JP7740231B2 - Multilayer film, its manufacturing method, metal clad laminate and printed wiring board manufacturing method - Google Patents

Description

The present invention relates to a multilayer film having a specified base film layer and specified buffer layers provided on both sides of the base film layer, a method for manufacturing the same, a metal clad laminate, and a method for manufacturing a printed wiring board.

Multilayer films that use a polyimide base film layer and have layers containing tetrafluoroethylene-based polymers on both sides of the base film layer are known as materials for electronic components such as coverlays, flexible flat cables, and printed wiring boards. One proposed embodiment of such multilayer films uses a polyimide with low water absorption and a low glass transition point and a tetrafluoroethylene-based polymer with a low melting temperature (Patent Document 1).

Japanese Patent Application Laid-Open No. 2015-176921

While such multilayer films have excellent moisture resistance and interlayer adhesion, they have issues with their high-temperature heat resistance. Therefore, when such multilayer films are subjected to processes that expose them to high temperatures—for example, when the multilayer film and metal foil are hot-pressed together to form a metal-clad laminate, or when a transmission circuit is formed on the metal foil of the metal-clad laminate and then subjected to a soldering process at 288°C or higher to form a printed wiring board—the multilayer film tends to wrinkle or blister, reducing production efficiency.

The present inventors have discovered that by disposing layers containing a tetrafluoroethylene-based polymer having a melting temperature equal to or higher than a predetermined temperature on both sides of a layer containing a polyimide having a glass transition temperature lower than a predetermined temperature, a multilayer film having heat resistance at even higher temperatures and being less susceptible to thermal expansion and contraction can be obtained. Such a multilayer film is ideal for high-temperature processing, such as processing into printed wiring boards.
An object of the present invention is to provide a multilayer film having the above physical properties, a method for producing the same, a metal clad laminate obtained from the same, and a method for producing a printed wiring board by processing the same.

The present invention has the following aspects.
<1> A multilayer film having a base film layer containing a polyimide having a glass transition temperature of less than 288°C, and buffer layers provided on both sides of the base film layer, the buffer layers containing a tetrafluoroethylene-based polymer containing units based on perfluoro(alkyl vinyl ether) and having a melting temperature of 288°C or higher.
<2> The multilayer film according to <1>, wherein the base film layer and each of the buffer layers are in direct contact with each other.
<3> The multilayer film according to <1> or <2>, wherein the thickness of the multilayer film is 50 μm or more.
<4> The multilayer film according to any one of <1> to <3>, wherein the thickness of the base film layer is 50 μm or less, and the thickness of each of the buffer layers is 5 μm or more.
<5> The multilayer film according to any one of <1> to <4>, wherein the ratio of the total thickness of the buffer layer to the thickness of the base film layer is 0.8 or more.

<6> The multilayer film according to any one of <1> to <5>, wherein the base film layer has a tensile modulus at 320°C of 0.2 GPa or more.
<7> The multilayer film of any one of <1> to <6>, wherein the tetrafluoroethylene-based polymer is a polymer containing units based on perfluoro(alkyl vinyl ether) and having a polar functional group, or a polymer containing 2.0 to 5.0 mol % of units based on perfluoro(alkyl vinyl ether) based on the total units and having no polar functional group.
<8> The multilayer film according to any one of <1> to <7>, wherein the buffer layer further contains an aromatic polymer.
<9> The multilayer film of any one of <1> to <8>, wherein the multilayer film has a thermal expansion/contraction rate of −2 to +1% as measured when heated at 150° C. for 30 minutes.

<10> The method for producing a multilayer film according to any one of <1> to <9>, wherein each of the buffer layers is formed from a liquid composition containing a powder of the tetrafluoroethylene-based polymer.
<11> The method for producing a multilayer film according to any one of <1> to <9>, wherein each of the buffer layers is formed from a film containing the tetrafluoroethylene-based polymer.

<12> A metal clad laminate comprising: a base film layer containing a polyimide having a glass transition point of less than 288°C; buffer layers provided on both sides of the base film layer, the buffer layers containing a tetrafluoroethylene-based polymer that contains units based on perfluoro(alkyl vinyl ether) and has a melting point of 288°C or higher; and a metal foil layer provided on at least one of the buffer layers on the side opposite to the base film layer.
<13> The metal clad laminate according to <12>, which is a printed wiring board material.
<14> The metal clad laminate according to <12> or <13>, having a soldering heat resistance temperature of 288°C or higher.
<15> A method for producing a printed wiring board, comprising etching the metal foil layer of the metal clad laminate according to any one of <12> to <14> to form a transmission circuit, thereby obtaining a printed wiring board.

According to the present invention, a multilayer film, a metal clad laminate, and a printed wiring board can be obtained, which have a base film layer containing a specified polyimide and buffer layers provided on both sides of the base film layer and containing a specified tetrafluoroethylene-based polymer.

The following terms have the following meanings: "Average particle size (D50)" is the volume-based cumulative 50% diameter of a target object (powder and filler) determined by laser diffraction/scattering. That is, the particle size distribution of the target object is measured by laser diffraction/scattering, and a cumulative curve is calculated with the total volume of the target particle population set to 100%, and the "average particle size (D50)" is the particle size at the point on the cumulative curve where the cumulative volume is 50%.
"D90" is the volume-based cumulative 90% diameter of the object, measured in the same manner.
"Melting temperature (melting point)" is the temperature corresponding to the maximum value of the melting peak of a polymer as measured by differential scanning calorimetry (DSC).
The "glass transition temperature (Tg)" is a value measured by analyzing a polymer using dynamic mechanical analysis (DMA) method.
The "viscosity" is a value determined by measuring the liquid composition using a Brookfield viscometer at 25° C. and 30 rpm. The measurement is repeated three times, and the average value of the three measurements is used.
The "thixotropy ratio" is a value calculated by dividing the viscosity of a liquid composition measured at a rotation speed of 30 rpm by the viscosity of a liquid composition measured at a rotation speed of 60 rpm.
The "tensile modulus" is a value measured by analyzing a film using a wide-range viscoelasticity measuring device at a measurement frequency of 10 Hz.
A "unit" in a polymer may be an atomic group formed directly from one monomer molecule by polymerization, or may be an atomic group formed by treating the obtained polymer with a predetermined method to convert a part of the structure of the atomic group formed from one monomer molecule. A unit based on monomer A contained in a polymer may also be simply referred to as a "monomer A unit."
The "ten-point average roughness" is a value determined by measuring a film in accordance with Annex JA of JIS B 0601:2013.

The multilayer film of the present invention comprises a base film layer and buffer layers provided on both sides of the base film layer. The base film layer contains a polyimide (hereinafter also referred to as "PI") having a glass transition temperature of less than 288°C, and the buffer layer contains a tetrafluoroethylene-based polymer (hereinafter also referred to as "F polymer") containing units based on perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE units") and having a melting temperature of 288°C or higher.
The multilayer film of the present invention has excellent interlayer adhesion and moisture resistance, as well as high-temperature resistance and low thermal expansion and contraction. The reason for this is not entirely clear, but is thought to be as follows.

PI with a low glass transition point has excellent moisture resistance and flexibility, but is prone to softening at high temperatures. In addition, tetrafluoroethylene-based polymers generally have a large linear expansion coefficient, so they tend to shrink when cooled from a high temperature state. Therefore, it was thought that in a multilayer film combining layers containing such polymers, the layer containing PI would deform when heated under high temperature exposure, and the layer containing tetrafluoroethylene-based polymer would shrink when cooled after high temperature exposure, resulting in wrinkles or swelling. However, the present inventors have discovered that a layer containing a tetrafluoroethylene-based polymer (F polymer) containing PAVE units functions as a thermal buffer layer due to its heat resistance, suppressing deformation of the layer containing PI, and its melt flowability buffers the shrinkage.
As a result, the multilayer film of the present invention has excellent interlayer adhesion and moisture resistance, as well as high-temperature resistance and low thermal expansion and contraction, and is therefore considered to be suitable for use as a material for applications requiring high-temperature processes for processing printed wiring boards, etc.

The F polymer in the present invention is a polymer containing units based on tetrafluoroethylene (TFE) (TFE units) and PAVE units.
The F polymer may further contain units based on other comonomers.
Preferred PAVEs are _CF2 = _CFOCF3 , _CF2 = _{CFOCF2CF3 and CF2} = _CFOCF2CF2CF3 ( _PPVE ), _with PPVE being more preferred.
The melting temperature of the F polymer is 288° C. or higher, and preferably 300° C. or higher. The melting temperature of the F polymer is preferably 350° C. or lower, and more preferably 320° C. or lower. In this case, the multilayer film tends to have better heat resistance and is less susceptible to thermal expansion and contraction.
The glass transition point of the F polymer is preferably 75 to 125°C, more preferably 80 to 100°C.

The F polymer preferably has a polar functional group (oxygen-containing polar group). The polar functional group may be contained in a unit in the F polymer or in a terminal group of the main chain of the F polymer. Examples of the latter include an F polymer having a polar functional group as a terminal group derived from a polymerization initiator, a chain transfer agent, etc., and an F polymer having a polar functional group obtained by subjecting an F polymer to plasma treatment or ionizing radiation treatment.
As the polar functional group, a hydroxyl group-containing group and a carbonyl group-containing group are preferred, and from the viewpoint of improving adhesion to the buffer layer, a carbonyl group-containing group is more preferred.
As the hydroxyl group-containing group, a group containing an alcoholic hydroxyl group is preferred, and —CF ₂ CH ₂ OH and —C(CF ₃ ) ₂ OH are more preferred.
The carbonyl group-containing group is a group containing a carbonyl group (>C(O)), and examples of the carbonyl group-containing group include a carboxyl group, an alkoxycarbonyl group, an amide group, an isocyanate group, a carbamate group (-OC(O)NH ₂ ), an acid anhydride residue (-C(O)OC(O)-), an imide residue (-C(O)NHC(O)-, etc.) and a carbonate group (-OC(O)O-), and an acid anhydride residue is more preferred.

When the F polymer has a carbonyl group-containing group, the number of carbonyl group-containing groups in the F polymer is preferably 100 to 5000, more preferably 300 to 3000, and even more preferably 800 to 1500 per 1 × ¹⁰ carbon atoms in the main chain. The number of carbonyl group-containing groups in the F polymer can be quantified by the composition of the polymer or the method described in WO 2020/145133.

The F polymer is preferably a polymer (1) that further contains a unit based on a monomer having a polar functional group and has a polar functional group, or a polymer (2) that contains 2.0 to 5.0 mol % of PAVE units based on all units and has no polar functional group, with polymer (1) being more preferred.
These F polymers tend to form small spherulites. A buffer layer containing such an F polymer tends to have excellent surface smoothness, and the buffer layer and base polymer layer are closely attached to each other, resulting in a multilayer film that tends to have excellent peel strength and water resistance.

The polymer (1) preferably contains, based on all units, 90 to 99 mol % of TFE units, 0.5 to 9.97 mol % of PAVE units, and 0.01 to 3 mol % of units derived from a monomer having a polar functional group.
Furthermore, as the monomer having a polar functional group, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic anhydride (also known as himic anhydride; hereinafter also referred to as "NAH") are preferred.
Specific examples of the polymer (1) include the polymers described in WO 2018/16644.

The polymer (2) is composed of only TFE units and PAVE units, and preferably contains 95.0 to 98.0 mol % of TFE units and 2.0 to 5.0 mol % of PAVE units based on the total units.
The content of PAVE units in the polymer (2) is preferably 2.1 to 5.0 mol %, more preferably 2.2 to 5.0 mol %, based on all units.
The phrase "polymer (2) has no polar functional groups" means that the number of polar functional groups that the polymer has per 1 × ¹⁰ carbon atoms constituting the polymer main chain is less than 500. The number of polar functional groups is preferably 100 or less, more preferably less than 50. The lower limit of the number of polar functional groups is usually 0.
Polymer (2) may be produced using a polymerization initiator, a chain transfer agent, or the like that does not generate a polar functional group as a terminal group of the polymer chain, or may be produced by fluorinating an F polymer having a polar functional group (e.g., an F polymer having a polar functional group derived from a polymerization initiator at the terminal group of the main chain of the polymer). Examples of fluorination treatment methods include a method using fluorine gas (see, for example, JP 2019-194314 A).

The content of the F polymer in the buffer layer in the present invention is preferably 50% by mass or more, more preferably 60% by mass or more, and the upper limit of the content of the F polymer is 100% by mass.
The buffer layer may further contain another resin (polymer), which may be a thermosetting resin or a thermoplastic resin.
The other resin is preferably an aromatic polymer, in which case the buffer layer has excellent UV absorption properties and the multilayer film tends to have excellent UV processability.
Examples of other resins include epoxy resins, maleimide resins, urethane resins, polyimides, polyamic acids, polyamideimides, polyphenylene ethers, polyphenylene oxides, liquid crystal polyesters, and fluoropolymers other than F polymers.

As the other resin, maleimide resin, polyimide, and polyamic acid are preferred. When at least one of these resins is contained, the multilayer film tends to have excellent flexibility and peel strength. As the other resin, maleimide resin, polyimide, and polyamic acid, all of which are aromatic, are more preferred.
The polyimide is preferably thermoplastic.
In this case, the total content of maleimide, polyimide, and polyamic acid in the buffer layer is preferably 0.1 to 30% by mass, more preferably 1 to 10% by mass, and the ratio of the total content of maleimide, polyimide, and polyamic acid to the content of the F polymer is preferably 1.0 or less, more preferably 0.01 to 0.5.

As the other resin, a fluoropolymer other than F polymer is also preferred, and non-thermofusible PTFE is more preferred, since in this case the physical properties (electrical properties such as low dielectric loss tangent) inherent to PTFE tend to be significantly exhibited in the multilayer film.
In this case, the content of non-thermofusible PTFE is preferably 1 to 30% by mass, more preferably 5 to 20% by mass. The ratio (mass ratio) of the content of non-thermofusible PTFE to the content of F polymer is preferably 1.0 or less, more preferably 0.1 to 0.4.
The buffer layer preferably further contains an inorganic filler from the viewpoint of further improving the low linear expansion and electrical properties of the multilayer film.

As the inorganic filler, nitride fillers and inorganic oxide fillers are preferred, with boron nitride fillers, beryllia fillers (beryllium oxide fillers), silicate fillers (silica fillers, wollastonite fillers, talc fillers), and metal oxide (cerium oxide, aluminum oxide, magnesium oxide, zinc oxide, titanium oxide, etc.) fillers being more preferred, and silica fillers being even more preferred.
The silica content in the inorganic filler is preferably 50% by mass or more, more preferably 75% by mass or more, and the upper limit of the silica content is 100% by mass.

The inorganic filler is preferably at least partially surface-treated. Examples of surface treatment agents used for such surface treatment include polyhydric alcohols (trimethylolethane, pentaerythritol, propylene glycol, etc.), saturated fatty acids (stearic acid, lauric acid, etc.), esters thereof, alkanolamines, amines (trimethylamine, triethylamine, etc.), paraffin wax, silane coupling agents, silicones, and polysiloxanes.
Preferred silane coupling agents are 3-aminopropyltriethoxysilane, vinyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane and 3-isocyanatopropyltriethoxysilane.

The average particle size of the inorganic filler is preferably 20 μm or less, more preferably 10 μm or less, and is preferably 0.1 μm or more, more preferably 1 μm or more.
The shape of the inorganic filler may be any of granular, needle-like (fibrous), and plate-like. Specific shapes of the inorganic filler include spherical, scale-like, layer-like, leaf-like, apricot-like, columnar, cockscomb-like, equiaxial, leaf-like, micaceous, block-like, flat, wedge-like, rosette-like, net-like, and prismatic shapes.
Specific examples of inorganic fillers include silica fillers (such as the "Adma Fine" series manufactured by Admatechs Co., Ltd.), zinc oxide surface-treated with esters such as propylene glycol dicaprate (such as the "FINEX" series manufactured by Sakai Chemical Industry Co., Ltd.), spherical fused silica (such as the "SFP" series manufactured by Denka Co., Ltd.), coated with polyhydric alcohol and inorganic substances (such as the "Tipaque" series manufactured by Ishihara Sangyo Kaisha, Ltd.), rutile-type titanium oxide surface-treated with alkylsilane (such as the "JMT" series manufactured by Teika Co., Ltd.), hollow Examples of suitable fillers include silica fillers (such as the "E-SPHERES" series manufactured by Taiheiyo Cement Corporation, the "SILINAX" series manufactured by Nittetsu Mining Co., Ltd., and the "Ecocospher" series manufactured by Emerson & Cumming Co.), talc fillers (such as the "SG" series manufactured by Nippon Talc Co., Ltd.), steatite fillers (such as the "BST" series manufactured by Nippon Talc Co., Ltd.), and boron nitride fillers (such as the "UHP" series manufactured by Showa Denko KK and the "Denka Boron Nitride" series ("GP" and "HGP" grades) manufactured by Denka Company, Ltd.).

The content of the inorganic filler in the buffer layer is preferably 1% by mass or more, more preferably 3% by mass or more, and is preferably 40% by mass or less, more preferably 30% by mass or less, and more preferably 20% by mass or less.
The ratio (mass ratio) of the inorganic filler content to the F polymer content in the buffer layer is preferably 0.01 or more, more preferably 0.1 or more, and is preferably 1 or less, more preferably 0.8 or less.
In addition to the components described above, the buffer layer may contain additives such as a silane coupling agent, a dehydrating agent, a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a brightening agent, a colorant, a conductive agent, a release agent, a surface treatment agent, and a flame retardant.

The glass transition point of the PI in the present invention is preferably lower than 288° C., more preferably lower than 275° C., and even more preferably 260° C. or lower. The glass transition point of the PI is preferably 200° C. or higher.
The PI is preferably obtained from a polyamic acid obtained by polymerizing an aromatic diamine component and a tetracarboxylic acid component.
Examples of the aromatic diamine component include paraphenylenediamine, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 4,4'-bis(aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, and 2,2'-bis(4-aminophenoxyphenyl)propane. Two or more types of aromatic diamine components may be used.

Examples of the tetracarboxylic acid component include pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, oxydiphthalic acid, 2,3',3,4'-biphenyltetracarboxylic acid, and 3,3',4,4'-benzophenonetetracarboxylic acid, and preferably pyromellitic acid, 3,3',4,4'-biphenyltetracarboxylic acid, oxydiphthalic acid, and dianhydrides of these tetracarboxylic acids. Two or more types of tetracarboxylic acid components may be used.
The base film layer in the present invention can be obtained by casting a solution containing polyamic acid obtained by polymerizing the above-mentioned aromatic diamine and the above-mentioned tetracarboxylic acid component into a film and thermally decyclizing and desolvating it, or by mixing a polyamic acid solution with a cyclization catalyst and a dehydrating agent, chemically decyclizing it to prepare a gel film, and then heating and desolvating it. The solution containing polyamic acid may further contain a cyclization catalyst (imidization catalyst), a dehydrating agent, a gelation retarder, etc.

Specific examples of the base film layer include "Kapton 50EN-S" (manufactured by DuPont-Toray Co., Ltd.), "Kapton 100EN" (manufactured by DuPont-Toray Co., Ltd.), "Kapton 100H" (manufactured by DuPont-Toray Co., Ltd.), "Kapton 100KJ" (manufactured by DuPont), "Kapton 100JP" (manufactured by DuPont-USA), and "Kapton 100LK" (manufactured by DuPont-Toray Co., Ltd.).
The base film layer may further contain plasticizers, other resins, colorants, various additives, etc. Examples of additives include antistatic agents, flame retardants, heat stabilizers, ultraviolet absorbers, lubricants, release agents, nucleating agents, and reinforcing agents (fillers).

The tensile modulus of the base film layer at 320° C. is preferably 0.2 GPa or more, more preferably 0.4 GPa or more, and is preferably 10 GPa or less, more preferably 5 GPa or less.
In this case, the multilayer film is less likely to deform when heated and cooled during processing, and is therefore easier to handle. In other words, if the tensile modulus of the base film layer is equal to or greater than the above-mentioned lower limit, the deformation of the buffer layer during heating and cooling during processing is effectively alleviated by the elasticity of the base film layer, making the multilayer film less likely to wrinkle, and the physical properties (surface smoothness, etc.) of the resulting processed product are likely to be improved. Even if the content of the F polymer in the buffer layer or the thickness of the buffer layer is high and the buffer layer is prone to deformation, a processed product with excellent physical properties is likely to be obtained. Furthermore, if the tensile modulus of the base film layer is equal to or less than the above-mentioned upper limit, the flexibility of the multilayer film is likely to be further improved.

The imide group density of the PI contained in the base film layer is preferably 0.20 to 0.35. If the imide group density is equal to or less than the upper limit, the water absorption rate of the base film layer is lowered, and changes in the dielectric properties of the multilayer film are more easily suppressed. If the imide group density is equal to or greater than the lower limit, the imide groups function as polar groups, which not only improves the adhesion between the base film layer and the buffer layer but also significantly reduces the water absorption rate.
Furthermore, if the imide group density is within this range, wrinkles tend not to occur during processing of the multilayer film.
The imide group density is a value obtained by dividing the molecular weight per unit of the imide group portion (140.1) in a polyimide obtained by imidizing a polyimide precursor by the molecular weight per unit of the polyimide. For example, the imide group density of a polyimide (molecular weight per unit: 382.2) obtained by imidizing a polyimide precursor consisting of two components, 1 mol of pyromellitic dianhydride (molecular weight: 218.1) and 1 mol of 3,4'-oxydianiline (molecular weight: 200.2), is 0.37, which is the value obtained by dividing 140.1 by 382.2.

In the multilayer film of the present invention, it is preferable that the base film layer and each buffer layer are in direct contact with each other. In this case, the multilayer film tends to have excellent heat resistance and water resistance. Furthermore, the occurrence of warping of the multilayer film is easily suppressed.
The thickness of the buffer layer in the multilayer film is preferably 5 μm or more, and more preferably 10 μm or more. The thickness of the buffer layer is preferably 200 μm or less, and more preferably 100 μm or less. The thickness of each buffer layer is preferably the same.
The thickness of the base film layer in the multilayer film is preferably 10 μm or more, more preferably 25 μm or more, and the thickness of the buffer layer is preferably 100 μm or less, more preferably 50 μm or less, which tends to result in excellent heat resistance and low linear expansion of the multilayer film.

The ratio of the total thickness of the buffer layer to the thickness of the base film layer is preferably 0.8 or more, more preferably 1.0 or more, and is preferably 10 or less, more preferably 5 or less.
When the thickness of the base film layer is 50 μm or less, the thickness of each buffer layer is preferably 5 μm or more, more preferably 10 μm or more, in which case the multilayer film tends to have excellent heat resistance.
The thickness of the multilayer film is preferably 50 μm or more, more preferably 75 μm or more, and is preferably 300 μm or less, more preferably 150 μm or less.
In this case, the multilayer film tends to have excellent heat resistance and low linear expansion.

The thermal expansion/contraction rate of the multilayer film is preferably −2 to +1%, more preferably −1.5 to 0%. Such a multilayer film is less likely to wrinkle when processed into a printed wiring board, allowing a high yield to be maintained. Furthermore, since such a multilayer film has excellent thermal shock resistance, it is highly resistant to thermal shock when forming through-holes or vias when processed into a printed wiring board, and as a result, a printed wiring board that is less likely to break is easily obtained.
The thermal expansion/contraction rate is determined by the following method.
At 25°C, a straight line approximately 10 cm long is drawn on a 12 cm square multilayer film, and the distance between the endpoints of the line is defined as the initial length _L. The multilayer film is then heat-treated at 150°C for 30 minutes and cooled to 25°C. After that, the linear distance _L between the endpoints of the line drawn on the multilayer film is measured, and the thermal expansion/contraction rate (%) is calculated using the following formula:
Thermal expansion/contraction rate (%) = (L ₁ /L ₀ -1) x 100

The surface of the multilayer film may be subjected to a surface treatment with a silane coupling agent or the like, or may be surface-modified by a corona treatment, a plasma treatment, etc. Furthermore, the surface of the multilayer film may be subjected to a roughening treatment or an annealing treatment.
In particular, if the surface of the multilayer film is plasma-treated, it is easy to laminate it with other materials. As the plasma treatment, atmospheric pressure plasma treatment and vacuum plasma treatment are preferred.

Methods for producing the multilayer film of the present invention include a method in which each buffer layer is formed from a liquid composition containing an F polymer powder (i.e., an aggregate of F polymer-containing particles) (hereinafter also referred to as "Method 1"), and a method in which each buffer layer is formed from a film containing an F polymer (hereinafter also referred to as "F film") (hereinafter also referred to as "Method 2").
The definitions and scope of the PI, base film layer, F polymer, buffer layer, and multilayer film in Method 1, including preferred embodiments, are the same as the definitions of the PI, base film layer, F polymer, buffer layer, and multilayer film in the multilayer film.

The content of the F polymer in the powder of the F polymer (hereinafter also referred to as "F powder") in Method 1 is preferably 80% by mass or more, more preferably 100% by mass.
Other components that can be contained in the particles of the F powder include resins other than the F polymer and inorganic fillers.
Examples of the resin include aromatic polyester, polyamideimide, thermoplastic polyimide, polyphenylene ether, and polyphenylene oxide.
Examples of inorganic fillers include silicon oxide (silica), metal oxides (beryllium oxide, cerium oxide, alumina, soda alumina, magnesium oxide, zinc oxide, titanium oxide, etc.), boron nitride, and magnesium metasilicate (steatite). At least a portion of the surface of the inorganic filler may be surface-treated.

The particles of F powder containing a resin other than the F polymer or an inorganic filler may have a core-shell structure in which the F polymer is the core and the resin other than the F polymer or the inorganic filler is the shell, or may have a core-shell structure in which the F polymer is the shell and the resin other than the F polymer or the inorganic filler is the core. Such F powder particles can be obtained, for example, by coalescence (collision, aggregation, etc.) of particles of the F polymer with particles of the resin other than the F polymer or the inorganic filler.
The D50 of the F powder is preferably 0.1 to 6 μm, and the D90 of the F powder is preferably 10 μm or less.
The content of the F powder in the liquid composition in Method 1 is preferably 5 to 40% by mass. In this case, the densely contained F powder easily forms a dense buffer layer, and the physical properties of the F polymer are easily exhibited in the buffer layer.

The liquid composition in Method 1 preferably contains a liquid dispersion medium. The liquid dispersion medium is a liquid compound that is inactive at 25°C and functions as a dispersion medium for the F powder.
The boiling point of the liquid dispersion medium is preferably 125 to 250°C.
The liquid dispersion medium is preferably one or more liquid compounds selected from the group consisting of water, amides, ketones and esters, and more preferably N-methyl-2-pyrrolidone (NMP), γ-butyrolactone, cyclohexanone and cyclopentanone.
The content of the liquid dispersion medium in the liquid composition is preferably 50 to 80% by mass.
The viscosity of the liquid composition is preferably 50 to 1000 mPa·s, and the thixotropy ratio of the liquid composition is preferably 1.0 to 2.0.

The liquid composition may contain an acetylene-based surfactant, a silicone-based surfactant, a fluorine-based surfactant, or the like, from the viewpoint of improving dispersion stability and handling properties.
From the viewpoint of further improving the low linear expansion and electrical properties of the buffer layer, the liquid composition preferably further contains an inorganic filler. The definition and range of the inorganic filler, including preferred embodiments, are the same as those of the inorganic filler in the multilayer film.
The liquid composition may further contain other resins (polymers). The definitions and ranges of the other resins, including preferred embodiments thereof, are the same as those of the other resins in the multilayer film.
The additives such as the surfactant and inorganic filler may be added to the F powder-containing liquid composition, or may be premixed with the F powder to form a mixture, which is then dispersed in a liquid dispersion medium. Alternatively, the surfactant and inorganic filler may be predispersed or dissolved in a liquid dispersion medium, which is then mixed with the F powder-containing liquid composition or the F powder.

A specific embodiment of Method 1 is to apply a liquid composition to one side of a base film layer, heat to remove the liquid dispersion medium, apply a liquid composition to the other side of the base film layer, heat to remove the liquid dispersion medium, and further heat to bake the F polymer, thereby forming each buffer layer and obtaining a multilayer film.
The temperature in the former heating is preferably 120 to 200° C. The temperature in the latter heating is preferably a temperature equal to or higher than the melting temperature of the F polymer, specifically preferably 280 to 400° C., more preferably 300 to 380° C.
The heating time for the latter is preferably 30 seconds to 5 minutes, more preferably 1 to 2 minutes.

Examples of the heating method include a method using an oven, a method using a ventilation drying furnace, and a method using heat rays such as infrared rays.
The heating atmosphere may be either normal pressure or reduced pressure, and may be any of an oxidizing gas (oxygen gas, etc.), a reducing gas (hydrogen gas, etc.), and an inert gas (rare gas, nitrogen gas).
The liquid composition can be applied to the base film layer by a coating method such as spraying, roll coating, spin coating, gravure coating, microgravure coating, gravure offset, knife coating, kiss coating, bar coating, die coating, fountain-meyer bar coating, or slot die coating.

The definitions and scope of the PI, base film layer, F polymer, buffer layer, and multilayer film in Method 2 are the same as those of the PI, base film layer, F polymer, buffer layer, and multilayer film in the multilayer film, including preferred embodiments.
The thickness of the F film in Method 2 is preferably 5 μm or more, more preferably 10 μm or more, and is preferably 200 μm or less, more preferably 100 μm or less.
The content of the F polymer in the F film is preferably 50% by mass or more, more preferably 60% by mass or more, and the upper limit of the content of the F polymer is 100% by mass.
The F film may further contain other resins (polymers), inorganic fillers, and additives. The definitions and ranges of the other resins, inorganic fillers, and additives, including preferred embodiments, are the same as those of the other resins, inorganic fillers, and additives in the multilayer film.

A specific embodiment of Method 2 is to sandwich a base film layer between two F films, which are then heat-pressed and bonded together to obtain a multilayer film.
The pressing temperature in the heat press is preferably 310 to 400°C.
The heat pressing is preferably carried out at a vacuum of 20 kPa or less in order to prevent the inclusion of air bubbles.
Furthermore, it is preferable to heat the heat press after the above-mentioned degree of vacuum is reached. If the temperature is raised before the above-mentioned degree of vacuum is reached, the buffer layer will be pressed in a softened state, i.e., in a state where it has a certain degree of fluidity and adhesiveness, which may cause bubbles.
The pressure in the heat press is preferably 0.2 to 10 MPa from the viewpoint of firmly adhering the buffer layer and the base film layer.
In particular, when the tensile modulus of the base film is equal to or greater than the above-mentioned lower limit, the occurrence of wrinkles due to heating and cooling in the heat press can be easily suppressed.

The metal clad laminate of the present invention has a base film layer containing PI, buffer layers containing an F polymer provided on both sides of the base film layer, and a metal foil layer provided on the surface of at least one of the buffer layers (the side opposite to the base film layer).
Examples of metals that can be used to form the metal foil layer include copper, copper alloys, stainless steel, nickel, nickel alloys (including alloy 42), aluminum, aluminum alloys, titanium, and titanium alloys.
As the metal foil for forming the metal foil layer, copper foil is preferred, with rolled copper foil with no front and back distinction and electrolytic copper foil with front and back distinction being more preferred, and rolled copper foil being even more preferred. Rolled copper foil has a small surface roughness, so that transmission loss can be reduced even when the metal clad laminate is processed into a printed wiring board. Furthermore, it is preferable to use rolled copper foil after immersing it in a hydrocarbon organic solvent to remove rolling oil.

The ten-point average roughness of the surface of the metal foil layer is preferably 0.01 to 4 μm, in which case the adhesion to the buffer layer is good and a printed wiring board with excellent transmission characteristics is easily obtained.
The surface of the metal foil layer may be roughened by a method such as forming a roughened layer, dry etching, or wet etching.
The thickness of the metal foil layer is preferably less than 20 μm, more preferably 2 to 15 μm.
The surface of the metal foil layer may be partially or entirely treated with a silane coupling agent.
A method for producing a metal clad laminate includes stacking a multilayer film and a metal foil so that the buffer layer of the multilayer film contacts the metal foil, and then hot pressing them together. The multilayer film has excellent high-temperature resistance, so warping and wrinkling are unlikely to occur when the multilayer film is laminated to the metal foil.
In this case, the surface of the F film may be subjected to a surface treatment from the viewpoint of improving the low linear expansion property and peel strength of the metal clad laminate.

The metal clad laminate of the present invention is excellent in various physical properties such as electrical properties, heat resistance such as solder reflow resistance, drilling workability, chemical resistance, surface smoothness, etc. Therefore, the metal clad laminate of the present invention is suitable as a printed wiring board material, and can be easily and efficiently processed into flexible printed wiring boards and rigid printed wiring boards.
A printed wiring board can be obtained by etching the metal foil layer of the metal clad laminate of the present invention to form a transmission circuit. That is, the method for producing a printed wiring board of the present invention is a method for obtaining a printed wiring board by etching the metal foil of the metal clad laminate to form a transmission circuit. Note that either wet etching or dry etching can be used for etching.
The metal clad laminate of the present invention includes the multilayer film of the present invention, and is resistant to deformation and warping even during etching.
In the manufacture of a printed wiring board, after forming a transmission circuit, an interlayer insulating film may be formed on the transmission circuit, and a conductor circuit may be further formed on the interlayer insulating film. The interlayer insulating film may be formed from the liquid composition described above.

The solder heat resistance temperature of the metal clad laminate of the present invention is preferably 288° C. or higher, more preferably 300° C. or higher, and even more preferably 320° C. or higher. The solder heat resistance temperature is preferably 380° C. or lower. The metal clad laminate of the present invention includes the multilayer film of the present invention and has excellent high-temperature resistance due to the above-mentioned mechanism of action, so that it is likely to exhibit a solder heat resistance temperature in this range.
By using the metal clad laminate of the present invention, it is possible to suppress the occurrence of swelling, warping, and peeling of the multilayer film, which is an electrical insulating layer, during the solder reflow process (a process in which solder paste is placed on a printed wiring board material and heated), which is a high-temperature process in the mounting process of printed wiring board manufacturing, and to manufacture printed wiring boards with high production efficiency.

The solder heat resistance temperature is measured by the following method.
The metal foil layer of a 4 cm square metal clad laminate is patterned into a 2.5 cm square, and the pattern is suspended in a solder bath with an initial temperature of 250°C to observe the presence or absence of external abnormalities (external defects such as expansion, bubbles, layer separation, etc.).
If no abnormalities are found in the appearance, the temperature of the solder bath is increased and the appearance is inspected in the same manner. The temperature of the solder layer is repeatedly increased and inspected, and the highest temperature at which no abnormalities occur is taken as the solder heat resistance temperature.
The printed wiring board of the present invention is less likely to develop bulging at the interface between the base film layer and the buffer layer or warpage of the printed wiring board, even when subjected to a solder reflow process (a process in which solder paste is placed on a printed wiring board material and heated), which is a high-temperature process in the mounting process of printed wiring board manufacturing.

1. Preparation of each component and part [F Polymer]
F Polymer 1: PFA-based polymer containing 98.0 mol%, 0.1 mol%, and 1.9 mol% of TFE units, NAH units, and PPVE units, in that order (melting temperature: 300°C)
F Polymer 2: PFA-based polymer containing 97.5 mol% TFE units and 2.5 mol% PPVE units, in that order (melting temperature: 305°C)
Non-F polymer 1: FEP-based polymer containing 40.8 mol%, 44.8 mol%, 13.9 mol%, and 0.5 mol% of TFE units, ethylene units, hexafluoropropene units, and perfluoro(1,1,5-trihydro-1-pentene) units, in this order (melting temperature: 162°C).
F Polymer 1 has 1000 carbonyl-containing groups per 1×10 ⁶ carbon atoms in the main chain, and F Polymer 2 has 40 carbonyl-containing groups.

[powder]
Powder 1: Powder made of F polymer 1, D50 of 2.1 μm Powder 2: Powder made of F polymer 2, D50 of 1.8 μm Powder 3: Powder made of non-F polymer 1, D50 of 2.2 μm [Surfactant]
Surfactant 1: A nonionic polymer which is a copolymer _{of CH2} =C( _CH3 )C(O) _OCH2CH2 ( _CF2 ) _6F and _CH2 =C( _CH3 ) _C (O)( _OCH2CH2 ) _23OH and has a fluorine content of 35% by mass [inorganic filler]
Filler 1: Silica filler with D50 of 0.4 μm

[varnish]
Varnish 1: Varnish in which thermoplastic aromatic polyimide (PI1) was dissolved in NMP (solid content: 18% by mass)
[Liquid dispersion medium]
NMP: N-methyl-2-pyrrolidone [base film]
Polyimide film 1: Aromatic polyimide film having a thickness of 50 μm, a glass transition point of 245° C., and a tensile modulus of elasticity of 0.3 GPa at 320° C. Polyimide film 2: Aromatic polyimide film having a thickness of 50 μm, a glass transition point of 330° C., and a tensile modulus of elasticity of 0.2 GPa at 320° C. [Copper foil]
Copper foil 1: 18 μm thick electrolytic copper foil

2. Example of Preparation of Dispersion (Liquid Composition) First, Powder 1, Varnish 1, Surfactant 1, and NMP were placed in a pot, and zirconia balls were then placed in. The pot was then rolled at 150 rpm for 1 hour to prepare a composition.
Filler 1, surfactant 1, and water were placed in a separate pot, and zirconia balls were then placed in. The pot was then rolled at 150 rpm for 1 hour to prepare a composition.
Furthermore, both compositions were placed in another pot, and zirconia balls were placed in. The pot was then rolled at 150 rpm for 1 hour to obtain Dispersion Liquid 1 (viscosity: 400 mPa s) containing Powder 1 (14 parts by mass), Filler 1 (7 parts by mass), PI 1 (1 part by mass), Surfactant 1 (3 parts by mass), and NMP (75 parts by mass).

Dispersions 2 to 5 were obtained in the same manner as Dispersion 1, except that the types and amounts of the powder, inorganic filler, and aromatic polyimide (PI1) were changed as shown in Table 1 below.

3. Example of manufacturing a multilayer film Dispersion 1 was applied to one side of polyimide film 1 by a small-diameter gravure reverse method, and the film was passed through a ventilation drying oven (oven temperature 150°C) for 3 minutes to remove NMP and form a dry film. Dispersion 1 was also similarly applied to the other side of polyimide film 1 and dried to form a dry film.
Next, the polyimide film 1 with the dry coating formed on both sides was passed through a far-infrared furnace (furnace temperature near the furnace entrance and exit: 300°C, furnace temperature near the center: 340°C) for 20 minutes to melt and bake the powder 1.
This resulted in the formation of buffer layers (thickness: 25 μm) containing F polymer 1 on both sides of polyimide film 1, and a film (multilayer film 1) in which the buffer layer, polyimide film layer, and buffer layer were directly formed in this order was obtained.
Multilayer films 2 to 5 were obtained in the same manner as multilayer film 1, except that dispersion 1 was changed to dispersions 2 to 5, respectively.
Multilayer film 6 was obtained in the same manner as multilayer film 1, except that the thickness of the buffer layer was changed to 12 μm.
Further, multilayer film 7 was obtained in the same manner as multilayer film 1, except that dispersion liquid 1 was changed to dispersion liquid 2 and polyimide film 1 was changed to polyimide film 2.

4. Example of Manufacturing a Copper-Clad Laminate Copper foil 1 was placed on each side of multilayer film 1 so as to be in contact with the buffer layer, and the multilayer film was vacuum hot-pressed (pressing temperature: 320°C, pressing pressure: 2 MPa, pressing time: 2 minutes) to obtain copper-clad laminate 1.
The copper clad laminate 1 is a laminate in which an electrolytic copper foil layer, a buffer layer, a base film layer, a buffer layer, and an electrolytic copper foil layer are laminated in this order.
Before vacuum hot pressing, the surface of the buffer layer of multilayer film 1 was treated by vacuum plasma treatment (vacuum degree: 20 Pa) using a mixed gas of 95% by volume of argon and 5% by volume of hydrogen (flow rate: 2000 sccm) under a high-frequency voltage of 40 kHz (discharge power density: 300 W·min/m ² ).
Copper clad laminates 2 to 7 were obtained in the same manner as copper clad laminate 1, except that multilayer film 1 was changed to multilayer films 2 to 7.

5. Example of Manufacturing a Printed Wiring Board The electrolytic copper foil layer of the copper clad laminate 1 was processed by etching to form a transmission circuit, thereby obtaining a printed wiring board 1. The transmission circuit in the printed wiring board 1 was formed so that 200 patterns each having a width of 100 μm and a length of 100 mm were formed.
Printed wiring boards 2 to 7 were obtained in the same manner as printed wiring board 1, except that copper clad laminate 1 was changed to copper clad laminates 2 to 7.

6. Evaluation of Multilayer Film 6-1. Evaluation of Adhesion A rectangular test piece measuring 100 mm in length and 10 mm in width was cut out from the resulting multilayer film. The base film layer and the buffer layer were then peeled from one end of the test piece in the longitudinal direction to a position 50 mm from one end. Next, the test piece was peeled at a 90-degree angle at a tensile speed of 50 mm/min using a tensile tester (manufactured by Orientec Co., Ltd.), with the position 50 mm from one end in the longitudinal direction as the center. The maximum load when peeled was taken as the peel strength (N/cm) and evaluated according to the following criteria.
○: Peel strength ≧10 N/cm
×: Peel strength < 10 N/cm

6-2. Evaluation of Adhesion After High-Temperature, High-Humidity Treatment A rectangular test piece 100 mm long and 10 mm wide was cut out from the obtained multilayer film and held in an atmosphere of 85°C and 85% relative humidity for 72 hours. After that, the peel strength was measured in the same manner as in 6-1 and evaluated according to the following criteria.
○: Peel strength ≧5 N/cm
△: 3N/cm<peel strength<5N/cm
×: Peel strength ≦ 3 N/cm

6-3. Evaluation of Thermal Expansion Properties The resulting multilayer film was cut into a 12 cm square sample, and the thermal expansion rate was determined by the following method.
At 25°C, a straight line approximately 10 cm long was drawn on the sample, and the distance between the endpoints of the line was taken as the initial length _L0 . The sample was then heat-treated at 150°C for 30 minutes and cooled to 25°C. The linear distance _L1 between the endpoints of the line drawn on the sample was then measured, and the thermal expansion/contraction rate (%) was calculated using the following formula:
Thermal expansion/contraction rate (%) = (L ₁ /L ₀ -1) x 100
The heat stretchability of the multilayer film was evaluated according to the following criteria.
◯: The thermal expansion/contraction rate is −1.5% or more and 0% or less.
△: The thermal expansion/contraction rate is −2% or more and less than −1.5%, or more than 0% and 1% or less.
×: The thermal expansion/contraction rate is less than −2% or more than 1%.

6-4. Evaluation of Electrical Properties A specimen measuring 10 cm in length and 5 cm in width was cut out from the obtained multilayer film, and the dielectric loss tangent (measurement frequency: 10 GHz) of the multilayer film was measured by the SPDR (split post dielectric resonance) method and evaluated according to the following criteria.
Good: The dielectric loss tangent is less than 0.0010.
Δ: The dielectric loss tangent is 0.0010 or more and 0.0025 or less.
×: The dielectric loss tangent is more than 0.0025.

7. Evaluation of Copper Clad Laminate 7-1. Evaluation of Solder Float Resistance The obtained copper clad laminate was cut into 5 cm squares and floated in a solder bath at 288°C for 5 seconds 5 times, and the appearance of the copper clad laminate was then evaluated according to the following criteria.
○: No swelling or peeling was observed.
Δ: No blistering was observed, but some peeling was observed.
×: Blisters and peeling were observed.

7-2. Evaluation of processability The copper clad laminate was irradiated with a UV-YAG laser having a wavelength of 355 nm in a circular pattern having a diameter of 100 μm using a laser processing machine, thereby forming circular through-holes in the copper clad laminate.
The laser output was 1.2 W, the laser focal diameter was 25 μm, the number of revolutions on the circumference was 20, and the oscillation frequency was 40 kHz.
Thereafter, a piece of the copper clad laminate containing the through-hole was cut out and hardened with a thermosetting epoxy resin, and then polished until the cross section of the through-hole was exposed. The cross section of the part where the through-hole was formed was observed under a microscope, and the periphery of the through-hole was visually inspected and evaluated according to the following criteria.
Good: No scraping or peeling was observed at the layer interface inside the through hole.
Δ: Scraping was observed at the layer interface inside the through-hole, but no peeling was observed.
×: Scraping and peeling were observed at the layer interface inside the through-hole.

8. Evaluation of Printed Wiring Boards 8-1. Evaluation of Disconnection Rate of Transmission Circuit Patterns The resistance values at both ends of each transmission circuit pattern of the obtained printed wiring board were measured and evaluated according to the following criteria.
Good: All patterns have a resistance value of 10 Ω or less (break rate: 0%).
Δ: There is one pattern showing a resistance value of more than 10 Ω (break rate: 0.5%).
×: There are two or more patterns showing a resistance value of more than 10Ω (break rate: 1% or more).

The results are summarized in Table 2.

According to the present invention, a multilayer film, a metal clad laminate and a printed wiring board can be obtained which have a base film layer containing a polyimide and buffer layers containing a tetrafluoroethylene-based polymer provided on both sides of the base film layer.
All of these are useful as printed wiring boards used for transmitting high frequency signals, and can also be processed into antenna parts, insulating layers for power semiconductors, aircraft parts, automobile parts, and the like.
The entire contents of the specification, claims and abstract of Japanese Patent Application No. 2020-062112, filed on March 31, 2020, are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims (14)

A multilayer film having a base film layer containing a polyimide having a glass transition temperature of less than 288°C and buffer layers provided on both sides of the base film layer, the buffer layer containing a unit based on perfluoro(alkyl vinyl ether) and comprising a tetrafluoroethylene-based polymer having a melting temperature of 288°C or higher and an aromatic polymer which is a maleimide resin or a polyimide , the content of the tetrafluoroethylene-based polymer in the buffer layer being 50% by mass or more, and the ratio of the total content of the aromatic polymers to the content of the tetrafluoroethylene-based polymer being 0.01 to 0.5. The multilayer film of claim 1, wherein the base film layer and each of the buffer layers are in direct contact. The multilayer film according to claim 1 or 2, wherein the thickness of the multilayer film is 50 μm or more. The multilayer film described in any one of claims 1 to 3, wherein the thickness of the base film layer is 50 μm or less, and the thickness of each of the buffer layers is 5 μm or more. The multilayer film according to any one of claims 1 to 4, wherein the ratio of the total thickness of the buffer layer to the thickness of the base film layer is 0.8 or greater. The multilayer film according to any one of claims 1 to 5, wherein the base film layer has a tensile modulus of elasticity at 320°C of 0.2 GPa or more. The multilayer film according to any one of claims 1 to 6, wherein the tetrafluoroethylene-based polymer is a polymer containing units based on perfluoro(alkyl vinyl ether) and having a polar functional group, or a polymer containing 2.0 to 5.0 mol% of units based on perfluoro(alkyl vinyl ether) relative to all units and having no polar functional group. The multilayer film according to any one of claims 1 to 7, wherein the thermal expansion/contraction rate measured when the multilayer film is heated at 150°C for 30 minutes is -2 to +1%. The method for producing a multilayer film according to any one of claims 1 to 8, wherein each of the buffer layers is formed from a liquid composition containing the tetrafluoroethylene-based polymer powder. A method for producing a multilayer film according to any one of claims 1 to 8, wherein each of the buffer layers is formed from a film containing the tetrafluoroethylene-based polymer. A metal clad laminate having a base film layer containing a polyimide having a glass transition point of less than 288°C, buffer layers provided on both sides of the base film layer, and a metal foil layer provided on the surface of at least one of the buffer layers opposite to the base film layer, wherein the buffer layer contains a unit based on perfluoro(alkyl vinyl ether), and the buffer layer contains a tetrafluoroethylene-based polymer having a melting point of 288°C or higher and an aromatic polymer which is a maleimide resin or a polyimide , the content of the tetrafluoroethylene-based polymer in the buffer layer being 50 mass% or higher, and the ratio of the total content of the aromatic polymers to the content of the tetrafluoroethylene-based polymers being 0.01 to 0.5. The metal clad laminate according to claim 11, which is a printed wiring board material. The metal clad laminate according to claim 11 or 12, having a soldering heat resistance temperature of 288°C or higher. A method for manufacturing a printed wiring board, comprising etching the metal foil layer of the metal clad laminate described in any one of claims 11 to 13 to form a transmission circuit, thereby obtaining a printed wiring board.