Fluororesin film, method for producing fluororesin film, copper-clad laminate, and circuit board

Abstract

Provided are: a fluororesin film having low thermal expansion properties and capable of maintaining low hygroscopicity even after a heat treatment; a method for producing a fluororesin film; a copper-clad laminate; and a circuit board.　This fluororesin film comprises a fluororesin and an inorganic filler, wherein the moisture absorption rate of said fluororesin film at a temperature of 40ºC and a relative humidity of 90% is less than 0.06 mass%.

Description

Fluororesin film, method for producing fluororesin film, copper-clad laminate, and circuit board

[0001] The present invention relates to a fluororesin film, a method for producing a fluororesin film, a copper-clad laminate, and a circuit board.

[0002] In recent years, with the progress of miniaturization, weight reduction, and space saving of electronic devices, there has been an increasing demand for flexible printed circuit boards (hereinafter sometimes referred to as "FPCs") that are thin, lightweight, flexible, and have excellent durability even when repeatedly bent. For the production of FPCs, flexible materials such as flexible copper-clad laminates (hereinafter sometimes referred to as "FCCLs") obtained by laminating an insulating film as a base material and a thin copper foil are used, and their applications are expanding to wirings in movable parts of mobile communication devices such as mobile phones and smartphones, network-related electronic devices such as these base station devices, server routers, and parts of large computers (for example, Patent Documents 1 to 3).

[0003] In these communication devices and network-related electronic devices, it is necessary to transmit and process a large amount of information with low loss and at high speed, and the electrical signals handled by FPCs are also becoming higher in frequency. When the transmission loss is large when transmitting high-frequency electrical signals, problems such as loss of electrical signals and increase in signal delay time occur. Therefore, the FCCL used for FPCs is required to have good electrical characteristics showing low dielectric characteristics (low relative dielectric constant, low dielectric tangent) and reduced transmission loss in high-frequency transmission.

[0004] JP-A-2014-160738 JP-A-2003-147320 JP-A-2016-69651

[0005] In particular, as a material for an insulating film used as a substrate for FPCs and the like for the sixth-generation mobile communication system (hereinafter sometimes referred to as "6G"), a material with less transmission loss during high-frequency communication is required. Examples of such materials include fluororesins showing low dielectric characteristics (low relative dielectric constant, low dielectric tangent).

[0006] However, because fluororesins expand easily with heat, using fluororesin alone as a substrate material for FPCs and the like can lead to problems such as the substrate warping when bonded to the copper foil, misalignment of the circuits laminated on the substrate, and circuits detaching from the substrate. Therefore, it is important that the substrate material has low thermal expansion properties.

[0007] Furthermore, during high-frequency communication, high humidity conditions can cause the substrate to absorb moisture, leading to a decrease in electrical properties such as the dielectric loss tangent (Df), potentially failing to meet the required electrical properties, particularly the dielectric loss tangent (Df). Therefore, it is preferable to use a low-hygroscopic material as the substrate. In particular, since fluororesin films undergo a heating process for firing during their manufacturing, it is crucial that the material maintains low hygroscopicity even after this heating treatment for use as a substrate or insulating film.

[0008] Therefore, the present invention aims to provide a fluororesin film that exhibits low thermal expansion and maintains low hygroscopicity even after heat treatment, a method for manufacturing a fluororesin film, a copper-clad laminate, and a circuit board.

[0009] To solve the above problems, the fluororesin film of the present invention is a fluororesin film comprising a fluororesin and an inorganic filler, wherein the moisture absorption rate of the fluororesin film at a temperature of 40°C and a relative humidity of 90% is less than 0.06% by mass.

[0010] The moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% may be 0.1% by mass or less.

[0011] The aforementioned fluororesin film may have a dielectric loss tangent Df of 0.001 or less at a frequency of 40 GHz under conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%.

[0012] The fluororesin film may have a dielectric loss tangent Df of 0.003 or less at a frequency of 40 GHz under conditions of a temperature of 85°C and a relative humidity of 85%.

[0013] The thermal expansion coefficient of the fluororesin film may be 120 ppm / K or less.

[0014] In the fluororesin film of the present invention, the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% after heating the inorganic filler to 340°C to 400°C may be 0.1% by mass or less.

[0015] In the fluororesin film of the present invention, the inorganic filler content may be 30% to 70% by volume.

[0016] In the fluororesin film of the present invention, the volume-average particle size of the inorganic filler may be 0.1 μm to 10 μm.

[0017] In the fluororesin film of the present invention, the inorganic filler may be any of alumina, titanium oxide, silica, barium sulfate, silicon carbide, boron nitride, silicon nitride, glass fiber, glass beads, and mica, or a combination of two or more of these.

[0018] In the fluororesin film of the present invention, the inorganic filler may be surface-treated silica whose surface has been hydrophobically treated.

[0019] The thickness of the fluororesin film may be between 25 μm and 500 μm.

[0020] The fluororesin may be PTFE, PFA, and FEP, or a combination of two or more of these.

[0021] Furthermore, in order to solve the above problems, the copper-clad laminate of the present invention is a copper-clad laminate in which the fluororesin film of the present invention and a copper foil layer are laminated, wherein the fluororesin film has a dielectric loss tangent Df of 0.003 or less at a frequency of 40 GHz under conditions of a temperature of 85°C and a relative humidity of 85%.

[0022] In the copper-clad laminate of the present invention, the fluororesin film may have a dielectric loss tangent Df of 0.001 or less at a frequency of 40 GHz under conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%.

[0023] In the copper-clad laminate of the present invention, the moisture absorption rate of the fluororesin film at a temperature of 40°C and a relative humidity of 90% may be less than 0.11% by mass.

[0024] In the copper-clad laminate of the present invention, the thermal expansion coefficient of the fluororesin film may be 120 ppm / K or less.

[0025] In the copper-clad laminate of the present invention, the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% after heating the inorganic filler to 340°C to 400°C may be 0.1% by mass or less.

[0026] In the copper-clad laminate of the present invention, the fluororesin contained in the fluororesin film may be any of PTFE, PFA, and FEP, or a combination of two or more of these.

[0027] Furthermore, in order to solve the above problems, the method for manufacturing a fluororesin film of the present invention is a method for manufacturing a fluororesin film of the present invention, comprising: a mixing step of uniformly mixing fluororesin powder and inorganic filler powder to obtain a mixture; a molding step of compacting and shaping the mixture; a heating step of heating the mixture after the molding step to melt the fluororesin powder; a cooling step of cooling the mixture after the heating step to crystallize the fluororesin; and a skiving step of skiving the mixture after the cooling step to form the fluororesin film.

[0028] Furthermore, in order to solve the above problems, the circuit board of the present invention comprises the fluororesin film of the present invention described above.

[0029] The present invention provides a fluororesin film that exhibits low thermal expansion and maintains low hygroscopicity even after heat treatment, a method for manufacturing a fluororesin film, a copper-clad laminate, and a circuit board.

[0030] The following describes an embodiment of the fluororesin film, a method for manufacturing the fluororesin film, a copper-clad laminate, and a circuit board according to the present invention.

[0031] [Fluororesin Film] The fluororesin film of the present invention comprises a fluororesin described later and an inorganic filler, and the moisture absorption rate of the fluororesin film at a temperature of 40°C and a relative humidity of 90% is less than 0.06% by mass.

[0032] (Thickness of fluororesin film) The thickness of the fluororesin film is preferably 25 μm to 500 μm. A film thickness of 25 μm or more ensures sufficient strength as a film and provides good handling properties. A film thickness of 500 μm or less provides sufficient flexibility.

[0033] The thickness of the fluororesin film can be appropriately selected according to its application and requirements, but for example, it may be 30 μm or more, 50 μm or more, 70 μm or more, or 100 μm or more, or it may be 400 μm or less, 300 μm or less, 200 μm or less, or 150 μm or less.

[0034] The thickness of the fluororesin film can be determined by using a film thickness measuring instrument such as a micrometer, and taking the average of the thickness measurements taken at any 10 points on the fluororesin film.

[0035] (Coefficient of thermal expansion (CTE)) If the thermal expansion of the fluororesin film is large, there is a risk that problems such as the substrate warping when the copper foil is bonded to the substrate, the arrangement of the circuits laminated on the substrate shifting, or the circuits peeling off from the substrate may occur.

[0036] For example, to create a multilayer FPC substrate, holes are sometimes drilled in FCCL using a UV-YAG laser, and then the inner walls of the holes are copper-plated to ensure conductivity between the layers of the substrate. In processing such multilayer substrates, the rate of volume expansion of the fluororesin film due to temperature rise is important from the perspective of preventing warping of the substrate and misalignment of circuits.

[0037] Specifically, the thermal expansion coefficient of the fluororesin film of the present invention is preferably 120 ppm / K or less, more preferably 100 ppm / K or less, and even more preferably 80 ppm / K or less. Ideally, the lower limit of the thermal expansion coefficient of the fluororesin film is 0 ppm / K, but as a guideline, the thermal expansion coefficient of the fluororesin film may be 20 ppm / K or more, 35 ppm / K or more, or 50 ppm / K or more.

[0038] (Dielectric loss tangent Df) For example, if the challenge is to minimize transmission loss during high-frequency communication, and considering the use of the fluororesin film of the present invention as a material for multilayer substrates for 6G, it is important that the film exhibits excellent low dielectric properties even under high temperature and high humidity conditions of 85°C and 85% relative humidity, as well as under normal temperature and humidity conditions of 20°C to 25°C and 50% to 65% relative humidity.

[0039] As a specific low dielectric property, the fluororesin film may have a dielectric loss tangent Df of 0.003 or less at a frequency of 40 GHz under conditions of a temperature of 85°C and a relative humidity of 85%, and it is more preferable that it be 0.0028 or less.

[0040] Furthermore, the fluororesin film may have a dielectric loss tangent Df of 0.001 or less at a frequency of 40 GHz under conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%, and it is more preferable that it be 0.0009 or less.

[0041] The dielectric loss tangent Df can be measured using an SCR dielectric resonator or similar device.

[0042] (Hygroscopicity) When communicating at high frequencies in a high-humidity environment, the dielectric loss tangent Df may decrease due to moisture absorption by the substrate, potentially failing to meet the required electrical characteristics. Therefore, it is preferable to use a low-hygroscopic material as the substrate. Specifically, the moisture absorption rate of a fluororesin film at a temperature of 40°C and a relative humidity of 90% is less than 0.06% by mass, and more preferably 0.05% by mass or less. By using such a fluororesin film as the substrate, the electrical characteristics can be met even in a high-humidity environment.

[0043] 〈Fluororesin〉 A fluororesin is a synthetic resin having excellent heat resistance, electrical insulation, non-stickiness, and weather resistance. A fluororesin film formed into a film shape is widely used in industrial fields such as chemical materials, electric and electronic components, semiconductors, and automobiles. And the fluororesin is also useful as a resin for a fluororesin film with little transmission loss during communication at high frequencies as in the present invention.

[0044] The fluororesin that can be used is not particularly limited as long as it is a resin capable of solving the problems of the present invention. For example, any one of PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer) or a mixture thereof can be used as the fluororesin.

[0045] The content of the fluororesin in the fluororesin film may be, for example, the remainder of the content of the inorganic filler in the fluororesin film. For example, when the content of the inorganic filler in the fluororesin film is 30% to 70% by volume as described later, the content of the fluororesin in the fluororesin film may be 30% to 70% by volume. If the content of the fluororesin is 30% by volume or more, good strength can be obtained as a fluororesin film. Also, if the content of the fluororesin is 70% by volume or less, sufficient flexibility can be obtained as a fluororesin film for a multilayer substrate.

[0046] 〈Inorganic filler〉 An inorganic filler is an important component together with the fluororesin and exists in a state of being dispersed in the fluororesin in the fluororesin film.

[0047] Since fluororesin melts under heat, if a fluororesin film is composed only of fluororesin, when manufacturing a multilayer substrate by joining a copper-clad laminate, for example, by hot pressing, the fluororesin may melt and flow out, resulting in a situation where wiring cannot be embedded properly or where misalignment of the wiring occurs. Also, as described above, there is a risk of problems such as warping of the substrate due to attaching the copper foil to the substrate, misalignment of the circuit arrangement laminated on the substrate, and peeling of the circuit from the substrate. Therefore, it is important for the substrate to be a material with low thermal expansion characteristics.

[0048] Therefore, by dispersing an inorganic filler in the fluororesin, the fluidity and viscosity of the fluororesin film can be adjusted even in the molten state of the fluororesin, achieving both good embedding properties of the wiring and prevention of wiring misalignment. The fluidity and viscosity of the fluororesin film can be appropriately adjusted in consideration of characteristics such as the volume average particle diameter of the inorganic filler, the content of the inorganic filler in the fluororesin film, the shape of the inorganic filler, and the dispersibility of the inorganic filler in the fluororesin film.

[0049] Specifically, as the inorganic filler, any one of alumina, titanium oxide, silica, barium sulfate, silicon carbide, boron nitride, silicon nitride, glass fiber, glass beads, and mica, or a combination of two or more of these can be used. In particular, in order to achieve both good embedding properties of the wiring and prevention of wiring misalignment when used in combination with fluororesin, silica, boron nitride, and alumina can be used alone or in combination.

[0050] The content of the inorganic filler in the fluororesin film is 30 to 70% by volume, and it may be 35% by volume or more, 40% by volume or more, 45% by volume or more, or may be 65% by volume or less, 60% by volume or less, 55% by volume or less.

[0051] If the inorganic filler content in the fluororesin film is 30% by volume or more, it is possible to prevent the wiring from tilting when manufacturing a multilayer substrate. Furthermore, if the inorganic filler content in the fluororesin film is 70% by volume or less, good embedding of the wiring can be achieved. However, if the inorganic filler content exceeds 70% by volume, for example, if the inorganic filler content is 90% by volume, in the process of producing a molded body by compression molding of the mixture of inorganic filler and fluororesin, the low amount of fluororesin reduces the bonding between the inorganic filler and fluororesin, which may significantly reduce the strength of the molded body and make it impossible to produce a fluororesin film.

[0052] The volume-average particle size of the inorganic filler can be appropriately selected for the desired thickness of the fluororesin film, but is preferably 0.1 μm to 10 μm, may be 0.2 μm or more, 0.3 μm or more, 0.5 μm or more, or 1 μm or more, and may be 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less.

[0053] By having a volume-average particle size of inorganic fillers in the range of 0.1 μm to 10 μm, aggregation of inorganic filler particles can be suppressed, allowing for uniform dispersion in the fluororesin, and reducing the proportion of coarse particles. Furthermore, having a volume-average particle size of inorganic fillers in the range of 0.1 μm to 10 μm suppresses the occurrence of through-holes (pinholes) in the fluororesin film, resulting in excellent elongation properties.

[0054] The shape of the inorganic filler is not particularly limited, but examples include spherical, plate-shaped, flaky, needle-shaped, etc. Furthermore, the interior of the inorganic filler may be solid or hollow.

[0055] During high-frequency communication, high humidity conditions can cause the dielectric loss tangent Df to decrease due to moisture absorption by the substrate, potentially preventing it from meeting the required electrical characteristics. Therefore, it is preferable to use a low-hygroscopic inorganic filler as the substrate material. Specifically, the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.03% by mass or less. Using such an inorganic filler as the substrate material allows the electrical characteristics to be met even in high-humidity conditions.

[0056] Since fluororesin films undergo a heating process for firing during their manufacturing, it is important that the material maintains low hygroscopicity even after this heat treatment in order to be used as a substrate or insulating film. Therefore, in the present invention, the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% after heating the inorganic filler to 340°C to 400°C is preferably 0.1% by mass or less, and more preferably 0.08% by mass or less.

[0057] (Surface-treated silica) As an inorganic filler, surface-treated silica with a hydrophobic surface can be used. Surface-treated silica is less likely to absorb moisture even in high-humidity environments, and as a result, the deterioration of the electrical properties of the fluororesin film in high-humidity environments can be suppressed.

[0058] Examples of such surface-treated silica include silica whose surface has been hydrophobically treated with a silane coupling agent. For example, with silane coupling agents such as hexamethyldisilazane or n-propyltrimethoxysilane, it is easy to perform a surface treatment on silica that imparts excellent heat resistance and hydrophobicity, such that the moisture absorption rate of silica at 40°C and 90% relative humidity after heating to 340°C to 400°C is 0.1% by mass or less. However, the surface treatment is not particularly limited as long as it can impart excellent heat resistance and hydrophobicity.

[0059] (Optional Components) The fluororesin film of the present invention may further contain optional components. Optional components are not particularly limited, but examples include flame retardants, flame retardant enhancers, pigments, antioxidants, reflective agents, opacities, lubricants, processing stabilizers, plasticizers, and foaming agents. When optional components are included, the total content of optional components in the fluororesin film may be 20% by mass or less, 10% by mass or less, or 5% by mass or less.

[0060] (Surface Modified Layer) The fluororesin film may have a surface modified layer having reactive functional groups on its surface. By providing a surface modified layer, the reactive functional groups can improve the adhesion between the fluororesin film and the copper foil layer. The surface modified layer can be formed by providing reactive functional groups through a surface treatment of the fluororesin surface by a plasma treatment process described later.

[0061] The fluororesin film of the present invention may or may not have an easily removable protective film layer to prevent scratches on the surface of the fluororesin film until immediately before use, and may or may not have any further layers. Furthermore, the fluororesin film may not contain an elastomer.

[0062] Furthermore, the fluororesin film of the present invention may consist only of fluororesin and inorganic filler, or it may further contain any additives such as the aforementioned optional components.

[0063] [Copper-clad laminate] The copper-clad laminate of the present invention is a copper-clad laminate having a structure in which the fluororesin film of the present invention described above and a copper foil layer are laminated together.

[0064] <Fluoropolymer Film> As fluoropolymer films have already been explained, detailed explanations will be omitted here, except for the following. For example, copper foil layers may be attached to both the front and back surfaces of the fluoropolymer film to form a copper-clad laminate, or copper foil layers may be attached to only one side of the fluoropolymer film to form a copper-clad laminate.

[0065] (Hygroscopicity) When communicating at high frequencies in a high-humidity atmosphere, the dielectric loss tangent Df may decrease due to moisture absorption by the substrate, potentially failing to meet the required electrical characteristics. Therefore, it is preferable to use a low-hygroscopic material as the substrate. Specifically, the moisture absorption rate of a fluororesin film at a temperature of 40°C and a relative humidity of 90% is preferably less than 0.11% by mass, more preferably less than 0.06% by mass, and even more preferably 0.05% by mass or less. By using such a fluororesin film as the substrate, the electrical characteristics can be met even in a high-humidity atmosphere.

[0066] Furthermore, when measuring the moisture absorption rate of a fluororesin film in a copper-clad laminate, the copper foil can be removed by immersing the copper-clad laminate in an etching solution, and the moisture absorption rate can be measured using the remaining fluororesin film as a sample.

[0067] <Copper foil layer> As the copper foil layer, an existing copper foil layer used in copper-clad laminates can be used. This layer mainly contains copper or a copper alloy, but may also contain metal components other than copper or a copper alloy.

[0068] The thickness of the copper foil layer is not particularly limited, but may be, for example, 1 μm to 50 μm, 2 μm to 40 μm, or 3 μm to 30 μm. A copper foil layer thickness of 1 μm or more provides excellent production stability as a copper-clad laminate and good handling properties. A copper foil layer thickness of 50 μm or less makes it easier to ensure the flexibility required for FPCs in flexible copper-clad laminates.

[0069] The overall thickness of the copper-clad laminate is not particularly limited, but may be between 10 μm and 500 μm, or between 60 μm and 450 μm. An overall thickness of 500 μm or less provides good flexibility suitable for FCCL, resulting in excellent handling during the manufacturing and use of circuit boards such as FPCs. Furthermore, an overall thickness of 10 μm or more provides sufficient strength for FCCL, resulting in excellent handling during the manufacturing and use of circuit boards such as FPCs.

[0070] The method for manufacturing copper-clad laminates is not particularly limited, and existing methods can be used. For example, a copper-clad laminate with a three-layer structure can be obtained by sandwiching both sides of a fluororesin film between copper foils and applying pressure, in the order of copper foil layer, fluororesin film, and copper foil layer. Pressure can be applied, for example, by sandwiching the fluororesin film, which is sandwiched between copper foils, with stainless steel plates. The pressure conditions are not particularly limited, but examples include a pressure of 1 MPa to 10 MPa, a temperature of 40°C to 400°C during pressure application, and hot pressing for typically 1 minute to 240 minutes.

[0071] (Dielectric loss tangent Df) For example, if the challenge is to minimize transmission loss during high-frequency communication, and considering the use of the copper-clad laminate of the present invention as a material for multilayer substrates for 6G, it is important that the material exhibits excellent low dielectric properties even under high temperature and high humidity conditions of 85°C and 85% relative humidity, as well as under normal temperature and humidity conditions of 20°C to 25°C and 50% to 65% relative humidity.

[0072] As a specific low dielectric property, the fluororesin film may have a dielectric loss tangent Df of 0.003 or less at a frequency of 40 GHz under conditions of a temperature of 85°C and a relative humidity of 85%, and it is more preferable that it be 0.0028 or less.

[0073] Furthermore, in the copper-clad laminate, the fluororesin film may have a dielectric loss tangent Df of 0.001 or less at a frequency of 40 GHz under conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%, and it is more preferable that it be 0.0009 or less.

[0074] The dielectric loss tangent Df can be measured using an SCR dielectric resonator or similar device.

[0075] In copper-clad laminates, the thermal expansion coefficient of the fluororesin film is preferably 120 ppm / K or less, more preferably 100 ppm / K or less, and even more preferably 80 ppm / K or less. Ideally, the lower limit of the thermal expansion coefficient of the fluororesin film should be 0 ppm / K, but as a guideline, the thermal expansion coefficient of the fluororesin film should be, for example, 20 ppm / K or more, 35 ppm / K or more, or 50 ppm / K or more.

[0076] In copper-clad laminates, the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% after heating the inorganic filler to 340°C to 400°C is preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.03% by mass or less.

[0077] In a copper-clad laminate, the fluororesin contained in the fluororesin film may be PTFE, PFA, FEP, or a combination of two or more of these.

[0078] [Circuit Board] The circuit board of the present invention comprises the fluororesin film of the present invention described above. Since the fluororesin film has already been described, its description is omitted here. For example, a circuit board such as an FPC can be obtained by patterning the copper foil layer of the copper-clad laminate of the present invention using a commonly used method such as etching to form a circuit. Other circuit boards besides FPCs are also included in the present invention.

[0079] [Method for Manufacturing Fluoropolymer Film] Next, the method for manufacturing the fluoropolymer film of the present invention will be described. This manufacturing method is a method for manufacturing the fluoropolymer film of the present invention described above, and includes the mixing step, molding step, heating step, cooling step, and skiving step described below. It may also include a plasma treatment step.

[0080] <Mixing Process> In the mixing process, the fluororesin powder and the inorganic filler powder are uniformly mixed to obtain a mixture. The fluororesin used as the powder is not particularly limited as long as it is a resin that can solve the problems of the present invention. For example, any one of PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer), or a combination of two or more of these, can be used.

[0081] The fluororesin powder used as a raw material has a particle shape, and its volume-average particle diameter can be 50% or less of the predetermined thickness of the fluororesin film, and can be appropriately selected according to the desired thickness of the fluororesin film. The volume-average particle diameter of the fluororesin powder is preferably 0.1 μm to 10 μm. By using fluororesin powder with a volume-average particle diameter in this range, a raw material composition can be obtained in which the inorganic filler powder particles and the fluororesin powder particles are uniformly dispersed. The volume-average particle diameter of the fluororesin powder may be 0.2 μm or more, 1 μm or more, or 5 μm or more. Furthermore, the volume-average particle diameter of the fluororesin powder may be 8 μm or less, or 5 μm or less.

[0082] Methods for setting the volume-average particle diameter of the fluororesin powder to 50% or less of the predetermined thickness of the fluororesin film, preferably in the range of 0.1 μm to 10 μm, include, for example, using a commercially available fluororesin particle dispersion (generally with a volume-average particle diameter in the range of 0.1 μm to 0.5 μm) in which fluororesin powder is dispersed in a solvent, or grinding commercially available powder-type fluororesin particles (generally with a volume-average particle diameter in the range of 200 μm to 600 μm) to achieve the above volume-average particle diameter. Details of the process using the fluororesin particles obtained by the above two methods will be described later.

[0083] The volume-average particle size of the fluororesin powder can be measured using a particle size distribution analyzer (Spectris Co., Ltd., "MS-3000") under a measurement air pressure of 1 Bar.

[0084] Specifically, the inorganic filler powder can be any of the following: alumina, titanium oxide, silica, barium sulfate, silicon carbide, boron nitride, silicon nitride, glass fiber, glass beads, and mica, or a combination of two or more of these. In particular, from the viewpoint of imparting high thermal stability (low thermal expansion) to the fluororesin film, silica, boron nitride, and alumina can be used alone or in combination. The inorganic filler powder has a particle shape, and the volume average particle diameter of the inorganic filler powder can be appropriately selected for the desired thickness of the fluororesin film as described above, but for example, it is preferably 0.1 μm to 10 μm, and may be 0.2 μm or more, 0.3 μm or more, 0.5 μm or more, or 1 μm or more, and may be 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less.

[0085] The volume-average particle size of the inorganic filler powder can be measured using a particle size distribution analyzer (Spectris Co., Ltd., "MS-3000") under a measurement air pressure of 1 Bar.

[0086] One embodiment of a method for obtaining a mixture by uniformly mixing fluororesin powder and inorganic filler powder is, for example, a dry mixing method of fluororesin powder and inorganic filler powder.

[0087] One method for uniformly mixing fluororesin powder and inorganic filler powder is to crush secondary particles formed by the aggregation of primary fluororesin particles to obtain fluororesin powder with a volume-average particle size of 0.1 μm to 10 μm, and then to mix the fluororesin powder and inorganic filler powder using a stirrer with blades or the like.

[0088] The particle size of the secondary particles of the fluororesin powder is not particularly limited, but may be, for example, 100 μm to 800 μm, 130 μm to 700 μm, or 150 μm to 600 μm. The method for crushing the secondary particles is not particularly limited, but examples include using a pulverizer such as a mixing pulverizer, an air jet pulverizer, or a freeze pulverizer.

[0089] The fluororesin powder and inorganic filler powder are blended in the raw material composition in a desired ratio such that the amounts of fluororesin powder and inorganic filler powder contained in the raw material composition are such that the fluororesin film of the present invention can be formed.

[0090] The stirring speed of the fluororesin powder and inorganic filler powder in dry mixing is not particularly limited, but may be, for example, 1000 rpm to 6000 rpm or 2000 rpm to 5000 rpm. The stirring time of the fluororesin powder and inorganic filler powder in dry mixing is not particularly limited, but may be, for example, 1 minute to 15 minutes or 2 minutes to 10 minutes.

[0091] Furthermore, fluororesin powder and inorganic filler powder can also be wet-mixed. For example, inorganic filler powder can be mixed and dispersed in the above-mentioned fluororesin particle dispersion.

[0092] <Molding Process> The molding process is a process of compressing and shaping the mixture obtained in the mixing process. For example, the mixture is molded into a cylindrical shape to form a molded body. One method of forming a molded body is to fill the mixture into a mold and compress it to form a cylindrical compressed molded body. The surface pressure during compression molding may be 10 MPa to 100 MPa, 20 MPa to 60 MPa, or 30 MPa to 50 MPa. By compressing the mixture, a compressed molded body is obtained in which the fluororesin powder and inorganic filler powder are uniformly dispersed.

[0093] <Heating Process> The heating process involves heating the mixture after the molding process to melt the fluororesin powder. Specifically, the compressed molded body obtained in the molding process is fired to obtain a billet. The firing temperature may be 340°C to 400°C, 350°C to 370°C, or 360°C to 370°C. The resulting billet is obtained as a molded body formed by the accumulation of fired mixtures. By firing the molded body, the individual fluororesin particles in the molded body melt and become one, with inorganic filler particles uniformly dispersed in the matrix. By firing the compressed molded body of the raw material composition, which is a mixture of fluororesin powder and inorganic filler powder, the formation of aggregates of inorganic filler powder can be suppressed, and a good billet with fewer coarse particles can be obtained.

[0094] <Cooling Process> The cooling process is a process in which the mixture after the heating process is cooled to crystallize the fluororesin. The cooling process allows the billet to be cooled from the firing temperature to room temperature and also allows the fluororesin to crystallize. For example, the billet can be cooled from 370°C to room temperature by leaving it undisturbed in the firing furnace.

[0095] From the standpoint of ease of performing the skiving process described later, the shape of the billet (molded body) is preferably cylindrical. When the billet (molded body) is cylindrical, the diameter of the cylinder may be, for example, 100 mm to 500 mm, or 150 mm to 500 mm.

[0096] <Skibing Process> The skiving process is a process in which the mixture after the cooling process is skived to form a fluororesin film. Specifically, the surface of the fired molded body, the billet, is cut to process it into a sheet. For example, if the billet (molded body) is cylindrical, the outer surface of the longitudinal direction of the fired cylindrical body is cut with a cutting blade, similar to peeling a radish, to obtain a sheet-like fluororesin film.

[0097] <Plasma Treatment Process> The plasma treatment process is a process in which, after the skiving process, the surface of the fluororesin film is treated with plasma to replace fluorine atoms with reactive functional groups and form a surface modified layer. The plasma treatment may be performed on only one side of the fluororesin film, or on both the front and back surfaces.

[0098] Examples of gases used in plasma treatment include nitrogen gas and hydrogen gas. Other gases that may be used include oxygen gas, argon gas, carbon dioxide gas, water vapor, helium gas, and ammonia gas. These gases may be used individually or in mixtures of two or more.

[0099] The preferred range of gas pressure in plasma processing varies depending on the type of gas used, but for example, when using a mixed gas of nitrogen and hydrogen, the gas pressure is preferably 1 Pa to 1000 Pa.

[0100] Plasma treatment can be performed by first evacuating a vacuum chamber containing a fluororesin film to a predetermined pressure, then, for example, introducing a plasma treatment gas into the vacuum chamber and generating a DC discharge plasma at an appropriate gas pressure.

[0101] The method for producing a fluororesin film according to the present invention may consist only of the above steps, or it may include a predetermined additional step in addition to the above steps.

[0102] The present invention will be described more specifically below with reference to examples. However, the present invention is not limited to the following examples.

[0103] [Measurement of moisture absorption rate of inorganic filler] The moisture absorption rate of the inorganic filler used in Examples 1-4 and Comparative Examples 1 and 2 was measured before and after heating. Silica, as shown in Table 1, was used as the inorganic filler.

[0104] (Measurement of moisture absorption rate before heating) A portion of the inorganic filler shown in Table 1 was sampled and degassed under vacuum at 23°C for 24 hours. The weight of the obtained sample was measured, and the amount of water vapor adsorbed was measured using a high-precision gas / vapor adsorption amount measuring device (BELSORP MAX X manufactured by Microtrac-Bell Co., Ltd.). The adsorption temperature was 40°C.

[0105] (Measurement of moisture absorption rate after heating) After measuring the moisture absorption rate before heating, each sample was heated at 365°C for 5 hours. After the heated samples were allowed to cool to room temperature, each sample was subjected to degassing treatment at 23°C under vacuum for 24 hours. The moisture absorption rate after heating was calculated based on the above.

[0106] [Preparation of Fluoropolymer Films] Fluoropolymer films of Examples 1 to 4 and Comparative Examples 1 and 2 were prepared according to the following procedure.

[0107] [Example 1] (Mixing process) PTFE powder (volume average particle size: 400 μm) and silica from Example 1 shown in Table 1 were mixed so that the silica content was 55 volume%, and the mixture was stirred for 5 minutes at a rotation speed of 3000 rpm using a rotary stirrer with blades to obtain a raw material composition which is a mixture containing PTFE powder and silica.

[0108] (Molding process, heating process, cooling process) 600 g of the raw material composition was filled into a cylindrical mold and compressed from above at a press pressure of 30 MPa for 3 minutes to obtain a cylindrical pre-molded body (outer diameter 67 mm x inner diameter 33 mm). The obtained pre-molded body was placed in a firing furnace and fired at 365°C for 5 hours. After that, the billet was left to stand in the firing furnace to cool from 365°C to room temperature to obtain the billet.

[0109] (Skibing process) The obtained billet (outer diameter 67 mm x inner diameter 33 mm) was skived using a skiving machine at a cutting speed of 8 m / min and a target thickness of 50 μm to produce a 50 μm thick fluororesin sheet that would become a fluororesin film.

[0110] [Example 2] A fluororesin film of Example 2 was manufactured in the same manner as in Example 1, except that the silica of Example 2 shown in Table 1 was used as the inorganic filler instead of the silica of Example 1.

[0111] [Example 3] The same silica used in Example 1 was used as the inorganic filler, and the PTFE powder and silica were mixed so that the silica content was 30% by volume. Otherwise, the fluororesin film of Example 3 was manufactured in the same manner as in Example 1.

[0112] [Example 4] Using the same silica as in Example 3, the fluororesin film of Example 4 was manufactured in the same manner as in Example 3, except that the silica was mixed with PTFE powder so that the silica content was 70% by volume.

[0113] [Comparative Example 1] A fluororesin film of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the silica of Comparative Example 1 shown in Table 1 was used as the inorganic filler instead of the silica of Example 1.

[0114] [Comparative Example 2] A fluororesin film of Comparative Example 2 was manufactured in the same manner as in Example 1, except that the silica of Comparative Example 2 shown in Table 1 was used as the inorganic filler instead of the silica of Example 1.

[0115] [Evaluation of physical properties of fluororesin films] The thermal expansion coefficient and hygroscopicity of the fluororesin films prepared in Examples 1-4 and Comparative Examples 1 and 2 were evaluated.

[0116] <Measurement of Thermal Expansion Coefficient (CTE)> A fluororesin film was cut into 40 mm x 3 mm sections, and the thermal expansion coefficient was measured using a thermomechanical measuring device (TMA) ("Q 400" manufactured by T.A. Instrument Japan Co., Ltd.). The thermal expansion coefficient was measured with a following load of 0.005 N and a measurement temperature from room temperature to 200 °C, with a heating rate of 5 °C / min. The thermal expansion coefficient was calculated from the amount of thermal expansion in the range of 50 °C to 150 °C during the measurement from room temperature to 200 °C.

[0117] <Measurement of moisture absorption rate> Rectangular test pieces, approximately 200 mm in length and 100 mm in width, were cut from the fluororesin film and used as samples. Degassing treatment was performed at 23°C under vacuum for 24 hours. The moisture absorption rate was calculated based on the above.

[0118] [Evaluation of physical properties of fluororesin film in copper-clad laminates] Copper-clad laminates were fabricated using the prepared fluororesin film. After removing the copper foil from the copper-clad laminates, the dielectric loss tangent Df of the remaining fluororesin film was measured.

[0119] <Preparation of Copper-Clad Laminate> A fluororesin film of Example 1 with a thickness of 50 μm was cut to a size of 100 mm x 100 mm. A low-roughness electrolytic copper foil (manufactured by Mitsui Mining & Smelting Co., Ltd., "TQ-M4-VSP", maximum height Rz: 0.6 μm) with a thickness of 18 μm, cut to 100 mm x 100 mm, was laminated onto this. The resulting laminated structure was sandwiched between SUS304 stainless steel plates (120 mm x 120 mm, thickness 5 mm), and a copper-clad laminate was obtained by hot pressing under conditions of a temperature of 360°C and a holding pressure of 4 MPa for 60 minutes.

[0120] Furthermore, copper-clad laminates were similarly prepared using the fluororesin films of Examples 2-4 and Comparative Examples 1 and 2.

[0121] <Removal of copper foil> The copper foil was removed from the fluororesin films of Examples 1 to 4 and Comparative Examples 1 and 2 by immersing the copper-clad laminate in an etching solution mainly composed of copper(II) chloride dihydrate.

[0122] <Measurement of Dielectric Loss Tangent Df> (Dielectric Loss Tangent Df before moisture absorption treatment) Rectangular test pieces measuring 35 mm in length and 45 mm in width were cut from the fluororesin films of Examples 1 to 4 and Comparative Examples 1 and 2. The dielectric loss tangent Df at a measurement frequency of 40 GHz was measured using the SCR (Split Cylinder Resonance) method with a testing machine under the following conditions. This measurement result was defined as the dielectric loss tangent Df before moisture absorption treatment.

[0123] (Test equipment) ・Network analyzer P5007A (Keysight Technologies, Inc.) (Used for measurements inside the constant temperature chamber) ・Network analyzer N5290A (Keysight Technologies, Inc.) (Used for measurements outside the constant temperature chamber) ・Split cylinder resonator (EM Labs Inc.) ・Constant temperature chamber SH-622 (ESPEC Corporation)

[0124] (Dielectric loss tangent Df during moisture absorption treatment) The sample was placed in a resonator inside a constant temperature bath (temperature 85°C, relative humidity 85%) and left to stand for 24 hours. After that, the dielectric loss tangent Df at a measurement frequency of 40 GHz was measured using the SCR method.

[0125] (Dielectric loss tangent Df after moisture absorption treatment) Immediately after removal from the constant temperature bath, the dielectric loss tangent Df was measured under standard conditions (JIS Z 8703-1983) of a temperature of 23°C, relative humidity of 50%, and atmospheric pressure of 86 kPa to 106 kPa, in the same manner as before moisture absorption treatment.

[0126] <Measurement of moisture absorption rate> Rectangular test pieces approximately 200 mm in length and 100 mm in width were cut from the fluororesin films of Examples 1 to 4 and Comparative Examples 1 and 2 to be used as samples, and degassing treatment was performed under vacuum at 23°C for 24 hours. The moisture absorption rate was calculated based on the above.

[0127] The results of the physical property evaluation are shown in Tables 1 and 2.

[0128]

[0129]

[0130] In Example 1, by using surface-treated silica with high heat resistance and low moisture absorption, there were no problems with the coefficient of thermal expansion, and the moisture absorption rate of the inorganic filler alone, the moisture absorption rate of the fluororesin film, and the dielectric loss tangent Df of the fluororesin film were all good (Tables 1 and 2). In particular, the dielectric loss tangent Df value was good even during moisture absorption.

[0131] Example 2 yielded similarly good results to Example 1 (Tables 1 and 2). Although the moisture absorption rate of the inorganic filler alone was inferior to that of Example 1 after heating at 365°C, there were no performance issues in the fluororesin film state.

[0132] In Example 3, although the thermal expansion coefficient of the fluororesin film was large due to the filler content being 30 volume%, the results for the moisture absorption rate of the inorganic filler alone, the moisture absorption rate of the fluororesin film, and the dielectric loss tangent Df of the fluororesin film were all good (Tables 1 and 2). In particular, the dielectric loss tangent Df value during moisture absorption was the best.

[0133] In Example 4, the filler content was 70% by volume, resulting in a smaller thermal expansion coefficient for the fluororesin film compared to Example 3. Furthermore, the moisture absorption rates of the inorganic filler alone, the fluororesin film, and the dielectric loss tangent Df of the fluororesin film were all favorable (Tables 1 and 2).

[0134] Comparative Example 1 used silica that had not undergone surface treatment, and the moisture absorption rate of the inorganic filler alone was higher compared to Examples 1 to 4 (Table 1). Although the use of this silica did not affect the moisture absorption rate of the fluororesin film, the dielectric loss tangent Df value decreased after and during moisture absorption, and in particular the dielectric loss tangent Df value during moisture absorption worsened compared to Examples 1 to 4 (Table 2).

[0135] Comparative Example 2 is an example using surface-treated silica, which has low hygroscopicity but poor heat resistance. The hygroscopicity of the inorganic filler alone was good before heating, but after heating, the hygroscopicity became higher than that of untreated silica (Table 1). In addition, discoloration was observed when the fluororesin film was manufactured, so it was judged to have an appearance defect, and the fluororesin film was not evaluated.

[0136] [Summary] Based on the above, it is clear that the present invention can provide a fluororesin film that has low thermal expansion and maintains low hygroscopicity even after heat treatment, a method for manufacturing a fluororesin film, a copper-clad laminate, and a circuit board, and is industrially useful.

Claims

1. A fluororesin film comprising a fluororesin and an inorganic filler, wherein the moisture absorption rate of the fluororesin film at a temperature of 40°C and a relative humidity of 90% is less than 0.06% by mass.

2. The fluororesin film according to claim 1, wherein the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% is 0.1% by mass or less.

3. The fluororesin film according to claim 1, wherein the dielectric loss tangent Df at a frequency of 40 GHz is 0.001 or less under the conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%.

4. The fluororesin film according to claim 1, wherein the dielectric loss tangent Df at a frequency of 40 GHz is 0.003 or less under the conditions of a temperature of 85°C and a relative humidity of 85%.

5. The fluororesin film according to claim 1, wherein the coefficient of thermal expansion of the fluororesin film is 120 ppm / K or less.

6. The fluororesin film according to claim 1, wherein the moisture absorption rate of the inorganic filler at a temperature of 40°C and relative humidity of 90% after heating the inorganic filler to 340°C to 400°C is 0.1% by mass or less.

7. The fluororesin film according to claim 1, wherein the inorganic filler content is 30% to 70% by volume.

8. The fluororesin film according to claim 1, wherein the volume-average particle size of the inorganic filler is 0.1 μm to 10 μm.

9. The fluororesin film according to claim 1, wherein the inorganic filler is any of alumina, titanium oxide, silica, barium sulfate, silicon carbide, boron nitride, silicon nitride, glass fiber, glass beads, and mica, or a combination of two or more of these.

10. The fluororesin film according to claim 1, wherein the inorganic filler is surface-treated silica whose surface is hydrophobic.

11. The fluororesin film according to claim 1, wherein the thickness of the fluororesin film is 25 μm to 500 μm.

12. The fluororesin film according to claim 1, wherein the fluororesin is any one of PTFE, PFA, and FEP, or a combination of two or more of these.

13. A copper-clad laminate comprising a fluororesin film according to claim 1 and a copper foil layer, wherein the fluororesin film has a dielectric loss tangent Df of 0.003 or less at a frequency of 40 GHz under conditions of a temperature of 85°C and a relative humidity of 85%.

14. The copper-clad laminate according to claim 13, wherein the fluororesin film has a dielectric loss tangent Df of 0.001 or less at a frequency of 40 GHz under conditions of a temperature of 20°C to 25°C and a relative humidity of 50% to 65%.

15. The copper-clad laminate according to claim 13, wherein the moisture absorption rate of the fluororesin film at a temperature of 40°C and a relative humidity of 90% is less than 0.11% by mass.

16. The copper-clad laminate according to claim 13, wherein the thermal expansion coefficient of the fluororesin film is 120 ppm / K or less.

17. The copper-clad laminate according to claim 13, wherein the moisture absorption rate of the inorganic filler at a temperature of 40°C and a relative humidity of 90% after the inorganic filler has been heated to 340°C to 400°C is 0.1% by mass or less.

18. The copper-clad laminate according to claim 13, wherein the fluororesin contained in the fluororesin film is any of PTFE, PFA, and FEP, or a combination of two or more of these.

19. A method for producing a fluororesin film according to claim 1, comprising: a mixing step of uniformly mixing fluororesin powder and inorganic filler powder to obtain a mixture; a molding step of compacting and shaping the mixture; a heating step of heating the mixture after the molding step to melt the fluororesin powder; a cooling step of cooling the mixture after the heating step to crystallize the fluororesin; and a skiving step of skiving the mixture after the cooling step to form the fluororesin film.

20. A circuit board comprising the fluororesin film described in claim 1.

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