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

Abstract

The present invention provides: a fluororesin film for a multilayer structure substrate, with which it is possible to produce, without using an adhesive, a multilayer structure substrate that has less transmission loss during high-frequency communication; a method for producing the fluororesin film for a multilayer structure substrate; a copper-clad laminate; a multilayer structure substrate; a method for producing a multilayer structure substrate; and a circuit board.　Disclosed is a fluororesin film for a multilayer structure substrate, which contains a fluororesin and an inorganic filler that is dispersed in the fluororesin, wherein the dielectric loss tangent Df is 0.0013 or less and the loss elastic modulus at a temperature of 350°C is 1.6 MPa or more. The fluororesin film for a multilayer structure substrate is used for the purpose of producing a multilayer structure substrate by bonding, without using an adhesive, a plurality of copper-clad laminates that have a two-layer structure in which a copper foil and the fluororesin film for a multilayer structure substrate are laminated.

Description

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

[0001] The present invention relates to a fluororesin film for a multilayer substrate, a method for producing a fluororesin film for a multilayer substrate, a copper-clad laminate, a multilayer substrate, a method for producing a multilayer substrate, 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, materials with low transmission loss during high-frequency communication are required as insulating film materials for FPCs for sixth-generation mobile communication systems (hereinafter sometimes referred to as "6G"). For example, multilayer substrates obtained by laminating multiple substrate materials such as single-layer LCP (liquid crystal polymer Df0.002) are essential technologies for smartphones, wearable devices, automotive radar, etc., because the multilayering of the substrate improves circuit density and saves space. Fabricating a multilayer substrate by laminating multiple single-layer substrate materials requires a process of laminating multiple single-layer LCPs, and conventionally, LCPs are bonded together with an adhesive to form a multilayer structure.

[0006] However, with conventional lamination methods, the electrical properties (dielectric constant Dk, dielectric loss tangent Df) of the multilayer structure substrate are poor due to the poor electrical properties (dielectric constant Dk, dielectric loss tangent Df) of the adhesive, and it has not been possible to obtain a multilayer structure substrate that satisfies the electrical property requirements for 6G. In addition, there have been problems such as variations in the thickness of the adhesive resulting in variations in the thickness of the individual multilayer structure substrates, an increase in the thickness of the multilayer structure substrate by the amount of the adhesive, and a hardening of the adhesive resulting in a hard multilayer structure substrate and poor flexibility.

[0007] Therefore, an object of the present invention is to provide a fluororesin film for a multilayer structure substrate, a method for producing a fluororesin film for a multilayer structure substrate, a copper-clad laminate, a multilayer structure substrate, a method for producing a multilayer structure substrate, and a circuit board, which are capable of producing a multilayer structure substrate with little transmission loss during high-frequency communication without using an adhesive.

[0008] In order to solve the above-mentioned problems, the fluororesin film for multilayer structure substrates of the present invention is a fluororesin film for multilayer structure substrates comprising a fluororesin and an inorganic filler dispersed in the fluororesin, wherein the fluororesin film for multilayer structure substrates has a dielectric dissipation factor Df of 0.0013 or less and a loss modulus at a temperature of 350°C of 1.6 MPa or more, and is a film used for manufacturing multilayer structure substrates by bonding, without adhesive, a plurality of copper-clad laminates each having a two-layer structure in which copper foil and the fluororesin film for multilayer structure substrates are laminated together.

[0009] 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.

[0010] The inorganic filler may have a volume average particle size of 0.1 to 10 μm.

[0011] The content of the inorganic filler may be 40% by volume to 70% by volume.

[0012] The fluororesin film may have a relative dielectric constant Dk of 3.0 or less at a frequency of 20 GHz.

[0013] The thickness of the fluororesin film may be 25 μm to 500 μm.

[0014] The fluororesin film may have a thermal expansion coefficient of 80 ppm / K or less.

[0015] The fluororesin film for multilayer structure substrates of the present invention may have a breaking strain of 100% or more.

[0016] The fluororesin may be any one of PTFE, PFA and FEP, or a combination of two or more of these.

[0017] Furthermore, in order to solve the above-mentioned problems, the method for producing a fluororesin film for a multilayer structure substrate of the present invention is the method for producing a fluororesin film for a multilayer structure substrate of the present invention, and includes the following steps: a mixing step of uniformly mixing a fluororesin powder and an inorganic filler powder 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.

[0018] In order to solve the above-mentioned problems, the copper-clad laminate of the present invention is a two-layer copper-clad laminate comprising the fluororesin film for a multilayer structure substrate of the present invention and a copper foil layer, in which the copper foil and the fluororesin film for a multilayer structure substrate are laminated together.

[0019] In addition, in order to solve the above-mentioned problems, the multilayer structure substrate of the present invention is a multilayer structure substrate in which two or more of the above-mentioned copper-clad laminates of the present invention are directly bonded together without an adhesive layer to form a multilayer structure.

[0020] In addition, in order to solve the above-mentioned problems, the method for manufacturing a multilayer structure substrate of the present invention is a method for manufacturing a multilayer structure substrate of the present invention, and includes a lamination step of directly laminating two or more copper-clad laminates of the present invention without using an adhesive to form a laminate structure, and a bonding step of bonding the laminate structure by hot pressing.

[0021] In order to solve the above problems, the circuit board of the present invention comprises the above-mentioned fluororesin film for multilayer structure substrates of the present invention.

[0022] The present invention can provide a fluororesin film for a multilayer structure substrate, a method for producing a fluororesin film for a multilayer structure substrate, a copper-clad laminate, a multilayer structure substrate, a method for producing a multilayer structure substrate, and a circuit board, which can produce a multilayer structure substrate with little transmission loss during high-frequency communication without using an adhesive.

[0023] 1 is a schematic cross-sectional view of copper-clad laminates 100 and 200. 2 is a schematic cross-sectional view of multilayer structure substrate 300. 3 is a schematic cross-sectional view of multilayer structure substrate 400. 4 is a schematic cross-sectional view of multilayer structure substrate 500. 5 is an optical microscope photograph of the cross section of a multilayer structure substrate produced using the fluororesin films of Examples 1 to 4 and Comparative Example 1. 6 is a measurement result of the S21 parameter.

[0024] Hereinafter, one embodiment of the fluororesin film for multilayer substrates, the method for producing a fluororesin film for multilayer substrates, the copper-clad laminate, the multilayer substrate, the method for producing a multilayer substrate, and the circuit board of the present invention will be described.

[0025] [Fluororesin Film for Multilayer Substrate] The fluororesin film for multilayer substrate of the present invention is a film used for manufacturing a multilayer substrate by bonding, without adhesive, a plurality of two-layer copper-clad laminates each having a copper foil and the fluororesin film for multilayer substrate laminated thereon. The fluororesin film comprises a fluororesin and an inorganic filler, as described below.

[0026] (Thickness of Fluororesin Film) The thickness of the fluororesin film is preferably 25 μm to 500 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 500 μm or less, sufficient flexibility can be obtained.

[0027] The thickness of the fluororesin film can be appropriately selected depending on the application and requirements, and may be, for example, 30 μm or more, 50 μm or more, 70 μm or more, or 100 μm or more, or 400 μm or less, 300 μm or less, 200 μm or less, or 150 μm or less.

[0028] 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.

[0029] (Relative dielectric constant 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 a multilayer structure substrate for 6G, it is preferable that the specific low dielectric properties of the fluororesin film are such that the dielectric loss tangent Df at a frequency of 20 GHz is 0.0013 or less, and the relative dielectric constant Dk at a frequency of 20 GHz is 3.0 or less.

[0030] The relative dielectric constant Dk and the dielectric loss tangent Df can be measured using an SPDR dielectric resonator or the like.

[0031] (Loss Modulus) The fluororesin film of the present invention has a loss modulus of 1.6 MPa or more at a temperature of 350°C. When producing a multilayer substrate, copper-clad laminates are bonded at a temperature of around 350°C, for example, by hot pressing. Therefore, the viscosity of the fluororesin film at a temperature of 350°C is important in producing a multilayer substrate, and the loss modulus is used as an indicator of this viscosity. A loss modulus of 1.6 MPa or more can achieve both good embedding properties for wiring and prevention of wiring misalignment. There is no particular upper limit to the loss modulus, but for a fluororesin film for a multilayer substrate containing a fluororesin and an inorganic filler, 6 MPa is a rough upper limit.

[0032] (Coefficient of thermal expansion, CTE) 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. 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.

[0033] 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.

[0034] (Breaking strain) The fluororesin film for multilayer substrates of the present invention has excellent handleability due to its breaking strain of 100% or more. That is, the film is easy to handle in the process of producing a multilayer substrate. The upper limit of the breaking strain is not particularly limited, but for a fluororesin film for multilayer substrates containing a fluororesin and an inorganic filler, 500% is a rough upper limit.

[0035] <Fluororesin> Fluororesin is a synthetic resin with excellent heat resistance, electrical insulation, non-stickiness, and weather resistance, and fluororesin films molded into film form are widely used in industrial fields such as chemical materials, electric and electronic components, semiconductors, automobiles, etc. Fluororesin is also useful as a resin for fluororesin films for multilayer structure substrates such as those of the present invention, which have low transmission loss during high-frequency communication.

[0036] 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.

[0037] The content of the fluororesin in the fluororesin film may be, for example, the remainder of the content of the inorganic filler in the fluororesin film. For example, when the content of the inorganic filler in the fluororesin film is 40% by volume to 70% by volume as described below, the content of the fluororesin in the fluororesin film may be 35% by volume to 65% by volume. If the content of the fluororesin is 35% by volume or more, the fluororesin film can have good strength. Furthermore, if the content of the fluororesin is 65% by volume or less, the fluororesin film can have sufficient flexibility for use in a multilayer structure substrate.

[0038] <Inorganic Filler> Inorganic fillers are important components along with fluororesin in order to achieve both good embedding properties for wiring and prevention of wiring misalignment when used in combination with fluororesin, and are present in a dispersed state in the fluororesin in the fluororesin film.

[0039] Fluororesin melts when exposed to heat. Therefore, if a fluororesin film is made solely from fluororesin, the fluororesin may melt and flow out when a copper-clad laminate is joined by, for example, hot pressing to produce a multilayer substrate, which may result in the wiring not being embedded properly or causing misalignment of the wiring.

[0040] Therefore, by dispersing an inorganic filler in the fluororesin, the fluidity and viscosity of the fluororesin film can be adjusted even when the fluororesin is in a molten state, thereby achieving both good embedding properties for wiring and prevention of wiring misalignment. In the present invention, the loss modulus is used as an index of viscosity, and factors that affect the loss modulus include the volume average particle size of the inorganic filler, the content of the inorganic filler in the fluororesin film, the shape of the inorganic filler, the dispersibility of the inorganic filler in the fluororesin film, and the molecular weight of the fluororesin. For example, as described below, the volume average particle size of the inorganic filler can be set to 0.1 to 10 μm, and the shape of the inorganic filler can be spherical, plate-like, scale-like, needle-like, or the like. The content of the inorganic filler in the fluororesin film necessary to achieve a desired loss modulus can be appropriately adjusted depending on the particle size, shape, molecular weight of the fluororesin, etc.

[0041] 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, silica, boron nitride, and alumina may be used alone or in combination with a fluororesin to achieve both good embedding properties for wiring and prevention of wiring misalignment.

[0042] The content of the inorganic filler in the fluororesin film is 40% by volume to 70% by volume, and may be 45% by volume or more, 50% by volume or more, or 55% by volume or more, or may be 65% by volume or less, 60% by volume or less, or 55% by volume or less.

[0043] When the inorganic filler content in the fluororesin film is 40% by volume or more, it is possible to prevent the wiring from tilting during the production of a multilayer substrate. Furthermore, when the inorganic filler content in the fluororesin film is 70% by volume or less, it is possible to satisfactorily embed the wiring. Note that, when the inorganic filler content exceeds 70% by volume, for example, when the inorganic filler content is 90% by volume, in the process of compression molding a mixture of the inorganic filler and the fluororesin to produce a molded body, the amount of fluororesin is so small that bonding between the inorganic filler and the fluororesin is reduced, and the strength of the molded body is significantly reduced, which may make it impossible to produce a fluororesin film.

[0044] The volume average particle diameter of the inorganic filler may be appropriately selected depending on the desired thickness of the fluororesin film, and is, for example, 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.

[0045] By making the volume average particle diameter of the inorganic filler in the range of 0.1 to 10 μm, aggregation of inorganic filler particles can be suppressed, the inorganic filler can be uniformly dispersed in the fluororesin, and the proportion of coarse particles can be reduced.Furthermore, by making the volume average particle diameter 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.

[0046] The volume average particle diameter 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 diameters (diameter or longest diameter) of 100 arbitrarily selected inorganic filler particles are measured, and the arithmetic mean value thereof can be used as the volume average particle diameter of the inorganic filler contained in the fluororesin film.

[0047] The shape of the inorganic filler is not particularly limited, but examples thereof include spherical, plate-like, scaly, needle-like, etc. The interior of the inorganic filler may be solid or hollow.

[0048] (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.

[0049] (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.

[0050] The fluororesin film of the present invention may or may not further comprise an easily removable protective film layer 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.

[0051] The fluororesin film of the present invention may be composed of only a fluororesin and an inorganic filler, or may further contain optional additives such as the optional components described above.

[0052] [Copper-clad laminate] The copper-clad laminate of the present invention is a two-layer copper-clad laminate comprising the fluororesin film for multilayer substrates of the present invention and a copper foil layer, in which copper foil and the fluororesin film for multilayer substrates are laminated together. The fluororesin film for multilayer substrates has already been described, so its description will be omitted here. For example, a copper-clad laminate may be formed by attaching copper foil layers to both the front and back surfaces of a fluororesin film for multilayer substrates, or a copper-clad laminate may be formed by attaching a copper foil layer to only one surface of a fluororesin film for multilayer substrates.

[0053] <Copper Foil Layer> As the copper foil layer, an existing copper foil layer used in 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.

[0054] 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 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 a flexible copper-clad laminate.

[0055] The total thickness of the 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 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 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.

[0056] The method for producing a copper-clad laminate is not particularly limited, and existing methods can be used. For example, by sandwiching both sides of a fluororesin film for a multilayer substrate with copper foil and applying pressure, a three-layer copper-clad laminate can be obtained in which a copper foil layer, a fluororesin film for a multilayer substrate, and a copper foil layer are laminated in this order. Pressurization can be performed, for example, by sandwiching both sides of the fluororesin film for a multilayer substrate with copper foil and then further sandwiching it with 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.

[0057] [Multilayer structure substrate] The multilayer structure substrate of the present invention is a substrate in which two or more of the above-mentioned copper-clad laminates of the present invention are directly bonded to each other without an adhesive layer. By providing the copper-clad laminate of the present invention with the fluororesin film for multilayer structure substrates of the present invention, it becomes possible to directly bond the copper-clad laminates to each other without an adhesive. There is no particular upper limit on the number of copper-clad laminates to be stacked, but from the viewpoint of miniaturization, weight reduction, and space saving of electronic devices, about 15 sheets is a reasonable upper limit.

[0058] Because conventional multilayer substrates include an adhesive layer, the electrical properties (dielectric constant Dk, dielectric loss tangent Df) of the multilayer substrate are poor due to the poor electrical properties of the adhesive (dielectric constant Dk, dielectric loss tangent Df), and the multilayer substrate does not satisfy the electrical property requirements for 6G. Furthermore, there are problems such as variations in the thickness of the adhesive layer resulting in variations in the thickness of the multilayer substrate, an increase in the thickness of the multilayer substrate by the amount of the adhesive layer, and a hardening of the adhesive layer resulting in a hardening of the multilayer substrate and poor flexibility.

[0059] The multilayer structure substrate of the present invention solves all of these problems because the multilayer structure is formed without using an adhesive, and is therefore a flexible multilayer structure substrate with excellent electrical properties and dimensional stability.

[0060] [Circuit Board] The circuit board of the present invention comprises the above-described fluororesin film for multilayer structure substrates of the present invention. The fluororesin film for multilayer structure substrates has already been described, so 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 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.

[0061] [Method for producing fluororesin film for multilayer structure substrate] Next, a method for producing the fluororesin film for multilayer structure substrate of the present invention will be described. This production method is a method for producing the fluororesin film for multilayer structure substrate 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.

[0062] <Mixing step> In the mixing step, the fluororesin powder and the inorganic filler powder are mixed uniformly to obtain a mixture. 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.

[0063] The fluororesin powder used as a raw material has a particulate shape, and its volume average particle diameter can be 50% or less of the predetermined thickness of the fluororesin film, and can be appropriately selected depending on the desired thickness of the fluororesin film. The volume average particle diameter of the fluororesin powder is preferably 0.1 μm to 10 μm. By using a fluororesin powder with a volume average particle diameter within this range, a raw material composition in which inorganic filler powder particles and fluororesin powder particles are uniformly dispersed can be obtained. The volume average particle diameter of the fluororesin powder may be 0.2 μm or more, 1 μm or more, or 5 μm or more. Furthermore, the volume average particle diameter of the fluororesin particles may be 8 μm or less, or 5 μm or less.

[0064] Methods for adjusting the volume average particle diameter 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 a volume average particle diameter in the range of 0.1 μm to 0.5 μm) in which fluororesin powder is dispersed in a solvent, and a method in which commercially available powder-shaped fluororesin particles (generally having a volume average particle diameter in the range of 200 μm to 600 μm) are pulverized to the above volume average particle diameter. Details of the process using the fluororesin particles obtained by the above two methods will be described later.

[0065] The volume 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.

[0066] 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 volume average particle diameter of the inorganic filler powder may be appropriately selected based on the desired thickness of the fluororesin film, as described above. For example, the volume average particle diameter 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.

[0067] The volume 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.

[0068] As an embodiment of a method for uniformly mixing the fluororesin powder and the inorganic filler powder to obtain a mixture, for example, a method of dry mixing the fluororesin powder and the inorganic filler powder may be used.

[0069] As a method for uniformly mixing the fluororesin powder and the inorganic filler powder, for example, a method may be mentioned in which secondary particles formed by agglomeration of primary particles of the fluororesin are crushed to obtain a fluororesin powder having a volume average particle diameter of 0.1 μm to 10 μm, and then the fluororesin powder and the inorganic filler powder are stirred and mixed using a bladed stirrer or the like.

[0070] 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.

[0071] The fluororesin powder and inorganic filler powder are blended in a desired ratio so that the contents of the fluororesin powder and inorganic filler powder contained in the raw material composition are each capable of forming the fluororesin film of the present invention.

[0072] The stirring speed of the fluororesin powder and the 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 and the inorganic filler powder in the dry mixing is not particularly limited, but may be, for example, 1 to 15 minutes, or 2 to 10 minutes.

[0073] Alternatively, the fluororesin powder and the inorganic filler powder may be wet mixed, for example, by mixing and dispersing the inorganic filler powder in the above-mentioned fluororesin particle dispersion.

[0074] <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 and the inorganic filler powder are uniformly dispersed can be obtained.

[0075] <Heating Step> 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 are uniformly dispersed. By firing a compression-molded body of a raw material composition obtained by mixing a fluororesin powder and an inorganic filler powder, it is possible to suppress the formation of aggregates of the inorganic filler powder, and a good billet with few coarse particles can be obtained.

[0076] <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.

[0077] 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.

[0078] <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.

[0079] <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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] [Method for manufacturing a multilayer structure substrate] Next, a method for manufacturing a multilayer structure substrate of the present invention will be described. This manufacturing method is a method for manufacturing the multilayer structure substrate of the present invention described above, and includes a laminating step and a bonding step, which will be described below.

[0085] <Lamination step> In the lamination step, two or more copper-clad laminates of the present invention are directly laminated without using an adhesive to form a laminate structure. The number of copper-clad laminates to be laminated is not particularly limited, but from the viewpoint of miniaturization, weight reduction, and space saving of electronic devices, about 15 sheets is a reasonable upper limit.

[0086] <Bonding Step> In the bonding step, the laminate structure obtained in the lamination step is bonded by heat pressing. This step fuses and fixes the layers of the laminate structure. The heat pressing conditions may be general conditions, such as heat pressing under vacuum conditions at a holding pressure of 1 MPa to 10 MPa for 10 minutes to 120 minutes. However, it is important to heat press the fluororesin film for multilayer structure substrates at a temperature of 330°C to 360°C to prevent gaps from forming between the layers of the laminate structure due to insufficient fusion.

[0087] The method for manufacturing a multilayer substrate of the present invention may consist of only the above steps, or may include certain additional steps in addition to the above steps. For example, to ensure electrical continuity between adjacent copper-clad laminates, through holes may be formed in the copper-clad laminates before the lamination step, and connecting conductors may be formed inside the through holes. Alternatively, a circuit pattern may be formed on the copper-clad laminates before the lamination step by etching or the like.

[0088] 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.

[0089] [Preparation of Fluororesin Film] Fluororesin films of Examples 1 to 4 and Comparative Example 1 were prepared according to the following procedure.

[0090] [Example 1] (Mixing step) PTFE powder (volume average particle diameter measured by a particle size distribution analyzer: 400 μm) and spherical silica (volume average particle diameter measured by SEM observation: 3 μm) as an inorganic filler were mixed in a volume ratio of PTFE powder:spherical silica = 60:40, and mixed using a rotary blade mixer at a rotation speed of 3000 rpm for 5 minutes to obtain a raw material composition that was a mixture containing PTFE powder and spherical silica.

[0091] (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 365°C to room temperature to obtain a billet.

[0092] (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.

[0093] [Example 2] A fluororesin film of Example 2 was produced in the same manner as in Example 1, except that the PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 50:50.

[0094] [Example 3] A fluororesin film of Example 3 was produced in the same manner as in Example 1, except that the PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 40:60.

[0095] [Example 4] The fluororesin film of Example 4 was produced in the same manner as in Example 1, except that the PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 30:70.

[0096] [Comparative Example 1] A fluororesin film of Comparative Example 1 was produced in the same manner as in Example 1, except that the PTFE powder and spherical silica were mixed in a volume ratio of PTFE powder:spherical silica = 70:30.

[0097] [Evaluation of Physical Properties of Fluororesin Film] The dielectric constant Dk, dielectric loss tangent Df, loss modulus, breaking strain, and handleability of the produced fluororesin film were evaluated. In addition, the coefficient of thermal expansion CTE of the raw material composition obtained in the mixing step was measured.

[0098] <Measurement of relative permittivity Dk and dielectric loss tangent Df> A rectangular test piece having a length of 30 mm and a width of 20 mm was cut out from a fluororesin film, and the relative permittivity 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.

[0099] (Test equipment) Vector network analyzer E8363C (Agilent Technologies) Dielectric constant measurement software 85071E (Agilent Technologies) Split post resonator (QWED)

[0100] <Measurement of Loss Modulus> A sample for measuring loss modulus was taken from the obtained fluororesin film so that the width of the measurement site was 5 mm and the distance between chucks was 5 mm. The loss modulus was measured using a DMA ("RSA-G2" manufactured by TA Instruments) under the conditions of a temperature range of 30 to 350°C (heating rate of 10°C / min), a frequency of 1 Hz, and a strain of 0.1%.

[0101] <Measurement of Breaking Strain> A sample for measuring breaking elongation was taken from the obtained sheet so that the width of the measurement site was 10 mm and the distance between chucks (distance between gauge marks) L0 was 22.25 mm. Using a tensile tester (manufactured by Shimadzu Corporation, "Ez-LX"), the sample was pulled at a pulling rate of 200 mm / min until break in an environment of 23°C and 50% RH, and the breaking elongation (breaking strain) was calculated from the distance L1 (mm) between the gauge marks at break using the following formula (1).

[0102] [Formula 1] Breaking elongation = (L1 - L0) / L0 × 100% (1)

[0103] <Evaluation of Handling Ease> Handling ease was evaluated as follows: as a result of measuring the breaking strain, a breaking elongation rate of 100% or more was evaluated as ◯ (good handling ease), and a breaking elongation rate of less than 100% was evaluated as × (poor handling ease).

[0104] <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.

[0105] The amount of spherical silica in the fluororesin film and the results of the physical property evaluation are shown in Table 1.

[0106]

[0107] The results in Table 1 show that there were no problems with Dk and Df for the fluororesin films of Examples 1 to 4 and Comparative Example 1. For the fluororesin film of Example 4, the loss modulus could not be measured and the breaking strain was less than 100%, resulting in poor handleability, but production of the fluororesin film was possible.

[0108] [Preparation of multilayer structure substrate] (Preparation of copper-clad laminates 100, 200) The fluororesin film of Example 1 having a thickness of 100 μm was cut into a size of 100 mm × 100 mm, and this film was placed in a vacuum plasma device. 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 performing plasma treatment on both sides of the fluororesin film.

[0109] On one of the plasma-treated surfaces of the plasma-treated fluororesin film 10, a copper foil 20 having a thickness of 18 μm and a low roughness (TQ-M4-VSP, manufactured by Mitsui Mining & Smelting Co., Ltd., maximum height Rz: 0.6 μm) cut to 100 mm × 100 mm was laminated, and the resulting laminate structure was sandwiched between SUS304 stainless steel plates (120 mm × 120 mm, thickness: 5 mm) and hot-pressed at a temperature of 360°C and a holding pressure of 4 MPa for 60 minutes, thereby obtaining a copper-clad laminate 100.

[0110] On both sides of the plasma-treated surface of the fluororesin film 10 after the plasma treatment, a copper foil 20 was laminated, each of which had a thickness of 18 μm and a low roughness (TQ-M4-VSP, manufactured by Mitsui Mining & Smelting Co., Ltd., maximum height Rz: 0.6 μm) cut into a size of 100 mm × 100 mm, to form a three-layer laminate structure of copper foil 20, fluororesin film 10, and copper foil 20. The resulting laminate structure was sandwiched between SUS304 stainless steel plates (120 mm × 120 mm, thickness: 5 mm) and hot-pressed at a temperature of 360°C and a holding pressure of 4 MPa for 60 minutes. Thereafter, only the copper foil 20 on one side was etched to form a circuit pattern 30, thereby obtaining a copper-clad laminate 200.

[0111] FIG. 1 shows a schematic cross-sectional view of copper clad laminates 100 and 200.

[0112] (Fabrication of Multilayer Structure Substrate) A copper-clad laminate 100 was laminated such that the fluororesin film 10 of the copper-clad laminate 100 was in contact with the surface of the copper-clad laminate 200 having the circuit pattern 30 (lamination process). The laminated structure was then sandwiched between SUS304 stainless steel plates (120 mm x 120 mm, 5 mm thick) and vacuum hot-pressed at a temperature of 360°C and a holding pressure of 4 MPa for 60 minutes (bonding process), thereby fabricating a multilayer structure substrate.

[0113] In the same manner as described above, a copper-clad laminate was prepared using the fluororesin films of Examples 2 to 4 and Comparative Example 1, and then a multilayer structure substrate was prepared by laminating copper foil 20, fluororesin film 10, circuit pattern 30, fluororesin film 10, and copper foil 20 in this order.

[0114] [Evaluation of Multilayer Structure Substrates] <Embeddability> The side surfaces of the multilayer structure substrates produced using the fluororesin films of Examples 1 to 4 and Comparative Example 1 were mechanically polished to expose cross sections, and the cross sections were observed with an optical microscope to evaluate whether the circuit pattern 30 was embedded without gaps in the fluororesin film. Figures 2 and 3 show schematic cross-sectional views of multilayer structure substrates 300 and 400. The embeddability was evaluated as ○ (pass) when the circuit pattern was embedded without gaps in the fluororesin film, as in the multilayer structure substrate 300 of Figure 2, and × (fail) when there were gaps between the circuit pattern and the fluororesin film, as in the multilayer structure substrate 400 of Figure 3.

[0115] <Wiring Misalignment> The side surfaces of the multilayer structure substrates produced using the fluororesin films of Examples 1 to 4 and Comparative Example 1 were mechanically polished to expose cross sections, and the wiring on the cross sections was observed with an optical microscope to evaluate wiring misalignment. The wiring misalignment evaluation was evaluated as ◯ (pass) when the wiring misalignment from the original position was less than 10 μm, and × (fail) when the misalignment from the original position was 10 μm or more.

[0116] <Wiring Tilt> The side surfaces of the multilayer structure substrates produced using the fluororesin films of Examples 1 to 4 and Comparative Example 1 were mechanically polished to expose cross sections, and the wiring on the cross sections was observed with an optical microscope to evaluate wiring tilt. Figure 4 shows a schematic cross-sectional view of a multilayer structure substrate 500. The multilayer structure substrate 500 has a circuit pattern 30 in which the wiring is tilted obliquely. The wiring tilt evaluation was rated as ◯ (pass) when the tilt from the original wiring was less than 2°, and x (fail) when the tilt from the original wiring was 2° or more.

[0117] The evaluation results of the multilayer structure substrates are shown in Table 2. Also, optical microscope photographs of the cross sections of the multilayer structure substrates produced using the fluororesin films of Examples 1 to 4 and Comparative Example 1 are shown in FIG.

[0118]

[0119] From the results in Table 2 and Figure 5, the multilayer substrates using the fluororesin films of Examples 1 to 4 and Comparative Example 1 all passed the evaluation results for embeddability and wiring misalignment. However, the multilayer substrate using the fluororesin film of Comparative Example 1, which contained a small amount of spherical silica, had a large tilt in the wiring and was judged to be unacceptable. As can be seen in Figure 5, the wiring in the circuit pattern 30 of the multilayer substrate using the fluororesin film of Comparative Example 1, which should have been horizontal, was tilted diagonally to the upper left.

[0120] [Evaluation of Transmission Loss] (Preparation of Multilayer Substrate of Comparative Example 2) A multilayer substrate was prepared using an adhesive and copper-clad laminates 100 and 200 manufactured using the fluororesin film of Example 1. First, an adhesive film (Resonac Corporation, "AS-400HS", relative permittivity εr (measured frequency: 10 GHz); 3.0, dielectric loss tangent tanδ (measured frequency: 10 GHz); 0.0025) was attached as an adhesive layer to the surface of copper-clad laminate 200 having circuit pattern 30. Next, copper-clad laminate 100 was laminated so that the adhesive layer and fluororesin film 10 of copper-clad laminate 100 were in contact with each other to form a laminate structure. Thereafter, the laminate structure was sandwiched between SUS304 stainless steel plates (120 mm x 120 mm, thickness 5 mm) and hot-pressed at a temperature of 180°C and a holding pressure of 2.5 MPa for 90 minutes to prepare a multilayer substrate of Comparative Example 2.

[0121] A multilayer structure substrate (without an adhesive layer) prepared using the fluororesin film of Example 1 and a multilayer structure substrate (with an adhesive layer) of Comparative Example 2 were pattern-etched to form striplines with a line width of approximately 0.06 mm and a length of 100 mm. Using these striplines, the S21 parameter was measured at a characteristic impedance of 50 Ω at frequencies up to 40 GHz using a network analyzer (Keysight Technologies, Inc., E8363B) and a probe (FormFactor, Inc., ACP40-GSG250).

[0122] Figure 6 shows the measurement results of the S21 parameter. A negative value for the S21 parameter indicates that there is transmission loss, and the smaller the absolute value of the S21 parameter, the smaller the transmission loss. As shown in Figure 6, the stripline without an adhesive layer had less transmission loss than the stripline with an adhesive layer.

[0123] [Summary] From the above, it is clear that the present invention can provide a fluororesin film for multilayer substrates, a method for producing a fluororesin film for multilayer substrates, a copper-clad laminate, a multilayer substrate, a method for producing a multilayer substrate, and a circuit board, which can produce multilayer substrates with low transmission loss during high-frequency communication without using an adhesive, and are therefore industrially useful.

[0124] REFERENCE SIGNS LIST 10 Fluororesin film 20 Copper foil 30 Circuit pattern 100 Copper-clad laminate 200 Copper-clad laminate 300 Multilayer structure substrate 400 Multilayer structure substrate 500 Multilayer structure substrate

Claims

1. A fluororesin film for multilayer structure substrates, comprising: a fluororesin; and an inorganic filler dispersed in the fluororesin, wherein the fluororesin film has a dielectric dissipation factor Df of 0.0013 or less and a loss modulus at a temperature of 350°C of 1.6 MPa or more, and the fluororesin film for multilayer structure substrates is a film used for manufacturing multilayer structure substrates by bonding, without adhesive, multiple two-layer copper-clad laminates each having a copper foil and the fluororesin film for multilayer structure substrates laminated together.

2. The fluororesin film for multilayer structure substrates 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.

3. The fluororesin film for multilayer structure substrates according to claim 1, wherein the volume average particle size of the inorganic filler is 0.1 to 10 μm.

4. The fluororesin film for multilayer structure substrates according to claim 1, wherein the content of the inorganic filler is 40 to 70% by volume.

5. The fluororesin film for multilayer structure substrates according to claim 1, wherein the relative dielectric constant Dk of said fluororesin film at a frequency of 20 GHz is 3.0 or less.

6. The fluororesin film for multilayer structure substrates according to claim 1, wherein the thickness of the fluororesin film is 25 μm to 500 μm.

7. The fluororesin film for multilayer structure substrates according to claim 1, wherein the thermal expansion coefficient of said fluororesin film is 80 ppm / K or less.

8. The fluororesin film for multilayer structure substrates according to claim 1, which has a breaking strain of 100% or more.

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

10. A method for producing a fluororesin film for a multilayer structure substrate according to claim 1, comprising: a mixing step of uniformly mixing a fluororesin powder and an inorganic filler powder 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 skiving step of skiving the mixture after the cooling step to form a fluororesin film.

11. A two-layer copper-clad laminate comprising the fluororesin film for multilayer substrates according to claim 1 and a copper foil layer, in which copper foil and the fluororesin film for multilayer substrates are laminated together.

12. A multilayer substrate in which two or more copper clad laminates according to claim 11 are directly bonded together without an adhesive layer to form a multilayer structure.

13. A method for manufacturing a multilayer structure substrate according to claim 12, comprising: a lamination step of directly laminating two or more copper-clad laminates according to claim 11 without using adhesive to form a laminate structure; and a bonding step of bonding the laminate structure by heat pressing.

14. A circuit board comprising the fluororesin film for multilayer structure boards according to claim 1.

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