Fluororesin sheet, method for producing same, and metal clad fluororesin substrate containing same

Abstract

A fluororesin sheet according to the present invention is a delipidated sheet containing a fluoropolymer and an inorganic filler. The fluoropolymer is aligned in the surface direction of the fluororesin sheet. The inorganic filler is surface-treated. A metal clad multilayer plate according to the present invention is obtained by affixing a metal foil on at least one surface of the fluororesin sheet. A method according to the present invention comprises: a step in which a surface-treated inorganic filler and an aqueous dispersion of a fluoropolymer are mixed, compounded, and press-molded to obtain a sheet; a step in which the sheets are layered and press-molded to obtain a sheet; a step in which the obtained sheet is rolled and then dried; and a step in which the sheet is delipidated at a temperature not lower than 200°C but lower than 300°C for 5-24 hours. As a result, a fluororesin sheet exhibiting good intrinsic handling properties is provided.

Description

Fluororesin sheet, its manufacturing method and metal-clad fluororesin substrate including the same

[0001] The present invention relates to a fluororesin sheet useful for printed wiring boards for high-speed communications using high frequencies such as millimeter waves and microwaves, a method for producing the same, and a method for producing a metal-clad fluororesin substrate including the same.

[0002] With the advancement of high-speed communications such as 5G, there is a strong demand for high-speed communication boards and antenna boards with low transmission loss even when using high frequencies such as millimeter waves. Furthermore, in information terminals such as smartphones, the trend toward high-density packaging and ultra-thin wiring boards is rapidly advancing. For high-speed communications such as 5G, printed circuit boards are widely used, which are made by laminating prepregs obtained by impregnating low-dielectric glass cloths such as D-glass, NE-glass, and L-glass with thermoplastic resins such as fluororesins and polyphenylene ethers, or thermosetting resins such as low-dielectric epoxy resins and low-dielectric maleimide resins, and then curing the prepregs under heat and pressure. Patent Document 1 proposes mixing fluororesin with low-molecular-weight polytetrafluoroethylene fine powder and an inorganic filler, and impregnating the mixture into glass fiber cloth to produce a fluororesin prepreg. Patent Document 2 proposes hydrophilizing the surface of a fluororesin prepreg impregnated into glass fiber cloth by adding amino and hydroxyl groups to the surface, followed by lamination with a metal foil. Patent Documents 3 to 7 propose laminating fluororesin sheets through multistage rolling.

[0003] Japanese Patent Application Laid-Open No. 2020-50860 Japanese Patent Application Laid-Open No. 2022-114351 International Publication No. 2023 / 013569 Japanese Patent Application Laid-Open No. 2021-061406 International Publication No. 2021 / 235276 Japanese Patent Application Laid-Open No. 2008-238828 Japanese Patent Application Laid-Open No. 2017-141345

[0004] However, fluororesin sheets have the problem of being easily torn and difficult to handle. In Patent Documents 1 and 2, glass fiber cloth is impregnated with the fluororesin to form a fluororesin prepreg, which improves handleability, but there has been a demand for a fluororesin sheet that does not use glass fiber cloth and that is easy to handle by itself, and a metal-clad fluororesin substrate including the same. In Patent Documents 3 to 7, there is a problem that the fluororesin sheet contains unnecessary organic matter, and there is a problem that defects due to air bubbles are likely to occur when the fluororesin sheet is heated to the hot press temperature in the copper foil laminating process.

[0005] In order to solve the above-mentioned conventional problems, the present invention provides a fluororesin sheet that is easy to handle as a single sheet and that is less likely to generate bubbles even when heated to the hot pressing temperature in the copper foil bonding process, a method for producing the same, and a metal-clad fluororesin substrate including the same.

[0006] One embodiment of the present invention relates to a fluororesin sheet comprising a fluoropolymer and an inorganic filler, wherein the fluoropolymer is oriented in the plane direction of the fluororesin sheet and the inorganic filler is surface-treated, and the fluororesin sheet is a degreased sheet.

[0007] Another embodiment of the present invention relates to a metal-clad laminate in which a metal foil is laminated to at least one surface of the fluororesin sheet.

[0008] Yet another embodiment of the present invention relates to a method for producing the fluororesin sheet, comprising: a first step of mixing an aqueous dispersion of a fluoropolymer with an inorganic filler that has been surface-treated in advance to form a compound, and press-molding the compound into a sheet; a second step of stacking the sheets and press-molding the stacked sheets into a sheet; a third step of rolling the obtained sheet and then drying it; and a fourth step of degreasing the sheet at a temperature of 200°C or higher and lower than 300°C for 5 to 24 hours.

[0009] The present invention provides a fluororesin sheet that is easy to handle by itself, a method for producing the same, and a metal-clad fluororesin substrate including the same, by containing a fluoropolymer and an inorganic filler, the fluoropolymer being oriented in the plane direction of the fluororesin sheet, the inorganic filler being surface-treated, and being a degreased sheet. This reduces water absorption (moisture absorption), and improves electrical stability. In particular, the use of a degreased sheet removes unnecessary organic matter, preventing defects due to bubbles even when heated to the hot press temperature (near the melting point of the fluororesin, 326°C) in the copper foil bonding process.

[0010] Fig. 1 is an SEM cross-sectional photograph (magnification 40x) of a fluororesin sheet of Example 1 of the present invention. Fig. 2 is an SEM cross-sectional photograph (magnification 40x) of a fluororesin sheet of Comparative Example 1. Fig. 3 is a schematic perspective view of a copper-clad fluororesin substrate of one embodiment of the present invention. Figs. 4A-D are schematic perspective views showing a method for producing a fluororesin sheet of one embodiment of the present invention.

[0011] The inventors have studied the problems with conventional fluororesin sheets laminated with glass fiber cloth and metal-clad fluororesin substrates containing such sheets, and have found that the conventional products have problems such as difficulty in forming thin films due to restrictions on the thickness of the glass fiber cloth, and also in terms of physical properties, the difference in volume balance between fluororesin and glass fiber cloth causes fluctuations in dielectric constant, which tends to result in poorer transmission characteristics at high frequencies compared to clothless substrates, and the high viscosity of fluororesin makes it difficult to impregnate deep inside, which can lead to the possibility of air entrapment.The present invention was completed based on these ideas.

[0012] The fluororesin sheet of the present invention contains a fluoropolymer and an inorganic filler, and the inorganic filler has been surface-treated (pretreated). The inorganic filler is preferably silicon oxide, aluminum oxide, titanium oxide, aluminum nitride, boron nitride, silicon nitride, barium titanate, barium sulfate, magnesium hydroxide, glass particles, ceramic particles, or a combination thereof. These inorganic fillers can enhance the reinforcing effect of the fluororesin sheet. The pretreatment refers to adhering a surface treatment agent to the surface of the inorganic filler before mixing the fluoropolymer and the inorganic filler.

[0013] The surface treatment agent is preferably a silane coupling agent, an aluminate coupling agent, or a titanate coupling agent. These surface treatment agents have a high affinity with fluoropolymers, and even when a large amount of inorganic filler is added to the fluoropolymer, they mix well and are easy to handle. Furthermore, high humidity can cause inorganic fillers to absorb moisture, resulting in an increase in the dielectric loss tangent. However, surface treatment of the inorganic filler can suppress moisture absorption and prevent the increase in the dielectric loss tangent. Specifically, moisture absorption can be prevented by blocking the hydroxyl groups (-OH groups) on the inorganic filler surface with a surface treatment agent. The water absorption rate is preferably 0.06% or less, more preferably 0.01 to 0.06%. This prevents a decrease in transmission characteristics at high frequencies even when humidity changes. Among surface treatment agents, silane coupling agents are preferred. Examples of silane coupling agents include R(CH3) a Si(OR') 4-a(R is an unsubstituted or substituted organic group having 1 to 20 carbon atoms, R' is an alkyl group having 1 to 4 carbon atoms, and a is 0 or 1), or a partial hydrolyzate thereof. Examples include methyltrimethoxysilane, n-propyltrimethoxysilane, hexyltrimethoxysilane, decyltrimethoxysilane, methyltriethoxysilane, n-propyltriethoxysilane, hexyltriethoxysilane, octyltriethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, 3-aminopropyltrimethoxysilane, and 3-aminopropyltriethoxysilane. Aluminate coupling agents include aluminum alkylacetoacetate diisopropylate, and titanate coupling agents include triisostearoyloxy-isopropoxytitanium. The above coupling agents can be used alone or in combination.

[0014] The surface treatment agent is preferably added in an amount of 0.05 to 4 parts by mass per 100 parts by mass of the inorganic filler, more preferably 0.07 to 3.5 parts by mass, and even more preferably 1 to 3 parts by mass, which ensures good mixing and improved handleability even when a large amount of the inorganic filler is added to the fluoropolymer.

[0015] In addition to the fluoropolymer and the surface-treated inorganic filler, further additives such as pigments, stabilizers, oils, alcohols, resins, etc. may also be added.

[0016] The fluoropolymer is oriented in the plane direction of the fluororesin sheet. This structure is achieved by repeating lamination and press molding multiple times, and by parallel lamination or cross lamination, when sheets of a compound (mixture) of a fluoropolymer and a surface-treated inorganic filler are laminated and press-molded to form a sheet. In addition, by repeating lamination and press molding multiple times, the fluoropolymer becomes entangled with the inorganic filler and coats it. This results in a structure in which interfacial delamination between the inorganic filler and the fluoropolymer is eliminated. The fluoropolymer is oriented in the plane direction and interfacial delamination between the inorganic filler and the fluoropolymer is eliminated, resulting in a fluororesin sheet with high tensile strength in multiple directions and easy to handle by itself, even without the use of glass fiber cloth. This structure results in high tensile strength in the plane direction and easy to handle by itself, even without the use of glass fiber cloth. Of course, the use of glass fiber cloth is not excluded, and it may be laminated at any location, but it is preferable to use a fluororesin sheet alone without using glass fiber cloth.

[0017] The fluororesin sheet preferably has a tensile strength of 5 MPa or more in both the length direction and the width direction, more preferably more than 5 MPa, and even more preferably 6 MPa or more. The higher the upper limit, the better, but practically it is preferably 100 MPa or less, and even more preferably 95 MPa or less. This provides high strength in various directions and improves handleability.

[0018] The fluororesin sheet preferably has a breaking elongation of 1% or more in both the length direction and the width direction, more preferably 5% or more, and even more preferably 10% or more. The upper limit is preferably 500% or less, and even more preferably 400% or less. This provides high elongation in various directions and improves handleability.

[0019] The fluoropolymer is preferably at least one selected from the group consisting of polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane polymer (PFA), and perfluoroethylenepropene copolymer (FEP). In particular, a combination of PTFE as the main component in an amount of 50% by mass or more and PFA and / or FEP as the secondary components in an amount of less than 50% by mass is preferred.

[0020] The fluororesin sheet preferably contains 10 to 4,000 parts by mass of inorganic filler per 100 parts by mass of fluororesin, more preferably 30 to 3,800 parts by mass, and even more preferably 50 to 3,500 parts by mass.

[0021] Each particle of the inorganic filler preferably has a D50 (median diameter) of 0.01 to 100 μm, more preferably 0.1 to 90 μm, and even more preferably 0.1 to 80 μm in cumulative particle size distribution on a volume basis as determined by a laser diffraction light scattering method. This improves reinforcing properties.

[0022] The thickness of the fluororesin sheet is preferably 0.05 to 10.0 mm, more preferably 0.1 to 9 mm, and even more preferably 0.12 to 8 mm, which allows it to be used with a variety of circuit boards.

[0023] The mass per unit area of ​​the fluororesin sheet is 80 to 40,000 g / m 2 and more preferably 160 to 30,000 g / m 2 and more preferably 200 to 20,000 g / m 2 This allows the production of circuit boards with different characteristics.

[0024] The fluororesin sheet can be bonded to metal foil with a surface roughness of Rz 0.85 or more and 2.0 or less, and the peel strength of the adhesive to the metal foil is preferably a maximum of 40 N / cm. More preferably, the peel strength is 5.0 to 40 N / cm, and even more preferably 5.3 to 40 N / cm. This allows it to be used with a variety of circuit boards. Furthermore, the solder heat resistance is preferably such that the copper foil does not peel or swell when a 50 mm square copper-clad fluororesin substrate sample is floated in a solder bath at 288°C for 10 minutes. This improves processability during soldering.

[0025] The metal-clad fluororesin substrate of the present invention has a metal foil laminated to at least one surface of any of the above-mentioned fluororesin sheets, preferably on both surfaces or in multiple layers, making it suitable for use in a wide variety of circuit boards.

[0026] The metal-clad fluororesin substrate preferably has a dielectric loss tangent of 0.0001 to 0.003 at a frequency of 10 GHz, and a relative dielectric constant of 1.5 to 20 at a frequency of 10 GHz, making it suitable for use as a high-frequency circuit board.

[0027] The metal foil is preferably copper foil, which allows it to be used for high frequency circuit boards.

[0028] The manufacturing method of the present invention includes the following steps. (1) Step 1: Mixing an aqueous dispersion of a fluoropolymer with an inorganic filler, compounding, and press-molding the mixture into a sheet. Here, "compound" has the same meaning as "green body." Press molding is preferably performed at room temperature and a pressure of 0.5 to 4.0 MPa. Compounding can be performed using a mixing method such as planetary mixing, kneading with a kneader, shaking, a three-roll mill, or a pot mill. (2) Step 2: Laminating the sheets and press-molding them into a sheet. Press molding is preferably performed at room temperature and a pressure of 0.15 to 2.5 MPa. Lamination and press molding may be repeated multiple times. "Multiple times" refers to 2 to 20 times, preferably 3 to 15 times. Furthermore, lamination may be performed in one direction (parallel lamination) or multiple directions (cross lamination). This results in a structure in which the fluoropolymer is laminated in the thickness direction of the fluororesin sheet and oriented in the plane direction. In addition, by repeating lamination and press molding multiple times, the fluoropolymer becomes entangled with the inorganic filler and coats it. This results in a structure that eliminates interfacial delamination between the inorganic filler and the fluoropolymer. As described above, the structure in which the fluororesin sheet is laminated in the thickness direction and oriented in the plane direction, combined with the structure in which the inorganic filler and the fluoropolymer do not undergo interfacial delamination, synergistically results in a fluororesin sheet with high tensile strength in multiple directions and easy handling even without the use of glass fiber cloth. (3) Step 3: Next, the obtained sheet is rolled and then dried. Rolling is preferred for rolling. Drying is preferably performed by natural drying (air drying at room temperature) or by heating at 150°C for 30 minutes. (4) Step 4: Next, the obtained sheet is degreased. The heating temperature during degreasing is 200°C or higher but lower than 300°C, preferably 210 to 280°C, and more preferably 220 to 260°C. At temperatures below 200°C, some parts remain that cannot be degreased, whereas at temperatures above 300°C, the surface treatment agent decomposes, reducing its effectiveness and increasing the amount of decomposed fluoropolymer. The treatment time for degreasing is preferably 10 minutes to 24 hours, more preferably 30 minutes to 20 hours, and even more preferably 1 to 16 hours.The size of the fluororesin sheet to be degreased is preferably 30 to 30,000 mm in length, 20 to 1,000 mm in width, and 70 to 11,000 μm in thickness. The degreasing process is preferably performed by exposing at least one side, preferably both sides, of the fluororesin sheet to air. This removes unwanted organic matter, such as stabilizers, oils, alcohols, and dispersants. If unwanted organic matter remains in the fluororesin sheet, it will form bubbles and cause defects when heated to the heat-pressing temperature (near the melting point of the fluororesin, 326°C) in the subsequent copper foil lamination process. Furthermore, the bubbles corrode the copper foil, reducing peel strength. The present invention removes unwanted organic matter by degreasing, preventing defects due to bubbles even when heated to the heat-pressing temperature (near the melting point of the fluororesin, 326°C) in the subsequent copper foil lamination process. The degreasing process may be performed under a reduced pressure of 0.1 to 10 kPa. Degreasing equipment includes an electric heating oven, a circulating air heating oven, an IR oven, etc., and may be a batch type or a continuous type. An IR oven can achieve a degreasing effect greater than that of an electric heating oven in a short time. An IR oven allows for continuous degreasing, and can also be used for long lengths. Since thick sheets require a long time to degrease, it is preferable to heat them in an electric heating oven and then reduce the pressure after they have been sufficiently heated.

[0029] The following description will be made with reference to the drawings. In the following drawings, the same reference numerals indicate the same objects. FIG. 1 is a scanning electron microscope (SEM) cross-sectional photograph (magnification 40x) of a fluororesin sheet 1 of Example 1 of the present invention. The fluoropolymer is oriented in the plane direction of the fluororesin sheet. In other words, it is the fluoropolymer that is aligned in the horizontal direction. It can also be observed that this fluororesin sheet has a structure in which the fluoropolymer is laminated in layers in the thickness direction. In other words, it has a laminated structure when viewed in the cross-sectional direction.

[0030] 2 is an SEM photograph (magnification: 40x) of the fluororesin sheet of Comparative Example 1. This fluororesin sheet was produced by a method that did not include the second step (lamination press step) of the method of the present invention, and no orientation of the fluoropolymer was observed.

[0031] 3 is a schematic perspective view of a copper-clad fluororesin substrate 1 according to one embodiment of the present invention. This copper-clad fluororesin substrate 1 has copper foils 3a and 3b attached to both sides of a fluororesin sheet 2. The fluororesin sheet 2 is heated to a temperature near its melting point of 326°C and is attached to the copper foils 3a and 3b by heat pressing. An adhesive may be used at this time.

[0032] Figures 4A-4D are schematic perspective views showing a method for producing a fluororesin sheet according to one embodiment of the present invention. Figure 4A shows a fluororesin sheet 4 obtained in the first step of the method of the present invention. Figure 4B shows an example in which this fluororesin sheet 4 is cross-laminated as indicated by arrows 5 and 6. The folding angle is optional. Figure 4C shows a fluororesin sheet 4 in parallel lamination as indicated by arrows 7 and 8. A mixed lamination may be formed by mixing the cross lamination of Figure 4B and the parallel lamination of Figure 4C. The fluororesin sheet 4 thus laminated is pressed with press plates 10 and 11 as shown in Figure 4D. Next, the fluororesin sheet of the present invention is obtained by the rolling, drying, and degreasing steps described above.

[0033] The following examples are provided for illustrative purposes only. The present invention is not limited to these examples. In the following examples, unless otherwise specified, parts refer to parts by mass, and % refers to mass %. Various parameters were measured using the following methods. <Effective relative dielectric constant, transmission loss> Transmission loss was measured using a network analyzer (manufactured by Keysight). <Peel strength (peel-off strength)> Peel-off strength was measured using a tensile tester (manufactured by Shimadzu Corporation) by peeling at a 90° angle in accordance with JIS C6481:1996. <Tensile strength, tensile elongation> Tensile strength tests and tensile elongation tests were performed using a tensile tester (manufactured by Shimadzu Corporation) in accordance with ASTM D638:1995. The tensile strength and tensile elongation were measured on a 5 cm wide sample and converted to values ​​per cm. Elongation refers to the elongation at break. <Thermal Resistance, Thermal Conductivity> Thermal resistance was measured according to a method compliant with ASTM D5470 (sandwiching a sample between aluminum blocks, applying a load of 5 kgf, measuring the thermal resistance value from the temperature difference between the top and bottom and the power, and calculating the thermal conductivity from the thermal resistance value). Thermal conductivity was calculated from the slope of the measured value. <Solder Heat Resistance> Solder heat resistance was confirmed by floating a 50 mm square copper-clad fluororesin substrate sample in a solder bath at 288°C for 10 minutes and checking whether the copper foil peeled or blistered. <Water Absorption> Water absorption was measured according to IPC-TM-650 2.6.2.1 (1986). <Presence of Bubbles During the Copper Foil Bonding Process> The presence of bubbles was visually observed when heated to the heat press temperature (near the melting point of the fluororesin, 326°C) during the copper foil bonding process. <Other Physical Properties> Measured according to industry standard tests.

[0034] Example 1 <Raw Materials> A liquid obtained by mixing 0.7 parts of trimethoxymethylsilane (DOWSIL SZ 6070 Silan manufactured by Toray Industries, Inc.) and 24 parts of normal propanol was added dropwise to 100 parts of silicon oxide (F-40: manufactured by Marukama Kamado Toryo Co., Ltd.) having a median diameter D50 of 5.6 μm in cumulative particle size distribution by laser diffraction light scattering method on a volume basis, and the mixture was mixed for 30 minutes in a Henschel mixer. The mixed filler was heated in an oven at 100° C. for 60 minutes to obtain a surface-treated filler. To 132 parts of the surface-treated filler, an aqueous dispersion (60% concentration) of polytetrafluoroethylene (31-JR: manufactured by Mitsui Chemours) was added so that the resin content was 95 parts by volume, and an aqueous dispersion (60% concentration) of perfluoroethylene (335-JR: manufactured by Mitsui Chemours) was added so that the resin content was 5 parts by volume, and the mixture was stirred to obtain a dispersion. <Mixing> The dispersion obtained above was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. Next, it was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. <Solidification> The dispersion obtained above was solidified. <Compounding (forming into a puddle)> The solidified dispersion was scooped out with a spatula or the like and kneaded. <Molding> The clay obtained above was placed in a metal frame with an inner frame of 14 cm square and a thickness of 1.5 cm, and press-molded at room temperature under a pressure of 2.0 MPa. <Lamination Pressing> The molded body soaked for a sufficient time as described above was placed in a metal frame with an inner frame of 21 cm square and a thickness of 0.65 cm, and press-molded at room temperature under a pressure of 1.5 MPa. Subsequently, the lamination shown in Figure 4A and the lamination shown in Figure 4B were performed once each, followed by press-molding at room temperature under a pressure of 1.0 MPa. <Rolling> The molded body obtained above was placed on a 100 μm thick polyethylene terephthalate (PET) film and rolled with a roll spacing of 3.5 mm. It is desirable to set a low reduction ratio during this process. Rolling was repeated until the desired thickness (approximately 0.16 mm) was reached. <Drying> The sheet obtained by rolling was dried while still on the PET film. <Degreasing> The dried sheet obtained was cut to a predetermined size and placed in an electric heating oven and heated at 250°C for 12 hours. The sheet size was 10,000 mm in length, 500 mm in width, and 100 µm in thickness. The degreasing treatment was carried out by exposing at least one side of the fluororesin sheet to air.<Hot Pressing> The long degreased sheet obtained in the degreasing step was cut to a predetermined size (e.g., 200 mm length x 300 mm width), and the copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name "CF-T4X-SV18", surface roughness Rz: 1.0 μm) / degreased sheet / copper foil were stacked in this order, gradually heated to a temperature of 350°C, and vacuum hot pressed at a vacuum degree of 0.9 kPa and a pressure of 8.0 MPa to form a laminate. FIG. 1 is a scanning electron microscope (SEM) cross-sectional photograph (magnification 40x) of fluororesin sheet 1 of Example 1. It can be seen that the fluoropolymer is oriented in the plane direction of the fluororesin sheet. In other words, it is the fluoropolymer that is aligned in the horizontal direction. It can also be observed that this fluororesin sheet has a structure in which the fluoropolymer is layered in the thickness direction. In other words, it has a layered structure when viewed in the cross-sectional direction.

[0035] Comparative Example 1 was carried out in the same manner as in Example 1, except that the inorganic filler was not surface-treated and was not degreased. Fig. 2 is an SEM photograph (magnification 40x) of the fluororesin sheet of Comparative Example 1. This fluororesin sheet was produced by a method that did not include the second step (lamination press step) of the method of the present invention, and no orientation of the fluoropolymer was observed.

[0036] Example 2 <Raw Materials> A liquid obtained by mixing 0.3 parts of trimethoxymethylsilane (DOWSIL SZ 6070 Silan manufactured by Toray Industries, Inc.) and 7 parts of normal propanol was added dropwise to 100 parts of silicon oxide (LS-44: manufactured by Marukama Kamado Toryo Co., Ltd.) having a median diameter D50 of 13.5 μm in cumulative particle size distribution by laser diffraction light scattering method on a volume basis, and the mixture was mixed for 30 minutes in a Henschel mixer. The mixed filler was heated in an oven at 100° C. for 60 minutes to obtain a surface-treated filler. To 132 parts of the surface-treated filler, an aqueous dispersion (60% concentration) of polytetrafluoroethylene (31-JR: manufactured by Mitsui Chemours) was added so that the resin content was 95 parts by volume, and an aqueous dispersion (60% concentration) of perfluoroethylene (335-JR: manufactured by Mitsui Chemours) was added so that the resin content was 5 parts by volume, and the mixture was stirred to obtain a dispersion. <Mixing> The dispersion obtained above was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. Next, it was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. <Solidification> The dispersion obtained above was solidified. <Compounding (forming into a puddle)> The solidified dispersion was scooped out with a spatula or the like and kneaded. <Molding> The clay obtained above was placed in a metal frame with an inner frame of 14 cm square and a thickness of 1.5 cm, and press-molded at room temperature under a pressure of 2.0 MPa. <Lamination Pressing> The molded body soaked for a sufficient time as described above was placed in a metal frame with an inner frame of 21 cm square and a thickness of 0.65 cm, and press-molded at room temperature under a pressure of 1.5 MPa. Subsequently, the lamination shown in Figure 4A and the lamination shown in Figure 4B were performed once each, followed by press-molding at room temperature under a pressure of 1.0 MPa. <Rolling> The molded body obtained above was placed on a 100 μm thick polyethylene terephthalate (PET) film and rolled with a roll spacing of 3.5 mm. It is desirable to set a low reduction ratio during this process. Rolling was repeated until the desired thickness (approximately 0.16 mm) was reached. <Drying> The sheet obtained by rolling was dried while still on the PET film. <Degreasing> The obtained dried sheet was cut to a predetermined size and placed in an electric heating oven and heated at 250°C for 12 hours. The sheet size was 10,000 mm in length, 500 mm in width, and 100 µm in thickness. The degreasing treatment was carried out by exposing at least one surface of the fluororesin sheet to air.<Hot Pressing> The long degreased sheet obtained in the degreasing step was cut to a predetermined size (for example, 200 mm length x 300 mm width), and the copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name "CF-T4X-SV18", surface roughness Rz: 1.0 μm) / degreased sheet / copper foil were stacked in this order, gradually heated to a temperature of 350° C., and vacuum hot pressed at a vacuum degree of 0.9 kPa and a pressure of 8.0 MPa to form a laminate.

[0037] Comparative Example 2 The same procedure as in Example 2 was carried out except that the surface of the inorganic filler was not treated and that degreasing was not performed.

[0038] Example 3 <Raw Materials> A liquid obtained by mixing 1.0 part of triisostearoyloxy-isopropoxytitanium (TTS: manufactured by Ajinomoto Fine-Techno Co., Ltd.) and 8 parts of xylene was added dropwise to 100 parts of silicon oxide (LS-44: manufactured by Marukama Kamado Toryo Co., Ltd.) having a median diameter D50 of 13.5 μm in cumulative particle size distribution by laser diffraction light scattering method on a volume basis, and the mixture was mixed for 30 minutes in a Henschel mixer. The mixed filler was heated in an oven at 150° C. for 60 minutes to obtain a surface-treated filler. To 132 parts of the surface-treated filler, an aqueous dispersion (60% concentration) of polytetrafluoroethylene (31-JR: manufactured by Mitsui Chemours) was added so that the resin content was 95 parts by volume, and an aqueous dispersion (60% concentration) of perfluoroethylene (335-JR: manufactured by Mitsui Chemours) was added so that the resin content was 5 parts by volume, and the mixture was stirred to obtain a dispersion. <Mixing> The dispersion obtained above was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. Next, it was stirred for 2 minutes using a propeller machine adjusted to a rotation speed of 60 to 70 rpm. <Solidification> The dispersion obtained above was solidified. <Compounding (forming into a puddle)> The solidified dispersion was scooped out with a spatula or the like and kneaded. <Molding> The molded body obtained above was placed in a metal frame with an inner frame of 14 cm square and 1.5 cm thickness, and press-molded at room temperature under a pressure of 2.0 MPa. <Lamination Pressing> The molded body that had been soaked for a sufficient time was placed in a metal frame with an inner frame of 21 cm square and 0.65 cm thickness, and press-molded at room temperature under a pressure of 1.5 MPa. The lamination shown in Figure 4A and the lamination shown in Figure 4B were then performed once each, followed by press-molding at room temperature under a pressure of 1.0 MPa. <Rolling> The molded body obtained above was placed on a 100 μm thick polyethylene terephthalate (PET) film and rolled with a roll spacing of 3.5 mm. It is desirable to set a low reduction ratio during this process. Rolling was repeated until the desired thickness (approximately 0.16 mm) was reached. <Drying> The sheet obtained by rolling was dried while still on the PET film. <Degreasing> The dried sheet obtained was cut to the desired dimensions and placed in an electric oven and heated at 250°C for 12 hours. The sheet had a length of 10,000 mm, a width of 500 mm, and a thickness of 100 μm. The degreasing treatment was carried out by exposing at least one surface of the fluororesin sheet to air.<Hot Pressing> The long degreased sheet obtained in the degreasing step was cut to a predetermined size (for example, 200 mm length x 300 mm width), and the copper foil (manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name "CF-T4X-SV18", surface roughness Rz: 1.0 μm) / degreased sheet / copper foil were stacked in this order, gradually heated to a temperature of 350° C., and vacuum hot pressed at a vacuum degree of 0.9 kPa and a pressure of 8.0 MPa to form a laminate.

[0039] Comparative Example 3 The same procedure as in Example 3 was carried out except that the inorganic filler was not subjected to surface treatment and was not degreased. The results are summarized in Table 1.

[0040]

[0041] As is clear from Table 1, in Example 1-3, no bubbles were generated during the copper foil attachment process, and a defect-free copper foil-clad laminate was obtained. It was also confirmed that the fluororesin sheet of Example 1-3, which was surface-treated with an inorganic filler, had low water absorption and was less susceptible to the effects of humidity. Furthermore, it was confirmed that the fluororesin sheet of Example 1-3 was easy to handle even without using glass fiber cloth, and had good physical and electrical properties.

[0042] The fluororesin sheet of the present invention and a metal-clad fluororesin substrate comprising the same are useful for wiring substrates such as IoT devices and wearable devices, which have low transmission loss even when using high frequencies such as millimeter waves, high-speed transmission FPCs, transceivers, high-speed communication boards, antenna boards, smartphones, smart watches, communication base station antennas, collision sensors, distance sensors, sensors in train monitoring systems, satellite communication antennas, intersection inspection sensors, security image sensors, runway foreign object detection systems, and river water level monitoring sensors.

[0043] REFERENCE SIGNS LIST 1 Copper-clad fluororesin substrate 2, 4 Fluororesin sheet 3a, 3b Copper foil 5, 6, 7, 8 Folding direction 9, 10 Press plate

Claims

1. A fluororesin sheet comprising a fluoropolymer and an inorganic filler, wherein the fluoropolymer is oriented in the plane direction of the fluororesin sheet, the inorganic filler is surface-treated, and the fluoropolymer is a degreased sheet.

2. The fluororesin sheet according to claim 1, wherein the inorganic filler is at least one type selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, aluminum nitride, boron nitride, silicon nitride, barium titanate, barium sulfate, magnesium hydroxide, glass particles, and ceramic particles other than the above.

3. The fluororesin sheet according to claim 1 or 2, wherein the surface treatment agent is at least one compound selected from the group consisting of silane coupling agents, aluminate coupling agents, and titanate coupling agents.

4. The fluororesin sheet according to any one of claims 1 to 3, wherein the surface treatment agent is applied in an amount of 0.05 to 4 parts by mass per 100 parts by mass of the inorganic filler.

5. The fluororesin sheet according to any one of claims 1 to 4, wherein the tensile strength of the fluororesin sheet is 5 MPa or more in both the length direction and width direction.

6. A fluororesin sheet according to any one of claims 1 to 5, wherein the fluoropolymer is at least one type selected from the group consisting of polytetrafluoroethylene: PTFE, perfluoroalkoxyalkane polymer: PFA, and perfluoroethylenepropene copolymer: FEP.

7. The fluororesin sheet according to any one of claims 1 to 6, wherein the fluororesin sheet contains 10 to 4,000 parts by mass of inorganic filler per 100 parts by mass of fluororesin.

8. A fluororesin sheet according to any one of claims 1 to 7, wherein each particle of the inorganic filler has a D50: median diameter of 0.01 to 100 µm in cumulative particle size distribution based on volume as measured by a laser diffraction light scattering method.

9. The fluororesin sheet according to any one of claims 1 to 8, wherein the thickness of the fluororesin sheet is 0.005 to 10.0 mm.

10. A fluororesin sheet according to any one of claims 1 to 9, which is capable of adhering to metal foil having a surface roughness: Rz of 0.6 or more and 2.0 or less, and has an adhesive strength with the metal foil of up to 40 N / cm in peel strength.

11. A metal-clad fluororesin substrate comprising the fluororesin sheet according to any one of claims 1 to 10, and a metal foil laminated to at least one surface of the sheet.

12. The metal-clad fluororesin substrate according to claim 11, wherein the metal-clad fluororesin substrate has a dielectric loss tangent of 0.0001 to 0.003 at a frequency of 10 GHz.

13. The metal-clad fluororesin substrate according to claim 11 or 12, wherein the metal-clad fluororesin substrate has a relative dielectric constant of 1.5 to 20 at a frequency of 10 GHz.

14. The metal-clad fluororesin substrate according to any one of claims 11 to 13, wherein the metal foil is a copper foil.

15. A method for producing a fluororesin sheet according to any one of claims 1 to 10, comprising: a first step of mixing an aqueous dispersion of a fluoropolymer with an inorganic filler that has been surface-treated in advance to form a compound, and press-molding the compound into a sheet; a second step of laminating the sheets and press-molding the laminate into a sheet; a third step of rolling and drying the sheet obtained; and a fourth step of degreasing the sheet at a temperature of 200°C or higher and lower than 300°C for 5 to 24 hours.

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