CN115397672A

CN115397672A - Multilayer film, method for producing same, metal-clad laminate, and method for producing printed wiring board

Info

Publication number: CN115397672A
Application number: CN202180026291.XA
Authority: CN
Inventors: 结城创太; 笠井涉; 光永敦美
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2020-03-31
Filing date: 2021-03-26
Publication date: 2022-11-25
Also published as: TW202206290A; WO2021200630A1; JPWO2021200630A1; KR20220162688A

Abstract

Provided are a multilayer film suitable for high-temperature processing such as processing for producing a printed wiring board, a method for producing the multilayer film, a metal-clad laminate obtained from the multilayer film, and a method for producing a printed wiring board by processing the multilayer film. The multilayer film and the copper-clad laminate each have a base film layer containing a polyimide having a glass transition temperature of less than 288 ℃ and buffer layers provided on both surfaces of the base film layer, and the buffer layers are buffer layers containing a tetrafluoroethylene polymer containing a perfluoro (alkyl vinyl ether) -based unit and having a melting temperature of 288 ℃ or higher.

Description

Multilayer film, method for producing same, metal-clad laminate, and method for producing printed wiring board

Technical Field

The present invention relates to a multilayer film having a predetermined base film layer and predetermined buffer layers provided on both surfaces of the base film layer, a method for producing the multilayer film, a metal-clad laminate, and a method for producing a printed wiring board.

Background

A multilayer film having a polyimide as a base film layer and having layers containing a tetrafluoroethylene polymer on both surfaces thereof is known to be useful as a material for electronic members such as a cover sheet, a flexible flat cable, and a printed wiring board. For such a multilayer film, it has been proposed to use a polyimide having a low water absorption rate and a low glass transition temperature and a tetrafluoroethylene polymer having a low melting temperature (patent document 1).

Documents of the prior art

Patent literature

Patent document 1: japanese patent laid-open No. 2015-176921

Disclosure of Invention

Technical problem to be solved by the invention

Although such a multilayer film is excellent in moisture resistance and interlayer adhesion, it has a problem in high-temperature heat resistance. Therefore, when such a multilayer film is subjected to a processing process exposed to high temperatures, for example, when the multilayer film and a metal foil are subjected to high-temperature pressure bonding processing to form a metal-clad laminate, or when a transmission circuit is further formed on the metal foil of the metal-clad laminate and a soldering process at 288 ℃.

The present inventors have found that when a layer containing a tetrafluoroethylene polymer having a melting temperature equal to or higher than a predetermined temperature is disposed on both surfaces of a layer containing a polyimide having a glass transition temperature lower than the predetermined temperature, a multilayer film having heat resistance at higher temperatures and less susceptible to expansion and contraction by heating can be obtained. The multilayer film is suitable for high-temperature processing such as processing for forming a printed wiring board.

The present invention aims to provide a multilayer film having the above physical properties, a method for producing the multilayer film, a metal-clad laminate obtained therefrom, and a method for producing a printed wiring board processed from the metal-clad laminate.

Technical scheme for solving technical problem

The present invention has the following technical contents.

< 1 > a multilayer film having a base film layer comprising a polyimide having a glass transition temperature of less than 288 ℃ and buffer layers disposed on both faces of the base film layer, the buffer layers being buffer layers comprising a tetrafluoroethylene-based polymer having a melting temperature of 288 ℃ or higher containing a perfluoro (alkyl vinyl ether) -based unit.

< 2 > the multilayer film as < 1 > wherein the base film layer is in direct contact with each of the buffer layers.

The multilayer film is < 3 > such as < 1 > or < 2 >, wherein the thickness of the multilayer film is more than 50 μm.

The multilayer film of any one of < 4 > to < 1 > -3 >, wherein the thickness of the base film layer is 50 μm or less, and the thickness of each buffer layer is 5 μm or more.

The multilayer film of any one of < 5 > to < 1 > - < 4 >, wherein a ratio of the total thickness of the buffer layer to the thickness of the base film layer is 0.8 or more.

The multilayer film described in any of < 6 > such as < 1 > to < 5 >, wherein the base film layer has a tensile elastic modulus of 0.2GPa or more at 320 ℃.

The multilayer film described in any one of < 7 > to < 6 >, wherein the tetrafluoroethylene-based polymer is a polymer having a polar functional group containing a perfluoro (alkyl vinyl ether) -based unit, or a polymer having no polar functional group containing 2.0 to 5.0 mol% of the perfluoro (alkyl vinyl ether) -based unit with respect to the whole units.

The multilayer film of any one of < 8 > such as < 1 > to < 7 >, wherein the buffer layer further comprises an aromatic polymer.

The multilayer film of any one of < 9 > to < 1 > < 8 >, wherein the multilayer film has a stretchability measured after heating at 150 ℃ for 30 minutes of-2 to 1%.

< 10 > a 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 > A method for producing a multilayer film, which is any one of < 1 > -to < 9 >, wherein each of the buffer layers is formed from a film comprising the tetrafluoroethylene-based polymer.

< 12 > a metal-clad laminate having a base film layer comprising a polyimide having a glass transition temperature of less than 288 ℃, buffer layers provided on both faces of the base film layer, and a metal foil layer provided on the face of at least one of the buffer layers opposite to the base film layer, wherein the buffer layer is a buffer layer comprising a tetrafluoroethylene-based polymer containing a perfluoro (alkyl vinyl ether) -based unit and having a melting point of 288 ℃ or higher.

< 13 > the metal-clad laminate as < 12 > which is a material for a printed wiring board.

The metal-clad laminate is < 14 > such as < 12 > or < 13 >, wherein the solder heat-resistant temperature is 288 ℃ or higher.

< 15 > a method for manufacturing a printed wiring board, comprising processing the metal foil layer of the metal-clad laminate of any one of < 12 > to < 14 > by etching to form a transmission circuit, thereby obtaining a printed wiring board.

Effects of the invention

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

Detailed Description

The following terms have the following meanings.

The "average particle diameter (D50)" is a cumulative 50% diameter on a volume basis of the object (powder and filler) determined by a laser diffraction scattering method. That is, the particle size distribution of the object is measured by a laser diffraction scattering method, and a cumulative curve is obtained with the total volume of the particle group of the object as 100%, and the particle size at a point where the cumulative volume reaches 50% on the cumulative curve.

"D90" is a particle diameter of 90% cumulative volume basis of the object measured in the same manner.

The "melting temperature (melting point)" means a temperature corresponding to the maximum value of the melting peak of the polymer measured by Differential Scanning Calorimetry (DSC).

"glass transition temperature (Tg)" is a value measured by analyzing a polymer by a dynamic viscoelasticity measurement (DMA) method.

The "viscosity" is a value obtained by measuring a liquid composition at 25 ℃ and 30rpm using a type B viscometer. The measurement was repeated 3 times, and the average of the 3 measurements was taken.

The "thixotropic ratio" is a value obtained by dividing the viscosity obtained by measuring the liquid composition at a rotation speed of 30rpm by the viscosity obtained by measuring the liquid composition at a rotation speed of 60 rpm.

The "tensile elastic modulus" is a value measured by analyzing a film at a measurement frequency of 10Hz using a wide-area viscoelasticity measuring apparatus.

The "unit" in the polymer may be a radical formed directly by polymerizing 1 molecule of a monomer, or may be a radical formed by converting a part of the structure of a radical formed from 1 monomer molecule by treating the obtained polymer by a predetermined method. The unit based on the monomer a contained in the polymer is also referred to simply as "monomer a unit".

The "ten-point average roughness" is a value specified by measuring a film in accordance with annex JA of JIS B0601.

The multilayer film of the present invention has a base film layer and buffer layers disposed on both sides of the base film layer. The base film layer contains polyimide (hereinafter also referred to as "PI") having a glass transition temperature of less than 288 ℃, and the buffer layer contains a tetrafluoroethylene polymer (hereinafter also referred to as "F polymer") containing a perfluoro (alkyl vinyl ether) -based unit (hereinafter also referred to as "PAVE unit") having a melting temperature of 288 ℃ or higher.

The multilayer film of the present invention has excellent interlayer adhesiveness and moisture resistance, and also has high-temperature heat resistance and low heat stretchability. The reason for this is not clear, but the following is considered.

A PI having a low glass transition temperature is in a state of being easily softened at a high temperature, although it is excellent in moisture resistance and flexibility. In general, tetrafluoroethylene polymers have a large linear expansion coefficient and therefore tend to shrink when cooled from a high-temperature state. Therefore, it is considered that a multilayer film in which layers including such a polymer are combined deforms the layer including PI when exposed to high temperature, and shrinks the layer including the tetrafluoroethylene-based polymer when exposed to high temperature and then cooled, and wrinkles and swelling are likely to occur. However, the present inventors have found that a layer containing a tetrafluoroethylene-based polymer (F polymer) containing a PAVE unit functions as a thermal buffer layer due to its heat resistance and suppresses deformation of the PI-containing layer, and shrinkage thereof is buffered by its melt fluidity.

As a result, the multilayer film of the present invention is considered to have high-temperature heat resistance and low thermal expansion and contraction properties in addition to excellent interlayer adhesiveness and moisture resistance, and thus can be advantageously used as a material for applications requiring a high-temperature process such as processing of printed wiring boards.

The F polymer of the present invention is a polymer containing Tetrafluoroethylene (TFE) -based units (TFE units) and PAVE units.

The F polymer may also contain units based on other comonomers.

As PAVE, CF is preferred ₂ ＝CFOCF ₃ 、CF ₂ ＝CFOCF ₂ CF ₃ And CF ₂ ＝CFOCF ₂ CF ₂ CF ₃ (PPVE), more preferably PPVE.

The melting temperature of the F polymer is 288 ℃ or higher, preferably 300 ℃ or higher. The melting temperature of the F polymer is preferably 350 ℃ or lower, more preferably 320 ℃ or lower. In this case, the multilayer film is likely to have more excellent heat resistance and is less likely to expand and contract by heating.

The glass transition temperature of the F polymer is preferably from 75 to 125 ℃ and more preferably from 80 to 100 ℃.

The F polymer preferably has a polar functional group (oxygen-containing polar group). The polar functional group may be included in the unit of the F polymer, and may also be included in the terminal group of the main chain of the F polymer. The latter form includes an F polymer having a polar functional group as an end group derived from a polymerization initiator, a chain transfer agent, or the like, and an F polymer having a polar functional group obtained by subjecting the F polymer to plasma treatment or ionizing radiation treatment.

The polar functional group is preferably a hydroxyl-containing group or a carbonyl-containing group, and more preferably a carbonyl-containing group from the viewpoint of improving adhesion to the buffer layer.

As the hydroxyl group-containing group, preferred is an alcoholic hydroxyl group, more preferred is-CF ₂ CH ₂ OH and-C (CF) ₃ ) ₂ OH。

The carbonyl-containing group is a group containing a carbonyl group (> C (O)), and the carbonyl-containing group is preferably a carboxyl group, an alkoxycarbonyl group, an amide group, an isocyanate group, or a carbamate group (-OC (O) NH) ₂ ) Acid anhydride residue (-C (O) OC (O) -) imide residues (-C (O) NHC (O) -, etc.) and carbonate groups (-OC (O) O-), anhydride residues are more preferred.

In the case where the F polymer has a carbonyl group-containing group, the number of carbonyl groups-containing groups in the F polymer is preferably 1X 10 per unit ⁶ The number of carbon atoms in the main chain is 100 to 5000, more preferably 300 to 3000, and still more preferably 800 to 1500. The number of carbonyl groups in the F polymer can be determined by the composition of the polymer or by the method described in International publication No. 2020/145133.

The F polymer is preferably a polymer (1) having a polar functional group, which further contains units based on a monomer having a polar functional group, and a polymer (2) having no polar functional group, which contains 2.0 to 5.0 mol% of PAVE units based on the whole units, and is more preferably a polymer (1).

These F polymers readily form spherulites. The cushion layer containing such an F polymer is likely to have excellent surface smoothness, and the cushion layer is likely to adhere to the polymer base layer, thereby easily achieving excellent peel strength and water resistance of the multilayer film.

The polymer (1) preferably contains, relative to the total 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 based on a monomer having a polar functional group, respectively.

Further, as the monomer having a polar functional group, itaconic anhydride, citraconic anhydride and 5-norbornene-2, 3-dicarboxylic anhydride (alias: nadic anhydride; hereinafter also referred to as "NAH") are preferable.

Specific examples of the polymer (1) include polymers described in International publication No. 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 whole units.

The content of PAVE units in the polymer (2) is preferably from 2.1 to 5.0 mol%, more preferably from 2.2 to 5.0 mol%, based on the total units.

The fact that the polymer (2) has no polar functional group means that the number of carbon atoms constituting the main chain of the polymer is 1X 10 ⁶ And the number of the polar functional groups of the polymer is less than 500. The number of the polar functional groups is preferably 100 or less, and more preferably less than 50. The lower limit of the number of the above polar functional groups is usually 0.

The polymer (2) may be produced by using a polymerization initiator or a chain transfer agent which does not generate a polar functional group as a terminal group of a polymer chain, or may be produced by subjecting an F polymer having a polar functional group (e.g., an F polymer having a polar functional group derived from a polymerization initiator in a terminal group of a polymer main chain) to a fluorination treatment. As a method of fluorination treatment, a method using fluorine gas is exemplified (see Japanese patent laid-open publication No. 2019-194314).

The content of the F polymer in the buffer layer of the present invention is preferably 50% by mass or more, and more preferably 60% by mass or more. The upper limit of the content of the F polymer is 100 mass%.

The buffer layer may further contain other resins (polymers). The other resin may be a thermosetting resin or a thermoplastic resin.

The other resin is preferably an aromatic polymer. In this case, the buffer layer is excellent in UV absorption, and the multilayer film is easily excellent in UV processability.

Examples of the other resin include epoxy resins, maleimide resins, polyurethane resins, polyimides, polyamide acids, polyamide imides, polyphenylene ethers, liquid crystal polyesters, and fluorine-containing polymers other than F polymers.

As the other resin, maleimide resin, polyimide, and polyamic acid are preferable. When at least one of these resins is contained, the multilayer film tends to be excellent in flexibility and peel strength. As the other resin, maleimide resins, polyimides, and polyamic acids, all of which are aromatic, are more preferable.

The polyimide is preferably thermoplastic.

In this case, the total content of the maleimide, polyimide and polyamic acid in the buffer layer is preferably 0.1 to 30% by mass, more preferably 1 to 10% by mass. The ratio of the total content of maleimide, polyimide and polyamic acid to the content of F polymer is preferably 1.0 or less, more preferably 0.01 to 0.5.

The other resin is preferably a fluoropolymer other than the F polymer, and more preferably non-heat-fusible PTFE. In this case, the physical properties (electrical properties such as low dielectric loss tangent) of PTFE tend to be remarkably exhibited in the multilayer film.

In this case, the content of the non-heat-fusible PTFE is preferably 1 to 30% by mass, and more preferably 5 to 20% by mass. The ratio (mass ratio) of the content of the non-heat-fusible PTFE to the content of the F polymer is preferably 1.0 or less, more preferably 0.1 to 0.4.

Further, from the viewpoint of further improving the low linear expansion property and the electrical characteristics of the multilayer film, the buffer layer preferably further contains an inorganic filler.

As the inorganic filler, a nitride filler and an inorganic oxide filler are preferable, a boron nitride filler, a beryllium oxide filler (beryllium oxide filler), a silicate filler (silica filler, wollastonite filler, talc filler) and a metal oxide (cerium oxide, aluminum oxide, magnesium oxide, zinc oxide, titanium oxide, etc.) filler are more preferable, and a silica filler is further preferable.

The content of silica in the inorganic filler is preferably 50% by mass or more, more preferably 75% by mass or more. The upper limit of the content of silica is 100 mass%.

It is preferable to surface-treat at least a part of the surface of the inorganic filler. Examples of the surface treatment agent used for the surface treatment include polyhydric alcohols (trimethylolethane, pentaerythritol, propylene glycol, and the like), saturated fatty acids (stearic acid, lauric acid, and the like), esters thereof, alkanolamines, amines (trimethylamine, triethylamine, and the like), paraffins, silane coupling agents, silicones, and polysiloxanes.

As the silane coupling agent, 3-aminopropyltriethoxysilane, vinyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane and 3-isocyanatopropyltriethoxysilane are preferable.

The average particle diameter of the inorganic filler is preferably 20 μm or less, more preferably 10 μm or less. The average particle diameter is preferably 0.1 μm or more, more preferably 0.1 μm or more.

The inorganic filler may be in any of a granular form, a needle-like form (fibrous form), and a plate-like form. Specific examples of the inorganic filler include spherical, scaly, lamellar, foliate, almond-shaped, columnar, chicken-crown-shaped, equiaxial, foliate, mica-shaped, massive, flat, wedge-shaped, rosette-shaped, mesh-shaped, and rectangular columnar shapes.

The inorganic fillers can be obtained by the methods described in the general formulae "A.mokayama (Tokyo 1245089, 1245094, 12473), the" A.124 (manufactured by SPK 1245 K.K.) The "electrochemical boron nitride (12587\\\12559\\1241250812590\\ 124521248812521\\\ 12489;" HGP "grade), etc.).

The content of the inorganic filler in the cushion layer is preferably 1% by mass or more, and more preferably 3% by mass or more. The content of the inorganic filler is preferably 40% by mass or less, more preferably 30% by mass or less, and further preferably 20% by mass or less.

The ratio (mass ratio) of the content of the inorganic filler to the content of the F polymer in the buffer layer is preferably 0.01 or more, and more preferably 0.1 or more. The above ratio is preferably 1 or less, more preferably 0.8 or less.

The buffer layer may contain additives such as a silane coupling agent, a dehydrating agent, a plasticizer, a weather resistant agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a whitening agent, a colorant, a conductive agent, a mold release agent, a surface treatment agent, a flame retardant, and the like, in addition to the above components.

The glass transition temperature of PI in the present invention is less than 288 deg.C, more preferably less than 275 deg.C, and still more preferably 260 deg.C or less. The glass transition temperature of PI is preferably above 200 ℃.

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 p-phenylenediamine, 4' -diaminodiphenyl ether, 3,4' -diaminodiphenyl ether, 1, 4-bis (4-aminophenoxy) benzene, 1, 3-bis (3-aminophenoxy) benzene, 4' -bis (aminophenoxy) biphenyl, 4' -bis (3-aminophenoxy) biphenyl, and 2,2' -bis (4-aminophenoxyphenyl) propane. The aromatic diamine component may be used in 2 or more kinds.

Examples of the tetracarboxylic acid component include pyromellitic acid, 3', 4' -biphenyltetracarboxylic acid, oxydiphthalic acid, 2,3',3,4' -biphenyltetracarboxylic acid, and 3,3', 4' -benzophenonetetracarboxylic acid, and pyromellitic acid, 3', 4' -biphenyltetracarboxylic acid, and oxydiphthalic acid, and dianhydrides of these tetracarboxylic acids can be cited. The tetracarboxylic acid component may be used in 2 or more kinds.

The base film layer in the present invention includes a method in which a solution containing polyamic acid obtained by polymerizing the aromatic diamine and the tetracarboxylic acid component is cast into a film form and heated to be subjected to ring-opening and solvent removal, and a method in which a ring-opening catalyst and a dehydrating agent are mixed in a polyamic acid solution to be subjected to chemical ring-opening to prepare a gel film, and the gel film is heated and solvent removed. The solution containing polyamic acid may further contain a cyclization catalyst (imidization catalyst), a dehydrating agent, a gelation retarder, and the like.

Specific examples of the base film layer include "Kapton 50EN-S" (manufactured by imperial du pont corporation (imperial bio-war 125241248712517125171250912531), kapton 100EN "(manufactured by tokyo dupont corporation)," Kapton 100H "(manufactured by tokyo dupont corporation)," Kapton 100KJ "(manufactured by tokyo 125171250931corporation), and" Kapton 100JP "(manufactured by tokyo 12487125125125171251731and.

The base film layer may further contain a plasticizer or other resin, a colorant, various additives, and the like. Examples of the additives include antistatic agents, flame retardants, heat stabilizers, ultraviolet absorbers, lubricants, mold release agents, crystallization nucleating agents, and reinforcing agents (fillers).

The tensile modulus of elasticity of the base film layer at 320 ℃ is preferably 0.2GPa or more, and more preferably 0.4GPa or more. The tensile modulus is preferably 10GPa or less, more preferably 5GPa or less.

In this case, even when heating and cooling are performed during processing of the multilayer film, the film is not easily deformed, and the workability is easily improved. That is, when the tensile elastic modulus of the base film layer is not less than the lower limit, deformation of the cushion layer is effectively relaxed by the elasticity of the base film layer during heating and cooling in processing, and the multilayer film is less likely to wrinkle, and the physical properties (surface smoothness, etc.) of the obtained processed product are easily improved. Even when the buffer layer is easily deformed due to a large content of the F polymer in the buffer layer or a large thickness of the buffer layer, a processed product having excellent physical properties can be easily obtained. When the tensile elastic modulus of the base film layer is not more than the above upper limit, the flexibility of the multilayer film is likely to be further improved.

The imide group density of PI contained in the base film layer is preferably 0.20 to 0.35. When the imide group density is not more than the above upper limit, the water absorption of the base film layer becomes lower, and it becomes easier to suppress a change in dielectric characteristics of the multilayer film. If the imide group density is not less than the lower limit, the imide group functions as a polar group, and not only the adhesion between the base film layer and the cushion layer is further improved, but also the water absorption rate is easily significantly reduced.

When the imide group density is in this range, the multilayer film is easily processed and wrinkles are less likely to occur.

The imide group density is a value obtained by dividing the molecular weight (140.1) per imide group unit portion in a polyimide obtained by imidizing a polyimide precursor by the molecular weight per polyimide unit. For example, the imide group density of a polyimide (molecular weight per unit: 382.2) obtained by imidizing a polyimide precursor comprising 1 mole of pyromellitic dianhydride (molecular weight: 218.1) and 1 mole of 3,4' -oxydianiline (molecular weight: 200.2) is a value of 140.1 divided by 382.2, which is 0.37.

In the multilayer film of the present invention, the base film layer and each buffer layer are preferably in direct contact. In this case, the multilayer film is likely to have excellent heat resistance and water resistance. Further, the occurrence of warpage of the multilayer film is easily suppressed.

The thickness of the buffer layer in the multilayer film is more preferably 5 μm or more, and still more preferably 10 μm or more. The thickness of the buffer layer is preferably 200 μm or less, 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. The thickness of the buffer layer is preferably 100 μm or less, more preferably 50 μm or less. In this case, the multilayer film is likely to have excellent heat resistance and low linear expansibility.

The ratio of the total thickness of the buffer layers to the thickness of the base film layer is preferably 0.8 or more, and more preferably 1.0 or more. The total thickness ratio 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 this case, the multilayer film is likely to have excellent heat resistance.

The thickness of the multilayer film is preferably 50 μm or more, more preferably 75 μm or more. The thickness of the multilayer film is preferably 300 μm or less, more preferably 150 μm or less.

In this case, the multilayer film is likely to have excellent heat resistance and low linear expansibility.

The multilayer film has a stretchability upon heating of preferably-2 to +1%, more preferably-1.5 to 0%. When processed into a printed wiring board, the multilayer film is less likely to cause wrinkles, and can maintain a high yield. Further, since such a multilayer film is excellent in thermal shock resistance, when processed into a printed wiring board, the printed wiring board is high in thermal shock resistance when forming a through hole or a through hole, and as a result, a printed wiring board in which disconnection is not easily generated is easily obtained.

The expansion/contraction ratio under heating can be determined by the following method.

Drawing 1 straight line with length of about 10cm on the 12cm square multilayer film at 25 deg.C, and setting the distance between the ends of the straight line as initial length L ₀ . Thereafter, the multilayer film was heat-treated at 150 ℃ for 30 minutes, cooled to 25 ℃ and then measured for the linear distance L between the end points of the line drawn on the multilayer film ₁ The thermal expansion/contraction ratio (%) was calculated according to the following formula.

Percent thermal expansion (%) = (L) ₁ /L ₀ -1)×100

The surface of the multilayer film may be surface-treated with a silane coupling agent or the like, or may be surface-modified by corona treatment, plasma treatment or the like. Further, the surface of the multilayer film may be roughened or annealed.

In particular, when the surface of the multilayer film is plasma-treated, the multilayer film is easily laminated with other materials. As the plasma treatment, atmospheric pressure plasma treatment and vacuum plasma treatment are preferable.

The method for producing the multilayer film of the present invention includes a method of forming each buffer layer from a liquid composition containing a powder of the F polymer (i.e., an assembly of particles of the F polymer) (hereinafter, also referred to as "present method 1"), and a method of forming each buffer layer from a film containing the F polymer (hereinafter, also referred to as "F film") (hereinafter, also referred to as "present method 2").

In method 1, the definitions and ranges, including preferred forms, of PI, base film layer, F polymer, buffer layer, and multilayer film are the same as those of 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 the present method 1 is preferably 80% by mass or more, more preferably 100% by mass.

Examples of other components that may 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 polyesters, polyamide-imides, thermoplastic polyimides, polyphenylene oxides, and polyphenylene ethers.

Examples of the inorganic filler include silica (silicon dioxide), metal oxides (beryllium oxide, cerium oxide, aluminum oxide, basic aluminum oxide, magnesium oxide, zinc oxide, titanium oxide, and the like), boron nitride, and magnesium metasilicate (talc). At least a part of the surface of the inorganic filler may be surface-treated.

The particles of the F powder containing the resin other than the F polymer or the inorganic filler may have a core-shell structure in which the F polymer is a core and the resin other than the F polymer or the inorganic filler is a shell, or a core-shell structure in which the F polymer is a shell and the resin other than the F polymer or the inorganic filler is a core. The particles of the F powder can be obtained, for example, by bonding (impacting, aggregating, etc.) the particles of the F polymer with particles of a resin other than the F polymer or an 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.

In the liquid composition of the present method 1, the content of the F powder is preferably 5 to 40% by mass. In this case, the dense buffer layer is more easily formed by the densely contained F powder, and the physical properties of the F polymer in the buffer layer are more easily expressed.

The liquid composition in the present method 1 preferably contains a liquid dispersion medium. The liquid dispersion medium functions as a dispersion medium for the F powder, and is a liquid compound inert at 25 ℃.

The boiling point of the liquid dispersion medium is preferably from 125 to 250 ℃.

As the liquid dispersion medium, 1 or more liquid compounds selected from water, amides, ketones, and esters are preferable, and N-methyl-2-pyrrolidone (NMP), γ -butyrolactone, cyclohexanone, and cyclopentanone are more preferable.

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 mPas or less, and the thixotropic ratio of the liquid composition is preferably 1.0 to 2.0 or less.

From the viewpoint of improving dispersion stability and workability, the liquid composition may further contain an acetylene-based surfactant, a silicone-based surfactant, a fluorine-based surfactant, and the like.

From the viewpoint of further improving the low linear expansion property and the electrical characteristics of the buffer layer, the liquid composition preferably further contains an inorganic filler. The definition and ranges of the inorganic filler, including preferred morphologies, are the same as the definition and ranges 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 morphologies, are the same as the definitions and ranges of the other resins in the multilayer film.

The additives such as the surfactant and the inorganic filler may be added to the liquid composition containing the F powder, or may be mixed with the F powder in advance to prepare a mixture, and the mixture may be dispersed in a liquid dispersion medium. Further, the surfactant, the inorganic filler, or the like may be dispersed or dissolved in a liquid dispersion medium in advance, and then mixed with the F powder-containing liquid composition or the F powder.

Specific examples of the method 1 include: that is, the liquid composition is applied to one surface of the base film layer and heated to remove the liquid dispersion medium, the liquid composition is applied to the other surface of the base film layer and heated to remove the liquid dispersion medium, and the polymer F is fired by heating to form each buffer layer, thereby obtaining a multilayer film.

The temperature of the former heating is preferably 120 to 200 ℃. The temperature of the latter heating is preferably a temperature higher than the melting temperature of the F polymer, more preferably 80 to 400 ℃, and still more preferably 300 to 380 ℃.

The latter heating time 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 forced air drying oven, and a method of irradiating heat rays such as infrared rays.

The atmosphere during heating may be either normal pressure or reduced pressure. The atmosphere may be any of an oxidizing gas (oxygen, etc.) atmosphere, a reducing gas (hydrogen, etc.) atmosphere, and an inert gas (rare gas, nitrogen) atmosphere.

The application of the liquid composition to the basement membrane layer can be carried out by spraying, roll coating, spin coating, gravure coating, microgravure coating, gravure offset coating, knife coating, kiss coating (12461124731241254088), bar coating, die coating, jet meyer rod coating (12501124491251251251251251251251251251251251251251251251253), slit die coating, etc..

In method 2, the definitions and ranges of PI, the base film layer, the F polymer, the buffer layer, and the multilayer film, including preferred forms, are the same as those of PI, the base film layer, the F polymer, the buffer layer, and the multilayer film.

The thickness of the F film in this method 2 is preferably 5 μm or more, more preferably 10 μm or more. The thickness of the F film 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. The upper limit of the content of the F polymer is 100 mass%.

The F film may further contain other resins (polymers), inorganic fillers, and additives. The definitions and ranges, including preferred morphologies, of the other resins, inorganic fillers, and additives are the same as the definitions and ranges for the other resins, inorganic fillers, and additives in the multilayer film.

As a specific embodiment of the method 2, a multilayer film can be obtained by laminating 2F films with a base film layer interposed therebetween by hot pressing.

The pressing temperature at the time of hot pressing is preferably 310 to 400 ℃.

From the viewpoint of suppressing the mixing of air bubbles, the hot pressing is preferably performed under a vacuum degree of 20kPa or less.

In the hot pressing, it is preferable to raise the temperature after the degree of vacuum is reached. If the temperature is raised before the degree of vacuum is reached, the cushion layer may be softened, that is, pressure-bonded in a state having a certain degree of fluidity and adhesion, and bubbles may be generated.

The pressure for hot pressing is preferably 0.2 to 10MPa from the viewpoint of firmly bonding the buffer layer and the base film layer.

In particular, if the tensile elastic modulus of the base film is not less than the lower limit, the occurrence of wrinkles due to heating and cooling during hot pressing can be easily suppressed.

The metal-clad laminate of the present invention has a base film layer containing PI, buffer layers containing F polymer provided on both surfaces of the base film layer, and a metal foil layer provided on a surface (a surface on the opposite side from the base film layer) of at least one buffer layer.

Examples of the metal constituting the metal foil layer include copper, copper alloy, stainless steel, nickel alloy (including 42 alloy), aluminum alloy, titanium, and titanium alloy.

The metal foil forming the metal foil layer is preferably a copper foil, more preferably a rolled copper foil having no difference in surface and back and an electrolytic copper foil having a difference in surface and back, and further preferably a rolled copper foil. Since the rolled copper foil has a small surface roughness, the transmission loss can be reduced even when the metal-clad laminate is processed into a printed wiring board. The rolled copper foil is preferably used after being immersed 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 this case, the adhesion with the buffer layer is good, and a printed wiring board having excellent transmission characteristics can be easily obtained.

The surface of the metal foil layer may be roughened. The roughening treatment may be performed by a method of forming a roughening treatment layer, a dry etching method, or a wet etching method.

The thickness of the metal foil layer is preferably less than 20 μm, more preferably 2 to 15 μm.

Further, a part or the whole of the surface of the metal foil layer may be treated with a silane coupling agent.

As a method for producing the metal-clad laminate, there is a method in which a multilayer film and a metal foil are laminated so that a buffer layer of the multilayer film is in contact with the metal foil, and the multilayer film and the metal foil are hot-pressed and bonded. Since the multilayer film has excellent high-temperature heat resistance, warping and wrinkles are less likely to occur when the multilayer film is bonded to a metal foil.

In this case, the surface of the F film may be subjected to a surface treatment in view of enhancing the low linear expansibility and the 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 reflow soldering resistance, hole-forming processability, chemical resistance, and surface smoothness. Therefore, the metal-clad laminate of the present invention is suitable as a material for a printed wiring board, and can be easily and efficiently processed into a flexible printed wiring board or a rigid printed wiring board.

When the metal foil layer of the metal-clad laminate of the present invention is etched to form a transmission circuit, a printed wiring board can be obtained. That is, the method for manufacturing a printed wiring board according to the present invention is a method for obtaining a printed wiring board by processing a metal foil of a metal-clad laminate by etching to form a transmission circuit. In addition, any of a wet etching method and a dry etching method can be used for etching.

The metal-clad laminate of the present invention includes the multilayer film of the present invention, and is not easily deformed and warped even when etched.

In the production of the printed wiring board, after the transfer circuit is formed, an interlayer insulating film may be formed on the transfer circuit, and then a conductor circuit may be formed on the interlayer insulating film. The interlayer insulating film may be formed of the liquid composition.

The solder heat-resistant temperature of the metal-clad laminate of the present invention is preferably 288 ℃ or higher, more preferably 300 ℃ or higher, and still more preferably 320 ℃ or higher. The solder heat-resistant temperature is preferably 380 ℃ or lower. The metal-clad laminate of the present invention includes the multilayer film of the present invention, and the above mechanism of action results in excellent high-temperature heat resistance, and therefore, the solder heat resistance temperature in this range is easily exhibited.

When the metal-clad laminate of the present invention is used, the occurrence of expansion, warpage, and peeling of the multilayer film as the electrical insulating layer in the reflow soldering step (step of placing solder paste on the printed wiring board material and heating) which is a high-temperature process in the mounting step of the printed wiring board production can be suppressed, and the printed wiring board can be produced with good production efficiency.

The solder heat resistance temperature was measured by the following method.

The metal foil layer of the metal-clad laminate having a square width of 4cm was patterned into a square width of 2.5cm, and the laminate was suspended in a solder bath at an initial temperature of 250 ℃ to observe the presence or absence of an abnormality (a defect in appearance such as swelling, bubbling, or layer separation) in the appearance.

In the case where there is no abnormality in appearance, the temperature of the solder bath is raised, and the presence or absence of abnormality in appearance is observed in the same manner. The temperature rise and observation of the solder bath were repeated, and the highest temperature at which no abnormality occurred was taken as the solder heat-resistant temperature.

The printed wiring board according to the present invention is less likely to cause expansion at the interface between the base film layer and the buffer layer and warpage of the printed wiring board even when subjected to a reflow soldering step (a step of placing and heating solder paste on a printed wiring board material) which is a high-temperature process in a mounting step for manufacturing the printed wiring board.

Examples

1. Preparation of the Components and Components

[ F Polymer ]

F Polymer 1: PFA-based polymer comprising TFE units, NAH units and PPVE units in the stated order at 98.0 mol%, 0.1 mol% and 1.9 mol% (melting temperature: 300 ℃ C.)

Polymer 2: PFA-based Polymer comprising, in order, 97.5 mol% and 2.5 mol% of TFE Unit and PPVE Unit (melting temperature 305 ℃ C.)

non-F polymer 1: FEP-based Polymer comprising, in this order, 40.8 mol%, 44.8 mol%, 13.9 mol%, 0.5 mol% of TFE units, ethylene units, hexafluoropropylene units and perfluoro (1, 5-trihydro-1-pentene) units (melting temperature: 162 ℃ C.)

In addition, F was added to the polymer 1 at 1X 10 intervals ⁶ The number of carbons in the main chain was 1000, and the number of the F polymer 2 was 40.

[ powder ]

Powder 1: powder of F Polymer 1 with a D50 of 2.1. Mu.m

Powder 2: powder of F Polymer 2 with a D50 of 1.8 μm

Powder 3: powder of non-F Polymer 1 with a D50 of 2.2 μm

[ surfactant ]

Surfactant 1: CH (CH) ₂ ＝C(CH ₃ )C(O)OCH ₂ CH ₂ (CF ₂ ) ₆ F and CH ₂ ＝C(CH ₃ )C(O)(OCH ₂ CH ₂ ) ₂₃ OH copolymer having a fluorine content of 35% by mass

[ inorganic Filler ]

Packing 1: silica filler with D50 of 0.4 μm

[ varnish ]

Varnish 1: varnish (solid content concentration: 18% by mass) obtained by dissolving thermoplastic aromatic polyimide (PI 1) in NMP

[ liquid Dispersion Medium ]

NMP: n-methyl-2-pyrrolidone

[ base film ]

Polyimide film 1: an aromatic polyimide film having a thickness of 50 μm and a tensile elastic modulus of 0.3GPa at a glass transition temperature of 245 ℃ and 320 DEG C

Polyimide film 2: an aromatic polyimide film having a thickness of 50 μm, a glass transition temperature of 330 ℃ and a tensile elastic modulus of 0.2GPa at 320 DEG

[ copper foil ]

Copper foil 1: electrolytic copper foil with thickness of 18 μm

2. Production example of Dispersion (liquid composition)

First, powder 1, varnish 1, surfactant 1 and NMP were put into a pot, and then zirconia balls were put into the pot. Then, the pot was rolled at 150rpm for 1 hour to prepare a composition.

The filler 1, the surfactant 1 and water were put into the other tank, and then the zirconia balls were put into the tank. Then, the pot was rolled at 150rpm for 1 hour to prepare a composition.

Two compositions were placed in a separate tank, and zirconia balls were placed therein. Thereafter, the pot was rolled at 150rpm for 1 hour to obtain a dispersion 1 (viscosity: 400 mPas) containing the powder 1 (14 parts by mass), the filler 1 (7 parts by mass), PI1 (1 part by mass), the 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 kinds and amounts of the powder, the inorganic filler and the aromatic polyimide (PI 1) were changed as shown in table 1 below.

[ Table 1]

Number of dispersion	1	2	3	4	5
						Powder of	Powder 1 (14)	Powder 2 (14)	Powder 1 (14)	Powder 1 (14)	Powder 3 (14)
Aromatic polyimide	P11(1)	PI1(1)	-	-	PI1(1)
						Inorganic filler	Filler 1 (7)	Filler 1 (7)	Filler 1 (7)	-	Filler 1 (7)
Surface active agent	Surfactant 1 (3)	Surfactant 1 (3)	Surfactant 1 (3)	Surfactant 1 (3)	Surfactant 1 (3)
						NMP	NMP(75)	NMP(75)	NMP(76)	NMP(83)	NMP(75)

Numbers in parentheses in the column for green powder, aromatic polyimide, inorganic filler, surfactant, and NMP indicate contents (% by mass)

3. Production example of multilayer film

The dispersion 1 was applied to one surface of the polyimide film 1 by a small-diameter gravure-reverse method, and the applied solution was passed through a through-air drying oven (oven temperature 150 ℃) for 3 minutes to remove NMP, thereby forming a dried film. The other side of the polyimide film 1 was also coated with the dispersion 1 in the same manner, and dried to form a dry film.

Thereafter, the polyimide film 1 having the dry coating films formed on both surfaces was passed through a far infrared oven (the oven temperature near the entrance and exit of the oven was 300 ℃ C., and the oven temperature near the center was 340 ℃ C.) for 20 minutes, and the powder 1 was melt-fired.

Thereby, buffer layers (thickness: 25 μm) containing the F polymer 1 were formed on both sides of the polyimide film 1, and a film (multilayer film 1) in which the buffer layers, the polyimide film layer, and the buffer layers were directly formed in this order was obtained.

Multilayer films 2 to 5 were obtained in the same manner as in the multilayer film 1 except that the dispersion liquid 1 was changed to the dispersion liquids 2 to 5, respectively.

The multilayer film 6 was obtained in the same manner as the multilayer film 1 except that the thickness of the buffer layer was changed to 12 μm.

Further, a multilayer film 7 was obtained in the same manner as the multilayer film 1 except that the dispersion liquid 1 was changed to the dispersion liquid 2 and the polyimide film 1 was changed to the polyimide film 2.

4. Production of copper-clad laminate

Copper foils 1 were placed on both sides of the multilayer film 1 in contact with the buffer layers, and then vacuum hot-pressed (pressing temperature: 320 ℃, pressing pressure: 2MPa, pressing time: 2 minutes) to obtain a 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 steel sheet was subjected to a high-frequency voltage of 40kHz (discharge power density: 300 W.min/m) ² ) IsUnder the conditions, the surface of the buffer layer of the multilayer film 1 was subjected to a surface treatment by a vacuum plasma treatment (degree of vacuum: 20 Pa) using a mixed gas (flow rate: 2000 sccm) of 95 vol% of argon gas and 5 vol% of hydrogen gas.

Copper-clad laminates 2 to 7 were obtained in the same manner as the copper-clad laminate 1, except that the multilayer film 1 was changed to multilayer films 2 to 7.

5. Production example of printed Wiring Board

The electrolytic copper foil layer of the copper-clad laminate 1 is processed by etching to form a transmission circuit, thereby obtaining the printed wiring board 1. Further, 200 transmission circuits in the printed wiring board 1 were formed with patterns having a width of 100 μm and a length of 100 mm.

Printed wiring boards 2 to 7 were obtained in the same manner as the printed wiring board 1, except that the copper-clad laminate 1 was changed to the copper-clad laminates 2 to 7.

6. Evaluation of multilayer film

6-1 evaluation of Adhesivity

Rectangular test pieces having a length of 100mm and a width of 10mm were cut out from the obtained film. Thereafter, the base film layer and the buffer layer were peeled off at a position of 50mm from one end of the test piece in the longitudinal direction. Then, the maximum load when peeling at 90 degrees at a tensile speed of 50 mm/min was defined as peel strength (N/cm) using a tensile tester (manufactured by orietec corporation \\125225612556124861248312463andevaluated as follows.

Good component: peel strength is not less than 10N/cm

X: peel strength < 10N/cm

6-2 evaluation of adhesion after high temperature and high humidity treatment

A rectangular test piece having a length of 100mm and a width of 10mm was cut out from the obtained multilayer film, and after holding the test piece in an atmosphere of 85 ℃ and a relative humidity of 85% for 72 hours, the peel strength was measured in the same manner as in 6-1, and evaluated according to the following criteria.

Good component: the peel strength is more than or equal to 5N/cm

And (delta): peel strength < 5N/cm < 3N/cm

X: peel strength less than or equal to 3N/cm

6-3 evaluation of Heat shrinkage

A sample cut into a 12cm square was cut out from the obtained multilayer film, and the thermal expansion/contraction ratio was determined by the following method.

Drawing 1 straight line having a length of about 10cm on the sample at 25 ℃ with the distance between the ends of the straight line as the initial length L ₀ . Then, the sample was subjected to a heat treatment at 150 ℃ for 30 minutes, cooled to 25 ℃ and then measured for the linear distance L between the end points of the straight line drawn on the sample ₁ The expansion/contraction ratio (%) was calculated according to the following formula.

Rate of thermal expansion (%) = (L) ₁ /L ₀ -1)×100

The multilayer film was evaluated for heat shrinkability according to the following criteria.

Good component: the heating shrinkage rate is more than-1.5% and less than 0%.

And (delta): the heating shrinkage rate is more than-2% and less than-1.5% or more than 0% and less than 1%.

X: the heating expansion rate is less than-2% or more than 1%.

6-4 evaluation of Electrical characteristics

A sample having a length of 10cm and a width of 5cm 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 dielectric resonance) method, and evaluated according to the following criteria.

Good component: the dielectric loss tangent is less than 0.0010.

And (delta): the dielectric loss tangent is 0.0010 to 0.0025 inclusive.

X: the dielectric loss tangent is greater than 0.0025.

7. Evaluation of copper-clad laminate

7-1 evaluation of reflow soldering resistance

The obtained copper-clad laminate was cut into 5cm squares, suspended 5 times in a solder bath at 288 ℃ for 5 seconds, and then the appearance of the copper-clad laminate was evaluated according to the following criteria.

Good component: no swelling and peeling was observed.

And (delta): no swelling was observed, but partial peeling was observed.

X: swelling and delamination were observed.

7-2 evaluation of processability

The obtained copper-clad laminate was irradiated with UV-YAG laser having a wavelength of 355nm by a laser beam machine so as to revolve around a circumference of 100 μm in diameter. Thereby, a circular through hole was formed in the copper-clad laminate.

The laser output power is 1.2W, the laser focal spot diameter is 25 μm, the number of revolutions on the circumference is 20, and the oscillation starting frequency is 40kHz.

Then, a copper-clad laminate including through holes was cut out and fixed with a thermosetting epoxy resin. Thereafter, the through-hole was polished until the cross-section thereof was exposed, and the cross-section of the portion where the through-hole was formed was observed with a microscope, and the periphery of the through-hole was visually observed, and evaluated according to the following criteria.

Good component: no occurrence of abrasion and peeling was observed at the layer interface inside the through-hole.

And (delta): the occurrence of abrasion was observed at the layer interface inside the through-hole, but the occurrence of peeling was not observed.

X: the occurrence of abrasion and peeling was confirmed at the layer interface inside the through-hole.

8. Evaluation of printed Wiring Board

8-1 evaluation of Pattern disconnection Rate of Transmission Circuit

The resistance values at both ends of each pattern of the transmission circuit of the obtained printed wiring board were measured, and evaluated according to the following criteria.

Good: the resistance values of all the patterns were 10. Omega. Or less (wire breakage rate: 0%).

And (delta): there were 1 pattern showing a resistance value of more than 10 Ω (wire breakage rate: 0.5%).

X: the number of patterns showing a resistance value of more than 10. Omega. Is 2 or more (wire breakage rate: 1% or more).

The results are summarized in Table 2.

[ Table 2]

Possibility of industrial utilization

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

These materials can be used as printed wiring boards for transmitting high-frequency signals, and can also be used as antenna components, insulating layers for power semiconductors, aircraft components, automobile components, and the like.

The entire contents of the specification, claims and abstract of japanese patent application No. 2020-062112 filed on the year 2020, month 03 and 31 are incorporated herein as disclosure of the present invention.

Claims

1. A multilayer film having a base film layer comprising a polyimide having a glass transition temperature of less than 288 ℃, and buffer layers provided on both faces of the base film layer, the buffer layers being buffer layers comprising a tetrafluoroethylene-based polymer containing a perfluoro (alkyl vinyl ether) -based unit having a melting temperature of 288 ℃ or higher.

2. The multilayer film of claim 1, wherein the base film layer is in direct contact with each of the buffer layers.

3. The multilayer film of claim 1 or 2, wherein the multilayer film has a thickness of 50 μ ι η or more.

4. The multilayer film according to 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 buffer layer is 5 μm or more.

5. The multilayer film according to any one of claims 1 to 4, wherein a ratio of the sum of thicknesses of the buffer layers to the thickness of the base film layer is 0.8 or more.

6. The multilayer film according to any one of claims 1 to 5, wherein the tensile elastic modulus of the base film layer at 320 ℃ is 0.2GPa or more.

7. The multilayer film according to any one of claims 1 to 6, wherein the tetrafluoroethylene-based polymer is a polymer having a polar functional group containing a perfluoro (alkyl vinyl ether) -based unit, or a polymer having no polar functional group containing 2.0 to 5.0 mol% of the perfluoro (alkyl vinyl ether) -based unit with respect to the whole units.

8. The multilayer film of any one of claims 1-7, wherein the buffer layer further comprises an aromatic polymer.

9. The multilayer film according to any one of claims 1 to 8, wherein the multilayer film has a stretchability measured after heating at 150 ℃ for 30 minutes of-2 to +1%.

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

11. A method for producing a multilayer film according to any one of claims 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 comprising a polyimide having a glass transition temperature of less than 288 ℃, buffer layers provided on both faces of the base film layer, and a metal foil layer provided on the face of at least one of the buffer layers opposite to the base film layer, wherein the buffer layer is a buffer layer comprising a tetrafluoroethylene-based polymer containing a perfluoro (alkyl vinyl ether) -based unit and having a melting point of 288 ℃ or higher.

13. The metal-clad laminate according to claim 12, which is a printed wiring substrate material.

14. The metal-clad laminate as claimed in claim 12 or 13, wherein the solder heat resistance temperature is 288 ℃ or higher.

15. A method for manufacturing a printed wiring board, wherein a transmission circuit is formed by etching the metal foil layer of the metal-clad laminate according to any one of claims 12 to 14, thereby obtaining a printed wiring board.