water absorption. The rate of change in dielectric tangent due to water absorption is then calculated for each sheet before and after densification treatment. In this case, the densified sheet has a smaller rate of change in dielectric tangent. Note that water absorption here is measured using the same method as the water absorption rate measurement method described above.

To fully obtain the benefits of the densification treatment, the ratio R2/R1 of the rate of change R2 of the dielectric tangent of the sheet before and after absorbing water to the rate of change R1 of the dielectric tangent of the sheet before and after absorbing water, is preferably 0.8 or less, and more preferably 0.7 or less. The lower the ratio R2/R1, the better, and there is no particular lower limit, but it may be, for example, 0.0 or more.

The pressure roll comes into direct contact with the sheet passing through the pressing mechanism and applies pressure to it. This is particularly preferred because it increases the density of the sheet. The pressure roll is preferably a rubber roll, a resin roll, or a metal roll. In particular, a pair of rolls, one of which is a metal roll and the other of which is a rubber-coated roll with a metal core, can be used to apply appropriate pressure. The material of the metal roll is not particularly limited, and examples include iron, stainless steel (SUS304, SUS430, SUS410, SUS403, etc.), copper, etc. Furthermore, the surface of the metal roll may be subjected to various surface treatments to improve durability and processability. Surface treatments are not particularly limited, and include plating treatments such as chrome plating, copper plating, nickel plating, or composite plating of these; mechanical treatments such as embossing and grooving; and treatments to improve release properties such as fluorine coating and silicone coating.

The linear pressure applied to the sheet by the pressure roll is preferably 30 to 500 kg/cm. More preferably, it is 40 to 400 kg/cm. Even more preferably, it is 50 to 300 kg/cm. This range is preferable because it makes it easier to produce a sheet with the specified density described above.

When applying pressure using the pressure roll, the electric furnace or the pressure roll itself may be heated. The temperature range of the sheet is preferably 10 to 250°C, more preferably 15 to 150°C, and even more preferably 20 to 100°C. By using such pressure conditions, the sheets can be more easily integrated, and the objectives of the present disclosure can be preferably achieved. The pressure rolls may have different diameters and rotation speeds on opposite sides. This design allows the shear force applied to the sheet to be varied.

The pressure roll may be any known means for pressing. More specifically, examples include a pair of rolls as described in Japanese Patent No. 6590350, a two-high rolling mill as described in Japanese Patent No. 5087646, a cluster mill as described in International Publication No. 2020/204070, and a planetary rolling mill as described in Japanese Patent Application Laid-Open No. 62-275508. The pressure roll may be equipped with a cleaning mechanism. The cleaning mechanism is not particularly limited, and examples include an adhesive roll.

The pressure press directly contacts the sheet with a press-fitting mechanism and applies pressure. This is particularly preferred because it increases the density of the sheet. The pressure press is preferably made of rubber, resin, or metal. Using a metal plate, in particular, allows for the application of appropriate pressure. The material of the metal plate is not particularly limited, and examples include iron, stainless steel (SUS304, SUS430, SUS410, SUS403, etc.), copper, etc. The plate surface may also be subjected to various surface treatments to improve durability and processability. Surface treatments are not particularly limited, and include plating treatments such as chrome plating, copper plating, nickel plating, or composite plating of these; mechanical treatments such as embossing and grooving; and treatments to improve release properties such as fluorine coating or silicone coating.

The pressure applied by the pressure press is preferably 1 to 200 MPa. More preferably, it is 5 to 100 MPa. Even more preferably, it is 10 to 80 MPa. This range is preferable because it makes it easier to produce a sheet with the specified density described above.

When applying pressure using the pressure press, the electric furnace or pressure plate itself may be heated. The temperature range of the sheets is preferably 10 to 250°C, more preferably 15 to 150°C, and even more preferably 20 to 100°C. By using these pressure conditions, the sheets can be more easily integrated, and the objectives of the present disclosure can be preferably achieved.

(Laminate)
The sheet-shaped resin composition of the present disclosure can be used by laminating it with other substrates as a sheet for printed wiring boards, i.e., it can be suitably used as an insulating material for circuit boards.

The present disclosure also relates to a metal clad laminate having a metal layer and the above-described sheet as essential layers. The metal clad laminate of the present disclosure is, for example, a metal clad laminate characterized by having a metal layer adhered to one or both sides of the above-described sheet (e.g., a fluororesin film). As described above, the fluororesin-containing film of the present disclosure is particularly suitable for use in printed wiring board applications, and can therefore be suitably used as such a metal clad laminate.

In the present disclosure, examples of metal species constituting the metal layer include copper (e.g., rolled copper, electrolytic copper, etc.), aluminum, SUS, nickel, gold, etc. Alloys of these can also be used. From the perspective of conductivity and circuit processability, copper is preferably used. A heat-resistant layer (nickel plating, titanium plating, etc.) or an anti-rust layer (chromate treatment layer, etc.) may be formed on the copper surface. Furthermore, the surface may be chemically treated with a silane coupling agent. Among these, it is preferable to use copper foil as the metal layer.

The copper foil preferably has an Rz of 1.6 μm or less. In other words, the sheet of the present disclosure also has excellent adhesion to highly smooth copper foil with an Rz of 1.6 μm or less. Furthermore, the copper foil only needs to have an Rz of 1.6 μm or less on at least the surface that adheres to the fluororesin film; the Rz value of the other surface is not particularly limited. The Rz value is the sum of the highest point (maximum peak height: Rp) and the deepest point (maximum valley depth: Rv). The surface roughness is the ten-point average roughness specified in JIS-B0601. In this specification, the Rz value is measured using a surface roughness meter (product name: Surfcom 470A, manufactured by Tokyo Seiki Co., Ltd.) with a measurement length of 4 mm.

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

The copper foil is not particularly limited, and specific examples include rolled copper foil and electrolytic copper foil.

There are no particular limitations on the copper foil with an Rz of 1.6 μm or less, and commercially available products can be used. Examples of commercially available copper foil with an Rz of 1.6 μm or less include electrolytic copper foil CF-T9DA-SV-18 (thickness 18 μm/Rz 0.85 μm) (manufactured by Fukuda Metal Foil & Powder Co., Ltd.).

The copper foil may be surface-treated to increase its adhesive strength with the fluororesin film of the present disclosure.

The surface treatment is not particularly limited, but examples include silane coupling treatment, plasma treatment, corona treatment, UV treatment, and electron beam treatment. The reactive functional group of the silane coupling agent is not particularly limited, but from the perspective of adhesion to resin substrates, it is preferable for the silane coupling agent to have at least one (e.g., one to four) terminal group selected from amino groups, (meth)acrylic groups, mercapto groups, and epoxy groups. Furthermore, the hydrolyzable group is not particularly limited, but examples include alkoxy groups such as methoxy groups and ethoxy groups. The copper foil used in this disclosure may have an anti-corrosion layer (such as an oxide film such as chromate), a heat-resistant layer, etc. formed thereon.

Surface-treated copper foil having a surface treatment layer made from the above-mentioned silane compound on the copper foil surface can be produced by preparing a solution containing the silane compound and then surface treating the copper foil with this solution.

The copper foil may have a roughened layer on the surface from the viewpoint of improving adhesion to the resin substrate.
If the roughening treatment is likely to degrade the performance required in the present disclosure, the amount of roughening particles electrodeposited on the copper foil surface may be reduced as needed, or the roughening treatment may not be performed at all.

In order to improve various properties, one or more layers selected from the group consisting of a heat-resistant treatment layer, a rust-proofing treatment layer, and a chromate treatment layer may be provided between the metal layer and the surface treatment layer. These layers may be a single layer or multiple layers.

The metal-clad laminate of the present disclosure may further include a layer other than the metal layer and the fluororesin film.
The layers other than the metal layer and the fluororesin film are preferably at least one type (e.g., 1 to 12 types) selected from the group consisting of polyimide, modified polyimide, liquid crystal polymer, polyphenylene sulfide, cycloolefin polymer, polystyrene, epoxy resin, bismaleimide, polyphenylene oxide, modified polyphenylene ether, polyphenylene ether, and polybutadiene.

These layers other than the metal layer and fluororesin film are not particularly limited as long as they are made of the resins mentioned above. Furthermore, it is preferable that the thickness of the layers other than the copper foil and fluororesin film be within the range of 12.5 to 260 μm.

In the metal-clad laminate of the present disclosure, the metal layer may be formed on one or both sides of the roll film. Methods for forming the metal layer include laminating (adhering) metal foil to the surface of the roll film, vapor deposition, and plating. Methods for laminating copper foil include heat pressing. The heat pressing temperature may be between the melting point of the dielectric film -150°C and the melting point of the dielectric film +40°C. The heat pressing time is, for example, 1 to 30 minutes. The laminate can be manufactured using a heat pressing pressure of 0.1 to 10 MPa.

The present disclosure also relates to a circuit board characterized by having the above-mentioned sheet and a metal layer. The above-mentioned metal-clad laminate is not particularly limited in its application and is used as a circuit board. Examples of circuit boards include printed circuit boards, laminated circuit boards, and high-frequency boards. A printed circuit board is a plate-shaped component that electrically connects electronic components such as semiconductors and capacitor chips while also arranging and fixing them in a limited space. There are no particular restrictions on the configuration of a printed circuit board formed from this metal-clad laminate. The printed circuit board may be a rigid board, a flexible board, or a rigid-flexible board. The printed circuit board may be a single-sided board, a board, a double-sided board, or a multilayer board (such as a built-up board). It is particularly suitable for use as a flexible board or a rigid board. It is particularly suitable for use as a high-frequency printed circuit board of 10 GHz or more.

The circuit board is not particularly limited and can be manufactured using the metal-clad laminate described above using a general method.

A laminate for a circuit board is also a laminate characterized by having a metal layer, the above-mentioned fluororesin film, and a substrate layer. There are no particular restrictions on the substrate layer, but it is preferable for it to have a fabric layer made of glass fiber and a resin film layer.

The glass fiber fabric layer is a layer made of glass cloth, glass nonwoven fabric, or the like.
Commercially available glass cloths can be used, preferably those treated with a silane coupling agent to enhance affinity with the fluororesin. Examples of glass cloth materials include E-glass, C-glass, A-glass, S-glass, D-glass, NE-glass, and low-dielectric-constant glass, with E-glass, S-glass, and NE-glass being preferred due to their ease of availability. The fiber weave may be plain or twill. The thickness of the glass cloth is typically 5 to 90 μm, preferably 10 to 75 μm, but it is preferable to use glass cloth that is thinner than the fluororesin film used.

The laminate may also use a glass nonwoven fabric as a fabric layer made of glass fibers. Glass nonwoven fabric is made by bonding short glass fibers with a small amount of a binder compound (resin or inorganic material), or by entangling the short glass fibers without the use of a binder compound to maintain its shape. Commercially available glass nonwoven fabrics can be used. The diameter of the short glass fibers is preferably 0.5 to 30 μm, and the fiber length is preferably 5 to 30 mm. Specific examples of binder compounds include resins such as epoxy resins, acrylic resins, cellulose, polyvinyl alcohol, and fluororesins, as well as inorganic materials such as silica compounds. The amount of binder compound used is typically 3 to 15% by mass of the short glass fibers. Examples of materials for the short glass fibers include E-glass, C-glass, A-glass, S-glass, D-glass, NE-glass, and low-dielectric-constant glass. The thickness of the glass nonwoven fabric is typically 50 to 1000 μm, preferably 100 to 900 μm. The thickness of the glass nonwoven fabric in this application refers to the value measured in accordance with JIS P8118:1998 using a digital gauge DG-925 (load 110 grams, face diameter 10 mm) manufactured by Ono Sokki Co., Ltd. The glass nonwoven fabric may be treated with a silane coupling agent to increase its affinity with the fluororesin.

Since most glass nonwoven fabrics have a very high porosity of over 80%, it is preferable to use a sheet that is thicker than a fluororesin sheet and compress it under pressure.

The glass fiber fabric layer may be a layer in which a glass cloth and a glass nonwoven fabric are laminated together, thereby combining the properties of both layers to obtain suitable properties.
The glass fiber fabric layer may be in the form of a prepreg impregnated with a resin.

In the laminate, the glass fiber fabric layer and the fluororesin film may be bonded at the interface, or the glass fiber fabric layer may be partially or entirely impregnated with the fluororesin film.
Furthermore, a prepreg may be prepared by impregnating a fabric made of glass fiber with a fluororesin composition. The prepreg thus obtained may be further laminated with the fluororesin film of the present disclosure. In this case, the fluororesin composition used to prepare the prepreg is not particularly limited, and the fluororesin film of the present disclosure may also be used.

The resin film used as the substrate layer is preferably a heat-resistant resin film or a thermosetting resin film. Examples of heat-resistant resin films include polyimide, modified polyimide, liquid crystal polymer, and polyphenylene sulfide. Examples of thermosetting resins include those containing epoxy resin, bismaleimide, polyphenylene oxide, modified polyphenylene ether, polyphenylene ether, and polybutadiene.
The heat-resistant resin film and the thermosetting resin film may contain reinforcing fibers. The reinforcing fibers are not particularly limited, but for example, glass cloth, particularly low-dielectric type, is preferred.
The dielectric properties, linear expansion coefficient, water absorption coefficient, and other properties of the heat-resistant resin film and the thermosetting resin film are not particularly limited, but for example, the dielectric constant at 20 GHz is preferably 3.8 or less, more preferably 3.4 or less, and even more preferably 3.0 or less. The dielectric loss tangent at 20 GHz is preferably 0.0030 or less, more preferably 0.0025 or less, and even more preferably 0.0020 or less. The linear expansion coefficient is preferably 100 ppm/°C or less, more preferably 70 ppm/°C or less, and even more preferably 40 ppm/°C or less. The water absorption is preferably 1.0% or less, more preferably 0.5% or less, and even more preferably 0.1% or less.

The present disclosure will now be described in detail based on examples. In the following examples, unless otherwise specified, "parts" and "%" represent "parts by mass" and "% by mass," respectively.

Each ingredient is as follows:
PTFE:
PTFE having the following properties was used:
Particle size: 500μm
Apparent density: 460g/L
Standard specific gravity: 2.17
Melting point: 327°C

Silica 1-6:
Silica 1, 3 to 6 used were SC-6500SQ (particle size 2.1 μm, spherical) manufactured by Admatechs Co., Ltd.
Silica 2 used was SC-2500SQ (particle size 0.5 μm, spherical) manufactured by Admatechs Co., Ltd.
The silica was subjected to the following surface treatment.
Silica 3: Aminopropyl, Treated at 1.0%
Silica 4: Aminopropyl, treatment amount 0.2%
Silica 5: Phenylamino, treatment amount 0.4%
Silica 6: Isocyanate, treated amount 0.2%

The treating agents used in the surface treatment are as follows:
Aminopropyl: 3-aminopropyltriethoxysilane Phenylamino: N-phenyl-3-aminopropyltrimethoxysilane Isocyanate: 3-isocyanatopropyltriethoxysilane

(Seat manufacturing)
According to the manufacturing method described in detail below, each sheet of the examples and comparative examples shown in Table 1 was manufactured.
The PTFE and silica were weighed out to the proportions shown in Table 1 and mixed in a mixer in the presence of dry ice. The temperature during mixing was −10°C or below. The resulting powder was left at room temperature for 2 hours, then placed in a plastic container, and 19 parts of a processing aid, hydrocarbon oil (product name: IP2028, manufactured by Idemitsu Kosan Co., Ltd.), were added (19 parts hydrocarbon oil per 100 parts of PTFE and silica combined). The mixture was mixed for 3 minutes, left in a thermostatic oven at 25°C for 2 hours, and then heated to 40°C using a mold with a flat outlet to extrude the mixture into a paste. The resulting sheet was rolled between two metal rolls to obtain a sample with a thickness of 125 μm, which was then dried at 200°C for 2 hours.

(High density sheet)
The dried sheet was rolled at room temperature (25°C) using a pair of rolls, one of which was a metal roll and the other of which was a rubber-coated roll with a metal core. The sheet was then baked at 360°C for 15 minutes to obtain a sheet. The resulting sheet was evaluated according to the following criteria. The results are shown in Table 1.

(specific gravity of sheet)
JIS Z 8807 (Method for measuring density and specific gravity of solids) 8 Measurement was carried out according to the method for measuring density and specific gravity by the submerged weighing method.

Furthermore, the rate of change in the specific gravity of the sheet before and after densification was also calculated.
The rate of change in the specific gravity of the sheet is
The rate of change was calculated as follows: (specific gravity after densification - specific gravity before densification) * 100 / specific gravity before densification.

(Water absorption test)
The sheet was cut into a 5 cm x 5 cm square. After wiping off any dirt on the surface of the sheet, it was dried at 110°C for one hour. Immediately after drying, it was allowed to cool naturally in a desiccator. When the sample had cooled to room temperature, it was weighed and this weight was designated W1. Next, it was immersed in water at room temperature for 24 hours to allow it to absorb water. After wiping off the moisture from the sheet after water absorption, its weight was measured (W2).
The water absorption rate is
Water absorption rate = [(W2-W1)/W1] x 100
was calculated as follows.

(dielectric tangent)
Df was measured at 25° C. and 10 GHz using a split cylinder type dielectric constant/dielectric loss tangent measuring device (manufactured by EM Lab).
The dielectric loss tangents of the normal sheet and the sheet after water absorption under the same conditions as in the water absorption test were measured. Furthermore, the rate of change in the dielectric loss tangent before and after water absorption was calculated.
The rate of change is
The change rate was calculated as follows: rate of change = (dielectric loss tangent after water absorption - dielectric loss tangent before water absorption) * 100 / dielectric loss tangent before water absorption.

(Coefficient of linear expansion)
TMA measurement was carried out in tension mode using a TMA-7100 (manufactured by Hitachi High-Tech Science Corporation). A sheet cut to a length of 20 mm, width of 5 mm, and thickness of 150 μm was used as a sample piece, and the distance between chucks was set to 10 mm. The sample was subjected to a load of 49 mN and the temperature was increased at a rate of 2°C/min from 0 to 150°C, and the displacement was determined from the amount of deformation.

The sheets of the present disclosure are particularly suitable for use in high-frequency printed circuit boards.

Claims (23)

A sheet containing fluororesin and filler, characterized by a specific gravity of 2.05 or more and 2.19 or less. The sheet according to claim 1, wherein the fluororesin is polytetrafluoroethylene resin. The sheet described in claim 1 or 2, wherein the filler is silica. The sheet described in claim 3, wherein the silica is spherical silica. The sheet described in claim 3 or 4, wherein the silica has a particle size of 0.1 to 3 μm. The sheet described in any one of claims 3 to 5, wherein the silica content is 20 to 80 mass% of the sheet weight. The sheet described in any one of claims 3 to 6, wherein the surface of the silica is treated with a surface treatment agent. the fluororesin is a polytetrafluoroethylene resin having a standard specific gravity of 2.0 to 2.3;
the filler is spherical silica, and the particle size of the spherical silica is 0.5 to 2.1 μm;
The content of the polytetrafluoroethylene resin is 40 to 60 mass% based on the weight of the sheet,
The sheet according to any one of claims 1 to 7, wherein the content of the spherical silica is 40 to 60 mass% based on the weight of the sheet. A sheet containing 20-80% by mass of polytetrafluoroethylene resin relative to the sheet weight and surface-treated spherical silica with a particle size of 0.1-3 μm, wherein the silica content is 20-80% by mass relative to the sheet weight and the specific gravity is 2.05 or more and 2.19 or less. The sheet described in any one of claims 1 to 9 has a water absorption rate of 0.10% or less. The sheet described in any one of claims 1 to 10 is obtained through a manufacturing process that includes a densification step that increases the specific gravity of the sheet by 1% or more. The sheet described in claim 11, wherein, when comparing the rate of change in dielectric tangent before and after water absorption, the rate of change in dielectric tangent after densification is lower than that before densification. The sheet described in any one of claims 1 to 12, having a linear expansion coefficient of 120 ppm/K or less. The sheet described in any one of claims 1 to 13 has a film thickness of 0.1 to 2 mm. The sheet described in any one of claims 1 to 14 is an insulating material for circuit boards. A metal clad laminate having a metal layer and a sheet according to any one of claims 1 to 15 as essential layers. The metal clad laminate of claim 16, wherein the metal layer is copper foil. A circuit board comprising the sheet according to any one of claims 1 to 15 and a metal layer. The circuit board of claim 18, wherein the metal constituting the metal layer is copper. The circuit board of claim 19, wherein the copper is rolled copper or electrolytic copper. The circuit board according to any one of claims 18 to 20, which is a printed circuit board, a laminated circuit board, or a high-frequency board. The method for manufacturing a sheet according to any one of claims 1 to 15, comprising: a step (1) of mixing the fluororesin and the filler and rolling the mixture into a sheet; and a step (2) of densifying the rolled sheet obtained by the step (1). In the densification step (2), the rolled sheet is pressed so that the specific gravity of the rolled sheet increases by 1% or more,
the fluororesin is a polytetrafluoroethylene resin having a standard specific gravity of 2.0 to 2.3;
the filler is spherical silica, and the particle size of the spherical silica is 0.5 to 2.1 μm;
The content of the polytetrafluoroethylene resin is 40 to 60 mass% based on the weight of the sheet,
The method for manufacturing a sheet according to claim 22, wherein the content of the spherical silica is 40 to 60 mass % based on the weight of the sheet.