JP7784385B2 - Method for using adhesive film, method for manufacturing cured body, and method for manufacturing structure - Google Patents

Description

The present invention relates to an adhesive film with excellent thermal conductivity and a method for manufacturing the same, an adhesive film with a support sheet, a cured body and a method for manufacturing the same, and a method for manufacturing a structure.

Heat dissipation materials with thermal conductivity have traditionally been used to dissipate heat generated in electronic devices such as thermoelectric conversion devices, photoelectric conversion devices, and semiconductor devices such as large-scale integrated circuits. For example, one method for efficiently dissipating heat generated by a semiconductor device to the outside is to place a sheet-like heat dissipation material (film, sheet) with excellent thermal conductivity between the semiconductor device and a heat sink.

As exemplified in Patent Document 1, such films or sheets are produced by applying a coating liquid of a heat-dissipating material containing an adhesive resin, an inorganic filler, a curing agent, and a solvent to a release sheet or substrate and then drying it. Examples of inorganic fillers that have been used include silica, alumina, glass, and titanium oxide.

JP 2015-67713 A

However, conventional films and sheets containing inorganic fillers have not always achieved the desired thermal conductivity. Therefore, there is a demand for films and sheets with even better thermal conductivity. However, when conventional films and sheets containing inorganic fillers are heavily loaded with inorganic fillers to achieve high thermal conductivity, they can become mechanically brittle, reducing flexibility and resulting in process defects such as the generation of broken pieces during use. Furthermore, the increased surface roughness of the sheet-shaped heat dissipation component can make it difficult to develop tack, which can prevent temporary adhesion when applied to an adherend. Furthermore, the sheet-shaped heat dissipation component can easily trap air when applied to an adherend, creating numerous voids at the adhesive interface between the sheet-shaped heat dissipation component and the adherend, or within the sheet-shaped heat dissipation component, resulting in reduced thermal conductivity.

The present invention was made in consideration of these circumstances, and aims to provide an adhesive film with excellent thermal conductivity and a method for manufacturing the same, an adhesive film with a support sheet, a cured body and a method for manufacturing the same, and a method for manufacturing a structure.

In order to achieve the above object, first, the present invention provides an adhesive film made of an adhesive resin composition containing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C), the adhesive film being used to undergo a heat treatment including a preheating step and a complete curing step of completely curing the adhesive film after the preheating step, wherein the preheating step is a step of maintaining the adhesive film at a temperature equal to or lower than the following temperature (T) for 30 minutes or more (Invention 1).
Temperature (T): The temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under the condition of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere.

The adhesive film of the above invention (Invention 1) has excellent thermal conductivity when it contains the above-mentioned specific thermally conductive filler (A) and is used for the above-mentioned purposes.

Secondly, the present invention provides an adhesive film made of an adhesive resin composition containing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C), wherein the adhesive film is heat-treated under the following conditions to obtain a cured body, the area ratio of voids in a cross section in the thickness direction of the cured body being 10% or less (Invention 2).
(conditions)
The heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step, and the preheating step is a step of holding the adhesive film for 30 minutes or more at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an atmospheric atmosphere.

The adhesive film of the above invention (Invention 2) has excellent thermal conductivity when it contains the above-mentioned specific thermally conductive filler (A) and is used to obtain the above-mentioned cured body by heat treatment under the above-mentioned conditions.

In the above inventions (Inventions 1 and 2), it is preferable that the content of the thermally conductive filler (A) is 5% by mass or more and 60% by mass or less (Invention 3).

In the above inventions (Inventions 1 to 3), it is preferable that the thermosetting component (B) is an epoxy resin (Invention 4).

In the above inventions (Inventions 1 to 4), it is preferable that the product is heat-pressed (Invention 5).

In the above inventions (Inventions 1 to 5), it is preferable that the arithmetic mean roughness (Ra) of at least one surface is 0.01 μm or more and 0.5 μm or less (Invention 6).

Third, the present invention provides a method for producing an adhesive film, characterized by mixing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride with a binder polymer (C) in a solvent, and then further mixing a thermosetting component (B) to obtain an adhesive resin composition, and then forming the obtained adhesive resin composition into a film (Invention 7).

In the above invention (Invention 7), it is preferable that the adhesive resin composition is formed into a film and then further heat-pressed (Invention 8).

In the above invention (Invention 8), it is preferable that the heating temperature of the heat press is lower than the curing reaction temperature of the thermosetting component (B) (Invention 9).

In the above inventions (Inventions 8 and 9), it is preferable that the arithmetic mean roughness (Ra) of at least one surface of the adhesive film after the heat pressing is 0.01 μm or more and 0.5 μm or less (Invention 10).

Fourthly, the present invention provides an adhesive film with a support sheet, which comprises the adhesive film (Inventions 1 to 6) and a support sheet laminated on at least one side of the adhesive film (Invention 11).

Fifth, the present invention provides a cured body obtained by subjecting an adhesive film made of an adhesive resin composition containing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C) to a heat treatment, wherein the cured body has an area ratio of voids in a thickness direction cross section of 10% or less (Invention 12).

In the above invention (Invention 12), it is preferable that the maximum area of the void portion in the thickness direction cross section is 100 μm ² or less (Invention 13).

In the above inventions (Inventions 12 and 13), the heat treatment preferably includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step, and the preheating step preferably includes a step of holding the adhesive film at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere for 30 minutes or more (Invention 14).

Sixth, the present invention provides a method for producing a cured body, in which an adhesive film made of an adhesive resin composition containing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C) is heat-treated to form a cured body, wherein the heat treatment includes a pre-heating step and a complete curing step in which the adhesive film is completely cured after the pre-heating step, and the pre-heating step includes a step of holding the adhesive film for 30 minutes or more at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere (Invention 15).

In the above invention (Invention 15), it is preferable to heat press the adhesive film before the heat treatment (Invention 16).

In the above inventions (Inventions 15 and 16), it is preferable that the area ratio of voids in the thickness direction cross section of the hardened body is 10% or less (Invention 17).

Seventh, the present invention provides a method for producing a structure, comprising: a lamination step of laminating at least a portion of a first member to at least a portion of a second member via an adhesive film made of an adhesive resin composition containing a thermally conductive filler (A) composed of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C); and a step of heat-treating the adhesive film after the lamination to form a cured body, thereby obtaining a structure in which at least a portion of the first member and at least a portion of the second member are bonded via the cured body, wherein the heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step, and the preheating step includes a step of maintaining the adhesive film at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere (Invention 18).

In the above invention (Invention 18), it is preferable to heat press the adhesive film before the heat treatment (particularly before the lamination process) (Invention 19).

In the above inventions (Inventions 18 and 19), it is preferable that the area ratio of voids in the thickness direction cross section of the hardened body is 10% or less (Invention 20).

In the above inventions (Inventions 18 to 20), it is preferable that in the lamination process, the lamination is carried out at a temperature equal to or higher than the temperature at which the loss tangent (tanδ) obtained by measuring the viscoelasticity of the adhesive film before any heat treatment is peaked (Invention 21).

In the above inventions (Inventions 18 to 20), the first member is a flexible sheet-like member, and a process is provided in which a laminate of the sheet-like member and the heat-pressed adhesive film is bonded to the second member, and it is preferable that the bonding is carried out at a temperature equal to or higher than the temperature at which the loss tangent (tanδ) peaks when measuring the viscoelasticity of the adhesive film before any heat treatment is performed.

The above inventions (Inventions 18 to 20) include a step of attaching the adhesive film to the first member or the second member, and it is preferable that the attachment is carried out at a temperature equal to or higher than the temperature at which the loss tangent (tan δ) peaks when measuring the viscoelasticity of the adhesive film before any heat treatment is performed.

The adhesive film, adhesive film with support sheet, cured body, and structure according to the present invention have excellent thermal conductivity. Furthermore, the method for manufacturing an adhesive film according to the present invention allows for the production of an adhesive film with excellent thermal conductivity. Furthermore, the method for manufacturing a cured body according to the present invention allows for the production of a cured body with excellent thermal conductivity, and the method for manufacturing a structure according to the present invention allows for the production of a structure with excellent thermal conductivity.

1 is a cross-sectional view of an adhesive film with a support sheet according to one embodiment of the present invention. 1 is a cross-sectional view of a structure according to one embodiment of the present invention. 1 is a scanning electron microscope (SEM) image of a cross section of a cured adhesive film (Example 1). 1 is a binarized image of a cross section of a cured adhesive film (Example 1) taken by a scanning electron microscope (SEM). 1 is a scanning electron microscope (SEM) image of a cross section of a cured adhesive film (Comparative Example 2). 1 is a binarized image of a cross section of a cured adhesive film (Comparative Example 2) taken by a scanning electron microscope (SEM).

Hereinafter, an embodiment of the present invention will be described.
[Adhesive film]
The adhesive film according to this embodiment comprises an adhesive resin composition (hereinafter sometimes referred to as "adhesive resin composition R") containing a thermally conductive filler (A) composed of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), and a binder polymer (C). The adhesive resin composition R preferably contains a curing agent (D) as desired, and further preferably contains a curing accelerator (E).

The adhesive film according to this embodiment is first used to undergo a heat treatment including a preheating step and a complete curing step of completely curing the adhesive film after the preheating step, and the preheating step preferably includes a step of maintaining the adhesive film at a temperature equal to or lower than the temperature (T) described below for 30 minutes or more. By performing such a heat treatment, the amount of voids inside the obtained cured body is reduced. Specifically, the area ratio of voids in the thickness direction cross section of the obtained cured body can be 10% or less. Details regarding the heat treatment will be described later.
Temperature (T): The temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under the condition of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere.

Secondly, the adhesive film according to this embodiment preferably has an area ratio of voids in the thickness direction cross section of a cured body obtained by heat-treating the adhesive film under the following conditions of 10% or less.
(conditions)
The heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step,
The preheating step is a step of holding the adhesive film, before any heat treatment, at a temperature not exceeding the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed under the condition of heating the adhesive film from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere for 30 minutes or more.

The thermally conductive filler (A), consisting of at least one of two-dimensional graphene and single-layer boron nitride, has a two-dimensional structure with a thickness on the order of tens of nanometers, characterized by a large specific surface area. Therefore, within the adhesive film, the thermally conductive filler (A) easily comes into contact with each other, forming a thermal conduction path for heat transfer. In particular, by performing the specified heat treatment described above, the voids within the resulting cured product can be reduced, thereby enabling excellent thermal conductivity to be achieved even with a small amount of thermally conductive filler (A). In other words, using the thermally conductive filler (A) as a filler in an adhesive resin composition can impart excellent thermal conductivity without high loading. Furthermore, because the thermally conductive filler (A) has a two-dimensional structure with a thickness on the order of tens of nanometers and is inherently flexible, adhesive films containing the thermally conductive filler (A) exhibit excellent flexibility. Furthermore, because a high loading of the thermally conductive filler (A) is not required, the adhesive film is less likely to become mechanically brittle.

When an adhesive film has excellent flexibility, it is less likely to trap air when applied to an adherend, preventing voids from forming at the interface between the adhesive film and the adherend and increasing the contact area between the adhesive film and the adherend. In other words, it is possible to prevent voids at the interface between the adhesive film and the adherend from increasing thermal resistance, thereby improving thermal conductivity between the adhesive film and the adherend. Furthermore, preventing the adhesive film from becoming mechanically brittle as described above reduces the likelihood of process defects, such as the generation of broken debris, during use.

Furthermore, when conventional inorganic fillers are heavily loaded to achieve the desired thermal conductivity, the surface roughness of the adhesive film increases, making it difficult for tack to develop. On the other hand, the thermally conductive filler (A) in this embodiment can achieve high thermal conductivity without the need for heavy loading, as described above, thereby reducing the surface roughness of the adhesive film. As a result, by adjusting the application temperature when applying the adhesive film to an adherend, it is possible to achieve suitable adhesive strength.

1. Components (1) Thermally conductive filler (A)
The thermally conductive filler (A) in this embodiment is composed of at least one of graphene and monolayer boron nitride having a two-dimensional structure. Graphene has a two-dimensional structure in which carbon atoms are regularly arranged in a hexagonal shape, and monolayer boron nitride has a two-dimensional structure in which boron atoms and nitrogen atoms are regularly arranged in a hexagonal shape, and is a two-dimensional compound originally composed of a single layer of atoms. The "graphene or monolayer boron nitride having a two-dimensional structure" in this specification may be a multilayer, and preferably has a thickness of 1/10 or less of the shortest length in a planar view. Note that, in this specification, graphene is defined as a material produced by thinly exfoliating graphite, and monolayer boron nitride also includes a material produced by thinly exfoliating boron nitride. In this specification, graphite itself does not fall under the category of "graphene having a two-dimensional structure" described above.

As mentioned above, graphene or single-layer boron nitride having a two-dimensional structure may be a single layer or multiple layers. In the case of multiple layers, the number of layers is typically about 2 to 1,000. There are no particular limitations on the planar shape of graphene or single-layer boron nitride having a two-dimensional structure.

The average particle size of the thermally conductive filler (A) is preferably 0.5 μm or more, more preferably 1.0 μm or more, particularly preferably 3.0 μm or more, and even more preferably 5.0 μm or more. This facilitates contact between the individual thermally conductive fillers (A) and facilitates the formation of thermal conduction paths, thereby utilizing the two-dimensional structure and resulting in an adhesive film with excellent thermal conductivity. Furthermore, the average particle size of the thermally conductive filler (A) is preferably 30 μm or less, particularly preferably 20 μm or less, and even more preferably 15 μm or less. This maintains the dispersion state in other materials such as the solvent and binder polymer (C), preventing segregation from preventing the formation of thermal conduction paths, resulting in an adhesive film with excellent thermal conductivity.

The thickness of the thermally conductive filler (A) is preferably 500 nm or less, more preferably 300 nm or less, particularly preferably 200 nm or less, and even more preferably 100 nm or less. This ensures that the flexibility of the resulting adhesive film is maintained. On the other hand, the lower limit of the thickness of the thermally conductive filler (A) is not particularly limited, but is typically 0.7 nm or more. From the viewpoint of thermal conductivity, it is preferably 5.0 nm or more, particularly preferably 10 nm or more, and even more preferably 15 nm or more.

The content of the thermally conductive filler (A) in the adhesive resin composition R is preferably 5% by mass or more, more preferably 10% by mass or more, particularly preferably 15% by mass or more, and even more preferably 20% by mass or more. When the lower limit of the content of the thermally conductive filler (A) is as described above, the thermally conductive fillers (A) are more likely to come into contact with each other, making it easier to form thermal conduction paths, resulting in an adhesive film with superior thermal conductivity.

The content of the thermally conductive filler (A) in the adhesive resin composition R is preferably 60% by mass or less, more preferably 55% by mass or less, particularly preferably 50% by mass or less, and even more preferably 40% by mass or less. By setting the upper limit of the content of the thermally conductive filler (A) as described above, the resulting adhesive film is prevented from becoming mechanically brittle and has excellent flexibility. In this embodiment, by using the thermally conductive filler (A) and performing the specified heat treatment as described above, the desired thermal conductivity can be obtained even with a relatively small content as described above.

(2) Thermosetting component (B)
The thermosetting component (B) in this embodiment is not particularly limited as long as it enables the adhesive resin composition R to be thermoset and exhibits adhesive properties upon thermoset. Examples include epoxy resins, phenolic resins, melamine resins, urea resins, polyester resins, urethane resins, acrylic resins, polyimide resins, benzoxazine resins, and phenoxy resins. These may be used alone or in combination of two or more. Among these, epoxy resins are preferred from the viewpoints of dispersibility and adhesiveness of the thermally conductive filler (A). For convenience, the term "epoxy resin" in this specification also includes unpolymerized or low-molecular-weight epoxy compounds.

Examples of epoxy resins include glycidyl ethers of phenols such as bisphenol A, bisphenol F, resorcinol, phenyl novolac, and cresol novolac; glycidyl ethers of alcohols such as butanediol, polyethylene glycol, and polypropylene glycol; glycidyl ethers of carboxylic acids such as phthalic acid, isophthalic acid, and tetrahydrophthalic acid; glycidyl or alkylglycidyl epoxy resins in which the active hydrogen bonded to the nitrogen atom is replaced with a glycidyl group, such as aniline isocyanurate; and so-called alicyclic epoxides in which epoxy groups have been introduced by, for example, oxidizing the carbon-carbon double bonds within the molecule, such as vinylcyclohexane diepoxide, 3,4-epoxycyclohexylmethyl-3,4-dicyclohexanecarboxylate, and 2-(3,4-epoxy)cyclohexyl-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane. Other epoxy resins that can be used include those having a biphenyl skeleton, a triphenylmethane skeleton, a dicyclopentadiene skeleton, a dicyclohexadiene skeleton, a naphthalene skeleton, an anthracene skeleton, etc., and polyfunctional epoxy resins. These epoxy resins can be used alone or in combination of two or more.

In this embodiment, from the viewpoint of dispersibility of the thermally conductive filler (A), it is preferable to use at least an epoxy resin having a π-conjugated mesogen skeleton as the thermosetting component (B). As an epoxy resin having a π-conjugated mesogen skeleton, an epoxy resin having a naphthalene skeleton or an epoxy resin having a biphenyl skeleton is preferred, and an epoxy resin having a naphthalene skeleton is particularly preferred. As an epoxy resin having a naphthalene skeleton, for example, one represented by the following formula (1) is preferred.

(In the formula, n is an integer of 0 or more.)

The epoxy equivalent of the epoxy resin having a π-conjugated mesogenic skeleton is preferably 100 g/eq or more, particularly preferably 150 g/eq or more, and even more preferably 180 g/eq or more. The epoxy equivalent is preferably 500 g/eq or less, particularly preferably 400 g/eq or less, and even more preferably 300 g/eq or less. This improves the dispersibility of the thermally conductive filler (A) and makes it easier to develop adhesive properties utilizing the epoxy groups. The epoxy equivalent in this specification is a value measured in accordance with JIS K7236.

The softening point of the epoxy resin having a π-conjugated mesogenic skeleton is preferably 40°C or higher, particularly preferably 50°C or higher, and even more preferably 60°C or higher. The softening point is also preferably 200°C or lower, particularly preferably 150°C or lower, and even more preferably 120°C or lower. This results in better dispersibility of the thermally conductive filler (A). The softening point in this specification is a value measured in accordance with the ring and ball method described in JIS K7234:1986.

In this embodiment, it is preferable to use a glycidyl ether of a phenol as the thermosetting component (B) in addition to the epoxy resin having a π-conjugated mesogenic skeleton described above, in order to adjust the tackiness and adhesiveness. Examples of glycidyl ethers of phenols include those listed above, but it is particularly preferable to use a bisphenol F epoxy resin.

The epoxy equivalent of the glycidyl ether of the above phenols is preferably 100 g/eq or more, particularly preferably 120 g/eq or more, and even more preferably 150 g/eq or more. Furthermore, the epoxy equivalent is preferably 500 g/eq or less, particularly preferably 400 g/eq or less, and even more preferably 300 g/eq or less. This results in superior tackiness and adhesion of the resulting adhesive film.

The content of thermosetting component (B) in adhesive resin composition R is preferably 5% by mass or more, more preferably 10% by mass or more, particularly preferably 15% by mass or more, and even more preferably 20% by mass or more. By setting the lower limit of the content of thermosetting component (B) as described above, adhesive resin composition R can be cured sufficiently, exhibiting superior mechanical strength and adhesive properties. Furthermore, the content is preferably 45% by mass or less, more preferably 40% by mass or less, particularly preferably 35% by mass or less, and even more preferably 30% by mass or less. By setting the upper limit of the content of thermosetting component (B) as described above, the content of other components can be ensured.

When an epoxy resin having a π-conjugated mesogenic skeleton is used in combination with a glycidyl ether of a phenol, the blending ratio (by mass) between them is preferably 20:80 to 95:5, more preferably 40:60 to 90:10, particularly preferably 50:50 to 85:15, and even more preferably 60:40 to 80:20. This allows for a good balance between the dispersibility of the thermally conductive filler (A) and the tackiness and adhesion of the adhesive film.

(3) Binder polymer (C)
The binder polymer (C) is blended for the purposes of forming the adhesive resin composition R into a film, imparting appropriate tack to the resulting adhesive film, etc. Examples of such binder polymers that can be used include acrylic polymers, polyester resins, phenoxy resins, urethane resins, silicone resins, and rubber-based polymers, with acrylic polymers being particularly preferred.

Examples of acrylic acid polymers include (meth)acrylic acid ester polymers obtained by polymerizing (meth)acrylic acid ester monomers. In this specification, (meth)acrylic acid refers to both acrylic acid and methacrylic acid. The same applies to other similar terms. Furthermore, the term "polymer" also includes the concept of "copolymer."

Examples of monomers constituting the (meth)acrylic acid ester polymer include (meth)acrylic acid alkyl esters in which the alkyl group has 1 to 18 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and butyl (meth)acrylate, as well as functional group-containing monomers that have a functional group within the molecule. Preferred examples of functional group-containing monomers include monomers that have a hydroxyl group within the molecule (hydroxyl group-containing monomers), monomers that have a carboxy group within the molecule (carboxy group-containing monomers), and monomers that have an amino group within the molecule (amino group-containing monomers). These may be used alone or in combination of two or more.

The (meth)acrylic acid ester polymer used as binder polymer (C) in this embodiment is preferably a copolymer of a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 18 carbon atoms and a functional group-containing monomer. The number of carbon atoms in the alkyl group in the (meth)acrylic acid alkyl ester is preferably 1 to 9, particularly preferably 1 to 6, and even more preferably 1 to 3. As the (meth)acrylic acid alkyl ester, methyl (meth)acrylate is particularly preferred, and methyl acrylate is most preferred.

As the functional group-containing monomer, a hydroxyl group-containing monomer is preferred. Examples of hydroxyl group-containing monomers include (meth)acrylic acid hydroxyalkyl esters such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. Of these, 2-hydroxyethyl (meth)acrylate is particularly preferred, and 2-hydroxyethyl acrylate is most preferred.

Using the above monomers makes it easier to disperse the thermally conductive filler (A) well in the adhesive resin composition R.

When a (meth)acrylic acid ester polymer obtained by copolymerizing the above-mentioned (meth)acrylic acid alkyl ester with a functional group-containing monomer is used as the binder polymer (C), the content of structural units derived from the functional group-containing monomer in the (meth)acrylic acid ester polymer is preferably in the range of 5 to 50% by mass, more preferably in the range of 8 to 30% by mass, and even more preferably in the range of 10 to 20% by mass.

The weight-average molecular weight of the acrylic polymer ((meth)acrylic acid ester polymer) used as the binder polymer (C) is preferably 50,000 or more, more preferably 100,000 or more, particularly preferably 150,000 or more, and even more preferably 200,000 or more. The weight-average molecular weight is preferably 1,000,000 or less, more preferably 700,000 or less, particularly preferably 500,000 or less, and even more preferably 400,000 or less. Having a weight-average molecular weight within the above range improves film-forming properties and adhesion, and also improves the dispersibility of the thermally conductive filler (A). The weight-average molecular weight used in this specification is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene.

The glass transition temperature (Tg) of the acrylic polymer ((meth)acrylic acid ester polymer) used as the binder polymer (C) is preferably -20°C or higher, more preferably -15°C or higher, particularly preferably -10°C or higher, and even more preferably -5°C or higher. The glass transition temperature (Tg) is preferably 60°C or lower, more preferably 50°C or lower, particularly preferably 40°C or lower, and even more preferably 35°C or lower. Having a glass transition temperature (Tg) within the above range results in good film-forming properties and adhesiveness, as well as improved dispersibility of the thermally conductive filler (A). The glass transition temperature (Tg) of the (meth)acrylic acid ester polymer used in this specification is a value calculated based on the FOX formula.

The content of binder polymer (C) in adhesive resin composition R is preferably 1% by mass or more, more preferably 2% by mass or more, particularly preferably 3% by mass or more, and even more preferably 4% by mass or more. Furthermore, the content is preferably 60% by mass or less, more preferably 50% by mass or less, particularly preferably 45% by mass or less, and even more preferably 40% by mass or less. By keeping the content of binder polymer (C) within the above range, the mechanical strength and adhesiveness of the cured adhesive film are maintained at a good level, while the film-forming properties and adhesiveness are improved, and the dispersibility of thermally conductive filler (A) is improved.

(4) Curing agent (D)
The adhesive resin composition R in this embodiment preferably further contains a curing agent (D), which allows the adhesive resin composition R to be cured satisfactorily.

The curing agent (D) is not particularly limited as long as it can cure the thermosetting component (B) by heating. Preferred examples include phenols, amines, and thiols, and the curing agent can be selected appropriately depending on the type of thermosetting component (B) described above. For example, when an epoxy resin is used as the thermosetting component (B), phenols are preferred from the viewpoint of reactivity with the epoxy resin. Furthermore, from the viewpoint of dispersibility of the thermally conductive filler (A), it is preferable to use a compound having a π-conjugated mesogenic skeleton, and it is particularly preferable to use phenols having a π-conjugated mesogenic skeleton.

Examples of phenols include bisphenol A, tetramethylbisphenol A, diallylbisphenol A, biphenol, bisphenol F, diallylbisphenol F, triphenylmethane-type phenol, tetrakisphenol, novolac-type phenol resin, cresol novolac resin, and biphenyl-type phenol resin. Phenols having a conjugated mesogenic skeleton include the biphenyl-type phenols listed above. These can be used alone or in combination of two or more.

Of the above, novolac-type phenolic resins or biphenyl-type phenolic resins are preferred, and it is particularly preferred to use both novolac-type phenolic resins and biphenyl-type phenolic resins in combination.

The hydroxyl equivalent of the novolac phenolic resin is preferably 70 g/eq or more, particularly preferably 80 g/eq or more, and even more preferably 90 g/eq or more. Furthermore, the hydroxyl equivalent is preferably 300 g/eq or less, particularly preferably 280 g/eq or less, and even more preferably 250 g/eq or less. This results in superior curing properties for the epoxy resin. Note that the hydroxyl equivalent in this specification is a value measured in accordance with JIS K0070.

Preferred examples of biphenyl-type phenolic resins include those represented by the following formulas (2) and (3):

(In the formula, n is an integer of 1 or more.)

The hydroxyl equivalent of the biphenyl-type phenolic resin is preferably 80 g/eq or more, particularly preferably 85 g/eq or more, and even more preferably 90 g/eq or more. Furthermore, the hydroxyl equivalent is preferably 300 g/eq or less, particularly preferably 280 g/eq or less, and even more preferably 250 g/eq or less. This prevents the inclusion of substances that inhibit the curing reaction, such as phenol alone, remaining as unreacted products during synthesis, and improves the curability of the epoxy resin.

The softening points of the novolac phenolic resins and biphenyl phenolic resins are preferably 60°C or higher, particularly preferably 80°C or higher, and even more preferably 90°C or higher. Furthermore, the softening points are preferably 200°C or lower, particularly preferably 150°C or lower, and even more preferably 130°C or lower. If the novolac phenolic resin or biphenyl phenolic resin does not soften, its sublimation temperature is preferably 270°C or higher. Furthermore, the sublimation temperature is preferably 330°C or lower. A resin with a high softening point or high sublimation temperature effectively exhibits π-electron interactions, resulting in better dispersibility of the thermally conductive filler (A).

The content of curing agent (D) in adhesive resin composition R is preferably 2% by mass or more, more preferably 4% by mass or more, particularly preferably 5% by mass or more, and even more preferably 8% by mass or more. Furthermore, the above content is preferably 40% by mass or less, more preferably 35% by mass or less, particularly preferably 30% by mass or less, and even more preferably 25% by mass or less. By ensuring that the content of curing agent (D) is within the above range, the curability of adhesive resin composition R is improved.

When a novolac-type phenolic resin and a biphenyl-type phenolic resin are used in combination, the blending ratio (by mass) between them is preferably 80:20 and 10:90, more preferably 70:30 and 20:80, particularly preferably 65:35 and 25:75, and even more preferably 60:40 and 30:70. This allows for a good balance between the curability of the adhesive resin composition R and the dispersibility of the thermally conductive filler (A).

(5) Curing accelerator (E)
The adhesive resin composition R in this embodiment preferably further contains a curing accelerator (E) that accelerates or adjusts the reaction between the thermosetting component (B) and the curing agent (D) described above.

Examples of the curing accelerator (E) include tertiary amines such as triethylenediamine, benzyldimethylamine, triethanolamine, dimethylaminoethanol, and tris(dimethylaminomethyl)phenol; imidazoles such as 2-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, and 2-phenyl-4-methyl-5-hydroxymethylimidazole; organic phosphines such as tributylphosphine, diphenylphosphine, and triphenylphosphine; and tetraphenylboron salts such as tetraphenylphosphonium tetraphenylborate and triphenylphosphine tetraphenylborate. These may be used alone or in combination of two or more.

When an epoxy resin is used as the thermosetting component (B) and a phenol is used as the curing agent (D), it is preferable to use an imidazole-based curing accelerator from the standpoint of the reactivity of the compounds, storage stability, physical properties of the cured product, and curing speed, and 2-phenyl-4,5-hydroxymethylimidazole is particularly preferred.

The content of the curing accelerator (E) in the adhesive resin composition R is preferably 0.0001% by mass or more, more preferably 0.001% by mass or more, particularly preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more. Furthermore, the content is preferably 1.0% by mass or less, more preferably 0.5% by mass or less, particularly preferably 0.1% by mass or less, and even more preferably 0.05% by mass or less. Having the content of the curing accelerator (E) within the above range improves the storage stability of the adhesive film and allows the adhesive resin composition R to cure well.

(6) Various Additives Various additives, such as a tackifier, a flame retardant, an antioxidant, a light stabilizer, a softener, and a rust inhibitor, may be added to the adhesive resin composition of the present embodiment, if desired.

2. Preparation of Adhesive Resin Composition The adhesive resin composition R in this embodiment can be obtained by thoroughly mixing a thermally conductive filler (A) consisting of at least one of graphene having a two-dimensional structure and single-layer boron nitride, a thermosetting component (B), a binder polymer (C), and, optionally, a curing agent (D), a curing accelerator (E), additives, and a solvent. Note that when any of the above components is used in a solid state, or when precipitation or the like occurs when mixed with other components in an undiluted state, that component may be dissolved or diluted alone in a solvent before being mixed with the other components.

In this embodiment, adhesive resin composition R is preferably prepared by first mixing thermally conductive filler (A) and binder polymer (C) in a solvent, followed by the addition of thermosetting component (B) and, if desired, curing agent (D), curing accelerator (E), additives, etc. Mixing thermally conductive filler (A) and binder polymer (C) before blending thermosetting component (B) improves the dispersibility of thermally conductive filler (A) and prevents segregation of thermally conductive filler (A) in the coating film. As a result, thermally conductive filler (A) is uniformly dispersed in the resulting adhesive film, resulting in an adhesive film with superior thermal conductivity.

Thermal conductive filler (A) and binder polymer (C) are preferably mixed in a solvent by stirring at a disperser rotation speed of 500 to 5,000 rpm for 10 minutes or more, and more preferably at a disperser rotation speed of 1,000 to 40,000 rpm for 20 minutes or more.

The solvent is not particularly limited and may be, for example, aliphatic hydrocarbons such as hexane, heptane, cyclohexane, etc.; aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methylene chloride and ethylene chloride; alcohols such as methanol, ethanol, propanol, butanol, 1-methoxy-2-propanol, etc.; ketones such as acetone, methyl ethyl ketone, 2-pentanone, isophorone, cyclohexanone, etc.; esters such as ethyl acetate and butyl acetate; cellosolve-based solvents such as ethyl cellosolve; N,N-dimethylformamide, trimethyl-2-pyrrolidone, butyl carbitol, etc., with methyl ethyl ketone being preferred.

The viscosity of the coating solution of adhesive resin composition R prepared in this manner is not particularly limited as long as it is within a coatable range and can be selected appropriately depending on the situation. The addition of a diluent solvent is not a necessary condition; if adhesive resin composition R has a coatable viscosity, it is not necessary to add a diluent solvent.

3. Production of Adhesive Film The adhesive film according to this embodiment is obtained by forming the adhesive resin composition R obtained above into a film. When forming the adhesive resin composition R into a film, it is preferable to use a release sheet as the coating target. For example, the adhesive film according to this embodiment can be easily produced by applying a coating liquid of the adhesive resin composition R to a release sheet and removing the dilution solvent by heating and drying.

Release sheets can be made of resin films, nonwoven fabrics, paper, etc., but resin films are commonly used. Examples of resin films include polyethylene films, polypropylene films, polybutene films, polybutadiene films, polymethylpentene films, polyvinyl chloride films, vinyl chloride copolymer films, polyethylene terephthalate films, polyethylene naphthalate films, polybutylene terephthalate films, polyurethane films, ethylene vinyl acetate films, ionomer resin films, ethylene-(meth)acrylic acid copolymer films, ethylene-(meth)acrylic acid ester copolymer films, polystyrene films, polycarbonate films, polyimide films, and fluororesin films. Crosslinked films of these films can also be used. Furthermore, laminated films of these films may also be used.

It is preferable that the release surface of the release sheet (the surface that comes into contact with the adhesive resin composition R) be subjected to a release treatment. Examples of release agents used for the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents. However, this release treatment is not always necessary.

There are no particular restrictions on the thickness of the release sheet, but it is usually around 20 to 150 μm.

The arithmetic mean roughness (Ra) of the release surface of the release sheet is preferably 0.1 μm or less, more preferably 0.05 μm or less, and especially preferably 0.01 μm or less. By having the arithmetic mean roughness (Ra) of the release surface of the release sheet be within the above range, even if the roughness of the release surface of the release sheet is transferred to the adhesive film, it becomes easier to adjust the arithmetic mean roughness (Ra) of the adhesive film to fall within the preferred range described below. The method for measuring the arithmetic mean roughness (Ra) in this specification is as shown in the test examples described below.

As an example of how to manufacture an adhesive film, a coating liquid of adhesive resin composition R is applied to the release surface of a release sheet. Coating methods that can be used include, for example, bar coating, knife coating, roll coating, blade coating, die coating, and gravure coating.

The coating of adhesive resin composition R is then dried to volatilize the dilution solvent and obtain an adhesive film. Drying conditions are preferably 90-150°C for 0.5-30 minutes, and particularly preferably 100-120°C for 1-10 minutes. The heating temperature for the drying process must be lower than the thermal curing temperature of adhesive resin composition R.

After the drying process, it is preferable to laminate another peelable protective film onto the exposed surface of the adhesive film to protect the adhesive film. In this case, the protective film is laminated so that the release surface of the peelable protective film contacts the exposed surface of the adhesive film. This results in a laminate consisting of a release sheet/adhesive film/protective film.

The protective film can be the same as the release sheet primarily made of resin film described above. The protective film can be either release-treated or not, as long as it is releasable from the adhesive film.

The adhesive film (laminate) obtained as described above is preferably heat-pressed. By heat-pressing the adhesive film, voids present inside the adhesive film can be reduced, resulting in better thermal conductivity. Specifically, heat-pressing makes it easier for the thermally conductive fillers (A) to come into contact with each other, making it easier to form thermal conduction paths and improving thermal conductivity. By performing such heat-pressing, the amount of thermally conductive filler (A) contained in the adhesive film can be further reduced, further improving the flexibility and adhesiveness of the adhesive film.

As described above, in the adhesive film of this embodiment, by carrying out a heat treatment including a preheating step in which the film is maintained at a temperature below a specific temperature (T) for 30 minutes or more, and a complete curing step in which the adhesive film is completely cured after the preheating step, the area ratio of voids in the thickness direction cross section of the resulting cured body can be easily reduced to 10% or less.However, by combining this with the above-mentioned heat pressing, it is even easier to reduce the area ratio of voids to 10% or less.

The heating temperature of the heat press should be below the curing reaction temperature of the thermosetting component (B). Specifically, it is preferably 30 to 90°C, more preferably 40 to 80°C, particularly preferably 45 to 70°C, and even more preferably 45 to 60°C.

The pressure of the heat press is preferably 0.5 to 15 MPa, more preferably 1 to 10 MPa, particularly preferably 1.5 to 5 MPa, and even more preferably 2 to 4 MPa.

The heat pressing time is preferably 0.5 to 60 minutes, more preferably 1 to 50 minutes, particularly preferably 2 to 40 minutes, and even more preferably 3 to 30 minutes.

By setting the heat pressing conditions as described above, it is easier to bring the physical properties of the cured adhesive film into the preferred ranges described below.

4. Physical Properties of the Adhesive Film (1) Thickness of the Adhesive Film The thickness (measured in accordance with JIS K7130) of the adhesive film according to this embodiment (including adhesive films that are not heat-pressed and adhesive films that are heat-pressed) is preferably 0.5 μm or more as a lower limit, more preferably 1 μm or more, particularly preferably 5 μm or more, and even more preferably 10 μm or more. When the lower limit of the adhesive film thickness is as described above, it is easy to exhibit good adhesive strength and adhesion.

The upper limit of the thickness of the adhesive film according to this embodiment is preferably 1000 μm or less, more preferably 500 μm or less, particularly preferably 200 μm or less, and even more preferably 100 μm or less. When the upper limit of the adhesive film thickness is as described above, the thermal conductivity is improved. The adhesive film may be formed as a single layer, or may be formed by laminating multiple layers.

(2) Arithmetic mean roughness (Ra)
The arithmetic mean roughness (Ra) of at least one surface of the adhesive film (including non-heat-pressed adhesive films and heat-pressed adhesive films) is preferably 0.5 μm or less, more preferably 0.4 μm or less, particularly preferably 0.35 μm or less, and even more preferably 0.3 μm or less. When the upper limit of the arithmetic mean roughness (Ra) of the adhesive film is as described above, the adhesiveness to the adherend, i.e., temporary adhesion, is improved, and handling is improved. In addition, the contact area with the adherend is increased, resulting in better thermal conductivity between the adhesive film and the adherend. If the arithmetic mean roughness (Ra) exceeds the upper limit, the thermally conductive filler (A) may segregate on the surface of the adhesive film, and the resin component may not be present at all on the adhesive film surface, or may be present only in trace amounts. As a result, there is a risk that the temporary adhesion may be insufficient when applied to the adherend.

On the other hand, the arithmetic mean roughness (Ra) of the adhesive film is preferably 0.01 μm or more, more preferably 0.02 μm or more, particularly preferably 0.03 μm or more, and even more preferably 0.05 μm or more. When the thermally conductive filler (A) segregates on the surface of the adhesive film, the arithmetic mean roughness (Ra) tends to increase. When the lower limit of the arithmetic mean roughness (Ra) of the adhesive film is as described above, it can be said that the thermally conductive filler (A) is present on the surface of the adhesive film, resulting in better thermal conductivity between the adhesive film and the adherend.

(3) Adhesion The adhesive strength of the adhesive film (including non-heat-pressed adhesive film and heat-pressed adhesive film) to a silicon wafer (arithmetic mean roughness (Ra): 0.02 μm or less) is preferably 0.1 mN/25 mm or more, more preferably 0.5 mN/25 mm or more, particularly preferably 0.8 mN/25 mm or more, and even more preferably 1.0 mN/25 mm or more. This allows for good adhesion to the adherend and excellent temporary adhesion.

While there are no particular limitations on the upper limit of the adhesive strength, it is generally preferred that it be 5.0 mN/25 mm or less, more preferably 3.0 mN/25 mm or less, particularly preferably 2.5 mN/25 mm or less, and even more preferably 2.0 mN/25 mm or less. This results in excellent reworkability. Note that adhesive strength in this specification basically refers to adhesive strength measured using the 180-degree peel method in accordance with JIS Z0237:2009, and the specific measurement method is as shown in the test examples below.

[Adhesive film with support sheet]
An adhesive film with a support sheet according to one embodiment of the present invention comprises the above-mentioned adhesive film (including adhesive films that are not heat-pressed and adhesive films that are heat-pressed) and a support sheet laminated on at least one side of the adhesive film. The support sheet may be peeled off from the adhesive film in the future.

By supporting the adhesive film on a support sheet, it is possible to improve the processability of the adherend, for example. For example, even if it is difficult to process the adherend using the adhesive film alone, it is possible to apply the adhesive film with the support sheet to one adherend, process it in that state, and then peel off the support sheet and apply the adhesive film to another adherend.

An example of an adhesive film with a support sheet according to this embodiment is shown in Figure 1. The adhesive film with a support sheet 2 shown in Figure 1 is composed of an adhesive film 1, a support sheet 11 laminated on one side of the adhesive film 1 (the upper side in Figure 1), and a release sheet 12 laminated on the other side of the adhesive film 1 (the lower side in Figure 1). The release sheet 12 is laminated on the adhesive film 1 so that its releasable side contacts the adhesive film 1. The release sheet 12 protects the adhesive film 1 until it is used, and may be omitted. Furthermore, in the adhesive film with a support sheet according to this embodiment, a protective film may be laminated in place of the release sheet 12.

The support sheet 11 is not particularly limited as long as it has sufficient mechanical strength to support the adhesive film 1. Materials that make up the support sheet 11 include, for example, resin film, nonwoven fabric, paper, etc., but resin film is commonly used.

Specific examples of resin films include polyethylene films such as low-density polyethylene (LDPE) film, linear low-density polyethylene (LLDPE) film, and high-density polyethylene (HDPE) film; polyolefin films such as polypropylene film, ethylene-propylene copolymer film, polybutene film, polybutadiene film, polymethylpentene film, ethylene-norbornene copolymer film, and norbornene resin film; ethylene copolymer films such as ethylene-vinyl acetate copolymer film, ethylene-(meth)acrylic acid copolymer film, and ethylene-(meth)acrylic acid ester copolymer film; polyvinyl chloride films such as polyvinyl chloride film and vinyl chloride copolymer film; polyester films such as polyethylene terephthalate film and polybutylene terephthalate film; polyurethane film; polyimide film; polystyrene film; polycarbonate film; and fluororesin films. Modified films such as crosslinked films and ionomer films can also be used. Furthermore, laminate films may be formed by laminating multiple of the above films, either the same or different types. The support sheet 11 may also be a release sheet. The support sheet 11 may be, for example, a sheet having a known adhesive layer provided on the above-mentioned resin film, nonwoven fabric, paper, or the like.

The thickness of the support sheet 11 is preferably 20 μm or more, particularly preferably 40 μm or more, and even more preferably 60 μm or more. Furthermore, the thickness is preferably 150 μm or less, particularly preferably 120 μm or less, and even more preferably 110 μm or less. Having a thickness of the support sheet 11 within the above range makes it easier for the support sheet 11 to have the desired mechanical strength, and improves the aforementioned adherend processability, etc.

The adhesive film 2 with support sheet may be a dicing/die-bonding sheet used in manufacturing semiconductor devices. In this case, the adhesive film 2 with support sheet can be used in the processes of dicing and die-bonding semiconductor elements, and the cured adhesive film functions as a thermal conductor to dissipate heat generated during operation of the semiconductor device to the outside. In this case, the support sheet 11 is preferably, for example, the aforementioned resin film with a known adhesive layer provided on the surface facing the adhesive film 1.

Examples of manufacturing an adhesive film 2 with a support sheet include peeling the release sheet from the aforementioned laminate consisting of a release sheet/adhesive film/protective film and laminating a support sheet, peeling the protective film from the aforementioned laminate consisting of a release sheet/adhesive film/protective film and laminating a support sheet, or using a support sheet instead of the protective film in the aforementioned adhesive film manufacturing method.

[Hardened body]
The cured product according to one embodiment of the present invention is obtained by curing the adhesive film described above through a heat treatment, and preferably by curing the adhesive film after the heat pressing described above through a heat treatment.

1. Physical Properties (1) Area Ratio of Void Portions In this embodiment, the area ratio of void portions in the thickness direction cross section of the cured body is preferably 10% or less, more preferably 7% or less, particularly preferably 5% or less, and even more preferably 4% or less. When the area ratio of the void portions is 10% or less, the thermally conductive fillers (A) are in closer contact with each other, the thermal conduction paths are formed at a high density, and the thermal conductivity is improved. The lower limit of the area ratio is not particularly limited, and 0% is most preferred. The method for deriving the area ratio of the void portions is as shown in the test examples described below.

(2) Maximum Area of Void Portions In this embodiment, the maximum area of void portions in the thickness direction cross section of the cured body is preferably 100 μm ² or less, more preferably 80 μm ² or less, particularly preferably 60 μm ² or less, and even more preferably 50 μm ² or less. When the area ratio of the void portions is 100 μm ² or less, the thermally conductive fillers (A) are in closer contact with each other, the thermal conduction paths are formed at a high density, and the thermal conductivity is improved. The lower limit of the maximum area is not particularly limited, and is most preferably 0 μm ^2. The method for deriving the maximum area of the void portions is as shown in the test examples described below.

(3) Thermal Conductivity The thermal conductivity of the cured body in this embodiment is preferably 4 W/mK or more, and particularly preferably 5 W/mK or more. This means that the cured body has excellent thermal conductivity. The cured body according to this embodiment can achieve such high thermal conductivity because the adhesive film has the above-described configuration. The method for measuring thermal conductivity in this specification is as shown in the test examples described below.

2. Method for Producing the Cured Product To produce the cured product according to this embodiment, the adhesive film (preferably the adhesive film after heat pressing) is subjected to a heat treatment, and is finally completely cured.

The heat treatment preferably includes a preheating step in which the adhesive film is held for 30 minutes or longer at a temperature (hereinafter sometimes referred to as the "preheating temperature") below which the adhesive film loses 0.5% weight when subjected to thermogravimetric analysis in an air atmosphere at a temperature rising rate of 10°C/min from 40°C to 400°C before any heat treatment is performed, and a complete curing step in which the adhesive film is completely cured after the preheating step. If the adhesive film is completely cured by rapid heat treatment, the low-molecular-weight components in the adhesive film tend to volatilize and foam, creating voids within the adhesive film. In contrast, if the heat treatment includes the above steps, the low-molecular-weight components are trapped within the matrix of the thermosetting component (B) or binder polymer (C) before volatilizing, thereby suppressing foaming and reducing voids in the adhesive film. The preheating step (and the aforementioned heat pressing) result in desirable void area ratios and maximum void areas, improving the thermal conductivity of the cured product.

The above preheating temperature is preferably 1 to 50°C lower than the temperature at which the adhesive film loses 0.5% weight, particularly preferably 10 to 40°C lower, and even more preferably 15 to 30°C lower. Furthermore, the lower limit of the above preheating temperature is preferably 80°C or higher, particularly preferably 90°C or higher, and even more preferably 100°C or higher.

Furthermore, it is preferable that the above pre-heating process be carried out for 30 minutes or more, particularly preferably for 30 to 120 minutes, and even more preferably for 30 to 60 minutes.

The above heat treatment preferably involves the above preheating step followed by a complete curing step at a heating temperature sufficient to completely cure the adhesive film. The heating temperature in the complete curing step must be higher than the above preheating temperature, and is preferably 5 to 100°C higher than the preheating temperature, more preferably 10 to 70°C higher, and even more preferably 20 to 50°C higher. Specifically, the heating temperature in the complete curing step is preferably 85 to 200°C, more preferably 100 to 190°C, and even more preferably 120 to 180°C.

Furthermore, the above-mentioned complete curing process is preferably carried out for 30 to 180 minutes, particularly preferably 45 to 150 minutes, and even more preferably 60 to 120 minutes.

The hardened body of this embodiment may exist on its own, but is typically in contact with one or more components to which heat conduction is desired.

[Structure]
A structure according to one embodiment of the present invention is formed by bonding at least a part of a first member and at least a part of a second member via the above-described hardened body.

An example structure in this embodiment is shown in Figure 2. The structure 3 shown in Figure 2 comprises a first member 31, a second member 32, and a hardened body 1A disposed between the first member 31 and the second member 32.

The cured body 1A is obtained by completely curing the aforementioned adhesive film (adhesive film 1; including both non-heat-pressed and heat-pressed adhesive films) through the aforementioned heat treatment. The first member 31 and the second member 32 are fixed to each other by the adhesive properties of the cured body 1A (adhesive film). The shapes of the first member 31 and the second member 32 in this embodiment are not particularly limited, and may be flexible sheets, plates, blocks, or the like.

The first member 31 (or second member 32) in this embodiment is not particularly limited, but is preferably, for example, a member that generates heat while performing a predetermined function and that requires suppression of temperature rise, or a member that requires control of the flow of heat generated by the member in a specific direction (heat-generating member). The second member 32 (or first member 31) is also not particularly limited, but is preferably, for example, a member that dissipates heat that it receives, or a member that transfers heat that it receives to another member (heat-transfer member). The hardened body 1A in this embodiment has excellent thermal conductivity, and therefore functions as a thermal conductor that, for example, transfers heat from the heated first member 31 to the second member 32 and releases the heat to the outside.

Examples of heat-generating components include semiconductor devices such as thermoelectric conversion devices, photoelectric conversion devices, and large-scale integrated circuits; electronic devices such as LED light-emitting elements, optical pickups, and power transistors; various electronic devices such as mobile terminals and wearable terminals; batteries; motors; and engines. Heat-transfer components are preferably made of highly conductive materials, such as metals such as aluminum, stainless steel, and copper, as well as graphite and carbon nanofibers. The form of the heat-transfer component is not particularly limited and may be a substrate, housing, heat sink, heat spreader, or the like.

To manufacture the structure 3 according to this embodiment, one side of the adhesive film described above is attached to the first member 31 (or the second member 32), and then the other side of the adhesive film is attached to the second member 32 (or the first member 31). When using the adhesive film 2 with the support sheet described above, the release sheet 12 is peeled off, and one side of the exposed adhesive film 1 is attached to the first member 31 (or the second member 32), and then the support sheet 11 is peeled off, and the other side of the exposed adhesive film 1 is attached to the second member 32 (or the first member 31).

The adhesive film used to manufacture the structure 3 according to this embodiment may be either a non-heat-pressed or heat-pressed adhesive film, but it is preferable to use a heat-pressed adhesive film. Alternatively, a non-heat-pressed adhesive film may be used, and the adhesive film may be heat-pressed after being attached to the first member 31 (or the second member 32) or after the first member 31 and the second member 32 are bonded together via the adhesive film. However, using a pre-heat-pressed adhesive film can prevent damage to the first member 31 and/or the second member 32 due to the heat press.

When bonding the first member 31 and the second member 32 together via the adhesive film, or when applying the adhesive film to the first member 31 or the second member 32, it is preferable to perform the bonding or application at a temperature (hereinafter sometimes referred to as the "application processing temperature") equal to or higher than the temperature at which the loss tangent (tan δ) peak obtained in a viscoelasticity measurement of the adhesive film before any heat treatment (hereinafter sometimes referred to as the "tan δ peak temperature"). By performing the bonding or application at such a temperature, the adhesive film becomes flexible, more effectively preventing air from being trapped between the adhesive film and the adherend, and improving thermal conductivity between the adhesive film and the adherend. The method for measuring the viscoelasticity of the adhesive film is as shown in the test examples described below.

When the first member 31 is a flexible sheet-like member and a laminate of the sheet-like member and adhesive film is attached to the second member 32, or when the adhesive film with support sheet 2 is attached to the first member 31 or the second member 32, the air entrapment suppression effect described above is even better. Because the laminate of the sheet-like member and adhesive film and the adhesive film with support sheet 2 are flexible and easily pliable, they can be gradually brought into close contact with the adherend from one direction to another, allowing them to be attached while pushing out air.

The above-mentioned application temperature is preferably 0 to 50°C higher than the tan δ peak temperature, particularly preferably 2 to 30°C higher, and even more preferably 5 to 20°C higher. Furthermore, the upper limit of the application temperature must be lower than the curing temperature of the adhesive film; specifically, it is preferably 120°C or lower, particularly preferably 100°C or lower, and even more preferably 90°C or lower.

As described above, after one side of the adhesive film is attached to the first member 31 (or the second member 32) and the other side of the adhesive film is attached to the second member 32 (or the first member 31), the heat treatment used to produce the cured adhesive film described above is carried out, completely curing the adhesive film to form the cured body 1A, thereby obtaining the structure 3 of this embodiment.

The above-described embodiments have been described to facilitate understanding of the present invention, and are not intended to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

For example, the release sheet 12 laminated to the adhesive film 1 in Figure 1 may be omitted. Furthermore, the shape of the first member and the first member in the structure is not limited to that shown in Figure 2, and may have various shapes.

The present invention will be explained in more detail below using examples, but the scope of the present invention is not limited to these examples.

Example 1
The following components (a) and (c) were mixed and diluted with methyl ethyl ketone to a solids concentration of 15% by mass, and the mixture was stirred in a disper at 3,000 rpm for 30 minutes or more to dissolve and disperse. To this mixture were added the following components (b-1), (b-2), (d-1), (d-2), and (e), and methyl ethyl ketone was further added to give a total solids concentration of 21% by mass. This mixture was stirred for 10 minutes in a planetary centrifugal mixer (Thinky Corporation, product name "AR-100") to obtain a coating solution of an adhesive resin composition.

The content of each component in this adhesive resin composition (converted to solids) was 30.01% by mass for component (a), 34.95% by mass for component (c), 16.56% by mass for component (b-1), 7.23% by mass for component (b-2), 5.61% by mass for component (d-1), 5.61% by mass for component (d-2), and 0.03% by mass for component (e).

(a) Thermally conductive filler: graphene (manufactured by ADEKA Corporation, product name "CNS-1A1", average particle size 12 μm, thickness 50 nm or less, Raman peak intensity ratio D/G = 0.1, when measured by X-ray diffraction using a CuKα radiation source (wavelength 0.15418 nm), peaks were detected at 2θ positions of 26.6° and 42.4°)

(b-1) Thermosetting component: a solid epoxy resin having a naphthalene skeleton represented by the following formula (1) (manufactured by Nippon Kayaku Co., Ltd., product name "NC-7000L", epoxy equivalent 223 to 238 g/eq, ICI viscosity (150°C) 0.50 to 1.00 Pa s, softening point 83 to 93°C) dissolved in methyl ethyl ketone (solid content concentration 70% by mass)

(In the formula, n is an integer of 0 or more.)

(b-2) Thermosetting component: Bisphenol F liquid epoxy resin (manufactured by Mitsubishi Chemical Corporation, product name "YL983U", epoxy equivalent 165-175 g/eq, viscosity (25°C) 3.0-6.0 Pa·s)

(c) Binder polymer: Acrylic ester polymer (manufactured by Mitsubishi Chemical Corporation, product name "Coponyl N-4617", a copolymer obtained by copolymerizing 85 parts by mass of methyl acrylate and 15 parts by mass of 2-hydroxyethyl acrylate, weight average molecular weight: 300,000, glass transition temperature: 6°C) dissolved in a 1:1 mixed solvent of ethyl acetate and toluene (solid concentration 36% by mass)

(d-1) Curing agent: Novolac phenolic resin (manufactured by Asahi Organic Materials Co., Ltd., product name "PAPS-PN4", hydroxyl group equivalent 104 g/eq, ICI viscosity (150°C) 3.0 Pa·s, softening point 111°C) dissolved in methyl ethyl ketone (solid concentration 60% by mass)

(d-2) Curing agent: A biphenyl-type phenol compound represented by the following formula (3) (manufactured by Honshu Chemical Industry Co., Ltd., product name "BP", hydroxyl group equivalent 93.1 g/eq, sublimation temperature 283°C) dissolved in methyl ethyl ketone (solid content concentration 10% by mass):

(e) Curing accelerator: 2-phenyl-4,5-hydroxymethylimidazole

The adhesive resin composition coating solution obtained in the above process was applied with an applicator to the release-treated surface of a release sheet (manufactured by Lintec Corporation, product name "SP-PET3811(S)"; arithmetic mean roughness (Ra): 0.002 μm) of a polyethylene terephthalate film, one side of which had been treated with a silicone-based release agent. The sheet was then heated and dried at 100°C for 2 minutes to form an adhesive film (thickness: 50 μm). The adhesive film with the release sheet was then bonded to the release-treated surface of a protective film (manufactured by Lintec Corporation, product name "SP-PET3811(S)"), one side of which had been treated with a silicone-based release agent, to obtain a laminate consisting of the release sheet, adhesive film (thickness: 50 μm), and protective film.

The laminate obtained above was cut into a 4 cm x 4 cm piece, and a pressure of 3.0 MPa was applied to the laminate at 50°C for 30 minutes using a screw-type heat press device to heat press the adhesive film.

Example 2
An adhesive film (after heat pressing) was produced in the same manner as in Example 1, except that in the heat pressing, the pressure applied by the screw-type heat pressing device was 2.5 MPa.

Example 3
In Test Example 4 described later, Example 3 was prepared by applying the adhesive film of Example 1 after heat pressing to the silicon wafer at a temperature of 75°C.

Comparative Example 1
In Test Example 6 described later, the adhesive film after heat pressing in Example 1 was directly subjected to a heat treatment at 175°C for 3 hours (complete curing step) without undergoing the preheating step, to prepare Comparative Example 1.

Comparative Example 2
An adhesive film (for convenience, also referred to as "adhesive film after heat pressing") was produced in the same manner as in Example 1, except that the pressure applied by the screw-type heat pressing device in the heat pressing was 0 MPa.

Comparative Example 3
(a) An adhesive film was produced in the same manner as in Example 1, except that spherical alumina particles (manufactured by Showa Denko K.K., product name "CB-P05J", average particle size 5.0 μm, aspect ratio: 1.1, specific gravity: 3.98 g/cm ³ ) were used as the thermally conductive filler instead of the graphene used in Example 1.

Comparative Example 4
An adhesive film was produced in the same manner as in Comparative Example 3, except that the amount of (a) spherical alumina particles (manufactured by Showa Denko K.K., product name "CB-P05J", average particle size 5.0 μm, aspect ratio: 1.1, specific gravity: 3.98 g/cm ³ ) used as a thermally conductive filler was changed to 81.4 mass %.

Comparative Example 5
In Test Example 6 described later, the adhesive film after heat pressing of Comparative Example 3 was directly subjected to heat treatment at 175°C for 3 hours (complete curing step) without undergoing the preheating step, to prepare Comparative Example 5.

Comparative Example 6
In Test Example 4 described later, Comparative Example 6 was prepared by applying the adhesive film after heat pressing of Comparative Example 3 to the silicon wafer at room temperature (RT).

[Test Example 1] <Thermogravimetric measurement (TG measurement)>
For the adhesive films before heat pressing obtained in each Example and Comparative Example, thermogravimetry was performed using a thermal analysis measuring device (Shimadzu Corporation, thermal analyzer TG/DTA simultaneous measuring device, product name "DTG-60") in an air atmosphere at a temperature increase rate of 10°C/min from 40°C to 400°C using approximately the same amount of alumina particles as the measurement sample as a reference sample, and the temperature at which the weight loss was 0.5% was determined. As a result, for the adhesive films before heat pressing obtained in Examples 1 to 3 and Comparative Examples 1 to 3, 5 to 6, the above temperature was 147°C, and for the adhesive film before heat pressing obtained in Comparative Example 4, the above temperature was 151°C.

[Test Example 2] <Dynamic viscoelasticity measurement>
The adhesive films obtained in each Example and Comparative Example before heat pressing were laminated to a thickness of 0.2 mm and cut into 20 mm x 10 mm pieces to serve as measurement samples. The obtained samples were mounted on a dynamic viscoelasticity measuring device (manufactured by TA Instruments, product name "DMA-Q800") with a measurement length (chuck distance) of 10 mm. The loss tangent (tan δ) was measured over a temperature range of 25 to 100 ° C under conditions of a frequency of 11 Hz, an amplitude of 5 μm, and a heating rate of 3 ° C/min. The temperature at which the loss tangent (tan δ) was maximized within the measurement temperature range was determined. As a result, the temperature was 73 ° C for the adhesive films obtained in Examples 1 to 3 and Comparative Examples 1 and 2 before heat pressing, 71 ° C for the adhesive films obtained in Comparative Examples 3, 5, and 6 before heat pressing, and 75 ° C for the adhesive film obtained in Comparative Example 4 before heat pressing.

Test Example 3: <Arithmetic mean roughness (Ra) measurement>
(1) Release-treated surface of release sheet The surface roughness of the release-treated surface of the release sheet used in each example and comparative example was measured in DFM mode using a scanning probe microscope (SPM) device (manufactured by Hitachi High-Tech Science Corporation, product name "SPA-300HV") over a 5 μm x 5 μm area on the sample surface, and the arithmetic mean roughness (Ra) was measured. The cantilever used was an "OMCL-AC240TS-C3" manufactured by Olympus Corporation (resonance frequency: 55-65 kHz, spring constant: approximately 2 N/m ² ). As a result, the arithmetic mean roughness (Ra) of the release-treated surface of the release sheet was 0.002 μm.

(2) Surface of adhesive film The surface roughness of the adhesive film surface (the surface that was in contact with the release-treated surface of the release sheet) after heat pressing obtained in each example and comparative example was measured using a shape measuring laser microscope (manufactured by Keyence Corporation, product name "3D laser microscope VK-9700") in accordance with JIS B0601:2001, with a cutoff of 2.5 mm, over an area of 700 x 500 ^μm2 , and the arithmetic mean roughness (Ra) was measured. The results are shown in Table 1.

[Test Example 4] <Adhesive Strength Measurement>
The coating liquid of the adhesive resin composition obtained in each Example and Comparative Example was coated on one side of a polyethylene terephthalate film having a thickness of 12 μm, and the film was dried by heating at 100° C. for 2 minutes to produce a laminate in which an adhesive film (thickness: 50 μm) was firmly bonded to the polyethylene terephthalate film.

Next, to protect the surface of the adhesive film, the release-treated side of a release film (manufactured by Lintec Corporation, product name "SP-PET381031", thickness 38 μm) was attached to the adhesive film side of the laminate. The resulting laminate, along with the release film, was cut into a 4 cm x 4 cm piece and hot-pressed under the conditions specified for each example. This was then cut together with the release film to obtain a sample for measuring adhesive strength, 25 mm wide and 40 mm long.

Separately, a silicon wafer (manufactured by Science and Technology Research Institute, diameter: 150 mm, thickness: 500 μm) whose surface had been chemically and mechanically polished until the arithmetic mean roughness (Ra) was 0.02 μm or less was prepared as an adherend. The release film was peeled off from the adhesive strength measurement sample, and the exposed surface of the adhesive film was attached to the treated surface of the silicon wafer, yielding a laminate consisting of the silicon wafer and the adhesive strength measurement sample. The temperature during attachment was 80°C (Examples 1-2, Comparative Examples 1-5), 75°C (Example 3), or room temperature (Comparative Example 6).

The resulting laminate was left for 20 minutes in an atmosphere of 23°C and 50% relative humidity, after which a 180° peel test was conducted in accordance with JIS Z0237:2000 using a universal tensile tester (Instron, product name "Model 5581 Tester") at a peel rate of 300 mm/min. The load during this 180° peel was measured, and this measurement value was taken as the adhesive strength (N/25 mm). The results are shown in Table 1.

[Test Example 5] <Evaluation of temporary adhesion>
The laminate obtained in Test Example 4 was observed, and the temporary adhesion was evaluated based on the following criteria. The results are shown in Table 1.
◯: The sample for adhesive strength measurement stuck to the adherend, and no air was entrapped at the interface.
Δ: The sample for adhesive strength measurement stuck to the adherend, and air entrapment was observed at the interface.
×: The sample for measuring adhesive strength did not stick to the adherend.

[Test Example 6] <Evaluation of voids in cross section of hardened body>
1. Area ratio of void portion The laminates after heat pressing obtained in Examples 1 to 3 and Comparative Examples 2 to 4, and 6 were heat treated at 125°C for 1 hour (preheating step), and then heat treated at 175°C for 2 hours (complete curing step) to completely cure the adhesive film after heat pressing and form a cured product. On the other hand, in Comparative Examples 1 and 5, the preheating step was not performed, and the adhesive film after heat pressing was directly heat treated at 175°C for 3 hours (complete curing step) to completely cure the adhesive film to form a cured product.

A cross section of the cured adhesive film was coated with a film approximately 30 nm thick using a sputtering device (manufactured by Vacuum Device Co., Ltd., product name "MSP-20-UM Magnetron Sputter") with a Pt-Pd target, and then antistatic treatment was performed. Six images of the cross section of the cured adhesive film were taken using a scanning electron microscope (SEM) device (manufactured by Keyence Corporation, product name "VE-9800"). The imaging conditions were an acceleration voltage of 8 kV and a magnification of 1000x. One of the SEM images (of the cured adhesive film of Example 1) is shown in Figure 3.

The six SEM images obtained were binarized to separate void areas from other areas. Image analysis software (ImageJ) was used for binarization, with a brightness threshold of 77-110. Figure 4 shows the binarized SEM image of the cross section of the cured adhesive film of Example 1 shown in Figure 3. In the figure, the black areas are void areas. The area percentage of the void areas was calculated using the binarized image by dividing the total area of the void areas by the area of the entire cross section of the adhesive film calculated from the SEM images, and then averaging the six SEM images. The results are shown in Table 1.

The brightness threshold value varies depending on the conditions set during SEM image observation, so the value was determined by comparing it with the actual SEM observation image. Furthermore, the binarized image was compared with the original SEM image, and any parts that did not correspond to voids were appropriately excluded.

For reference, Figure 5 shows an SEM image of a cross section of the cured adhesive film of Comparative Example 2. Similarly, Figure 6 shows the SEM image shown in Figure 5 after being binarized.

2. Maximum Area of Void Portions Using the binarized images of the six SEM images, the largest area of continuous void portions was determined as the maximum area (μm ² ) of the void portions. The results are shown in Table 1.

[Test Example 7] <Thermal diffusivity measurement evaluation>
The cured adhesive film obtained in Test Example 6 was cut into square samples with sides of 5 mm. The thermal diffusivity of the cured adhesive film was measured using a thermal conductivity measuring device (manufactured by aiphase, product name "aiphase mobile 1u"). The thermal diffusivity was then multiplied by the specific gravity and specific heat to calculate the thermal conductivity (W/mK) of the cured adhesive film. The results are shown in Table 1.

As can be seen from Table 1, the cured adhesive films produced in Examples 1 to 3 had excellent thermal conductivity. Furthermore, the adhesive films in Examples 1 to 3 also had excellent temporary adhesion.

The adhesive film and cured product of the present invention can be suitably used, for example, by being interposed between a heat-generating electronic device and a heat-dissipating substrate or heat sink to cool the electronic device. Furthermore, the structure of the present invention is useful, for example, as a structure comprising a heat-generating electronic device and a heat-dissipating substrate or heat sink.

DESCRIPTION OF SYMBOLS 1... adhesive film 11... support sheet 12... release sheet 2... adhesive film with support sheet 3... structure 1A... cured body of adhesive film 31... first member 32... second member

Claims (13)

A thermally conductive filler (A) comprising at least one of graphene having a two-dimensional structure and single-layer boron nitride;
a thermosetting component (B); and
A method for using an adhesive film made of an adhesive resin composition containing a binder polymer (C), comprising:
It is used to perform heat treatment after heat pressing,
the heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step,
The preheating step is a step of maintaining the temperature at or below the following temperature (T) for 30 minutes or more:
A method for using an adhesive film.
Temperature (T): The temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under the condition of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere. 2. The method for using an adhesive film according to claim 1 , wherein the content of the thermally conductive filler (A) is 5% by mass or more and 60% by mass or less. 3. The method for using an adhesive film according to claim 1 , wherein the thermosetting component (B) is an epoxy resin. 4. A method for using an adhesive film according to claim 1 , wherein the adhesive film has an arithmetic mean roughness (Ra) of at least one surface of the adhesive film of 0.01 μm or more and 0.5 μm or less. 5. The method for using the adhesive film according to claim 1, wherein the heating temperature of the heat press is lower than the curing reaction temperature of the thermosetting component (B). A method for using an adhesive film according to any one of claims 1 to 5, characterized in that the arithmetic mean roughness (Ra) of at least one surface of the adhesive film after the heat pressing is 0.01 μm or more and 0.5 μm or less. A method for using the adhesive film according to any one of claims 1 to 6 , comprising:
A method for using an adhesive film , characterized in that the adhesive film has a support sheet laminated on at least one side thereof. A thermally conductive filler (A) comprising at least one of graphene having a two-dimensional structure and single-layer boron nitride;
a thermosetting component (B); and
A method for producing a cured product, comprising heat-treating an adhesive film made of an adhesive resin composition containing a binder polymer (C) to form a cured product,
a step of heat-pressing the adhesive film before the heat treatment,
the heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step,
A method for producing a cured body, characterized in that the pre-heating step includes a step of holding the adhesive film at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetric measurement is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere for 30 minutes or more. 9. The method for producing a hardened body according to claim 8 , wherein the area ratio of voids in a cross section of the hardened body in the thickness direction is 10% or less. The maximum area of the void portion in the thickness direction cross section of the cured body is 100 μm ² The method for producing a cured body according to claim 8 or 9, wherein the method is as follows: A thermally conductive filler (A) comprising at least one of graphene having a two-dimensional structure and single-layer boron nitride;
a thermosetting component (B); and
a lamination step of laminating at least a part of a first member to at least a part of a second member via an adhesive film made of an adhesive resin composition containing a binder polymer (C);
and after the lamination, heat-treating the adhesive film to form a cured body, thereby obtaining a structure in which at least a part of the first member and at least a part of the second member are bonded via the cured body,
a step of heat-pressing the adhesive film before the heat treatment,
the heat treatment includes a preheating step and a complete curing step of completely curing the adhesive film after the preheating step,
A method for producing a structure, characterized in that the pre-heating step includes a step of holding the adhesive film at a temperature below the temperature at which the adhesive film loses 0.5% weight when thermogravimetry is performed on the adhesive film before any heat treatment under conditions of heating from 40°C to 400°C at a heating rate of 10°C/min in an air atmosphere for 30 minutes or more. The method for manufacturing a structure according to claim 11 , wherein the area ratio of voids in a cross section of the cured body in the thickness direction is 10% or less. 13. The method for manufacturing a structure according to claim 11 or 12, characterized in that in the lamination step, the lamination is carried out at a temperature equal to or higher than the temperature at which the loss tangent (tanδ) peaks as obtained by measuring the viscoelasticity of the adhesive film before any heat treatment.