JP7553751B1 - Fluororesin film, method for producing fluororesin film, sheet-like attachment film for flexible copper-clad laminates, flexible copper-clad laminates and circuit boards - Google Patents

Abstract

Provided are 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 the fluororesin film, a sheet-like attachment film for flexible copper-clad laminates, a flexible copper-clad laminate, and a circuit board. The fluororesin film contains a fluororesin, an inorganic filler dispersed in the fluororesin, and an ultraviolet absorbing agent dispersed in the fluororesin, wherein the inorganic filler content is 35% to 70% by volume, the ultraviolet absorbing agent has a dielectric loss tangent Df of 0.004 or less at a frequency of 20 GHz, and the fluororesin film has an ultraviolet absorbing rate of 20% or more, and the fluororesin film can be drilled by irradiation with a UV-YAG laser with a wavelength of 355 nm.

Description

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.

In recent years, with the progress of miniaturization, weight saving, and space saving of electronic devices, there is 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 and a thin copper foil, are used to manufacture FPCs, and their applications are expanding to wiring of 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 parts of large computers (for example, Patent Documents 1 to 3).

These communications devices and network-related electronic devices need to transmit and process large volumes of information at high speed with little loss, and the electrical signals handled by FPCs are also becoming increasingly high frequency. If there is a large transmission loss when transmitting high-frequency electrical signals, problems such as electrical signal loss and increased signal delay time will occur. For this reason, the FCCL used in FPCs is required to have low dielectric properties (low dielectric constant, low dielectric tangent) and good electrical properties with reduced transmission loss in high-frequency transmission.

JP 2014-160738 A JP 2003-147320 A JP 2016-69651 A

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

In addition, to make FPC boards multi-layered, holes are drilled in the FCCL using a UV-YAG laser or similar, and the inside walls of the holes are then copper-plated to ensure electrical continuity between the layers of the multi-layered boards. For this reason, there is a demand for insulating films for FPCs that can be drilled with a UV-YAG laser.

In addition, to make FPC boards multi-layered, it is necessary that the film does not change in size during the heat pressing process for lamination, in addition to the workability in the above-mentioned drilling process. Therefore, materials with a low coefficient of thermal expansion (CTE) are required for insulating films for multi-layer FPCs.

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

Therefore, the present invention aims to provide a fluororesin film that has low transmission loss during high-frequency communication and can be drilled with a UV-YAG laser, a manufacturing method for the fluororesin film, a sheet-like attachment film for flexible copper-clad laminates, a flexible copper-clad laminate, and a circuit board.

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% 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 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.

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

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.

The thermal expansion coefficient of the fluororesin film may be 80 ppm/K or less.

The fluororesin film may have a surface modification layer having reactive functional groups on its surface.

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

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

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.

The number average particle diameter of the inorganic filler may be 0.1 to 10 μm.

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

In addition, in order to solve the above 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.

The method for producing a fluororesin film of the present invention may include a plasma treatment step in which, after the skive processing step, the surface of the fluororesin film is plasma-treated to replace fluorine atoms with reactive functional groups to form a surface modification layer.

In addition, 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.

In addition, 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.

In addition, to solve the above problems, the circuit board of the present invention is provided with the fluororesin film of the present invention.

The present invention can provide a fluororesin film that has low transmission loss during high-frequency communication and can be drilled with a UV-YAG laser, a method for manufacturing the fluororesin film, a sheet-like attachment film for flexible copper-clad laminates, flexible copper-clad laminates, and circuit boards.

1 is a schematic cross-sectional view showing an example of a fluororesin film of the present invention. 1 is an enlarged electronic image of a cross section of a fluororesin film of Example 6. 1 is an enlarged electronic image of a cross section of a fluororesin film of Example 6. FIG. 3 is an enlarged electronic image of a cross section of the fluororesin film of Example 6 at a higher magnification than FIGS. 2A and 2B . FIG. 3 is an enlarged electronic image of a cross section of the fluororesin film of Example 6 at a higher magnification than FIGS. 2A and 2B .

Below, we will explain one embodiment of the fluororesin film, method for producing the fluororesin film, sheet-like attachment film for flexible copper-clad laminates, flexible copper-clad laminates, and circuit boards of the present invention.

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

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

The thickness of the fluororesin film can be selected appropriately 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.

The thickness of the fluororesin film can be determined by using a film thickness measuring device such as a micrometer to obtain the average value of thickness measurements taken at any ten points on the fluororesin film.

(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.

A 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. Compared to liquid lasers such as _CO2 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.

The wavelengths of YAG lasers include the fundamental wavelength (1,064 nm), second harmonic (532 nm), third harmonic (355 nm), and fourth harmonic (266 nm), and in this invention, a UV-YAG laser using the third harmonic (355 nm) as the laser light source is irradiated onto a fluororesin film to perform hole drilling. The diameter of the hole when performing this drilling is, for example, about 5 μm to 150 μm, and it is common to use a UV-YAG laser to drill holes with a diameter of 50 μm to 100 μm.

There is no particular limitation on the UV-YAG laser device, but for example, ESI's "MODEL 5330xi" and "MODEL 5335" can be used.

(Dielectric constant Dk, dielectric 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 6G FPCs, specific low dielectric properties of the fluororesin film are preferably such that the relative dielectric constant Dk at a frequency of 20 GHz is 3.0 or less, and the dielectric loss tangent Df at a frequency of 20 GHz is 0.0010 or less.

The dielectric constant Dk and dielectric tangent Df can be measured using an SPDR dielectric resonator, etc.

(Coefficient of thermal expansion CTE)
As described above, in order to make the FPC board multi-layered, holes are drilled in the FCL using a UV-YAG laser, and then the inner walls of the holes are copper-plated to create a multi-layered board. In order to process multi-layered boards in this way, it is necessary to prevent the board from warping and the circuit from shifting out of position. The rate of volume expansion due to the

Specifically, when considering the processing of multilayer substrates such as FPCs, it is preferable that the thermal expansion coefficient of the fluororesin film of the present invention is 80 ppm/K or less.

<Ultraviolet light absorption rate>
The fluororesin film of the present invention has an ultraviolet absorption rate of 20% or more, so that the fluororesin film irradiated with a UV-YAG laser having a wavelength of 355 nm generates heat and opens. With an ultraviolet absorption rate of 50% or more, heat is generated more efficiently, so that the process time for the hole-making process can be shortened. The upper limit of the ultraviolet absorption rate of the fluororesin film is theoretically 100%, but the upper limit of the measured value is approximately 95%.

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

UV absorbance (%) = 100 - UV reflectance (%) - UV transmittance (%) (1)

<Fluorine resin>
Fluororesins are synthetic resins with excellent heat resistance, electrical insulation, non-adhesiveness, and weather resistance, and fluororesin films molded into films are widely used in industrial fields such as chemical materials, electrical and electronic parts, semiconductors, automobiles, etc. Fluororesins are also useful as resins for fluororesin films that have little transmission loss during high-frequency communication as in the present invention and that can be drilled with a UV-YAG laser.

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 of these, can be used as the fluororesin.

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 ultraviolet absorber in the fluororesin film. For example, since the content of the inorganic filler in the fluororesin film is 35% by volume to 70% by volume as described below, when the content of the ultraviolet absorber is 0.3% by volume to 25% by volume, the content of the fluororesin in the fluororesin film may be 29.7% by volume to 64.7% by volume, or may be 40% by volume or more, 50% by volume or more, or 60% by volume or more, 60% by volume or less, 50% by volume or less, or 40% by volume or less. If the content of the fluororesin is 29.7% by volume or more, a good strength as a fluororesin film can be obtained. Also, if the content of the fluororesin is 64.7% by volume or less, the fluororesin film has sufficient flexibility as a fluororesin film for FCCL, and the thermal expansion coefficient is suppressed low by the inorganic filler contained in the fluororesin film, and excellent thermal stability is obtained.

<Inorganic filler>
The inorganic filler is an important component along with the ultraviolet absorber in order to enable the drilling of holes in the fluororesin film by irradiation with a UV-YAG laser with a wavelength of 355 nm, and is present in the fluororesin film in a dispersed state.

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 created in the film even if it is irradiated with a UV-YAG laser with a wavelength of 355 nm.

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

Specific examples of inorganic fillers that can be used include 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 content in the fluororesin film is 35% to 70% by volume, and may be 40% by volume or more, 50% by volume or more, 60% by volume or more, or 60% by volume or less, 50% by volume or less, 40% by volume or less.

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

The average particle size of the inorganic filler may be appropriately selected according to 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, and may be 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less. By having the average particle size of the inorganic filler in the range of 0.1 to 10 μm, aggregation of inorganic filler particles can be suppressed, and the inorganic filler can be uniformly dispersed in the fluororesin, and the proportion of coarse particles can be reduced, thereby improving the processing accuracy of the hole drilling process by irradiation with a UV-YAG laser. In other words, regardless of which area of the fluororesin film is to be drilled, it is necessary for the inorganic filler to be present in the area to be drilled. For this reason, it is important that the average particle size of the inorganic filler is smaller than the typical hole diameter of 50 μm to 100 μm when drilling holes with a UV-YAG laser, and therefore the average particle size of the inorganic filler is preferably 0.1 to 10 μm. Furthermore, by having the average particle size of the inorganic filler in 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.

The average particle diameter of the inorganic filler can be determined using a scanning electron microscope (such as "SU8220" manufactured by Hitachi High-Tech Corporation) by measuring the particle diameter (diameter or longest diameter) of each of 100 arbitrarily selected inorganic filler particles in a scanning electron microscope image obtained by observing the surface of a fluororesin film over an area of 100 μm horizontal x 100 μm vertical at an acceleration voltage of 5 kV and a magnification of 1000 times, and then measuring the arithmetic average value thereof to obtain the number average particle diameter of the inorganic filler contained in the fluororesin film.

<Ultraviolet absorber>
The ultraviolet absorber is an important component along with the inorganic filler in order to enable the perforation processing of the fluororesin film by irradiation with a UV-YAG laser having a wavelength of 355 nm, and is present in the fluororesin film in a dispersed state.

Fluororesin has no absorption band at 355 nm and has low thermal conductivity, so if a fluororesin film is made only 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. Also, although inorganic fillers have a 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 created in the film when it is irradiated with a UV-YAG laser with a wavelength of 355 nm.

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, when the UV absorber generates heat in a fluororesin film in which the UV absorber and inorganic filler are sufficiently dispersed, the heat is transmitted through the inorganic filler to the inside 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 irradiated with a UV-YAG laser with a wavelength of 355 nm to drill holes.

The present invention aims 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 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.

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 can efficiently perform hole drilling processing of fluororesin films. In addition, because they are heat resistant, their melting and solidification behavior can be easily matched to that of fluororesin, and no problems occur in the heating and cooling processes of the manufacturing method for fluororesin films described below.

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 molecular structure that is difficult to bend. As there is little molecular entanglement, which is typical of polymeric substances, the viscosity is low in the molten state and they can exhibit high fluidity when molded. In other words, liquid crystal polymers can achieve both heat resistance and fluidity.

More specifically, examples of liquid crystal polymers include wholly aromatic polyesters obtained by condensing monomers such as parahydroxybenzoic acid, biphenol, and terephthalic acid. Other examples include 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.

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

If the content of ultraviolet absorber in the fluororesin film is 0.3% by volume or more, it becomes possible to drill holes in the fluororesin film by irradiating it with a 355 nm wavelength UV-YAG laser when used in combination with an inorganic filler. Also, if the content of ultraviolet absorber in the fluororesin film is 25% by volume or less, the strength of the fluororesin film can be sufficiently maintained, and good handling properties can be obtained.

(Optional ingredients)
The fluororesin film of the present invention may further contain optional components. The optional components are not particularly limited, but may include, for example, flame retardants, flame retardant assistants, pigments, antioxidants, reflectivity imparting agents, masking agents, lubricants, processing stabilizers, plasticizers, foaming agents, etc. When the 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.

(Surface Modification Layer)
The fluororesin film may have a surface modification layer having a reactive functional group on the surface. By providing the surface modification layer, the reactive functional group can enhance the adhesion between the fluororesin film and the copper foil layer. The surface modification layer can be formed by providing the reactive functional group through a surface treatment of the fluororesin surface by a plasma treatment process described later.

Figure 1 is a schematic cross-sectional view showing an example of the fluororesin film of the present invention. The fluororesin film 100 shown in Figure 1 is a single-layer film that does not have a surface modification layer, but when a surface modification layer is provided, the surface modification layer is disposed on both or either of the front surface 11 and back surface 12 of the fluororesin film 100.

In addition, in Figure 1, for convenience of explanation, different symbols are given to the front surface 11 and the back surface 12 of the fluororesin film 100, but there is no need to distinguish between the front surface and the back surface.

The fluororesin film of the present invention may or may not further comprise an easily removable 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 may not contain an elastomer.

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

[Sheet-type 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. The film is 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 the sheet-like form 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.

In addition, the sheet-like adhesive film for flexible copper-clad laminates may or may not have a configuration other than the fluororesin film. For example, it may further include an easily removable protective film layer 200 to prevent the surface 11 of the fluororesin film 100 from being scratched until immediately before it is attached to the copper foil.

[Flexible copper-clad laminate]
The flexible copper-clad laminate of the present invention comprises the above-mentioned fluororesin film of the present invention and a copper foil layer. The fluororesin film has already been described, so its 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 a copper foil layer to both the front and back surfaces of the fluororesin film, or may be obtained by attaching a copper foil layer to only one surface of the fluororesin film.

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

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. When the thickness of the copper foil layer is 1 μm or more, the flexible copper-clad laminate has excellent production stability and good handling properties. Furthermore, when the thickness of the copper foil layer is 50 μm or less, the flexible copper-clad laminate can easily ensure the flexibility required for an FPC.

In the copper foil layer, the surface roughness (maximum height) Rz of the surface that adheres 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.

The overall 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 overall thickness of the flexible copper-clad laminate is 500 μm or less, it is possible to obtain good flexibility suitable for FCCL, and it is easy to handle during the manufacture and use of circuit boards such as FPCs. Furthermore, when the overall thickness of the flexible copper-clad laminate is 10 μm or more, it is possible to obtain sufficient strength as an FCCL, and it is easy to handle during the manufacture and use of circuit boards such as FPCs.

The manufacturing method of the 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 both sides of the fluororesin film with copper foil and then further sandwiching it with a stainless steel plate. The pressing conditions are not particularly limited, but examples include a pressing pressure of 1 MPa to 10 MPa, a temperature during pressing of 40°C to 400°C, and heat pressing for usually 1 minute to 240 minutes.

[Circuit board]
The circuit board of the present invention includes the above-mentioned fluororesin film of the present invention. Since the fluororesin film has already been described, the 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 by a commonly used method such as etching to form a circuit. Circuit boards other than FPCs are also included in the present invention.

[Method of manufacturing 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 a mixing step, a molding step, a heating step, a cooling step, and a skiving step, which will be described below. It may also include a plasma treatment step.

<Mixing process>
In the mixing step, the fluororesin powder, the inorganic filler powder, and the UV absorber are mixed uniformly 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.

The fluororesin powder used as a raw material has a particle shape, and its average particle size can be 50% or less of the predetermined thickness of the fluororesin film, and may be appropriately selected according to 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 in this range, a raw material composition in which the inorganic filler powder particles, the ultraviolet absorber particles, and the fluororesin powder particles are uniformly dispersed can be obtained. The average particle size of the fluororesin powder may be 0.2 μm or more, 1 μm or more, or 5 μm or more. The average particle size of the fluororesin particles may be 8 μm or less, or 5 μm or less.

Methods for making the average particle size of the fluororesin powder 50% or less of the specified 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 with an average particle size in the range of 0.1 μm to 0.5 μm) in which the fluororesin powder is dispersed in a solvent, and a method of grinding commercially available powder-shaped fluororesin particles (generally with an average particle size in the range of 200 μm to 600 μm) to the above average particle size. Details of the process using the fluororesin particles obtained by the above two methods will be described later.

The average particle diameter of the fluororesin powder can be measured using a particle size distribution measuring device (Spectris Inc., MS-3000) at a measurement air pressure of 1 Bar.

As the inorganic filler powder, specifically, 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, 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 average particle diameter of the inorganic filler powder may be appropriately selected according to the desired thickness of the fluororesin film as described above, but is preferably 0.1 to 10 μm, for example, 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.

The average particle diameter of the inorganic filler powder can be measured using a particle size distribution measuring device (Spectris Inc., MS-3000) at a measurement air pressure of 1 Bar.

Specific examples of the ultraviolet absorbent include liquid crystal polymers, polyether ether ketones, and polyphenylene sulfides, or a combination of two or more of these. In particular, liquid crystal polymers having excellent ultraviolet absorption and low dielectric tangent Df can be used. The ultraviolet absorbent has a particulate shape, and the average particle size of the ultraviolet absorbent may be appropriately selected based 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 9 μm or less, 8 μm or less, 5 μm or less, or 3 μm or less.

By setting the average particle size of the ultraviolet absorber in the range of 0.1 to 10 μm, the aggregation of the ultraviolet absorber particles can be suppressed, the ultraviolet absorber can be uniformly dispersed in the fluororesin, and the proportion of coarse particles can be reduced, so that the processing accuracy of the hole drilling by irradiation with a UV-YAG laser can be improved. In other words, regardless of which area of the fluororesin film is to be drilled, the ultraviolet absorber must be present in the area to be drilled. For this reason, it is important that the average particle size of the ultraviolet absorber is smaller than the diameter of a typical hole when drilling with a UV-YAG laser, which is 50 μm to 100 μm, so that the average particle size of the ultraviolet absorber is preferably 0.1 to 10 μm.

In addition, by having the average particle size of the UV absorber in the range of 0.1 to 10 μm, the occurrence of through holes (pinholes) in the fluororesin film can be suppressed, resulting in excellent elongation properties.

The average particle diameter of the UV absorber can be measured using a particle size distribution measuring device (Spectris Inc., MS-3000) at a measurement air pressure of 1 Bar.

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.

One method for uniformly mixing the fluororesin powder, inorganic filler powder, and UV absorber is to crush secondary particles formed by agglomerations of primary particles of the fluororesin to obtain fluororesin powder with an average particle size of 0.1 μm to 10 μm, and then stir and mix the fluororesin powder, inorganic filler powder, and UV absorber using a bladed stirrer or the like.

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, airflow crusher, or freeze crusher.

The fluororesin powder, inorganic filler powder, and UV absorber are blended in the raw material composition in the desired ratios so that the contents of the fluororesin powder, inorganic filler powder, and UV absorber are each capable of forming the fluororesin film of the present invention.

The stirring speed of the fluororesin powder and inorganic filler powder in the 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, inorganic filler powder, and ultraviolet absorber in the dry mixing is not particularly limited, but may be, for example, 1 to 15 minutes, or 2 to 10 minutes.

The fluororesin powder, inorganic filler powder, and UV absorber can also be wet mixed. For example, the inorganic filler powder and UV absorber can be mixed and dispersed in the above-mentioned fluororesin particle dispersion.

<Molding process>
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. For example, a method of forming a molded body includes a method of filling a mold with the mixture and compression molding it to form a cylindrical compression 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 compression molding the mixture, a compression molded body in which the fluororesin powder, inorganic filler powder, and ultraviolet absorber are uniformly dispersed is obtained.

<Heating process>
The heating step is a step of heating the mixture after the molding step 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 the ultraviolet absorber are uniformly dispersed. By firing the compression molded body of the raw material composition in which the fluororesin powder, the inorganic filler powder, and the ultraviolet absorber are mixed, the generation of aggregates of the inorganic filler powder and the ultraviolet absorber can be suppressed, and a good billet with few coarse particles can be obtained.

<Cooling process>
The cooling step is a step of cooling the mixture after the heating step to crystallize the fluororesin. The billet can be cooled from the sintering temperature to room temperature by the cooling step, and the fluororesin can be crystallized. For example, the billet can be cooled from 370° C. to room temperature by leaving it still in a sintering furnace.

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

<Skib processing 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 billet, which is a fired molded body, is cut and processed into a sheet. For example, when the billet (molded body) is a cylindrical body, a cutting blade is applied to the longitudinal outer peripheral surface of the fired cylinder to cut it, as if peeling a radish, to obtain a sheet-like fluororesin film.

<Plasma treatment process>
The plasma treatment step is a step of performing plasma treatment on the surface of the fluororesin film 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.

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

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

Plasma treatment can be performed, for example, by drawing a vacuum inside a vacuum device in which a fluororesin film is installed until a predetermined pressure is reached, and then introducing a gas for plasma treatment into the vacuum device and generating a direct current discharge plasma at an appropriate gas pressure.

The method for producing the fluororesin film of the present invention may consist of only the steps described above, or may include further specified steps in addition to the steps described above.

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

A fluororesin film was prepared according to the following procedure, and the relative dielectric constant Dk, dielectric loss tangent Df, and UV absorption rate of the prepared fluororesin film were measured. The processability for drilling holes using a UV-YAG laser was also evaluated. The thermal expansion coefficient CTE of the raw material composition obtained in the mixing process was also measured.

[Example 1]
<Production of fluororesin film>
The fluororesin film of Example 1 was produced by the following steps.

(Mixing process)
PTFE powder (average particle size measured by a particle size distribution analyzer: 400 μm), spherical silica as an inorganic filler (number average particle size 3 μm by SEM observation), and liquid crystal polymer (LCP) as a UV additive that is an ultraviolet absorber (LF-31P, number average particle size 7 μm, manufactured by ENEOS Liquid Crystal Corporation) 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 that is a mixture containing the PTFE powder, spherical silica, and liquid crystal polymer (LCP).

(molding process, heating process, cooling process)
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 billet was placed in a sintering furnace and sintered at 365° C. for 6 hours, and then the billet was left stationary in the sintering furnace to cool from 370° C. to room temperature, thereby obtaining a billet.

(Skib processing process)
The obtained billet (outer diameter 67 mm × inner diameter 33 mm) was skived using a skive processing 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.

(Plasma treatment process)
The fluororesin sheet was cut into a size of 100 mm x 100 mm and subjected to plasma treatment according to the following procedure: First, the fluororesin sheet was placed in a vacuum plasma device, and the inside of the vacuum device was evacuated, after which 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 in the device, thereby forming a surface modified layer by performing plasma treatment on both the front and back surfaces of the fluororesin sheet.

[Example 2]
The fluororesin film of Example 2 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=59:40:1.

[Example 3]
The fluororesin film of Example 3 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=57:40:3.

[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.

[Comparative Example 1]
The 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.

[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.

[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.

[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.

[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.

[Comparative Example 2]
The 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.

[Comparative Example 3]
The fluororesin film of Comparative Example 3 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=49.9:50:0.1.

[Example 9]
The 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.

[Comparative Example 4]
The 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.

[Comparative Example 5]
The 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.

[Comparative Example 6]
The 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.

[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.

[Example 11]
The fluororesin film of Example 11 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=39:60:1.

[Example 12]
The fluororesin film of Example 12 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=37:60:3.

[Example 13]
The fluororesin film of Example 13 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=35:60:5.

[Comparative Example 7]
The 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 the PTFE powder, spherical silica, and PI were mixed in a volume ratio of PTFE powder:spherical silica:PI = 49:50:1.

[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.

[Evaluation of Fluororesin Film]
<Measurement of relative 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 relative dielectric constant Dk and the dielectric loss tangent Df at a measurement frequency of 20 GHz were measured by the split post dielectric resonance (SPDR) method using a tester under the following conditions. Df of the UV additive was also measured.

(Test machine)
- Vector network analyzer E8363C (Agilent Technologies, Inc.)
・Dielectric constant measurement software 85071E (Agilent Technologies, Inc.)
Split-post resonator (QWED)

<Measurement of coefficient of thermal expansion (CTE)>
The raw material composition obtained in the mixing step was filled into a mold of 5 mm length x 5 mm width, and compression molded for 1 minute at a molding surface pressure (press pressure) of 30 MPa to obtain a cubic molded body with one side of 5 mm. This molded body was fired at 360 degrees for 6 hours, and the thermal expansion coefficient of the fired body (test body for measuring thermal expansion coefficient) was measured using a thermomechanical measuring device (TMA) (manufactured by TA Instruments Japan Co., Ltd., "Q 400"). The thermal expansion coefficient was measured with a follow-up load of 0.05 N, a measurement temperature from room temperature to 200 ° C, and a temperature rise 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 in the measurement performed from room temperature to 200 ° C.

<Measurement of ultraviolet absorptivity>
A rectangular test piece 50 mm long, 20 mm wide, and 50 μm thick was cut out from the fluororesin film, and the transmittance and reflectance of ultraviolet light at a wavelength of 355 nm were measured using a UV-Visible Spectrophotometer (Hitachi High-Tech Science UH-4150). The ultraviolet absorptance of the fluororesin film was calculated using the following formula (1).

UV absorbance (%) = 100 - UV reflectance (%) - UV transmittance (%) (1)

<Drilling with UV-YAG laser>
A fluororesin film was irradiated with a UV-YAG laser having a wavelength of 355 nm so as to circle around a circumference with a diameter of 50 μm using a laser processing machine (esi5335), and a process was performed to form circular through-holes in the fluororesin film. The irradiation conditions of the UV-YAG laser were a laser output of 2.0 W, a laser focus diameter of 15 μm, and an oscillation frequency of 40 kHz. The number of holes was 200. Thereafter, the laser irradiated surface was observed with an optical microscope to confirm whether through-holes were formed in the fluororesin film and whether there were through-holes. The case where through-holes were formed in all 200 places irradiated with the laser was judged as ◯, and the case where no through-hole was formed in even one place was judged as ×.

[Pass/fail decision]
(1) The ultraviolet absorption rate of the fluororesin film was 20% or more, (2) the relative dielectric constant Dk at a frequency of 20 GHz was 3.0 or less, (3) the dielectric loss tangent Df at a frequency of 20 GHz was 0.0010 or less, (4) the coefficient of thermal expansion (CTE) was 80 ppm/K or less, and (5) the hole drilling process using a UV-YAG laser was evaluated as ◯, and (6) any one of (1) to (5) that did not satisfy the conditions was evaluated as ×. The results are shown in Table 1.

From Table 1, it was found that a certain amount of UV additive was required to satisfy the UV processability requirements. In addition, when PI or 5% or more by volume of PEEK was used as a UV additive, the Df value increased and exceeded 0.0010.

For the fluororesin film of Example 6, the 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 diagram in which LCP400 is not enclosed, and Figure 2B is a diagram in which LCP400 is enclosed to clarify where LCP400 is located. Figure 3A is a diagram in which LCP400 is not enclosed at a higher magnification than Figure 2A, and Figure 3B is a diagram in which LCP400 is enclosed at a higher magnification than Figure 2B to clarify where LCP400 is located.

Figures 2A, 2B, 3A, and 3B show that spherical silica 300 is dispersed in the fluororesin film, and that LCP 400 is not dissolved in the fluororesin but is dispersed as particles with a particle diameter of 1 to 6 μm.

[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.

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

Claims (13)

Fluorine resin and
an inorganic filler dispersed in the fluororesin;
an ultraviolet absorber dispersed in the fluororesin;
A fluororesin film comprising:
The content of the inorganic filler is 35% by volume 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 absorbing rate of the fluororesin film is 20% or more,
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;
The ultraviolet absorber is any one of a liquid crystal polymer, a polyether ether ketone, and a polyphenylene sulfide, or a combination of two or more of these.
A fluororesin film that can be drilled by irradiating it with a UV-YAG laser with a wavelength of 355 nm. The fluororesin film according to claim 1, wherein the thickness of the fluororesin film is 25 μm to 200 μm. The fluororesin film according to claim 1, wherein the thermal expansion coefficient of the fluororesin film is 80 ppm/K or less. The fluororesin film according to claim 1, wherein the fluororesin film has a surface-modified layer having reactive functional groups on the surface. The fluororesin film according to claim 1, wherein the content of the ultraviolet absorber is 0.3% by volume to 25% by volume. 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. The fluororesin film according to claim 1, wherein the number average particle size of the inorganic filler is 0.1 to 10 μm. 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. A method for producing the fluororesin film according to claim 1,
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;
a skive processing step of skiving the mixture after the cooling step to form a fluororesin film;
A method for producing a fluororesin film comprising the steps of: The method for producing a fluororesin film according to claim 9 , further 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. A sheet-like adhesive film for flexible copper-clad laminates comprising the fluororesin film according to claim 1. A flexible copper-clad laminate comprising the fluororesin film of claim 1 and a copper foil layer. A circuit board comprising the fluororesin film according to claim 1.

Images