Fluororesin film, fluororestin film manufacturing method, sheet-shaped adhesive film for flexible copper clad laminate, flexible copper clad laminate, and circuit board

Abstract

Provided are a fluororesin film, a fluororesin film manufacturing method, a sheet-shaped adhesive film for a flexible copper clad laminate, a flexible copper clad laminate, and a circuit board, wherein there is little transmission loss during high-frequency communication, and UV-YAG laser drilling can be performed.　This fluororesin film includes: a fluororesin; an inorganic filler that has been dispersed in the fluororesin; and an ultraviolet absorbant that has been dispersed in the fluororesin, wherein the inorganic filler content is 35 vol% to 70 vol%, the dielectric loss tangent Df of the ultraviolet absorbant at a frequency of 20 GHz is 0.004 or less, the ultraviolet absorption rate of the fluororesin film is 20% or greater, and drilling can be performed by irradiation with a wavelength of 355 nm by a UV-YAG laser.

Description

Fluororesin film, manufacturing method of fluororesin film, sheet-like attachment film for flexible copper-clad laminate, flexible copper-clad laminate and circuit board

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

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

[0003] These communication devices and network-related electronic devices require the transmission and processing of large volumes of information at high speed with low loss, and the electrical signals handled by FPCs are becoming increasingly higher in frequency. If transmission loss is large when transmitting high-frequency electrical signals, problems such as electrical signal loss and increased signal delay time occur. For this reason, FCCL used in FPCs is required to have low dielectric properties (low dielectric constant, low dielectric dissipation factor) and good electrical properties with reduced transmission loss in high-frequency transmission.

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

[0005] In particular, there is a demand for insulating film materials for FPCs intended for sixth-generation mobile communication systems (hereinafter sometimes referred to as "6G") that have low transmission loss during high-frequency communication.

[0006] Furthermore, in order to create multi-layer FPC boards, holes are drilled in the FCCL using a UV-YAG laser or similar, and the inner walls of the holes are then copper-plated to ensure electrical continuity between the layers of the multi-layer board. Therefore, there is a demand for insulating films for FPCs that can be drilled with a UV-YAG laser.

[0007] Furthermore, in order to make FPC boards multilayered, it is necessary that the film not only be easy to process during the hole drilling process, but also that the film not change in size during the heat pressing process for lamination, so materials with a low coefficient of thermal expansion (CTE) are required for insulating films for multilayer FPCs.

[0008] However, although there has been demand for such materials in the past, there has been no material suitable for 6G that has low transmission loss during high-frequency communication and can be drilled with a UV-YAG laser.

[0009] Therefore, an object of the present invention is to provide a fluororesin film that has little transmission loss during high-frequency communication and that can be drilled with a UV-YAG laser, a method for manufacturing a fluororesin film, a sheet-like adhesive film for flexible copper-clad laminates, a flexible copper-clad laminate, and a circuit board.

[0010] In order to solve the above problems, the fluororesin film of the present invention is a fluororesin film comprising a fluororesin, an inorganic filler dispersed in the fluororesin, and an ultraviolet absorber dispersed in the fluororesin, wherein the content of the inorganic filler is 35% by volume to 70% by volume, the dielectric loss tangent Df of the ultraviolet absorber at a frequency of 20 GHz is 0.004 or less, the ultraviolet absorptance of the fluororesin film is 20% or more, and the fluororesin film can be drilled by irradiation with a UV-YAG laser at a wavelength of 355 nm.

[0011] The thickness of the fluororesin film may be 25 μm to 200 μm.

[0012] The fluororesin film of the present invention may have a relative dielectric constant Dk of 3.0 or less at a frequency of 20 GHz and a dielectric loss tangent Df of 0.0010 or less at a frequency of 20 GHz.

[0013] The fluororesin film may have a thermal expansion coefficient of 80 ppm / K or less.

[0014] The fluororesin film may have a surface-modifying layer having reactive functional groups on the surface.

[0015] The ultraviolet absorber may be any one of liquid crystal polymer, polyether ether ketone, and polyphenylene sulfide, or a combination of two or more of these.

[0016] The content of the ultraviolet absorber may be 0.3% by volume to 25% by volume.

[0017] The inorganic filler may be 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.

[0018] The inorganic filler may have a number average particle size of 0.1 to 10 μm.

[0019] The fluororesin may be any one of PTFE, PFA and FEP, or a combination of two or more of these.

[0020] Furthermore, in order to solve the above-mentioned problems, the manufacturing method of the present invention is a manufacturing method of the fluororesin film of the present invention, and includes a mixing step of uniformly mixing a fluororesin powder, an inorganic filler powder, and an ultraviolet absorber to obtain a mixture, a molding step of compressing and molding 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 a fluororesin film.

[0021] The method for producing a fluororesin film of the present invention may include, after the skiving step, a plasma treatment step of plasma-treating the surface of the fluororesin film to replace fluorine atoms with reactive functional groups to form a surface-modified layer.

[0022] In order to solve the above problems, the sheet-like attachment film for flexible copper-clad laminates of the present invention comprises the fluororesin film of the present invention.

[0023] In order to solve the above problems, the flexible copper-clad laminate of the present invention comprises the fluororesin film of the present invention and a copper foil layer.

[0024] In order to solve the above problems, the circuit board of the present invention comprises the fluororesin film of the present invention.

[0025] The present invention can provide a fluororesin film that has little transmission loss during high-frequency communication and that can be drilled with a UV-YAG laser, a method for producing a fluororesin film, a sheet-like attachment film for flexible copper-clad laminates, a flexible copper-clad laminate, and a circuit board.

[0026] 2A and 2B . FIG. 2B is a schematic cross-sectional view showing an example of the fluororesin film of the present invention. FIG. 3 is an enlarged electronic image of the cross section of the fluororesin film of Example 6. FIG. 4 is an enlarged electronic image of the cross section of the fluororesin film of Example 6. FIG. 5 is an enlarged electronic image of the cross section of the fluororesin film of Example 6 at a higher magnification than that of FIGS. 2A and 2B . FIG. 6 is an enlarged electronic image of the cross section of the fluororesin film of Example 6 at a higher magnification than that of FIGS. 2A and 2B .

[0027] Hereinafter, one embodiment of the fluororesin film, the method for producing a fluororesin film, the sheet-like attachment film for flexible copper-clad laminates, the flexible copper-clad laminates, and the circuit board of the present invention will be described.

[0028] [Fluororesin Film] The fluororesin film of the present invention comprises a fluororesin, an inorganic filler, and an ultraviolet absorber, which will be described later.

[0029] (Thickness of Fluororesin Film) The thickness of the fluororesin film is preferably 25 μm to 200 μm. When the film thickness is 25 μm or more, the strength of the film can be sufficiently maintained and good handling properties can be obtained. Furthermore, when the film thickness is 200 μm or less, sufficient flexibility can be obtained.

[0030] The thickness of the fluororesin film can be appropriately selected depending on the application and requirements, but may be, for example, 30 μm or more, 50 μm or more, 70 μm or more, or 100 μm or more, or 150 μm or less, or 100 μm or less.

[0031] The thickness of the fluororesin film can be determined by measuring the thickness at any ten positions on the fluororesin film using a film thickness measuring device such as a micrometer, and averaging the measured values.

[0032] (Drilling Processability) The fluororesin film of the present invention is a film that can be drilled by irradiation with a UV-YAG laser at a wavelength of 355 nm. As described above, insulating films for FPCs are required to be materials that can be drilled with a UV-YAG laser, and the fluororesin film of the present invention is a suitable film for use as an insulating film for FPCs.

[0033] The YAG laser is a solid-state laser obtained by adding a small amount of neodymium to a garnet-structured crystal composed of a composite oxide of yttrium and aluminum. ２ Compared to liquid lasers, it has advantages such as high light focusing ability, stable oscillation without deterioration over time, low running costs, and ease of introduction into processing equipment.

[0034] YAG laser wavelengths include the fundamental wavelength (1,064 nm), second harmonic (532 nm), third harmonic (355 nm), and fourth harmonic (266 nm), and in the present invention, a UV-YAG laser using the third harmonic (355 nm) as the laser light source is used to irradiate a fluororesin film, thereby enabling drilling. The diameter of the holes produced in this drilling process is, for example, about 5 μm to 150 μm, and it is common to drill holes with diameters of 50 μm to 100 μm using a UV-YAG laser.

[0035] The UV-YAG laser device is not particularly limited, but for example, "MODEL 5330xi" and "MODEL 5335" manufactured by ESI can be used.

[0036] (Relative permittivity Dk, dielectric loss tangent Df) An object of the present invention is to reduce transmission loss during high-frequency communication. In particular, assuming that the fluororesin film of the present invention is used as a material for an insulating film for a 6G FPC, specific low dielectric properties of the fluororesin film are such that the relative permittivity Dk at a frequency of 20 GHz is preferably 3.0 or less, and the dielectric loss tangent Df at a frequency of 20 GHz is preferably 0.0010 or less.

[0037] The relative dielectric constant Dk and the dielectric loss tangent Df can be measured using an SPDR dielectric resonator or the like.

[0038] (Coefficient of thermal expansion CTE) As mentioned above, in order to make multi-layer FPC boards, holes are drilled in the FCCL using a UV-YAG laser such as a UV-YAG laser, and then the inner walls of the drilled holes are copper-plated to ensure electrical continuity between the layers of the multi-layer board. In order to process multi-layer boards in this way, it is necessary to prevent warping of the board and misalignment of the circuits, so the rate of volume expansion of the fluororesin film due to temperature rise is important.

[0039] Specifically, in consideration of processing multilayer substrates such as FPCs, the fluororesin film of the present invention preferably has a thermal expansion coefficient of 80 ppm / K or less.

[0040] <Ultraviolet Absorption Rate> The fluororesin film of the present invention has an ultraviolet absorption rate of 20% or more, so that when irradiated with a UV-YAG laser having a wavelength of 355 nm, the fluororesin film generates heat and opens. An ultraviolet absorption rate of 50% or more allows for more efficient heat generation, thereby shortening the process time for the hole-making process. Theoretically, the upper limit of the ultraviolet absorption rate of the fluororesin film is 100%, but the upper limit of the measured value is approximately 95%.

[0041] The ultraviolet absorptance of the fluororesin film can be obtained from the results of measuring the transmittance and reflectance at 355 nm using a UV-visible spectrophotometer. For example, the ultraviolet absorptance of the fluororesin film can be calculated using the following formula (1):

[0042] UV absorbance (%) = 100 - UV reflectance (%) - UV transmittance (%) (1)

[0043] <Fluororesin> Fluororesin is a synthetic resin with excellent heat resistance, electrical insulation, non-stickiness, and weather resistance, and fluororesin films molded into films are widely used in industrial fields such as chemical materials, electrical and electronic components, semiconductors, automobiles, etc. Fluororesin is also useful as a resin for fluororesin films that have low transmission loss during high-frequency communication and can be drilled with a UV-YAG laser, as in the present invention.

[0044] The fluororesin that can be used 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 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 total content of the inorganic filler and the UV absorber in the fluororesin film. For example, since the content of the inorganic filler in the fluororesin film is 35 vol% to 70 vol%, as described below, when the content of the UV absorber is 0.3 vol% to 25 vol%, the content of the fluororesin in the fluororesin film may be 29.7 vol% to 64.7 vol%, or may be 40 vol% or more, 50 vol% or more, or 60 vol% or less, 50 vol% or less, or 40 vol% or less. When the content of the fluororesin is 29.7 vol% or more, the fluororesin film has good strength. Furthermore, when the content of the fluororesin is 64.7 vol% or less, the fluororesin film has sufficient flexibility for use in FCCLs, and the inorganic filler contained in the fluororesin film keeps the thermal expansion coefficient low, resulting in excellent thermal stability.

[0046] <Inorganic Filler> The inorganic filler is an important component, along with the ultraviolet absorber, for enabling perforation of the fluororesin film by irradiation with a UV-YAG laser at a wavelength of 355 nm, and is present in the fluororesin film in a dispersed state.

[0047] Fluororesin has no absorption band at 355 nm and has low thermal conductivity, so if a fluororesin film is made solely of fluororesin, no holes will be created in the film even if it is irradiated with a UV-YAG laser with a wavelength of 355 nm.

[0048] On the other hand, inorganic fillers have a higher thermal conductivity than fluororesin, and therefore can sufficiently transmit the heat generated by the ultraviolet absorber when it absorbs ultraviolet rays. Therefore, in a fluororesin film in which inorganic fillers are sufficiently dispersed, when the ultraviolet absorber generates heat, the heat is transmitted through the inorganic filler to the interior of the fluororesin film. Because the transmitted heat is sufficient to melt the fluororesin, the fluororesin film in which inorganic fillers are dispersed can be drilled by irradiating it with a UV-YAG laser with a wavelength of 355 nm.

[0049] Specifically, the inorganic filler may be 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. In particular, from the viewpoint of imparting high thermal stability (low thermal expansion) to the fluororesin film, silica, boron nitride, and alumina may be used alone or in combination.

[0050] The content of the inorganic filler in the fluororesin film is 35% by volume to 70% by volume, and may be 40% by volume or more, 50% by volume or more, or 60% by volume or more, or may be 60% by volume or less, 50% by volume or less, or 40% by volume or less.

[0051] When the content of the inorganic filler in the fluororesin film is 35% by volume or more, the coefficient of thermal expansion of the fluororesin film is suppressed to a small value, for example, 100 ppm / K or less, and the fluororesin film has excellent thermal stability. Also, when the content of the inorganic filler in the fluororesin film is 70% by volume or less, the strength of the fluororesin film can be sufficiently maintained and good handleability can be obtained.

[0052] The average particle size of the inorganic filler can be selected appropriately depending on the desired thickness of the fluororesin film, but is preferably 0.1 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, or 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less. By ensuring that the average particle size of the inorganic filler is in the range of 0.1 to 10 μm, aggregation of inorganic filler particles can be suppressed, allowing the inorganic filler to be uniformly dispersed in the fluororesin. Furthermore, the proportion of coarse particles can be reduced, thereby improving the processing accuracy of drilling by UV-YAG laser irradiation. In other words, regardless of which region of the fluororesin film is to be drilled, the inorganic filler must be present in the region to be drilled. For this reason, it is important that the average particle size of the inorganic filler is smaller than the 50 μm to 100 μm diameter of typical holes drilled with a UV-YAG laser, and therefore the average particle size of the inorganic filler is preferably 0.1 to 10 μm. Furthermore, by ensuring that the average particle size of the inorganic filler is in the range of 0.1 to 10 μm, the generation of through holes (pinholes) in the fluororesin film can be suppressed, and excellent elongation properties can be obtained.

[0053] The average particle size of the inorganic filler can be determined using a scanning electron microscope (such as "SU8220" manufactured by Hitachi High-Technologies Corporation). In a scanning electron microscope image obtained by observing the surface of a fluororesin film over an area of ​​100 μm wide x 100 μm long at an acceleration voltage of 5 kV and a magnification of 1000, the particle size (diameter or longest diameter) of each of 100 arbitrarily selected inorganic filler particles is measured, and the arithmetic mean value thereof can be used as the number average particle size of the inorganic filler contained in the fluororesin film.

[0054] <Ultraviolet Absorber> The ultraviolet absorber is an important component, along with the inorganic filler, for enabling perforation processing of the fluororesin film by irradiation with a UV-YAG laser at a wavelength of 355 nm, and is present in the fluororesin film in a dispersed state.

[0055] Because fluororesin has no absorption band at 355 nm and has low thermal conductivity, if a fluororesin film is made only of fluororesin, no holes will be formed in the film even if it is irradiated with a UV-YAG laser at a wavelength of 355 nm. Furthermore, although inorganic fillers have higher thermal conductivity than fluororesin, the inorganic fillers themselves do not generate sufficient heat when irradiated with ultraviolet light, so even if a fluororesin film is made by dispersing only inorganic fillers in fluororesin, no holes will be formed in the film when it is irradiated with a UV-YAG laser at a wavelength of 355 nm.

[0056] On the other hand, the UV absorber generates heat by absorbing UV rays, and this heat melts the fluororesin, making it possible to drill holes in the fluororesin film. Therefore, in a fluororesin film in which the UV absorber and inorganic filler are sufficiently dispersed, when the UV absorber generates heat, the heat is transmitted through the inorganic filler to the interior of the fluororesin film. Because the transmitted heat is sufficient to melt the fluororesin, the fluororesin film in which the UV absorber and inorganic filler are dispersed can be drilled by irradiating it with a UV-YAG laser with a wavelength of 355 nm.

[0057] An object of the present invention is to reduce transmission loss during high-frequency communication, and assuming that the fluororesin film of the present invention is used as an insulating film material for 6G FPCs in particular, the dielectric loss tangent Df of the ultraviolet absorber at a frequency of 20 GHz is set to be 0.004 or less. If the dielectric loss tangent Df exceeds 0.004, the value of the dielectric loss tangent Df of the fluororesin film will be large, which may result in increased transmission loss during high-frequency communication.

[0058] Specifically, the UV absorber may be any one of liquid crystal polymer (LCP), polyether ether ketone (PEEK), polyphenylene sulfide (PPS), or a combination of two or more of these. These UV absorbers are useful because they allow efficient perforation of fluororesin films. In addition, because they are heat-resistant, their melting and solidification behavior can be easily matched to that of fluororesin, and they do not cause problems during the heating and cooling steps of the fluororesin film manufacturing method described below.

[0059] Liquid crystal polymers are, for example, thermoplastic polymers that exhibit liquid crystallinity when melted. Examples of liquid crystal polymers that can be used as UV absorbers include aromatic polyesters that have an absorption band in the UV region, and have a rigid, unbending molecular structure. Because of the low molecular entanglement typical of polymeric substances, they have low viscosity in the molten state and can exhibit high fluidity during molding. In other words, liquid crystal polymers can achieve both heat resistance and fluidity.

[0060] More specific examples of liquid crystal polymers include wholly aromatic polyesters obtained by condensing monomers such as parahydroxybenzoic acid, biphenol, terephthalic acid, etc. Also included are polycondensates of ethylene terephthalate and parahydroxybenzoic acid, polycondensates of phenol and phthalic acid and parahydroxybenzoic acid, and polycondensates of 2,6-hydroxynaphthoic acid and parahydroxybenzoic acid.

[0061] The content of the ultraviolet absorber in the fluororesin film is 0.3 vol % to 25 vol %, and may be 0.5 vol % or more, 1 vol % or more, or 3 vol % or more, or may be 20 vol % or less, 15 vol % or less, or 10 vol % or less.

[0062] When the content of the ultraviolet absorber in the fluororesin film is 0.3% by volume or more, the fluororesin film can be drilled by irradiating it with a UV-YAG laser at a wavelength of 355 nm when used in combination with an inorganic filler. Also, when the content of the ultraviolet absorber in the fluororesin film is 25% by volume or less, the strength of the fluororesin film can be sufficiently maintained, and good handleability can be obtained.

[0063] (Optional Components) The fluororesin film of the present invention may further contain optional components. Examples of optional components include, but are not limited to, flame retardants, flame retardant assistants, pigments, antioxidants, reflectivity imparting agents, opacifying agents, lubricants, processing stabilizers, plasticizers, and foaming agents. When optional components are contained, the total content of the optional components in the fluororesin film may be 20% by mass or less, 10% by mass or less, or 5% by mass or less.

[0064] (Surface Modification Layer) The fluororesin film may have a surface modification layer having reactive functional groups on its surface. By providing the surface modification layer, the reactive functional groups can enhance the adhesion between the fluororesin film and the copper foil layer. The surface modification layer can be formed by providing reactive functional groups through surface treatment of the fluororesin surface by a plasma treatment step described below.

[0065] Fig. 1 is a schematic cross-sectional view showing an example of the fluororesin film of the present invention. The fluororesin film 100 shown in Fig. 1 is a single-layer film not provided with a surface-modifying layer, but when provided with a surface-modifying layer, the surface-modifying layer is disposed on both or either of the front surface 11 and the back surface 12 of the fluororesin film 100.

[0066] In addition, for convenience of explanation, in FIG. 1, the front surface 11 and the back surface 12 of the fluororesin film 100 are denoted by different reference numerals, but the front surface and the back surface do not need to be distinguished from each other.

[0067] The fluororesin film of the present invention may or may not further comprise an easily detachable protective film layer 200 to prevent the surface of the fluororesin film from being scratched until immediately before use, and may or may not optionally comprise additional layers. In addition, the fluororesin film does not need to contain an elastomer.

[0068] The fluororesin film of the present invention may be composed of only a fluororesin, an inorganic filler, and an ultraviolet absorber, or may further contain optional additives such as the optional components described above.

[0069] [Sheet-like adhesive film for flexible copper-clad laminates] The sheet-like adhesive film for flexible copper-clad laminates of the present invention comprises the fluororesin film of the present invention described above. This film is a film for flexible copper-clad laminates, and is a film to be attached to copper foil as an insulating layer to form a flexible copper-clad laminate. Examples of sheet-like forms include a tape wound around a core, a tape wound without a core, and a sheet cut to any size, such as A4 size or A3 size. Note that the fluororesin film has already been described, so a description thereof will be omitted here.

[0070] The sheet-like adhesive film for flexible copper-clad laminates may or may not include components other than the fluororesin film. For example, the adhesive film may further include an easily removable protective film layer 200 to prevent scratches on the surface 11 of the fluororesin film 100 until just before it is attached to the copper foil.

[0071] [Flexible Copper-Clad Laminate] The flexible copper-clad laminate of the present invention comprises the above-described fluororesin film of the present invention and a copper foil layer. The fluororesin film has already been described, so a detailed description will be omitted here. For example, the flexible copper-clad laminate of the present invention is obtained by attaching the sheet-like attachment film for flexible copper-clad laminate of the present invention to a copper foil layer. The flexible copper-clad laminate may be obtained by attaching copper foil layers to both the front and back surfaces of the fluororesin film, or by attaching a copper foil layer to only one surface of the fluororesin film.

[0072] <Copper Foil Layer> As the copper foil layer, an existing copper foil layer used in flexible copper-clad laminates can be adopted, and it is a layer mainly containing copper or a copper alloy, but may also contain metal components other than copper or a copper alloy.

[0073] 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. Having a copper foil layer thickness of 1 μm or more provides excellent production stability as a flexible copper-clad laminate and good handling properties. Furthermore, having a copper foil layer thickness of 50 μm or less makes it easier to ensure the flexibility required for an FPC in the flexible copper-clad laminate.

[0074] The surface roughness (maximum height) Rz of the copper foil layer on the surface to be bonded to the fluororesin film of the present invention is, for example, 1.3 μm or less, preferably 1.0 μm or less, and more preferably 0.8 μm or less. The lower limit of the surface roughness (maximum height) Rz is not particularly limited, but is, for example, 0.01 μm or more or 0.1 μm or more. The surface roughness (maximum height) Rz can be a value obtained by measuring the maximum depth when observing the cross section of the flexible copper-clad laminate.

[0075] The total thickness of the flexible copper-clad laminate is not particularly limited, but may be 10 μm to 500 μm, or 60 μm to 450 μm. When the total thickness of the flexible copper-clad laminate is 500 μm or less, good flexibility suitable for FCCL can be obtained, and handling is excellent during the manufacture and use of circuit boards such as FPCs. Furthermore, when the total thickness of the flexible copper-clad laminate is 10 μm or more, sufficient strength as an FCCL can be obtained, and handling is excellent during the manufacture and use of circuit boards such as FPCs.

[0076] The method for producing a flexible copper-clad laminate is not particularly limited, and existing methods can be used. For example, by sandwiching both sides of a fluororesin film with copper foil and applying pressure, a three-layer flexible copper-clad laminate can be obtained in which a copper foil layer, a fluororesin film, and a copper foil layer are laminated in this order. Pressurization can be performed, for example, by sandwiching the fluororesin film with copper foil on both sides and further sandwiching it between stainless steel plates. The pressurization conditions are not particularly limited, but examples include a pressure of 1 MPa to 10 MPa, a temperature during pressing of 40°C to 400°C, and heat pressing for typically 1 minute to 240 minutes.

[0077] [Circuit Board] The circuit board of the present invention comprises the above-described fluororesin film of the present invention. Since the fluororesin film has already been described, further description will be omitted here. For example, a circuit board such as an FPC can be obtained by patterning the copper foil layer of the flexible copper-clad laminate of the present invention using a commonly used method such as etching to form a circuit. Circuit boards other than FPCs are also included in the present invention.

[0078] [Method for producing fluororesin film] Next, a method for producing the fluororesin film of the present invention will be described. This method is a method for producing the fluororesin film of the present invention, and includes the following steps: mixing, molding, heating, cooling, and skiving. It may also include a plasma treatment step.

[0079] <Mixing step> In the mixing step, a mixture is obtained by uniformly mixing a fluororesin powder, an inorganic filler powder, and an ultraviolet absorber. The fluororesin powder 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.

[0080] The fluororesin powder used as a raw material has a particulate shape, and its average particle size can be 50% or less of the predetermined thickness of the fluororesin film, and can be appropriately selected depending on the desired thickness of the fluororesin film. The average particle size of the fluororesin powder is preferably 0.1 μm to 10 μm. By using a fluororesin powder with an average particle size within this range, a raw material composition can be obtained in which inorganic filler powder particles, UV absorber particles, and fluororesin powder particles are uniformly dispersed. The average particle size of the fluororesin powder may be 0.2 μm or more, 1 μm or more, or 5 μm or more. Furthermore, the average particle size of the fluororesin particles may be 8 μm or less, or 5 μm or less.

[0081] Methods for adjusting the average particle size of the fluororesin powder to 50% or less of the predetermined thickness of the fluororesin film, preferably within the range of 0.1 μm to 10 μm, include, for example, a method using a commercially available fluororesin particle dispersion (generally having an average particle size in the range of 0.1 μm to 0.5 μm) in which fluororesin powder is dispersed in a solvent, and a method in which commercially available powder-shaped fluororesin particles (generally having an average particle size in the range of 200 μm to 600 μm) are pulverized to the above average particle size. Details of the process using the fluororesin particles obtained by the above two methods will be described later.

[0082] The average particle size of the fluororesin powder can be measured using a particle size distribution measuring device (manufactured by Spectris Inc., "MS-3000") under the condition of a measurement air pressure of 1 Bar.

[0083] Specifically, the inorganic filler powder may be 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. In particular, from the viewpoint of imparting high thermal stability (low thermal expansion) to the fluororesin film, silica, boron nitride, and alumina may be used alone or in combination. The inorganic filler powder has a particulate shape, and the average particle size of the inorganic filler powder may be appropriately selected based on the desired thickness of the fluororesin film, as described above. For example, the average particle size is preferably 0.1 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, or 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less.

[0084] The average particle size of the inorganic filler powder can be measured using a particle size distribution measuring device (manufactured by Spectris Inc., "MS-3000") under the condition of a measurement air pressure of 1 Bar.

[0085] Specifically, the ultraviolet absorber may be any one of liquid crystal polymer, polyether ether ketone, and polyphenylene sulfide, or a combination of two or more of these. In particular, a liquid crystal polymer having excellent ultraviolet absorption and a low dielectric loss tangent Df can be used. The ultraviolet absorber has a particulate shape, and the average particle size of the ultraviolet absorber may be appropriately selected depending on the desired thickness of the fluororesin film. For example, the average particle size is preferably 0.1 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, or may be 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less.

[0086] By setting the average particle size of the UV absorber in the range of 0.1 to 10 μm, aggregation of UV absorber particles can be suppressed, allowing the UV absorber to be uniformly dispersed in the fluororesin, and the proportion of coarse particles can be reduced, thereby improving the processing accuracy of drilling by UV-YAG laser irradiation. In other words, regardless of which region of the fluororesin film is to be drilled, the UV absorber must be present in the region to be drilled. To achieve this, it is important that the average particle size of the UV absorber is smaller than the 50 to 100 μm diameter of typical holes drilled with a UV-YAG laser. Therefore, the average particle size of the UV absorber filler is preferably 0.1 to 10 μm.

[0087] Furthermore, by making the average particle size of the ultraviolet absorber within the range of 0.1 to 10 μm, the occurrence of through holes (pinholes) in the fluororesin film can be suppressed, and excellent elongation properties can be obtained.

[0088] The average particle size of the ultraviolet absorber can be measured using a particle size distribution measuring device (manufactured by Spectris, Inc., "MS-3000") under the condition of a measurement air pressure of 1 Bar.

[0089] As an embodiment of a method for uniformly mixing a fluororesin powder, an inorganic filler powder, and an ultraviolet absorber to obtain a mixture, for example, a method of dry-mixing the fluororesin powder, the inorganic filler powder, and the ultraviolet absorber may be used.

[0090] Examples of a method for uniformly mixing the fluororesin powder, inorganic filler powder, and ultraviolet absorber include a method in which secondary particles formed by agglomeration of primary particles of the fluororesin are crushed to obtain a fluororesin powder having an average particle size of 0.1 μm to 10 μm, and then the fluororesin powder, inorganic filler powder, and ultraviolet absorber are stirred and mixed using a bladed stirrer or the like.

[0091] 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 methods using a crusher such as a mixer crusher, an airflow crusher, or a freeze crusher.

[0092] The fluororesin powder, inorganic filler powder, and ultraviolet absorber are blended in the raw material composition in desired proportions so that the contents of the fluororesin powder, inorganic filler powder, and ultraviolet absorber can be respectively used to form the fluororesin film of the present invention.

[0093] The stirring speed of the fluororesin powder and the inorganic filler powder in the dry mixing is not particularly limited, and may be, for example, 1000 rpm to 6000 rpm, or 2000 rpm to 5000 rpm. The stirring time of the fluororesin powder, the inorganic filler powder, and the ultraviolet absorber in the dry mixing is not particularly limited, and may be, for example, 1 to 15 minutes, or 2 to 10 minutes.

[0094] Alternatively, the fluororesin powder, inorganic filler powder, and ultraviolet absorber may be wet mixed together, for example, by mixing and dispersing the inorganic filler powder and ultraviolet absorber in the above-mentioned fluororesin particle dispersion.

[0095] <Molding Step> The molding step is a step of compressing and molding the mixture obtained in the mixing step. For example, the mixture is molded into a cylindrical shape to form a molded body. As a method for forming a molded body, for example, a method of filling a mold with the mixture and compression molding it to form a cylindrical compression molded body can be mentioned. 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 compression molding the mixture, a compression molded body in which the fluororesin powder, inorganic filler powder, and UV absorber are uniformly dispersed can be obtained.

[0096] <Heating Step> The heating step is a step in which the mixture obtained after the molding step is heated to melt the fluororesin powder. Specifically, the compression-molded body obtained in the molding step is fired to obtain a billet. The firing temperature may be 100°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 accumulating the fired mixture. By firing the molded body, the individual fluororesin particles in the molded body are melted and integrated into a matrix in which the inorganic filler particles and UV absorber are uniformly dispersed. By firing a compression-molded body of a raw material composition obtained by mixing a fluororesin powder, an inorganic filler powder, and a UV absorber, it is possible to suppress the formation of aggregates of the inorganic filler powder and the UV absorber, and a good billet with few coarse particles can be obtained.

[0097] <Cooling Step> The cooling step is a step in which the mixture obtained after the heating step is cooled to crystallize the fluororesin. The cooling step 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 being left stationary in a firing furnace.

[0098] From the viewpoint of ease of carrying out the skiving process described below, the billet (formed body) is preferably cylindrical in shape. When the billet (formed body) is cylindrical, the diameter of the cylindrical body may be, for example, 100 mm to 500 mm, or may be 150 mm to 500 mm.

[0099] <Skiving 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, a billet, is cut into a sheet. For example, when the billet (molded body) is a cylindrical body, a cutting blade is applied to the longitudinal outer surface of the fired cylinder, as if peeling a radish, to obtain a sheet-like fluororesin film.

[0100] <Plasma Treatment Step> In the plasma treatment step, the surface of the fluororesin film is plasma-treated after the skiving step to replace fluorine atoms with reactive functional groups to 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 sides.

[0101] Examples of gas species used in the plasma treatment include nitrogen gas and hydrogen gas. Other gas species that may be used include oxygen gas, argon gas, carbon dioxide gas, water vapor, helium gas, and ammonia gas. These gases may be used alone or in combination of two or more.

[0102] The preferred range of gas pressure in plasma treatment varies depending on the type of gas used. For example, when a mixed gas of nitrogen gas and hydrogen gas is used, the gas pressure is preferably 1 Pa to 1000 Pa.

[0103] The plasma treatment can be carried out by evacuating a vacuum chamber containing a fluororesin film to a predetermined pressure, and then introducing a gas for plasma treatment into the vacuum chamber and generating a direct current discharge plasma at an appropriate gas pressure.

[0104] The method for producing a fluororesin film of the present invention may consist of only the above-mentioned steps, or may include a predetermined step in addition to the above-mentioned steps.

[0105] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0106] A fluororesin film was prepared by the following procedure, and the dielectric constant Dk, dielectric loss tangent Df, and UV absorptivity of the prepared fluororesin film were measured. The drilling processability with a UV-YAG laser was also evaluated. The coefficient of thermal expansion CTE of the raw material composition obtained in the mixing step was also measured.

[0107] Example 1 <Production of Fluororesin Film> A fluororesin film of Example 1 was produced by the following steps.

[0108] (Mixing step) PTFE powder (average particle size measured with a particle size distribution analyzer: 400 μm), spherical silica as an inorganic filler (number average particle size 3 μm as determined by SEM observation), and liquid crystal polymer (LCP) as a UV additive (LF-31P manufactured by ENEOS Liquid Crystal Corporation, number average particle size 7 μm) which is an ultraviolet absorber were mixed in a volume ratio of PTFE powder:spherical silica:LCP=59.5:40:0.5, and mixed for 5 minutes at a rotation speed of 3000 rpm using a rotary blade mixer to obtain a raw material composition which was a mixture containing the PTFE powder, spherical silica, and liquid crystal polymer (LCP).

[0109] (Forming step, heating step, cooling step) 600 g of the raw material composition was filled into a cylindrical mold and compression-molded from above at a pressure of 30 MPa for 3 minutes to obtain a cylindrical preform (outer diameter 67 mm × inner diameter 33 mm). The obtained preform was placed in a firing furnace and fired at 365°C for 6 hours. Thereafter, the billet was left to stand in the firing furnace and cooled from 370°C to room temperature to obtain a billet.

[0110] (Skiving Step) The obtained billet (outer diameter 67 mm × inner diameter 33 mm) was skived using a skiving device 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 the fluororesin film.

[0111] (Plasma Treatment Step) The fluororesin sheet was cut into a size of 100 mm × 100 mm and subjected to plasma treatment according to the following procedure: First, the fluororesin sheet was placed in a vacuum plasma device, and after evacuating the inside of the vacuum device, a mixed gas of nitrogen gas and hydrogen gas was introduced, and plasma treatment was performed for 10 seconds using 2.45 GHz microwaves in a mixed gas atmosphere with a gas pressure of 5 Pa inside the device, thereby forming a surface-modified layer on both the front and back surfaces of the fluororesin sheet.

[0112] [Example 2] A fluororesin film of Example 2 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 59:40:1.

[0113] [Example 3] A fluororesin film of Example 3 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 57:40:3.

[0114] [Example 4] The fluororesin film of Example 4 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 55:40:5.

[0115] Comparative Example 1 A fluororesin film of Comparative Example 1 was produced in the same manner as in Example 1, except that no liquid crystal polymer (LCP) was used, and PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 60:40.

[0116] [Example 5] The fluororesin film of Example 5 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 49.5:50:0.5.

[0117] [Example 6] The fluororesin film of Example 6 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 49:50:1.

[0118] [Example 7] The fluororesin film of Example 7 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 45:50:5.

[0119] [Example 8] The fluororesin film of Example 8 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 40:50:10.

[0120] Comparative Example 2 A fluororesin film of Comparative Example 2 was produced in the same manner as in Example 1, except that no liquid crystal polymer (LCP) was used, and PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 50:50.

[0121] Comparative Example 3 A fluororesin film of Comparative Example 3 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP=49.9:50:0.1.

[0122] Example 9 A fluororesin film of Example 9 was produced in the same manner as in Example 1, except that polyether ether ketone (PEEK) (Polypla-Evonik, VESTAKEEP (registered trademark) 2000 UFP10) was used instead of liquid crystal polymer (LCP) as a UV additive, and the PTFE powder, spherical silica, and PEEK were mixed in a volume ratio of PTFE powder:spherical silica:PEEK = 49:50:1.

[0123] Comparative Example 4 A fluororesin film of Comparative Example 4 was produced in the same manner as in Example 9, except that the PTFE powder, spherical silica, and PEEK were mixed in a volume ratio of PTFE powder:spherical silica:PEEK = 49.5:50:0.5.

[0124] Comparative Example 5 A fluororesin film of Comparative Example 5 was produced in the same manner as in Example 9, except that the PTFE powder, spherical silica, and PEEK were mixed in a volume ratio of PTFE powder:spherical silica:PEEK = 45:50:5.

[0125] Comparative Example 6 A fluororesin film of Comparative Example 6 was produced in the same manner as in Example 9, except that the PTFE powder, spherical silica, and PEEK were mixed in a volume ratio of PTFE powder:spherical silica:PEEK = 40:50:10.

[0126] [Example 10] The fluororesin film of Example 10 was produced in the same manner as in Example 1, except that the PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 39.5:60:0.5.

[0127] [Example 11] A fluororesin film of Example 11 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 39:60:1.

[0128] [Example 12] A fluororesin film of Example 12 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 37:60:3.

[0129] [Example 13] A fluororesin film of Example 13 was produced in the same manner as in Example 1, except that PTFE powder, spherical silica, and liquid crystal polymer (LCP) were mixed in a volume ratio of PTFE powder:spherical silica:LCP = 35:60:5.

[0130] Comparative Example 7 A fluororesin film of Comparative Example 7 was produced in the same manner as in Example 1, except that polyimide (PI) (P84NT manufactured by Daicel-Evonik Co., Ltd.) was used instead of liquid crystal polymer (LCP) as a UV additive, and PTFE powder, spherical silica, and PI were mixed in a volume ratio of PTFE powder:spherical silica:PI = 49:50:1.

[0131] Comparative Example 8 The fluororesin film of Comparative Example 8 was produced in the same manner as in Example 1, except that no liquid crystal polymer (LCP) was used, and PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 40:60.

[0132] [Evaluation of Fluororesin Film] <Measurement of Dielectric Constant Dk and Dielectric Loss Tangent Df> A rectangular test piece 30 mm long and 20 mm wide was cut out from the fluororesin film, and the dielectric constant Dk and dielectric loss tangent Df at a measurement frequency of 20 GHz were measured by the SPDR (Split Post Dielectric Resonance) method using a testing machine under the following conditions. Df of the UV additive was also measured.

[0133] (Test equipment) Vector network analyzer E8363C (Agilent Technologies) Dielectric constant measurement software 85071E (Agilent Technologies) Split post resonator (QWED)

[0134] <Measurement of Thermal Expansion Coefficient (CTE)> The raw material composition obtained in the mixing process was filled into a 5 mm x 5 mm mold and compression molded at a molding surface pressure (press pressure) of 30 MPa for 1 minute to obtain a cubic molded body with a side length of 5 mm. This molded body was fired at 360°C for 6 hours, and the thermal expansion coefficient of the resulting fired body (test specimen for thermal expansion coefficient measurement) was measured using a thermomechanical measuring apparatus (TMA) (manufactured by TA Instruments Japan, Inc., "Q 400"). The thermal expansion coefficient was measured with a follow-up load of 0.05 N, from room temperature to 200°C, and at a heating rate of 5°C / min. The thermal expansion coefficient was calculated from the amount of thermal expansion in the range of 50 to 150°C during measurements performed from room temperature to 200°C.

[0135] <Measurement of UV Absorbance> A rectangular test piece measuring 50 mm in length, 20 mm in width, and 50 μm in thickness was cut out from the fluororesin film, and the transmittance and reflectance of UV light at a wavelength of 355 nm were measured using a UV-visible spectrophotometer (Hitachi High-Tech Science UH-4150). The UV absorbance of the fluororesin film was calculated using the following formula (1).

[0136] UV absorbance (%) = 100 - UV reflectance (%) - UV transmittance (%) (1)

[0137] <Drilling with UV-YAG Laser> A fluororesin film was irradiated with a 355 nm UV-YAG laser in a circular pattern with a diameter of 50 μm using a laser processing machine (esi5335) to form circular through-holes in the fluororesin film. The UV-YAG laser irradiation conditions were a laser output of 2.0 W, a laser focal diameter of 15 μm, and an oscillation frequency of 40 kHz. 200 holes were drilled. The laser-irradiated surface was then observed with an optical microscope to confirm whether through-holes had been formed in the fluororesin film and whether they were present or not. A score of ◯ was given when through-holes were formed in all 200 locations irradiated with the laser, and a score of × was given when no through-holes were formed in even one location.

[0138] [Pass / Fail Judgment] A test was judged as good if it met the following criteria: (1) the ultraviolet absorption rate of the fluororesin film is 20% or more, (2) the relative permittivity Dk at a frequency of 20 GHz is 3.0 or less, (3) the dielectric loss tangent Df at a frequency of 20 GHz is 0.0010 or less, (4) the coefficient of thermal expansion (CTE) is 80 ppm / K or less, and (5) the drilling process using a UV-YAG laser. A test was judged as bad if any one of the criteria (1) to (5) was not met. The results are shown in Table 1.

[0139]

[0140] Table 1 shows that a certain amount of UV additive is required to satisfy the UV processability requirements. Furthermore, when PI or 5% by volume or more of PEEK is used as the UV additive, the Df value increases and exceeds 0.0010.

[0141] For the fluororesin film of Example 6, a cross section was mechanically polished and a magnified electronic image was taken using a scanning electron microscope (Hitachi High-Technologies, SU3500). Magnified electronic images are shown in Figures 2A, 2B, 3A, and 3B. Figure 2A is a view in which LCP 400 is not enclosed, and Figure 2B is a view in which LCP 400 is enclosed to clarify where LCP 400 is located. Furthermore, Figure 3A is a view in which LCP 400 is not enclosed at a higher magnification than Figure 2A, and Figure 3B is a view in which LCP 400 is enclosed at a higher magnification than Figure 2B to clarify where LCP 400 is located.

[0142] 2A, 2B, 3A, and 3B, it can be seen that spherical silica 300 is dispersed in the fluororesin film, and that LCP 400 is not melted into the fluororesin but is dispersed as particles with a particle diameter of 1 to 6 μm.

[0143] [Summary] From the above, it is clear that the present invention can provide a fluororesin film that has little transmission loss during high-frequency communication and that can be drilled with a UV-YAG laser, a method for producing a fluororesin film, a sheet-like attachment film for flexible copper-clad laminates, a flexible copper-clad laminate, and a circuit board, and is therefore industrially useful.

[0144] 11 Surface 12 Back surface 100 Fluorine resin film 200 Protective film layer 300 Spherical silica 400 LCP

Claims

1. A fluororesin film comprising: a fluororesin; an inorganic filler dispersed in the fluororesin; and an ultraviolet absorber dispersed in the fluororesin, wherein the inorganic filler content is 35% to 70% by volume; the ultraviolet absorber has a dielectric loss tangent Df of 0.004 or less at a frequency of 20 GHz; the ultraviolet absorptivity of the fluororesin film is 20% or more; and the fluororesin film can be drilled by irradiation with a UV-YAG laser at a wavelength of 355 nm.

2. The fluororesin film according to claim 1, wherein the thickness of the fluororesin film is 25 μm to 200 μm.

3. The fluororesin film according to claim 1, wherein the fluororesin film has a relative dielectric constant Dk of 3.0 or less at a frequency of 20 GHz and a dielectric loss tangent Df of 0.0010 or less at a frequency of 20 GHz.

4. The fluororesin film according to claim 1, wherein the thermal expansion coefficient of the fluororesin film is 80 ppm / K or less.

5. The fluororesin film according to claim 1, which is provided with a surface-modified layer having reactive functional groups on its surface.

6. The fluororesin film according to claim 1, wherein the ultraviolet absorbing agent is any one of a liquid crystal polymer, polyether ether ketone, and polyphenylene sulfide, or a combination of two or more of these.

7. The fluororesin film according to claim 1, wherein the content of the ultraviolet absorbing agent is 0.3 to 25 volume %.

8. The fluororesin film according to claim 1, wherein the inorganic filler is 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.

9. The fluororesin film according to claim 1, wherein the inorganic filler has a number average particle size of 0.1 to 10 μm.

10. 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.

11. A method for producing a fluororesin film according to claim 1, comprising: a mixing step of uniformly mixing a fluororesin powder, an inorganic filler powder, and an ultraviolet absorber to obtain a mixture; a molding step of compressing 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 skive processing step of skiving the mixture after the cooling step to form a fluororesin film.

12. A method for producing a fluororesin film as described in claim 11, comprising a plasma treatment step of plasma treating the surface of the fluororesin film after the skiving step to replace fluorine atoms with reactive functional groups to form a surface modified layer.

13. A sheet-like attachment film for flexible copper-clad laminates, comprising the fluororesin film according to claim 1.

14. A flexible copper-clad laminate comprising the fluororesin film according to claim 1 and a copper foil layer.

15. A circuit board comprising the fluororesin film according to claim 1.

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