JP7699424B2 - Method for producing laminated film for flexible printed wiring board and flexible printed wiring board - Google Patents

Description

The present invention relates to a method for producing a laminate film for flexible printed wiring boards and a flexible printed wiring board.

In ubiquitous computing, wearable computers are of great significance as terminals that end users come into direct contact with. For example, Patent Document 1 discloses a wearable device equipped with a camera, display, microphone, etc., in a glasses-type frame. In order to attach such a wearable device to the user's body, the wearable device must be flexible. For this reason, flexible printed circuit boards are often used as the internal substrate of wearable devices.

In rigid printed wiring boards, the miniaturization of circuits has progressed by using circuit formation methods such as the semi-additive (SAP) method or the modified semi-additive (MSAP) method, and in recent years, circuit formation with a circuit width (L)/circuit spacing (S) [hereinafter simply referred to as L/S] of 10 μm or less/10 μm or less has been achieved. On the other hand, the subtractive method, which is the mainstream method of circuit formation for flexible printed wiring boards, has limitations in strict control of circuit width and fine circuit formation, so that the miniaturization of circuits for flexible printed wiring boards has not progressed, and currently, only circuits with L/S of about 30 μm/30 μm are formed, and although some miniaturization is being considered, the reality is that the formation of fine circuits with L/S of 15 μm or less/15 μm or less has not yet been achieved. This is because in the subtractive method, circuits are formed by etching copper, but the etching proceeds not only in the thickness direction of the copper but also in the width direction, so the cross-sectional shape of the circuit becomes trapezoidal rather than rectangular.

Therefore, the use of circuit formation methods such as the SAP method or MSAP method, which selectively grow a plating film to form a circuit, has also been considered for flexible printed wiring boards (see, for example, Patent Document 2). If a circuit formation method such as the SAP method or MSAP method is used for flexible printed wiring boards, it is expected that the circuit will be finer than when a subtractive method is used.

JP 2015-149552 A JP 2017-204538 A

Patent Document 2 describes laminating a thin metal film on a transparent insulating resin film via a resin layer, and this configuration enhances adhesion between the transparent insulating resin film and the thin metal film.
Therefore, the present inventors began to study a method for laminating a thermosetting resin film with a copper foil and an insulating resin film so that the thermosetting resin film of the thermosetting resin film with the copper foil is in contact with the insulating resin film, thereby ensuring adhesion between the insulating resin film and the copper foil, and forming a fine circuit by a circuit formation method such as the SAP method or the MSAP method using the laminated film thus obtained. As a result, it was found that since the thermosetting resin film of the thermosetting resin film with the copper foil is already cured, it cannot be easily laminated to the insulating resin film, and it is necessary to heat the film to a predetermined temperature or higher in order to laminate a thermosetting resin film to, for example, a polyimide film.
The inventors further investigated and found that when a thermosetting resin film with a copper foil is heated to a predetermined temperature or higher, wrinkles tend to occur in the polyimide film in the resulting laminated film. When visually inspected, the wrinkles are not necessarily large, and may simply look like a pattern. However, even if such wrinkles exist in the polyimide film in the laminated film, the mere presence of such wrinkles tends to make it difficult to form a fine circuit with L/S=15 μm or less/15 μm or less (see Reference Example 1).

The inventors have conducted extensive research and found that if a thermosetting resin film with a copper foil can be welded to a polyimide film at 200°C or less, wrinkles are unlikely to occur in the polyimide film in the resulting laminated film. However, further research has revealed the following. The laminated film obtained by welding does not yet have a very high adhesion, so it is subsequently subjected to a heat curing process (hereinafter referred to as a curing process) to improve the adhesion between the polyimide film and the thermosetting resin film. However, it was found that if the laminated film obtained by welding is directly heat cured at 185°C for 60 minutes, voids are generated in the laminated film, causing a deterioration in appearance, or the adhesion between the polyimide film and the thermosetting resin film becomes insufficient. In other words, it was found that if a method is adopted in which a thermosetting resin film with a copper foil is welded to a polyimide film at 200°C or less in order to suppress the occurrence of wrinkles, it is difficult to improve the adhesion of the laminated film obtained in this way while suppressing the occurrence of voids.

In view of these circumstances, the present invention aims to provide a method for producing a laminate film for flexible printed wiring boards that suppresses the occurrence of wrinkles and voids, has high adhesion, and enables the formation of fine circuits with L/S = 15 μm or less/15 μm or less, and a flexible printed wiring board that contains the laminate film for flexible printed wiring boards obtained by the production method.

As a result of intensive research by the inventors to solve the above problems, they discovered that the above problems could be solved by carrying out a multi-stage curing process after welding a thermosetting resin film with a copper foil to a polyimide film at 200°C or less and setting the thermosetting temperature of each curing process within a predetermined range, which led to the completion of the present invention. The present invention was completed based on this knowledge.

The present invention relates to the following [1] to [13].
[1] A method for producing a laminate film for flexible printed wiring boards, comprising the step [1] of subjecting (1) a polyimide film and (2) a copper foil-attached thermosetting resin film to thermocompression bonding at 200° C. or less in such a manner that the thermosetting resin film faces the polyimide film, thereby fusing the thermosetting resin film to the polyimide film,
The method for producing a laminate film for flexible printed wiring boards includes a first curing step (i) of thermally curing the film at 115 to 155°C after the step [1], and a second curing step (ii) of further thermally curing the film at 170°C or higher after the first curing step (i).
[2] The method for producing a laminate film for flexible printed wiring boards according to the above [1], wherein the heat curing time in the first curing step (i) is 10 to 100 minutes.
[3] The method for producing a laminate film for flexible printed wiring boards according to the above [1] or [2], wherein the heat curing temperature in the second curing step (ii) is 170 to 200° C.
[4] The method for producing a laminate film for flexible printed wiring boards according to any one of [1] to [3] above, wherein the heat curing time in the second curing step (ii) is 20 to 100 minutes.
[5] The method for producing a laminate film for flexible printed wiring boards according to any one of [1] to [4] above, wherein the thermal curing in the first curing step (i) and the second curing step (ii) is carried out under normal pressure.
[6] The method for producing a laminate film for flexible printed wiring boards according to any one of the above [1] to [5], wherein in the step [1], the thermocompression bonding is performed by a roll lamination method.
[7] The method for producing a laminate film for flexible printed wiring boards according to any one of [1] to [6] above, wherein the thermosetting resin film of the copper foil-attached thermosetting resin film (2) contains (A) an epoxy resin, (B) an epoxy resin curing agent, and (C) a curing accelerator, and the (C) curing accelerator is formed from a thermosetting resin composition containing a strongly basic compound.
[8] The method for producing a laminate film for flexible printed wiring boards according to the above [7], wherein the strongly basic compound has a pKa of 10 or more.
[9] The method for producing a laminate film for flexible printed wiring boards according to the above [7] or [8], wherein the strongly basic compound is at least one selected from the group consisting of 1,8-diazabicyclo[5.4.0]undecene-7 (DBU), 1,5-diazabicyclo[4.3.0]nonene-5 (DBN), tetramethylguanidine (TMG), 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,5,7-triazabicyclo[4.4.0]decene-5 (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]decene-5 (MTBD).
[10] The method for producing a laminate film for flexible printed wiring boards according to any one of [7] to [9] above, wherein the content of the strongly basic compound is 0.6 parts by mass or more per 100 parts by mass of the epoxy resin (A).
[11] The method for producing a laminate film for flexible printed wiring boards according to any one of [7] to [10] above, wherein the (B) epoxy resin curing agent contains a phenol-based curing agent.
[12] A flexible printed wiring board comprising a laminate film for flexible printed wiring boards obtained by the manufacturing method according to any one of [1] to [11] above.
[13] The flexible printed wiring board according to the above [12], having a circuit with a circuit width (L)/circuit spacing (S) of 15 μm or less/15 μm or less.

The present invention can provide a method for producing a laminate film for flexible printed wiring boards that suppresses the occurrence of wrinkles and voids, has high adhesion, and enables the formation of fine circuits with L/S = 15 μm or less/15 μm or less. It can also provide a flexible printed wiring board having a fine circuit with L/S = 15 μm or less/15 μm or less.

1 is a schematic diagram of one embodiment of a method for producing a laminate film for flexible printed wiring boards of the present invention. 1 is a schematic diagram showing one embodiment of a laminate film for flexible printed wiring boards obtained by the present invention. FIG. 2 is a schematic diagram showing another embodiment of the laminate film for flexible printed wiring boards obtained by the present invention. FIG. 2 is a schematic diagram for explaining a method for measuring the welding temperature defined in the present invention. 1 is a photograph showing the state of a polyimide film in a laminate film A for flexible printed wiring boards produced in Example 1 (the state in which voids are suppressed). 1 is a photograph showing the state of a polyimide film (the state in which voids are generated) in a laminate film G for flexible printed wiring boards produced in Comparative Example 1. 1 is a photograph showing the state (without wrinkles) of a polyimide film in a laminate film A for flexible printed wiring boards produced in Example 1. 1 is a scanning electron microscope (SEM) photograph of a circuit with L/S=8 μm/8 μm of the flexible printed wiring board A produced in Example 1. The line in the lower right corner is 20 μm per division, and 200 μm per 10 divisions. 1 is a photograph showing the state (with wrinkles) of a polyimide film in a laminate film X for flexible printed wiring boards produced in Reference Example 1.

In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the values shown in the examples. In addition, the lower and upper limits of the numerical ranges may be arbitrarily combined with the lower or upper limit of other numerical ranges.
Furthermore, in this specification, the content of each component in the thermosetting resin composition means the total content of the multiple substances present in the thermosetting resin composition when multiple substances corresponding to each component are present, unless otherwise specified. In addition, the solid content means the residue in the thermosetting resin composition excluding volatile components.
The present invention also includes any combination of the features described in this specification.
In this specification, room temperature refers to 5 to 35° C., and preferably 15 to 25° C. In addition, in this specification, the average primary particle size refers to the average particle size of unagglomerated particles, not the so-called secondary particle size of aggregated particles. The average primary particle size is a value measured by a laser diffraction particle size distribution analyzer.

[Method of manufacturing laminate film for flexible printed wiring boards]
The manufacturing method of the present invention utilizes the characteristic of the copper foil-attached thermosetting resin film described below, that is, the thermosetting resin film is welded to the polyimide film at 200° C. or less.
Specifically, the present invention relates to a method for producing a laminate film for flexible printed wiring boards, the method comprising the step [1] of thermocompression bonding (1) a polyimide film and (2) a thermosetting resin film with a copper foil at 200° C. or less (the set temperature of a heat source, which is preferably 140 to 200° C., more preferably 140 to 195° C., and even more preferably 140 to 190° C., and the lower limit of the temperature range may be 150° C., 160° C., or 165° C.) so that the thermosetting resin film faces the polyimide film, thereby fusing the thermosetting resin film to the polyimide film,
The method for producing a laminate film for flexible printed wiring boards includes a first curing step (i) of thermally curing the film at 115 to 155°C after the step [1], and a step curing step (ii) of further thermally curing the film at 170°C or higher after the first curing step (i).
Hereinafter, the "laminate film for flexible printed wiring boards" may be abbreviated to "laminate film".

The laminate film obtained by the production method of the present invention is one in which the occurrence of wrinkles in the polyimide film is suppressed. By using this laminate film and a circuit formation method such as the SAP method or the MSAP method, it is possible to produce a flexible printed wiring board having a fine circuit, in particular a circuit with L/S = 15 μm or less/15 μm or less (preferably 10 μm or less/10 μm or less).
Furthermore, the laminated film obtained by the production method of the present invention has improved adhesion while suppressing the generation of voids, and has an excellent appearance.
Each step will be described below in order.

(Step [1])
First, step [1] will be described in detail.
The (1) polyimide film used in step [1] is preferably one that has a suitable strength and does not cause breakage. For example, so-called aromatic polyimides in which aromatic compounds are directly linked by imide bonds are preferred from the viewpoint of film strength. The polyimide film may be one that has been surface-treated to enhance adhesion.

(1) The polyimide film is not particularly limited, but can be produced, for example, by reacting (polycondensing) a known acid anhydride with a known diamine compound to form a polyamic acid, and then by dehydrating and cyclizing the polyamic acid to form a film. The imidization rate of the polyimide film is preferably 50% or more, more preferably 80% or more, even more preferably 90% or more, and particularly preferably 95% or more. The imidization rate is measured by Fourier transform infrared spectroscopy (FT-IR method).
The acid anhydride is not particularly limited, and examples thereof include aromatic tetracarboxylic dianhydrides such as pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride, etc. Among these, biphenyltetracarboxylic dianhydride is preferred as the acid anhydride.
The diamine compound is not particularly limited, but examples thereof include aromatic diamine compounds such as oxydianiline, p-phenylenediamine, and benzophenonediamine.

(1) As the polyimide film, commercially available products can be used. Examples of commercially available polyimide films include Upilex R, Upilex S, Upilex SGA (all manufactured by Ube Industries, Ltd., "Upilex" is a registered trademark), Kapton H, Kapton V, Kapton E, Kapton EN, Kapton ENZT (all manufactured by DuPont-Toray Co., Ltd., "Kapton" is a registered trademark), Apical AH, Apical NPI (all manufactured by Kaneka Corporation, "Apical" is a registered trademark), and the like.
The thickness of the polyimide film (1) is preferably from 5 to 250 μm, more preferably from 10 to 175 μm, further preferably from 15 to 100 μm, and particularly preferably from 20 to 50 μm, from the viewpoint of film handling properties.

The details of the (2) thermosetting resin film with copper foil used in step [1] will be described later. Examples of the thermosetting resin film with copper foil include (A) "thermosetting resin film/copper foil" and (B) "thermosetting resin film/ultra-thin copper foil/peeling layer/carrier copper foil".

The method for producing the laminated film of the present invention is not particularly limited, but from the viewpoint of industrial implementation, a production method using a roll-to-roll method is preferred. In addition, although there is no particular limit to the process [1], it is preferred to use a roll lamination method from the viewpoint of suppressing the intrusion of voids and from the viewpoint of simplicity.
FIG. 1 shows a schematic diagram of one embodiment of the manufacturing method of the laminated film of the present invention using the roll-to-roll method and the roll lamination method. FIG. 1 shows a diagram in which the thermosetting resin film with copper foil is thermocompressed to both sides of the polyimide film, but the thermosetting resin film with copper foil may be thermocompressed to only one side. A polyimide film 1 is fed from a polyimide film feed device 8, and a thermosetting resin film with copper foil 4 is fed from a copper foil thermosetting resin film feed device 9. At this time, the thermosetting resin film 4 with copper foil is fed in a state in which the thermosetting resin film 3 is located on the polyimide film 1 side. After each film passes through a free roll 10, the thermosetting resin film 3 of the thermosetting resin film with copper foil is laminated to the polyimide film 1 while being welded by a laminating roll 11 at 200° C. or less (preferably in the above-mentioned temperature range), to form a laminated film 7.
The linear speed at which each film is fed is preferably 0.05 to 5 m/min, more preferably 0.1 to 4.5 m/min, even more preferably 0.1 to 4 m/min, particularly preferably 0.3 to 3.5 m/min, and most preferably 0.3 to 3 m/min.
The surface pressure of the laminating roll 11 is preferably 0.3 to 1.2 MPa, and more preferably 0.5 to 1.0 MPa.

(Step [2])
Next, step [2] will be described in detail.
In the method for producing a laminated film of the present invention, after the step [1], a step cure step [2] is performed, which includes a first cure step (i) of thermally curing at 115 to 155° C., and a second cure step (ii) of further thermally curing at 170° C. or higher after the first cure step (i). The step cure step [2] suppresses the generation of voids during thermal curing, and a laminated film having high adhesion between the thermosetting resin film and the polyimide film can be obtained.
The step cure process [2] may be composed of two processes, a first cure process (i) and a second cure process (ii), or may further include another cure process, but is preferably composed of two processes, a first cure process (i) and a second cure process (ii). In addition, when it further includes another cure process, the other cure process may be composed of one process or may be composed of two or more processes. In addition, from the viewpoint of fully exhibiting the effects of the present invention, it is preferable that the other cure process is between the first cure process (i) and the second cure process (ii) or after the second cure process (ii).
The heating device used to thermally cure the laminated film in each step of the step cure process [2] is not particularly limited, and examples thereof include a furnace such as a quartz tube furnace; a heat treatment device such as a hot plate or a rapid thermal annealing (RTA) device; a dryer, etc. Both the heating device and the dryer may be of a batch type or a continuous type. For example, one embodiment in which the step cure process [2] is carried out using a batch type dryer is also one of the preferred embodiments.

<First curing step (i)>
In the first curing step (i), the laminated film obtained in step [1] is thermally cured at 115 to 155°C. By subjecting the laminated film obtained in step [1] to the first curing step (i) before the second curing step (ii), the generation of voids during thermal curing is suppressed, and a laminated film having high adhesion between the thermosetting resin film and the polyimide film can be obtained. In this specification, the temperature during thermal curing in the step curing step [2] may be referred to as the "thermal curing temperature".
Voids occur in the laminated film whether the heat curing temperature in the first curing step (i) is lower than 115° C. or higher than 155° C. Furthermore, when the heat curing temperature in the first curing step (i) is lower than 115° C., the adhesion is also likely to be insufficient.
The heat curing temperature in the first curing step (i) is preferably 120 to 150°C, more preferably 130 to 145°C, and even more preferably 135 to 145°C, from the viewpoint of suppressing voids and improving adhesion.
The heat curing time in the first curing step (i) is preferably 10 to 100 minutes, more preferably 10 to 80 minutes, even more preferably 15 to 70 minutes, particularly preferably 30 to 70 minutes, and most preferably 50 to 70 minutes, from the viewpoint of suppressing voids and improving adhesion.
The first curing step (i) is not particularly limited, but is preferably carried out under normal pressure in order to avoid unevenness caused by applying pressure.

<Second curing step (ii)>
In the second curing step (ii), the laminated film obtained in the first curing step (i) is further heat-cured at a temperature of 170° C. or higher. From the viewpoint of improving adhesion, the heat-curing temperature in the second curing step (ii) is preferably 170 to 200° C., more preferably 175 to 190° C., and even more preferably 180 to 190° C.
From the viewpoint of improving adhesion, the heat curing time in the second curing step (ii) is preferably 20 to 100 minutes, more preferably 30 to 90 minutes, further preferably 40 to 80 minutes, and particularly preferably 50 to 70 minutes.
The second curing step (ii) is not particularly limited, but is preferably carried out under normal pressure in order to avoid unevenness caused by pressurization. Therefore, it is preferable to carry out both the first curing step (i) and the second curing step (ii) under normal pressure, and further, it is preferable to carry out the entire step curing step [2] under normal pressure.

Examples of the laminated film obtained in this way include laminated films having the structures shown in Figures 2 and 3. Figure 2 has a structure of "polyimide film 1/thermosetting resin film 3/copper foil 2", and Figure 3 has a structure of "polyimide film 1/thermosetting resin film 3/ultra-thin copper foil 12/peeling layer 13/carrier copper foil 14".

The thermosetting resin film of the (2) copper foil-attached thermosetting resin film used in the step [1] will be described in detail below.
(Thermosetting resin film)
The thermosetting resin film of the (2) copper foil-attached thermosetting resin film used in the step [1] is preferably a thermosetting resin film having a welding temperature, as defined below, to a polyimide film of 200° C. or less. The definition of the welding temperature will be explained below with reference to FIG. 4 as appropriate.
(Definition of welding temperature)
First, a copper foil-attached thermosetting resin film 4 having a copper foil 2 and a thermosetting resin film 3 is superimposed on a polyimide film 1 having a thickness of 25 μm so that the thermosetting resin film 3 and the polyimide film 1 are in contact with each other, and the polyimide film 1 is placed on the hot plate 5 so that the hot plate 5 is the side of the hot plate 5. Next, a 50 g weight 6 is placed on the copper foil-attached thermosetting resin film 4 and left to stand at a predetermined temperature for 60 seconds to form a laminated film 7. Thereafter, the hot plate 5 and the weight 6 are removed, and the formed laminated film 7 is allowed to cool to room temperature. When the polyimide film 1 and the thermosetting resin film 3 are peeled off from the obtained laminated film 7, if the thermosetting resin film 3 is attached to 80% or more of the surface of the polyimide film 1, the thermosetting resin film 3 is considered to be welded to the polyimide film 1, and the lowest temperature at which the film is welded is taken as the welding temperature.
"80%" means that when the surface of the polyimide film 1 is visually observed, the area of the portion of the surface of the polyimide film 1 which is not visible is 80% or more.

In the thermosetting resin film, the welding temperature for the polyimide film is 200° C. or less. By using a welding temperature of 200° C. or less, the thermosetting resin film with copper foil and the polyimide film can be thermally bonded at 200° C. or less, so that the occurrence of wrinkles in the polyimide film in the obtained laminated film can be effectively suppressed. As described above, the wrinkles are not necessarily large wrinkles, and may be wrinkles that look like simple patterns when visually confirmed. However, even if there are such wrinkles, the mere presence of the wrinkles in the polyimide film in the laminated film makes it difficult to form a fine circuit with L/S=15 μm or less/15 μm or less in the flexible printed wiring board. Therefore, in order to form a fine circuit for a flexible printed wiring board, it is important to form a laminated film while suppressing the occurrence of wrinkles.
For example, when the thermosetting resin film with copper foil and the polyimide film are thermocompressed at 230°C or higher, the wrinkles tend to occur in the polyimide film in the laminated film. This is particularly noticeable at 250°C or higher. The reason for this is thought to be that the copper foil and the polyimide film have almost the same thermal expansion coefficient at 200°C or lower, but at high temperatures of 230°C or higher, the thermal expansion coefficients of the copper foil are significantly different. For example, the thermal expansion coefficient of the copper foil is about 15 ppm/°C from 0°C to 300°C, which does not change significantly, while the thermal expansion coefficient of the polyimide film is about 15 ppm/°C from 0°C to 200°C, but increases to about 28 ppm/°C at 250°C, so the difference in the thermal expansion coefficients of the two becomes large, and the degree of expansion and contraction during thermal expansion and thermal contraction becomes different, making it easier for wrinkles to occur. This is presumably the reason.

There is no particular lower limit to the welding temperature of the thermosetting resin film, but since it is practically difficult to weld the film to a polyimide film at a low temperature, the lower limit is usually 140° C. or higher.
From the above viewpoints, the welding temperature of the thermosetting resin film is preferably 140 to 200°C, more preferably 140 to 195°C, and even more preferably 140 to 190°C, and the lower limit of the welding temperature may be 150°C, 160°C, or 165°C.

The thermosetting resin film is not particularly limited as long as it has the above-mentioned welding temperature. From the viewpoint of setting the welding temperature for a polyimide film at 200° C. or less, the thermosetting resin film is preferably formed from a thermosetting resin composition containing (A) an epoxy resin, (B) an epoxy resin curing agent, and (C) a curing accelerator, wherein the (C) curing accelerator contains a strongly basic compound.
Each component that the thermosetting resin composition may contain will be described in detail below.

<(A) Epoxy resin>
The epoxy resin (A) is an epoxy resin having two or more epoxy groups in one molecule. Here, the epoxy resin (A) is classified into glycidyl ether type epoxy resin, glycidyl amine type epoxy resin, glycidyl ester type epoxy resin, etc. Among these, the glycidyl ether type epoxy resin is preferred.
[0023] (A) Epoxy resins are classified into various epoxy resins depending on the difference in the main skeleton, and each of the above types of epoxy resins is further classified into bisphenol type epoxy resins such as bisphenol A type epoxy resins, bisphenol F type epoxy resins, and bisphenol S type epoxy resins; alicyclic epoxy resins; aliphatic chain epoxy resins; novolac type epoxy resins such as phenol novolac type epoxy resins, cresol novolac type epoxy resins, bisphenol A novolac type epoxy resins, and bisphenol F novolac type epoxy resins; phenol aralkyl type epoxy resins; stilbene type epoxy resins; dicyclopentadiene type epoxy resins; naphthalene skeleton-containing epoxy resins such as naphthol novolac type epoxy resins and naphthol aralkyl type epoxy resins; biphenyl type epoxy resins; biphenyl aralkyl type epoxy resins; xylylene type epoxy resins; dihydroanthracene type epoxy resins; dicyclopentadiene type epoxy resins, etc.
Among these, from the viewpoint of heat resistance, aromatic epoxy resins are preferred, novolac type epoxy resins, naphthalene skeleton-containing epoxy resins, biphenyl type epoxy resins, and biphenylaralkyl type epoxy resins are more preferred, and naphthalene skeleton-containing epoxy resins, biphenyl type epoxy resins, and biphenylaralkyl type epoxy resins are even more preferred from the viewpoints of low thermal expansion, desmear resistance, etc. Furthermore, dicyclopentadiene type epoxy resins are preferred from the viewpoint of adhesive strength with copper foil and electroless plating.
The epoxy resin (A) may be used alone or in combination of two or more kinds from the viewpoints of insulation reliability and heat resistance.

The epoxy equivalent of the (A) epoxy resin is preferably 50 to 3,000 g/eq, more preferably 80 to 2,000 g/eq, even more preferably 100 to 1,000 g/eq, and particularly preferably 200 to 400 g/eq. Here, the epoxy equivalent is the mass (g/eq) of the resin per equivalent of epoxy group, and can be measured according to the method specified in JIS K 7236. Specifically, using an automatic titration device "GT-200 type" (manufactured by Mitsubishi Chemical Analytech Co., Ltd.), 2 g of epoxy resin is weighed into a 200 ml beaker, 90 ml of methyl ethyl ketone is dropped, and after dissolving in an ultrasonic cleaner, 10 ml of glacial acetic acid and 1.5 g of cetyltrimethylammonium bromide are added, and the epoxy equivalent is determined by titrating with a 0.1 mol/L perchloric acid/acetic acid solution.

(A) Commercially available epoxy resins include, for example, bisphenol A type epoxy resins "jER1001", "jER828EL", and "YL980" (manufactured by Mitsubishi Chemical Corporation, "jER" is a registered trademark), bisphenol F type epoxy resins "jER806H" and "YL983U" (all manufactured by Mitsubishi Chemical Corporation), phenol novolac type epoxy resin "N770" (manufactured by DIC Corporation), phenol biphenyl aralkyl type epoxy resin "NC-3000H" (manufactured by Nippon Kayaku Co., Ltd.), cresol novolac type epoxy resin "EPICLON N-660" (manufactured by DIC Corporation, "EPICLON" is a registered trademark), and naphthalene skeleton-containing epoxy resin "HP-9900" (manufactured by DIC Corporation).

Here, from the viewpoint of flexibility, the epoxy resin (A) may be an epoxy resin having two or more epoxy groups in one molecule and having a structural unit derived from an alkylene glycol having three or more carbon atoms. The structural unit derived from an alkylene glycol having three or more carbon atoms is preferably present in the main chain of the epoxy resin.
In addition, "having a structural unit derived from an alkylene glycol having 3 or more carbon atoms" may be one obtained by using an alkylene glycol having 3 or more carbon atoms as a monomer, or may be one obtained by using a compound having an alkylene glycol skeleton having 3 or more carbon atoms.

The alkylene glycol having 3 or more carbon atoms is preferably an alkylene glycol having 4 or more carbon atoms. The upper limit of the carbon number is not limited, but is preferably 15 or less, more preferably 10 or less, and even more preferably 8 or less. The (A) epoxy resin may be, for example, a bisphenol A type epoxy resin having a structural unit derived from hexanediol in the main chain.
A specific example of an epoxy resin having a structural unit derived from an alkylene glycol having 3 or more carbon atoms in the main chain is an epoxy resin obtained by reacting a vinyl ether compound represented by the following general formula (I) with a bifunctional phenol compound represented by the following general formula (II) and then epoxidizing the resulting mixture. More specifically, the epoxy resin can be produced by reacting a vinyl ether compound represented by the following general formula (I) with a bifunctional phenol compound represented by the following general formula (II) and then epoxidizing the terminals with an epihalohydrin such as epichlorohydrin. The epoxy resin can also be said to have a structure derived from an alkylene diol represented by HO[-R ¹ -O]n-H (R ¹ and n are the same as those in general formula (I)) and a structure derived from a bifunctional phenol compound represented by the following general formula (II).

In general formula (I), ^R1 represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms. In general formula (II), ^R2 represents an aliphatic hydrocarbon group having 1 to 10 carbon atoms, preferably an aliphatic hydrocarbon group having 1 to 5 carbon atoms, and more preferably an aliphatic hydrocarbon group having 1 to 3 carbon atoms. In addition, n represents an integer of 1 to 15, preferably an integer of 1 to 10, more preferably an integer of 1 to 5, even more preferably an integer of 1 to 3, and particularly preferably 1.
Examples of the aliphatic hydrocarbon group having 1 to 10 carbon atoms represented by ^R1 include alkylene groups having 1 to 10 carbon atoms, such as methylene, 1,2-dimethylene, 1,2-trimethylene, 1,3-trimethylene, 1,4-tetramethylene, 1,5-pentamethylene, 1,6-hexamethylene, and 1,9-nonamethylene. The alkylene group having 1 to 10 carbon atoms is preferably an alkylene group having 2 to 10 carbon atoms, more preferably an alkylene group having 2 to 7 carbon atoms, and even more preferably a 1,6-hexamethylene group.
Examples of the aliphatic hydrocarbon group having 1 to 10 carbon atoms represented by ^R2 include alkylene groups having 1 to 10 carbon atoms, such as methylene, 1,2-dimethylene, 1,3-trimethylene, 1,4-tetramethylene, and 1,5-pentamethylene; and alkylidene groups having 2 to 10 carbon atoms, such as ethylidene, propylidene, isopropylidene, butylidene, isobutylidene, pentylidene, and isopentylidene. The alkylene group having 1 to 10 carbon atoms is preferably an alkylene group having 1 to 5 carbon atoms, and more preferably an alkylene group having 1 to 3 carbon atoms. The alkylidene group having 2 to 10 carbon atoms is preferably an alkylidene group having 2 to 5 carbon atoms, and more preferably an alkylidene group having 2 to 3 carbon atoms, and even more preferably an isopropylidene group.
^R2 is preferably attached at the 4-position of the phenol.

When the thermosetting resin composition contains (A) an epoxy resin, the content is preferably 10 to 85 mass %, more preferably 30 to 80 mass %, and even more preferably 45 to 75 mass %, based on the solid content of the thermosetting resin composition, from the viewpoint of the balance between various properties such as adhesive strength with copper foil and electroless plating and heat resistance.

<(B) Epoxy Resin Curing Agent>
Examples of the epoxy resin curing agent (B) include phenol-based curing agents, cyanate ester-based curing agents, and acid anhydride-based curing agents. Among these, from the viewpoint of lowering the welding temperature to the polyimide film, it is preferable that the epoxy resin curing agent (B) contains a phenol-based curing agent. The epoxy resin curing agent (B) may be used alone or in combination of two or more kinds.

The phenol-based curing agent is not particularly limited, and preferred examples thereof include cresol novolac-type curing agents, biphenyl-type curing agents, phenol novolac-type curing agents, naphthylene ether-type curing agents, triazine skeleton-containing phenol-based curing agents, etc. Among these, cresol novolac-type curing agents, biphenyl-type curing agents, and triazine skeleton-containing phenol-based curing agents are more preferred.
Examples of commercially available phenol-based curing agents include cresol novolac-type curing agents such as Phenolite KA-1160, Phenolite KA-1163, Phenolite KA-1165, and EXB-9829 (all manufactured by DIC Corporation, "Phenolite" is a registered trademark); biphenyl-type curing agents such as MEH-7700, MEH-7810, and MEHC-7851 (all manufactured by Meiwa Kasei Co., Ltd.); phenol novolac-type curing agents such as Phenolite TD2090 (manufactured by DIC Corporation); naphthylene ether-type curing agents such as EXB-6000 (manufactured by DIC Corporation); and triazine skeleton-containing phenol-based curing agents such as LA3018, LA7052, LA7054, and LA1356 (all manufactured by DIC Corporation).

The cyanate ester curing agent is not particularly limited, but examples thereof include bisphenol A dicyanate, polyphenol cyanate [oligo(3-methylene-1,5-phenylene cyanate)], 4,4'-methylenebis(2,6-dimethylphenyl cyanate), 4,4'-ethylidene diphenyl dicyanate, hexafluorobisphenol A dicyanate, 2,2-bis(4-cyanate)phenylpropane, 1,1-bis(4-cyanate phenylmethane), bis(4-cyanate-3,5-dimethylphenyl)methane, 1,3-bis(4-cyanate phenyl-1-(methylethylidene))benzene, bis(4-cyanate phenyl)thioether, bis(4-cyanate phenyl)ether, etc.

The acid anhydride curing agent is not particularly limited, but examples thereof include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylnadic anhydride, hydrogenated methylnadic anhydride, trialkyltetrahydrophthalic anhydride, dodecenyl succinic anhydride, 5-(2,5-dioxotetrahydro-3-furanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, trimellitic anhydride, and pyromellitic anhydride.

When the thermosetting resin composition contains (B) an epoxy resin curing agent, the content is preferably 10 to 100 parts by mass, more preferably 20 to 85 parts by mass, and even more preferably 40 to 70 parts by mass, per 100 parts by mass of (A) epoxy resin, from the viewpoints of adhesive strength with copper foil and electroless plating and heat resistance.

<(C) Curing Accelerator>
From the viewpoint of setting the welding temperature for the polyimide film at 200° C., it is preferable that the curing accelerator (C) contains a strongly basic compound, and it is also preferable that the strongly basic compound is contained in a predetermined amount as described below.
The strongly basic compound is preferably a strongly basic compound having a pKa of 10 or more, more preferably a strongly basic compound having a pKa of 11 or more, still more preferably a strongly basic compound having a pKa of 11 to 16, and particularly preferably a strongly basic compound having a pKa of 11 to 14. In the present invention, pKa is the pKa in water. The strongly basic compound is preferably a nitrogen-containing cyclic compound having the above-mentioned pKa, more preferably at least one selected from the group consisting of 1,8-diazabicyclo[5.4.0]undecene-7 (DBU), 1,5-diazabicyclo[4.3.0]nonene-5 (DBN), tetramethylguanidine (TMG), 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,5,7-triazabicyclo[4.4.0]decene-5 (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]decene-5 (MTBD), and even more preferably at least one selected from the group consisting of DBU and DBN.

When the (C) curing accelerator contains a strongly basic compound, the content is preferably 0.6 parts by mass or more, and may be 0.8 parts by mass or more, per 100 parts by mass of the (A) epoxy resin. By containing 0.6 parts by mass or more of the strongly basic compound per 100 parts by mass of the (A) epoxy resin, a thermosetting resin film having a welding temperature to a polyimide film of 200° C. or less can be obtained. However, this is merely one example of a method for adjusting the welding temperature to the above-mentioned level, and the present invention is not particularly limited to this method.
Generally, increasing the content of the curing accelerator such as a strongly basic compound increases the gelation rate of the thermosetting composition, and therefore the content of the strongly basic compound should be avoided to be 0.6 parts by mass or more per 100 parts by mass of the epoxy resin (A). However, in the present invention, the content was intentionally increased, and an effect of lowering the welding temperature to the polyimide film was obtained.

Although the reason why the inclusion of 0.6 parts by mass or more of a strongly basic compound per 100 parts by mass of (A) epoxy resin allows the resulting thermosetting resin film to be welded to a polyimide film at 200° C. or less has not been clarified, it is speculated that there are the following possibilities: For example, (1) the inclusion of the specified amount of a strongly basic compound may shorten the molecular chain length of the polymer component in the formed thermosetting resin film, or may include low-molecular-weight compounds, which function as adhesive components for the polyimide film, and (2) when a phenol-based curing agent is used as the (B) epoxy resin curing agent, the phenol-based curing agent and the strongly basic compound may form a salt, which may improve the adhesion to the polyimide film.
There is no particular upper limit to the content of the strongly basic compound, but from the viewpoint of maintaining a low gelation rate of the thermosetting resin composition, the content is preferably 2.8 parts by mass or less, more preferably 2.5 parts by mass or less, even more preferably 2.0 parts by mass or less, particularly preferably 1.7 parts by mass or less, and most preferably 1.5 parts by mass or less, and in any case, the lower limit can be considered to be 0.6 parts by mass or more, or can also be considered to be 0.8 parts by mass or more.

The (C) curing accelerator may contain a curing accelerator other than the strongly basic compound. Examples of such curing accelerators include imidazoles and their derivatives; organic phosphorus compounds such as phosphines and phosphonium salts, and adducts of tertiary phosphines and quinones; secondary amines, tertiary amines, and quaternary ammonium salts (excluding the strongly basic compounds). As the (C) curing accelerator, one type may be used alone, or two or more types may be used in combination, but as described above, it is preferable to use at least a strongly basic compound.

As the curing accelerator other than the strongly basic compound, for example, from the viewpoint of adhesion strength to copper foil and electroless plating and flame retardancy, imidazole compounds and derivatives thereof are preferred. In other words, it can be said that a combination of the strongly basic compound and an imidazole compound or a derivative thereof as the curing accelerator (C) is also a preferred embodiment.
Specific examples of the imidazole compound and its derivatives include 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 1,2-dimethylimidazole, 2-ethyl-1-methylimidazole, 1,2-diethylimidazole, 1-ethyl-2-methylimidazole, 2-ethyl-4-methylimidazole, 4-ethyl-2-methylimidazole, 1-isobutyl-2-methylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2,3-dihydro-1H-pyrrolo[1,2- a] benzimidazole, 2,4-diamino-6-[2'-methylimidazolyl-(1')]ethyl-s-triazine, 2,4-diamino-6-[2'-undecylimidazolyl-(1')]ethyl-s-triazine, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]ethyl-s-triazine and other imidazole compounds; 2-methylimidazoline, 2-ethyl-4-methylimidazoline, 2-undecylimidazoline, 2-phenyl-4-methylimidazoline, Examples of the imidazole compound include an imidazoline compound such as ethyl imidazoline, an addition reaction product of the imidazole compound (preferably 1-cyanoethyl-2-phenylimidazole) with trimellitic acid, an addition reaction product of the imidazole compound with isocyanuric acid, an addition reaction product of the imidazole compound (preferably 2-ethyl-4-methylimidazole) with a diisocyanate compound (preferably hexamethylene diisocyanate), an addition reaction product of the imidazole compound with hydrobromic acid, etc. The imidazole compound may be used alone or in combination of two or more kinds.

When the thermosetting resin composition contains a curing accelerator other than the strongly basic compound, the content is preferably 0.1 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, even more preferably 0.1 to 1 part by mass, and particularly preferably 0.1 to 0.5 parts by mass, relative to 100 parts by mass of the epoxy resin (A) from the viewpoints of heat resistance and storage stability. In addition, the composition may not contain any curing accelerator other than the strongly basic compound.

<(D) Polymer component>
The thermosetting resin composition may further contain a polymer component (D). However, the polymer component (D) does not include an epoxy resin. The polymer component (D) may be used alone or in combination of two or more kinds.
Examples of the polymer component (D) include (D1) crosslinked rubber particles, (D2) at least one selected from the group consisting of polyvinyl acetal resins and carboxylic acid-modified polyvinyl acetal resins, etc. From the viewpoint of adhesive strength to copper foil and electroless plating, it is preferable to use the component (D1) in combination with the component (D2).

((D1) Crosslinked Rubber Particles)
The (D1) crosslinked rubber particles are preferably those having excellent affinity with the (A) epoxy resin, and are not particularly limited, but include acrylonitrile butadiene crosslinked rubber particles, carboxylic acid modified acrylonitrile butadiene crosslinked rubber particles, and core-shell type crosslinked rubber particles, etc. By using the (D1) component, the adhesive strength with copper foil and electroless plating tends to be increased.
The crosslinked rubber particles (D1) may be used alone or in combination of two or more kinds.

The acrylonitrile-butadiene crosslinked rubber particles are obtained by partially crosslinking acrylonitrile and butadiene during copolymerization to obtain particulate form. Also, carboxylic acid such as acrylic acid, methacrylic acid, etc. can be copolymerized together to obtain carboxylic acid modified acrylonitrile-butadiene crosslinked rubber particles.
The core-shell type crosslinked rubber particles are crosslinked rubber particles having a core layer and a shell layer. For example, they may have a two-layer structure in which the outer shell layer is made of a glassy polymer and the inner core layer is made of a rubbery polymer; or a three-layer structure in which the outer shell layer is made of a glassy polymer, the middle layer is made of a rubbery polymer, and the core layer is made of a glassy polymer. The glass layer is made of, for example, a polymer of methyl methacrylate, and the rubbery polymer layer is made of, for example, an acrylic rubber, a butadiene rubber, or the like.
The size of the (D1) crosslinked rubber particles can be set to 50 nm to 1 μm in terms of average primary particle diameter.

As the crosslinked rubber particles (D1), a commercially available product may be used. The commercially available product may be in the form of an "epoxy resin containing crosslinked rubber particles."
Commercially available acrylonitrile butadiene rubber (NBR) particles include the XER-91 series (manufactured by JSR Corporation).
Commercially available core-shell type rubber particles include Staphyloid AC3832, AC3816N (manufactured by Aica Kogyo Co., Ltd., "Staphyloid" is a registered trademark), Metablen KW-4426, W300A, W450A (manufactured by Mitsubishi Chemical Corporation, "Metablen" is a registered trademark), EXL-2655 (manufactured by Dow Chemical Japan Co., Ltd.), butadiene-alkyl methacrylate-styrene copolymer, Paraloid EXL-2655 (manufactured by Kureha Corporation, "Paraloid" is a registered trademark), acrylic acid ester-methacrylic acid ester copolymer, Staphyloid AC-3355, TR-2122 (manufactured by Aica Kogyo Co., Ltd., "Staphyloid" is a registered trademark), and butyl acrylate-methyl methacrylate copolymer, Kane Ace MX series (manufactured by Kaneka Corporation, "Kane Ace" is a registered trademark).

When the thermosetting resin composition contains the (D1) component, the content thereof is preferably 0.5 to 25 parts by mass, more preferably 1 to 20 parts by mass, even more preferably 1 to 10 parts by mass, and particularly preferably 3 to 8 parts by mass, per 100 parts by mass of the (A) epoxy resin, from the viewpoint of adhesive strength with copper foil and electroless plating.

((D2) Polyvinyl acetal resin, carboxylic acid modified polyvinyl acetal resin)
There is no particular restriction on the type, amount of hydroxyl groups, and amount of acetyl groups of the polyvinyl acetal resin, but the degree of polymerization is preferably 1000 to 2500. If the degree of polymerization is within the above range, the heat resistance tends to be excellent. Here, the degree of polymerization of the polyvinyl acetal resin can be determined from the number average molecular weight of the raw material polyvinyl acetate (measured using a standard polystyrene calibration curve by gel permeation chromatography). In addition, the polyvinyl acetal resin can be modified with a carboxylic acid to be used as a carboxylic acid-modified polyvinyl acetal resin. By using the component (D2), the coatability of the thermosetting resin composition becomes good, and the adhesive strength with copper foil and electroless plating tends to be high.
The component (D2) may be used alone or in combination of two or more types.

As component (D2), commercially available products may be used. Examples of commercially available products that can be used include S-LEC BX-1, BX-2, BX-5, BX-55, BX-7, BH-3, BH-S, KS-3Z, KS-5, KS-5Z, KS-8, and KS-23Z (all manufactured by Sekisui Chemical Co., Ltd.), and Denka Butyral 4000-2, 5000A, 6000C, and 6000EP (all manufactured by Denka Co., Ltd.).

When the thermosetting resin composition contains the (D2) component, the content thereof is preferably 0.5 to 25 parts by mass, more preferably 1 to 20 parts by mass, even more preferably 1 to 10 parts by mass, and particularly preferably 3 to 8 parts by mass, per 100 parts by mass of the (A) epoxy resin, from the viewpoint of adhesive strength with copper foil and electroless plating.

<(E) Flame Retardant>
The thermosetting resin composition may further contain (E) a flame retardant.
The (E) flame retardant is not particularly limited, but examples thereof include chlorine-based flame retardants, bromine-based flame retardants, phosphorus-based flame retardants, hydrated metal compound-based flame retardants, etc. From the viewpoint of environmental compatibility, phosphorus-based flame retardants and hydrated metal compound-based flame retardants are preferred.
The flame retardant (E) may be used alone or in combination of two or more kinds.

Examples of the chlorine-based flame retardants include chlorinated paraffin.
Examples of the brominated flame retardant include brominated epoxy resins such as brominated bisphenol A type epoxy resins and brominated phenol novolac type epoxy resins; unsaturated double bond group-containing brominated flame retardants such as tribromophenylmaleimide, tribromophenylacrylate, tribromophenylmethacrylate, tetrabromobisphenol A type dimethacrylate, pentabromobenzylacrylate, and brominated styrene; hexabromobenzene, pentabromotoluene, ethylene bis(pentabromophenyl), ethylene bistetrabromophthalimide, 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane, tetrabromocyclooctane, hexabromocyclododecane, bis(tribromophenoxy)ethane, brominated polyphenylene ether, brominated polystyrene, and 2,4,6-tris(tribromophenoxy)-1,3,5-triazine.

The phosphorus-based flame retardant is not particularly limited as long as it contains phosphorus atoms among those generally used as flame retardants, and may be an inorganic phosphorus-based flame retardant or an organic phosphorus-based flame retardant. From the viewpoint of environmental compatibility, one that does not contain halogen atoms may be selected. From the viewpoints of adhesion strength with copper foil and electroless plating, heat resistance, glass transition temperature, thermal expansion coefficient, and flame retardancy, an organic phosphorus-based flame retardant may be used.
Examples of inorganic phosphorus-based flame retardants include red phosphorus; ammonium phosphates such as monoammonium phosphate, diammonium phosphate, triammonium phosphate, and ammonium polyphosphate; inorganic nitrogen-containing phosphorus compounds such as phosphoric acid amide; phosphoric acid; and phosphine oxide.
Examples of organic phosphorus-based flame retardants include aromatic phosphoric acid esters, mono-substituted phosphonic acid diesters, di-substituted phosphinic acid esters, metal salts of di-substituted phosphinic acid, organic nitrogen-containing phosphorus compounds, cyclic organic phosphorus compounds, etc. Here, examples of metal salts include lithium salts, sodium salts, potassium salts, calcium salts, magnesium salts, aluminum salts, titanium salts, zinc salts, etc., and may also be aluminum salts. Among organic phosphorus-based flame retardants, aromatic phosphoric acid esters can be selected.

Examples of the hydrated metal compounds include aluminum hydroxide and magnesium hydroxide. These may be used alone or in combination of two or more. The hydrated metal compounds may also fall under the category of (F) inorganic fillers described below, but materials that can impart flame retardancy are classified as flame retardants.

When the thermosetting resin composition contains (E) a flame retardant, the content is preferably 0.5 to 5 mass %, more preferably 1 to 3 mass %, and even more preferably 1.5 to 2.5 mass %, based on the solid content of the thermosetting resin composition, from the viewpoint of flame retardancy.

<(F) Inorganic filler>
The thermosetting resin composition may further contain (F) an inorganic filler.
(F) inorganic filler, for example, silica, alumina, barium sulfate, talc, mica, kaolin, boehmite, beryllia, barium titanate, potassium titanate, strontium titanate, calcium titanate, aluminum carbonate, magnesium hydroxide, aluminum hydroxide, aluminum borate, aluminum silicate, calcium carbonate, calcium silicate, magnesium silicate, zinc borate, zinc stannate, zinc oxide, titanium oxide, silicon carbide, silicon nitride, boron nitride, clay (calcined clay), and glass such as glass short fiber, glass powder and hollow glass beads, and at least one selected from the group consisting of these is preferably used.As glass, E glass, T glass, D glass, etc. are preferably mentioned.Among these, silica and alumina are preferred, and silica is more preferred from the viewpoint of reducing the thermal expansion coefficient of the resin layer.
Examples of the silica include precipitated silica produced by a wet method and having a high water content, and dry method silica produced by a dry method and containing almost no bound water, etc. Dry method silica further includes crushed silica, fumed silica, and fused silica (fused spherical silica) depending on the production method, and among these, fumed silica and fused silica (fused spherical silica) are preferred.
The inorganic filler may be surface-treated with a surface treatment agent such as a silane coupling agent in order to improve moisture resistance, and may be hydrophobized in order to improve dispersibility.

(F) Inorganic filler can be appropriately selected depending on the purpose. From the viewpoint of forming a fine circuit, for example, the specific surface area is preferably 10 m ² /g or more, more preferably 30 to 250 ^{m 2} /g, even more preferably 50 to 200 ^{m 2} /g, and particularly preferably 70 to 160 m ² /g. The specific surface area of the inorganic filler can be determined by a measurement method usually performed by a person skilled in the art, for example, by the BET method. The BET method is a method in which molecules with known adsorption occupancy areas are adsorbed on the powder particle surface at the temperature of liquid nitrogen, and the specific surface area of the sample is determined from the amount of the molecules adsorbed. The most commonly used method for specific surface area analysis is the BET method using an inert gas such as nitrogen.

From the viewpoint of reducing the surface shape after roughening treatment in the plating process, for example, the average primary particle size of the inorganic filler is preferably 100 nm or less, more preferably 1 to 80 nm, even more preferably 1 to 50 nm, and still more preferably 5 to 30 nm.
Commercially available inorganic fillers having an average primary particle size of 100 nm or less include, for example, AEROSIL R972 (specific surface area = 110 ± 20 m ² /g, average primary particle size = about 16 nm), AEROSIL R202 (specific surface area = 100 ± 20 m ² /g, average primary particle size = about 14 nm) [all manufactured by Nippon Aerosil Co., Ltd., "AEROSIL" is a registered trademark]; PL-1 (specific surface area = 181 m ² /g, average primary particle size = 15 nm), PL-7 (specific surface area = 36 m ² /g, average primary particle size = 75 nm) [all manufactured by Fuso Chemical Co., Ltd.]; and AL-A06 (specific surface area = 55 m ² /g, average primary particle size = 31 nm) [manufactured by CIK Nanotech Co., Ltd.].

When the thermosetting resin composition contains an inorganic filler (F), the content thereof is preferably 20% by mass or less, more preferably 15% by mass or less, further preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on the solid content of the thermosetting resin composition. If the content of the inorganic filler is 20% by mass or less, the thermosetting resin film tends to be able to maintain a good surface shape, and the deterioration of plating characteristics and insulation reliability tends to be easily prevented.
In addition, an embodiment in which the thermosetting resin composition does not contain the inorganic filler (F) is also preferable.

<Other ingredients>
The thermosetting resin composition may further contain additives such as a leveling agent, an antioxidant, a thixotropy imparting agent, a thickener, and a flow adjuster, if necessary.
In the case where the thermosetting resin composition contains other components, the content of each of the other components is preferably 20 parts by mass or less, may be 10 parts by mass or less, may be 0.1 to 5 parts by mass, or may be 0.1 to 3 parts by mass, relative to 100 parts by mass of the epoxy resin (A).

<Organic Solvent>
The thermosetting resin composition can be made into a resin varnish by adding an organic solvent, and then a thermosetting resin film can be formed. Examples of the organic solvent include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; acetate esters such as ethyl acetate, butyl acetate, cellosolve acetate, propylene glycol monomethyl ether acetate, and carbitol acetate; carbitols such as cellosolve, methyl carbitol, and butyl carbitol; aromatic hydrocarbons such as toluene and xylene; dimethylformamide, dimethylacetamide (hereinafter also referred to as "DMAc"), N-methylpyrrolidone (hereinafter also referred to as "NMP"), diethylene glycol dimethyl ether, and propylene glycol monomethyl ether. The organic solvent may be used alone or in combination of two or more kinds.
The solid content concentration of the resin varnish is preferably from 10 to 50% by mass, and more preferably from 20 to 45% by mass, from the viewpoints of the solubility of the thermosetting resin composition, the mixed state, and the workability of application and drying.

(Method of manufacturing thermosetting resin film with copper foil)
The resin varnish is applied onto a copper foil and dried to form a so-called copper foil-attached thermosetting resin film having a copper foil on a thermosetting resin film. The drying temperature is preferably 90 to 210° C., more preferably 120 to 200° C., and even more preferably 160 to 190° C. The drying time is preferably 1 to 60 minutes, more preferably 1 to 30 minutes, and even more preferably 1 to 10 minutes.

The thermosetting resin film in the copper foil-attached thermosetting resin film functions as a primer layer to increase the adhesive strength between the polyimide film and the copper foil or electroless plating, and to enable the formation of fine circuits. Therefore, the layer formed by the thermosetting resin film can also be called a primer layer.

The thickness of the thermosetting resin film in the copper foil-attached thermosetting resin film is not particularly limited, but is preferably 0.5 to 6 μm, more preferably 1 to 3 μm, and even more preferably 1.5 to 2.5 μm.
The thickness of the copper foil in the copper foil-attached thermosetting resin film is not particularly limited, but is preferably 10 to 40 μm, more preferably 15 to 30 μm, and even more preferably 15 to 25 μm.

Examples of the thermosetting resin film with copper foil include the embodiment (A) of "thermosetting resin film/copper foil" and the embodiment (B) of "thermosetting resin film/ultra-thin copper foil/peeling layer/carrier copper foil". In this specification, the ultra-thin copper foil/peeling layer/carrier copper foil in the embodiment (B) may be collectively referred to as copper foil.
The above-mentioned embodiment (A) can be mainly used in the SAP method, and the above-mentioned embodiment (B) can be mainly used in the MSAP method, or may be used in the SAP method, since the ultra-thin copper foil remains on the thermosetting resin film and functions as a seed layer as it is. The thickness of the ultra-thin copper foil in embodiment (B) is usually 0.5 to 3 μm. The release layer in the above-mentioned embodiment (B) enables smooth peeling at the interface between the release layer and the ultra-thin copper foil when peeling the carrier copper foil. The release layer is not particularly limited as long as it can be smoothly peeled at the interface between the release layer and the ultra-thin copper foil, and a known layer can be adopted. The release layer may be an inorganic release layer or an organic release layer.

[Flexible printed wiring board]
The present invention also provides a flexible printed wiring board containing the laminate film for flexible printed wiring board obtained by the manufacturing method of the present invention. The flexible printed wiring board of the present invention can have a fine circuit, particularly a circuit with L/S = 15 μm or less/15 μm or less (preferably 10 μm or less/10 μm or less). The finer the L/S, the more preferable it is, but considering the current limit of miniaturization, in the circuit of the flexible printed wiring board of the present invention, L/S = 3 μm or more/3 μm or more, 5 μm or more/5 μm or more, or 7 μm or more/7 μm or more.

When manufacturing a flexible printed wiring board having a fine circuit, the laminate film obtained by the above manufacturing method can be subjected to a process for forming via holes and a process for treating the laminate with an oxidizing roughening liquid (hereinafter, this process is referred to as a desmear process) as necessary, and then a fine circuit can be formed by a circuit formation method such as a semi-additive (SAP) method or a modified semi-additive (MSAP) method.

A specific method using the SAP method will be briefly described below.
First, the copper foil of the laminate film (and the ultra-thin copper foil if present) is removed, and then a plating seed layer is formed by electroless plating, and a mask pattern is formed thereon to expose a part of the plating seed layer corresponding to the desired circuit pattern. After forming a metal layer by electrolytic plating on the exposed plating seed layer, the mask pattern is removed. Thereafter, the unnecessary plating seed layer is removed by etching or the like to obtain the flexible printed wiring board of the present invention.

Next, a specific method using the MSAP method will be briefly described.
First, in the MSAP method, a thermosetting resin film with a copper foil having the above-mentioned configuration (B), i.e., a configuration of "thermosetting resin film/ultra-thin copper foil/peeling layer/carrier copper foil", is used as the thermosetting resin film with a copper foil, and a laminate film having this configuration is utilized.
The carrier copper foil of the outermost layer of the laminated film is peeled off together with the release layer, and the ultra-thin copper foil is left on the thermosetting resin film and used as a seed layer. A flexible printed wiring board can be manufactured in the same manner as in the SPA method, except that the plating seed layer prepared by the SAP method is used as the seed layer prepared by the above method.

The present invention will now be described in more detail with reference to the following examples, which are not intended to limit the present invention in any way.
Using the copper foil-attached thermosetting resin film produced in each example, the temperature at which it could be welded to a polyimide film was investigated according to the following method.
Furthermore, the laminated film for flexible printed wiring boards produced in each example was evaluated or measured for the presence or absence of wrinkles, the presence or absence of voids, and adhesion according to the following methods. Also, the formation of a fine circuit in each example was evaluated according to the following criteria.

[Evaluation method]
(1. Method for investigating welding temperature)
The thermosetting resin film with copper foil obtained in each example was superimposed on a 25 μm thick polyimide film "UPILEX (registered trademark)-SGA" (manufactured by Ube Industries, Ltd.) so that the thermosetting resin film and the polyimide film were in contact with each other, and the film was placed on a hot plate so that the polyimide film was on the hot plate side. Next, a 50 g weight was placed on the thermosetting resin film with copper foil, and the film was left to stand for 60 seconds at a hot plate temperature in the range of 160 to 300° C. to form a laminated film.
After that, the hot plate and the weight were removed, and the laminated film formed was allowed to cool to room temperature. When the polyimide film and the thermosetting resin film were peeled off from the laminated film obtained, if the thermosetting resin film was attached to 80% or more of the surface of the polyimide film, the thermosetting resin film was deemed to be welded to the polyimide film, and the lowest temperature at which the film was welded was taken as the welding temperature. The welding temperature was investigated by carrying out the same operation several times while increasing the hot plate temperature by 10°C each time.
The results are shown in Table 1.

(2. Method for evaluating the presence or absence of wrinkles)
The laminated film produced in each example was visually observed from the polyimide film side and evaluated according to the following evaluation criteria. The results are shown in Table 1.
A: Nothing observed.
C: Striped patterns are observed.

(3. Method for Evaluating the Presence or Absence of Voids)
The laminated film after the first curing step (i) and the laminated film after the second curing step (ii) were visually observed from the polyimide film side and evaluated according to the following evaluation criteria. In order to easily understand the state of the occurrence of voids, the polyimide film side of the laminated film after the second curing step (ii) was photographed with a digital camera in Example 1 and Comparative Example 1, and the results are shown in Figures 5 and 6. Figure 5 corresponds to the following evaluation A, and Figure 6 corresponds to the following evaluation C.
A: The occurrence of voids is suppressed, and the appearance is good.
C: A large number of voids were observed, and the appearance was poor.

(4. Method for Evaluating Adhesion)
The laminated film after the first curing step (i) and the laminated film after the second curing step (ii) were peeled off from the carrier copper foil, and the remaining ultra-thin copper foil was plated up to form a conductor layer (ultra-thin copper foil and plating) having a thickness of 15 to 20 μm. The polyimide film of the laminated film thus obtained was bonded to a support plate "MCL-700G (R) etched product" (thickness 0.6 mm, manufactured by Hitachi Chemical Co., Ltd.) with an adhesive "epoxy adhesive Araldite Rapid" (manufactured by Nichiban Co., Ltd.). Next, a 5 mm wide peel evaluation line was formed on the conductor layer, and the peeling angle was kept at 90°. The peel strength (kN/m) between the polyimide film and the thermosetting resin film was measured when peeling at a speed of 50 mm/min using a tensile tester, and used as an index of adhesion between the polyimide film and the thermosetting resin film.
The laminated film after the first curing step (i) preferably has a compressive strength of 0.50 kN/m or more, and the laminated film after the second curing step (ii) preferably has a compressive strength of 1.0 kN/m or more.

(5. Possibility of forming fine circuits)
In each example, whether or not a fine circuit could be formed was evaluated according to the following evaluation criteria.
A: A circuit with L/S=8 μm/8 μm was formed.
C: A circuit with L/S=8 μm/8 μm could not be formed.

[Example 1]
(1) Preparation of Thermosetting Resin Composition A (Resin Varnish A) Thermosetting resin composition A (resin varnish A) was prepared by blending the following components.
Naphthalene-type epoxy resin "NC7000L" (manufactured by Nippon Kayaku Co., Ltd.) 100 parts by weight Cresol novolac-type phenolic resin "PHENOLITE (registered trademark) KA1165" (manufactured by DIC Corporation) 41 parts by weight Aminotriazine skeleton-containing phenolic resin "PHENOLITE (registered trademark) LA7054" (manufactured by DIC Corporation) 18 parts by weight 1,8-diazabicyclo[5.4.0]undecene-7 (DBU) 0.7 parts by weight 2-phenylimidazole 0.3 parts by weight Carboxylic acid-modified polyvinyl acetal resin "S-LEC (registered trademark) KS-23Z" (manufactured by Sekisui Chemical Co., Ltd.) 4.7 parts by weight Flow adjuster "Modaflow" (manufactured by CBC Corporation) 0.6 parts by weight Methyl ethyl ketone (organic solvent) 231 parts by weight

(2) Preparation of Thermosetting Resin Film A with Copper Foil Resin varnish A was applied to a copper foil "Micro Thin MT18EX" (manufactured by Mitsui Smelting & Co., Ltd.) having a configuration of ultra-thin copper foil/peeling layer/carrier copper foil [ultra-thin copper foil thickness: 1.5 μm, carrier copper foil thickness: 18 μm] using a die coater, and dried at 180° C. for 2 minutes to form a thermosetting resin layer having a thickness of 2 μm on the copper foil.

(3) Preparation of Laminated Film A for Flexible Printed Wiring Boards As shown in FIG. 1, laminated film A for flexible printed wiring boards was prepared by a roll-to-roll method.
Specifically, a polyimide film "UPILEX (registered trademark)-SGA" (manufactured by Ube Industries, Ltd.) having a thickness of 25 μm was fed from the polyimide film feeding device, and a copper foil-attached thermosetting resin film A was fed from the copper foil-attached thermosetting resin film feeding device. At this time, the copper foil-attached thermosetting resin film A was fed in a state in which the thermosetting resin film was located on the polyimide film side.
(Step [1])
The thermosetting resin film of the copper foil-attached thermosetting resin film A was thermocompression bonded to the polyimide film using a laminating roll preset to the welding temperature of the thermosetting resin film determined by the above investigation.
(Step cure process [2])
Next, the film was thermally cured at 140°C for 60 minutes using a batch dryer (first curing step (i)), and further thermally cured at 185°C for 60 minutes (second curing step (ii)), thereby sufficiently increasing the adhesion between the thermosetting resin film and the polyimide film, and a laminate film A for flexible printed wiring boards was produced.
The prepared laminate film A for flexible printed wiring boards was photographed from the polyimide film side using a digital camera. The results are shown in Figure 7. It can be seen from Figure 7 that no wrinkles were formed.

(4) Preparation of flexible printed wiring board A (4-1. Step of forming via holes)
A via hole was formed by a laser method on the thermosetting resin film of the laminate film A for flexible printed wiring boards. A UV laser device (product name: 5335esi, manufactured by ESI Japan Co., Ltd.) was used as the laser.
(4-2. Desmear Treatment Step)
The entire sample was immersed in a desmearing solution to remove the smear caused by the laser drilling.
The desmear treatment was carried out by first immersing the specimen in a swelling solution containing a conditioner (trade name: MLB Conditioner 211, manufactured by Rohm and Haas Electronic Materials Co., Ltd.) at 60° C. for 5 minutes, then immersing the specimen in a desmear solution containing a promoter (trade name: MLB Promoter 213, manufactured by Rohm and Haas Electronic Materials Co., Ltd.) at 70° C. for 1 minute, and finally immersing the specimen in a neutralizing solution (trade name: MLB Neutralizer 216-2, manufactured by Dow Chemical Japan Ltd.) at 40° C. for 5 minutes to reduce and remove manganese remaining on the surface.
(4-3. Formation of fine circuits by SAP method)
After the carrier copper foil was peeled off together with the release layer, the ultra-thin copper foil remaining on the thermosetting resin film was etched off with a ferric chloride solution, and a circuit was formed by the SAP method according to the following procedure.
First, as a pretreatment for electroless plating, the laminated film obtained by the above procedure was immersed in a conditioner solution "Conditioner Neoganth MV" (manufactured by Atotech Japan Co., Ltd.) at 60 ° C. for 4 minutes, then washed with water, and immersed in a pre-dip solution "Pre-dip Neoganth MV" (manufactured by Atotech Japan Co., Ltd.) at room temperature for 1 minute. Next, the laminated film was immersed in an activator solution "Activator Neoganth MV" (manufactured by Atotech Japan Co., Ltd.) at 40 ° C. for 4 minutes, then immersed in a reducer solution "Reducer Neoganth MV" (manufactured by Atotech Japan Co., Ltd.) at 30 ° C. for 3 minutes, washed with water, and immersed in an electroless copper plating solution "Printganth MV TP1" (manufactured by Atotech Japan Co., Ltd.) at 34 ° C. for 20 minutes to perform electroless plating, forming a seed layer with a thickness of 0.5 μm on the surface of the thermosetting resin film. At this time, a seed layer is also formed in the via hole.
Next, a plating resist was patterned on the surface of the seed layer, and then electrolytic plating was performed. The plating resist and unnecessary portions of the seed layer were then removed to produce a flexible printed wiring board A having an L/S=8 μm/8 μm circuit. A scanning electron microscope (SEM) photograph of the L/S=8 μm/8 μm circuit of the flexible printed wiring board A is shown in FIG. 8.

[Reference example 1]
Thermosetting resin composition X (resin varnish X) was prepared in the same manner as in preparation of thermosetting resin composition A (resin varnish A) in Example 1, except that the amount of DBU was changed from 0.7 parts by mass to 0.15 parts by mass.
The thermosetting resin film X with copper foil and the laminate film X for flexible printed wiring boards were produced by performing the same operation as in Example 1. The produced laminate film X for flexible printed wiring boards was photographed from the polyimide film side. The result is shown in FIG. 9. It can be seen from FIG. 9 that wrinkles were generated. As shown in Table 1, since the welding temperature of the thermosetting resin film to the polyimide film was high, a high temperature was required when producing the laminate film X for flexible printed wiring boards, and as a result, wrinkles were generated in the polyimide film in the laminate film X for flexible printed wiring boards due to the difference in the thermal expansion coefficient between the copper foil and the polyimide film. When an attempt was made to produce a flexible printed wiring board using this laminate film X for flexible printed wiring boards in the same manner as in Example 1, the circuit with L/S = 8 μm/8 μm had collapsed, and it was not possible to form a fine circuit using the SAP method.

[Examples 2 to 6]
In the step cure process [2] of Example 1, the same operation was performed except that the conditions of the first cure process (i) were changed as shown in Table 1, to prepare laminate films B to F for flexible printed wiring boards.

[Comparative Example 1]
A laminate film G for flexible printed wiring boards was produced in the same manner as in Example 1, except that in the step cure process [2], the first cure process (i) was not performed and the second cure process (ii) was performed.

[Comparative Examples 2 to 5]
In the step cure process [2] of Example 1, the same operation was carried out except that the conditions of the first cure process (i) were changed as shown in Table 1, to prepare laminate films H to K for flexible printed wiring boards.

From Table 1, in Reference Example 1, in which the welding temperature of the thermosetting resin film to the polyimide film exceeded 200° C., wrinkles occurred in the polyimide film in the laminated film, and even if the SAP method was used, a flexible printed wiring board having a circuit of L/S=8 μm/8 μm could not be manufactured. On the other hand, in the Examples, the welding temperature of the thermosetting resin film to the polyimide film was 200° C. or less, and welding was possible at that temperature, so that the generation of wrinkles in the polyimide film in the laminated film could be suppressed.
Furthermore, in the examples in which the first curing step (i) was followed by the second curing step (ii), the generation of voids was suppressed, the appearance was good, and sufficient adhesion was obtained. However, in Comparative Example 1 in which the first curing step (i) was not performed, and Comparative Examples 2 to 5 in which the temperature of the first curing step (i) was not within the range of 115 to 155°C, voids were significantly generated, the appearance was poor, and the adhesion also tended to be insufficient.

By using the laminated film obtained by the manufacturing method of the present invention, it is possible to form fine circuits even in flexible printed wiring boards, and the film is useful as an internal substrate (flexible printed wiring board) for wearable devices equipped with a camera, display, microphone, etc., in a frame such as a watch type, eyeglass type, finger ring type, shoe type, pocket type, or pendant type.

Reference Signs List 1 Polyimide film 2 Copper foil 3 Thermosetting resin film 4 Thermosetting resin film with copper foil 5 Hot plate 6 Weight 7 Laminated film 8 Polyimide film delivery device 9 Copper foil-attached thermosetting resin film delivery device 10 Free roll 11 Laminate roll 12 Ultra-thin copper foil 13 Release layer 14 Carrier copper foil

Claims (7)

A method for producing a laminate film for flexible printed wiring boards, comprising a step [1] of subjecting (1) a polyimide film and (2) a copper foil-attached thermosetting resin film to thermocompression bonding at 200° C. or less in such a manner that the thermosetting resin film faces the polyimide film, thereby fusing the thermosetting resin film to the polyimide film,
The (1) polyimide film is a polyimide film of the Upilex (registered trademark) series having a thickness of 15 to 100 μm,
The (2) thermosetting resin film with a copper foil is obtained by applying a resin varnish containing a thermosetting resin composition and an organic solvent onto a copper foil having a thickness of 10 to 40 μm and drying the applied resin varnish at 160 to 210° C., and the thermosetting resin film of the thermosetting resin film with a copper foil contains (A) an epoxy resin, (B) an epoxy resin curing agent, and (C) a curing accelerator, and the (C) curing accelerator is a thermosetting resin film formed from a thermosetting resin composition containing a strongly basic compound, and the (A) component contains a naphthalene-type epoxy resin, and the (B) the (A) component contains a phenol-based curing agent, the content of the (A) component is 30 to 80 mass% based on the solid content of the thermosetting resin composition, the content of the (B) component is 20 to 85 mass parts based on 100 mass parts of the (A) epoxy resin, the content of the (C) component other than the strongly basic compound is 0.1 to 5 mass parts based on 100 mass parts of the (A) epoxy resin, the content of the strongly basic compound is 0.6 to 2.8 mass parts based on 100 mass parts of the (A) epoxy resin, and the thickness of the thermosetting resin film is 0.5 to 6 μm,
The method for producing a laminate film for flexible printed wiring boards includes a first curing step (i) of thermally curing the film at 115 to 155°C for 15 to 70 minutes after the step [1], and a second curing step (ii) of further thermally curing the film at 180 to 190°C for 40 to 80 minutes after the first curing step (i). 2. The method for producing a laminate film for flexible printed wiring boards according to claim 1 , wherein the thermal curing in the first curing step (i) and the second curing step (ii) is both carried out under normal pressure. 3. The method for producing a laminate film for a flexible printed wiring board according to claim 1 or 2 , wherein in the step [1], the thermocompression bonding is performed by a roll lamination method. The method for producing a laminate film for flexible printed wiring boards according to any one of claims 1 to 3 , wherein the strongly basic compound has a pKa of 10 or more. The method for producing a laminate film for flexible printed wiring boards according to any one of claims 1 to 4, wherein the strongly basic compound is at least one selected from the group consisting of 1,8-diazabicyclo[5.4.0]undecene-7 (DBU), 1,5-diazabicyclo[4.3.0]nonene-5 (DBN), tetramethylguanidine (TMG), 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,5,7-triazabicyclo[4.4.0]decene-5 (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0 ]decene-5 (MTBD). A method for producing a flexible printed wiring board, comprising the steps of forming a via hole in a laminate film for flexible printed wiring board obtained by the production method according to any one of claims 1 to 5 , treating the laminate film with an oxidizing roughening solution, and then forming a circuit thereon . The method for producing a flexible printed wiring board according to claim 6 , wherein a circuit having a circuit width (L)/circuit spacing (S) of 15 μm or less/15 μm or less is provided .