Polyimide film, method for manufacturing the same, metal-clad laminate, circuit board, multilayer circuit board, electronic device and electronic equipment

Abstract

By changing the surface state of the insulating resin layer, a single-sided circuit board is provided in which adhesion of the insulating resin layer is suppressed. [Solution] The polyimide film 10 has a three-layer laminated structure consisting of a polyimide layer 11, a polyimide layer 12 laminated adjacent to the polyimide layer 11, and a polyimide layer 13 laminated adjacent to the polyimide layer 12. The polyimide layer 11 has a modified layer and a modified surface MS exposed to the outside. The fluorine atom concentration of the modified surface MS of the polyimide film 10 is in the range of 25 to 50 atm% when measured by X-ray photoelectron spectroscopy.

Description

Technical Field

[0001] The present invention relates to a polyimide film useful as a circuit board material, a method for producing the same, a metal-clad laminate using the polyimide film as an insulating resin layer, a circuit board, a multilayer circuit board, an electronic device, and an electronic apparatus using these.

Background Art

[0002] With the progress of miniaturization, weight reduction, and space saving of electronic devices, the demand for flexible printed wiring boards (FPCs) that are thin, lightweight, flexible, and have excellent durability even when repeatedly bent has been increasing. Since FPCs enable three-dimensional and high-density mounting even in limited spaces, their applications are expanding to various electronic components.

[0003] In recent years, portable electronic devices such as smartphones using touch panels have been used in a wide range of fields. Among them, for example, the market for electronic devices (so-called "foldable devices") having flexibility such that a hinge portion is formed in a display area such as a display and can be folded has been expanding in recent years. In the bent portion of such a foldable device, a multilayer circuit board having an Air-gap structure in which the insulating resin layer sides of a pair of single-sided circuit boards are partially bonded through an adhesive layer called a bonding sheet is employed. When such a multilayer circuit board having an Air-gap structure is bent, the insulating resin layers facing each other through the Air-gap may come into contact and adhesion (sticking) may occur, and countermeasures have been demanded.

[0004] In order to suppress the adhesion of the insulating resin layer in a circuit board, a modification treatment for changing the surface state is considered to be effective. As a technology related to circuit board materials, Patent Document 1 discloses a metal-clad laminate having appropriate adhesiveness and being easily peeled off. In Patent Document 1, an inorganic filler is contained in the resin layer forming the bonding surface to balance adhesiveness and peelability.

[0005] Patent Document 2 proposes preventing adhesion at bends and maintaining flexibility by setting the 10-point average surface roughness of the polyimide layer, which serves as the electrical insulating layer of a flexible printed circuit board, to 1.5 μm or more and less than 2.0 μm, and the contact angle to 60° or more and less than 120°. In Patent Document 2, the surface roughness and contact angle of the polyimide layer are controlled by "not performing" surface treatments such as adding fillers or plasma treatment.

[0006] Although not relating to surface modification, Patent Document 3 discloses a method of treating polyimide films used as interlayer insulating materials and passivation materials for multilayer wiring in semiconductor devices with fluorine-containing plasma to fluorinate the interior of the film. However, the fluorine-containing plasma treatment described in Patent Document 3 aims to lower the dielectric constant. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6774285 [Patent Document 2] Patent No. 4954111 [Patent Document 3] Patent No. 2626559 [Overview of the project] [Problems that the invention aims to solve]

[0008] Patent documents 1 and 2 describe altering the surface state of an insulating resin layer by incorporating inorganic fillers. However, as electronic devices become smaller, the insulating resin layers of circuit boards are also becoming thinner, limiting the high-concentration incorporation of inorganic fillers. For example, if inorganic fillers are incorporated at high concentrations into a very thin insulating resin layer, defects such as breakage are more likely to occur when bent, significantly reducing the reliability of electronic devices. Furthermore, increasing the amount of inorganic fillers added can lead to adverse effects such as impaired dimensional stability and reduced drillability, as well as concerns about line contamination during the circuit board manufacturing process.

[0009] On the other hand, another approach to changing the surface state of the insulating resin layer is known, for example, by performing surface treatment such as plasma treatment. However, plasma treatment is generally known as a method to improve adhesion by introducing hydrophilic functional groups such as carboxyl groups and hydroxyl groups to the surface of the insulating resin layer (for example, Patent Document 2, paragraph 0044), and its effect on suppressing adhesion of the insulating resin layer has not been verified.

[0010] The objective of the present invention is to provide a circuit board in which adhesion of the insulating resin layer is suppressed by changing the surface state of the insulating resin layer. [Means for solving the problem]

[0011] As a result of diligent research to solve the above problems, the inventors of the present invention have found that forming a very thin modified layer on the surface of the insulating resin layer by plasma treatment containing fluorine atoms has the effect of suppressing adhesion of the insulating resin layer, and have completed the present invention.

[0012] In other words, the polyimide film of the present invention is a polyimide film comprising a single or multiple polyimide layers, The polyimide film has a modified layer containing fluorine atoms, The modified layer is characterized by having a modified surface exposed to the outside, and the concentration of fluorine atoms measured on the modified surface by X-ray photoelectron spectroscopy being in the range of 25 to 50 atm%.

[0013] The polyimide film of the present invention includes a polyimide layer having the modified layer, and the polyimide layer having the modified layer may contain inorganic filler particles. In this case, the average particle size of the inorganic filler particles may be in the range of 0.3 to 1.5 μm, and the content of the inorganic filler particles in the polyimide layer having the modified layer may be in the range of 1 to 10 volume%.

[0014] The polyimide film of the present invention may have a structure in which a first thermoplastic polyimide layer having the modified layer, a non-thermoplastic polyimide layer, and a second thermoplastic polyimide layer are laminated in this order.

[0015] The polyimide film of the present invention may have a maximum thickness of 30 nm or less in the depth direction from the surface.

[0016] The method for producing the polyimide film of the present invention is as follows: Steps i) and ii) below); Step i) A step of preparing a polyimide film containing one or more polyimide layers, And, Step ii) A step of treating the surface of the polyimide film with a plasma of a gas containing fluorine, It is characterized by including.

[0017] The metal-clad laminate of the present invention comprises an insulating resin layer and a metal layer laminated on one side of the insulating resin layer, The insulating resin layer is characterized in that it contains the polyimide film of the present invention, and the modified layer in the polyimide film is exposed on the side opposite to the metal layer.

[0018] The circuit board of the present invention is a circuit board including an insulating resin layer and a circuit wiring layer laminated on one side surface of the insulating resin layer, wherein the insulating resin layer includes the polyimide film of the present invention, and the modified layer of the polyimide film is exposed on the side opposite to the circuit wiring layer.

[0019] The multilayer circuit board of the present invention is a multilayer circuit board in which a plurality of circuit boards are laminated, and at least includes a first circuit board, a second circuit board, and an adhesive layer that is partially interposed between the first circuit board and the second circuit board to bond the two, wherein both the first circuit board and the second circuit board are single-sided circuit boards each having an insulating resin layer and a circuit wiring layer laminated on one side surface of the insulating resin layer, in a region where the adhesive layer does not exist, the surface of the insulating resin layer side of the first circuit board and the surface of the insulating resin layer side of the second circuit board face each other and are arranged to be separated from each other, and one or both of the first circuit board and the second circuit board are the circuit board of the present invention.

[0020] In the multilayer circuit board of the present invention, a void portion may be formed between the two insulating resin layers using the adhesive layer as a spacer, and repeated bending operations may be repeated using the void portion as a bending site.

[0021] The electronic device of the present invention includes the multilayer circuit board of the present invention.

[0022] The electronic apparatus of the present invention includes the multilayer circuit board of the present invention.

Advantages of the Invention

[0023] The polyimide film of the present invention has a modified layer containing a predetermined concentration of fluorine atoms, thereby suppressing adhesion of the polyimide film. Therefore, in metal-clad laminates and circuit boards using the polyimide film of the present invention as an insulating resin layer, yield can be improved and reliability can be ensured. Accordingly, the polyimide film of the present invention is particularly suitable for use as a circuit board material such as FPC in electronic devices and electronic equipment. [Brief explanation of the drawing]

[0024] [Figure 1] This is a schematic diagram showing the cross-sectional structure in the thickness direction of a polyimide film according to a preferred embodiment of the present invention. [Figure 2] This is a schematic diagram showing the cross-sectional structure in the thickness direction of a metal-clad laminate according to a preferred embodiment of the present invention. [Figure 3] This is a schematic diagram showing the cross-sectional structure in the thickness direction of a circuit board according to a preferred embodiment of the present invention. [Figure 4] This is a schematic diagram showing the cross-sectional structure in the thickness direction of a multilayer circuit board to which the circuit board of the present invention is applied. [Modes for carrying out the invention]

[0025] Embodiments of the present invention will be described with reference to the drawings as appropriate.

[0026] A polyimide film according to one embodiment of the present invention is a polyimide film comprising one or more polyimide layers. The polyimide film may be a film (sheet), or it may be laminated on, for example, an inorganic material substrate or another resin substrate.

[0027] The polyimide film is not particularly limited as long as it contains polyimide as the main component of the resin component, preferably at a rate of 70% by weight or more, more preferably at a rate of 80% by weight or more, and most preferably at a rate of 90-100% by weight of the resin component. Here, "main component of the resin component" means a component that is present in more than 50% by weight of the total resin component. In this invention, "polyimide" refers to resins made of polymers having imide groups in their molecular structure, including polyimide, polyamideimide, polyetherimide, polyesterimide, polysiloxaneimide, and polybenzimidazoleimide.

[0028] Figure 1 shows a cross-sectional configuration of a polyimide film 10 according to a preferred embodiment of the present invention. In Figure 1, as a preferred example, the polyimide film 10 is shown to have a three-layer laminated structure comprising a polyimide layer 11, a polyimide layer 12 laminated adjacent to the polyimide layer 11, and a polyimide layer 13 laminated adjacent to the polyimide layer 12. The polyimide layers 11 and 13 may be adhesive thermoplastic polyimide layers that mainly ensure adhesion to the metal layer, and the polyimide layer 12 may be a non-thermoplastic polyimide layer that mainly ensures mechanical properties as a base resin layer. The polyimide film 10 has a modified layer containing fluorine atoms. The modified layer is extremely thin and is therefore not shown in Figure 1, but it is formed on the exposed side of the polyimide layer 11. The modified layer has a modified surface MS that is exposed to the outside in the polyimide film 10.

[0029] It should be noted that Figure 1 is merely an example, and the polyimide film of the present invention does not need to have a three-layer structure; it may have one, two, or four or more layers. Furthermore, the polyimide film of the present invention may include a resin layer made of any material other than polyimide, as long as it does not impair the effects of the invention. Moreover, the polyimide layer 13 may have a modified layer, or both the polyimide layer 11 and the polyimide layer 13 may have a modified layer. In addition, the polyimide layer having a modified layer does not need to be a thermoplastic polyimide layer; it may be a non-thermoplastic polyimide layer.

[0030] The thickness of the modified layer can be very thin. For example, the maximum thickness of the modified layer is preferably 30 nm or less in the depth direction from the modified surface MS, more preferably 25 nm or less, and even more preferably 20 nm or less, 15 nm or less, 10 nm or less, and 5 nm or less, in that order. As will be described later, in the present invention, the surface treatment is performed with fluorine-containing plasma, so the fluorine atom concentration is highest near the surface of the polyimide film 10, and the relative fluorine atom concentration decreases with increasing depth. Therefore, the maximum thickness of the modified layer means the maximum depth to which fluorine atoms can be detected. In the present invention, from the viewpoint of suppressing adhesion of the polyimide film 10, it is sufficient that only the extreme surface of the polyimide film 10 is modified, and it is not necessary for fluorine atoms to be diffused to the depths. From this viewpoint, the lower limit of the thickness of the modified layer is 1 nm or more in the depth direction from the modified surface MS. Furthermore, in the polyimide film 10, in areas deeper than the modified layer in the thickness direction, fluorine atoms due to the modification are substantially absent. Here, "substantially absent" means that the concentration of fluorine atoms due to the modification is below the detection limit.

[0031] The polyimide film 10 of the present invention has a fluorine atom concentration in the range of 25 to 50 atm% when the modified surface mass spectrometer is measured by X-ray photoelectron spectroscopy. If the fluorine atom concentration is less than 25 atm%, the modification effect is not obtained, and the effect of suppressing adhesion of the polyimide film 10 (or insulating resin layer) is not sufficiently obtained. On the other hand, if the fluorine atom concentration exceeds 50 atm%, the adhesiveness decreases drastically, and it may not be possible to obtain sufficient adhesive strength even when it is desired to bond to an adhesive layer such as a bonding sheet. From the above viewpoint, the lower limit of the fluorine atom concentration when the modified surface MS is measured by X-ray photoelectron spectroscopy is preferably 26 atm% or higher, more preferably 27 atm% or higher, and even more preferably 28 atm% or higher. The upper limit of the fluorine atom concentration is preferably 45 atm% or lower, more preferably 40 atm% or lower, and even more preferably 35 atm% or lower.

[0032] Furthermore, if a monomer containing fluorine atoms is used as a raw material for polyimide 10, fluorine atoms may be detected in deeper layers other than the modified layer because they are present in the main chain or side chains of the polyimide. In this case, the thickness of the modified layer can be estimated by the difference in fluorine atom concentration between the modified layer and the deeper layers outside the modified layer. In other words, when a monomer containing fluorine atoms is used as a raw material for polyimide, the difference (F1-F0) between the fluorine atom concentration (F1) on the modified surface MS and the fluorine atom concentration (F0) deeper than the modified layer (i.e., the unmodified part) should be within the above concentration range. As shown in the examples and comparative examples below, fluorine atoms derived from the raw material monomer of polyimide exist bonded to the main chain or side chain of the polyimide, and have been confirmed to have almost no effect on suppressing adhesion of the polyimide film 10 (or insulating resin layer). Therefore, in order to suppress adhesion, it is considered important to diffuse fluorine atoms only to the extreme surface of the polyimide film 10 by fluorine-containing plasma treatment.

[0033] Regarding the contact angle of the modified surface MS, while not particularly limiting, it is preferable that the contact angle with pure water be 90 degrees or more, more preferably 100 degrees or more, and even more preferably within the range of 100 to 120 degrees. When the contact angle with pure water is 90 degrees or more, the surface free energy of the polyimide film is generally lowered, reducing adhesion and thus effectively suppressing adhesion. However, as shown in the examples below, the fluorine atom concentration of the modified surface MS is considered to be the dominant factor in suppressing adhesion, and the contact angle is considered to be only a secondary factor. Furthermore, similar to the contact angle, the surface roughness of the modified surface MS is only a secondary factor in suppressing adhesion of the polyimide film 10 (or insulating resin layer), and it is considered that the fluorine atom concentration of the modified surface MS has a dominant effect on suppressing adhesion.

[0034] In the polyimide film 10, the polyimide layer 11 having a modified layer preferably contains silica particles 14, which are an inorganic filler. The silica particles 14 are dispersed almost uniformly in the polyimide layer 11. By including the silica particles 14, it becomes easier to control the thermal expansion coefficient of the polyimide layer 11 within a desired range. In addition, the silica particles 14 also have the function of reducing the adhesion of the polyimide layer 11.

[0035] The preferred average particle size of the silica particles 14 is closely related to the thickness of the polyimide layer 11 having the modified layer. For example, when the thickness of the polyimide layer 11 is in the range of 0.5 to 2.5 μm, the average particle size of the silica particles 14 is preferably in the range of 0.3 to 1.5 μm, more preferably in the range of 0.7 to 1.5 μm, and even more preferably in the range of 0.9 to 1.3 μm. If the average particle size is less than 0.3 μm, effects such as improved adhesion and control of the coefficient of thermal expansion cannot be sufficiently obtained, and if it exceeds 1.5 μm, the film tends to become brittle, and this tendency is particularly pronounced when the thickness of the polyimide layer 11 is thinned to the above range. The average particle size of silica particles can be measured, for example, by laser diffraction, but it can also be analyzed from scanning electron microscope images of the cross-section of a polyimide film.

[0036] The preferred content of silica particles 14 is closely related to the thickness of the polyimide layer 11 having the modified layer. For example, when the thickness of the polyimide layer 11 is in the range of 0.5 to 2.5 μm, the content of silica particles 14 in the polyimide layer 11 is preferably in the range of 1 to 10 volume%. If the content of silica particles 14 is less than 1 volume%, the effects such as improved adhesion and control of the coefficient of thermal expansion cannot be sufficiently obtained, and if it exceeds 10 volume%, adverse effects such as weakening of the polyimide film 10, decreased dimensional stability, decreased processability, and line contamination due to the shedding of silica particles 14 during the circuit processing process are likely to occur.

[0037] In the polyimide film 10, the adhesion of the modified surface MS is controlled not only by the diffusion of fluorine atoms but also by the dispersed silica particles 14. Thus, the advantages of pre-dispersing silica particles 14 in addition to the diffusion of fluorine atoms into the polyimide layer 11 are as follows. As thin-film thinning progresses, there are limits to the high-concentration incorporation of inorganic fillers, making it difficult to control the surface state of the polyimide film (insulating resin layer) solely by dispersing inorganic fillers, as in conventional technology. In other words, high concentrations of inorganic fillers in thin films tend to cause problems such as weakening, decreased dimensional stability, reduced processability, and line contamination. On the other hand, considering environmental regulations, it is required to keep the concentration of fluorine atoms introduced into the polyimide film as low as possible. In a preferred embodiment of the present invention, by treating the polyimide layer 11 in which silica particles 14 are dispersed with a fluorine-containing plasma, the concentration of fluorine atoms can be suppressed compared to when the silica particles 14 are not dispersed. Specifically, when silica particles 14 are dispersed, the upper limit of the concentration of fluorine atoms in the polyimide layer 11, when the modified surface MS is measured by X-ray photoelectron spectroscopy, may preferably be 40 atm% or less, more preferably 35 atm% or less. Thus, in a preferred embodiment of the present invention, by performing both the diffusion of fluorine atoms by plasma treatment and the dispersion of silica particles 14, the amount of fluorine atom diffusion is suppressed on the one hand, while the silica particle content is also reduced on the other hand, while the adhesion of the modified surface MS is appropriately controlled.

[0038] Furthermore, the polyimide layer 13 may also contain silica particles 14, similar to the polyimide layer 11. This is because silica particles 14 affect the coefficient of thermal expansion, and if the silica particle content of the polyimide layer 11 and the polyimide layer 13 differs significantly, it can cause warping of the polyimide film 10. Therefore, it is preferable that the average particle size and content of silica particles 14 in the polyimide layer 13 be the same as those in the polyimide layer 11.

[0039] Although silica particles 14 have been described above as a representative example of an inorganic filler, the polyimide film 10 of the present invention can also use other inorganic fillers instead of silica particles 14, or other inorganic fillers can be used in combination with silica particles 14, as long as the effects of the invention are not impaired. The type of inorganic filler other than silica particles is not particularly limited, but for example, aluminum oxide (alumina), magnesium oxide (magnesia), beryllium oxide, niobium oxide, titanium oxide, boron nitride, aluminum nitride, silicon nitride, aluminum fluoride, calcium fluoride, magnesium fluoride, potassium silicofluoride, talc, glass, barium titanate, etc. are preferred. Two or more of these may be used in combination. When using inorganic fillers other than silica particles, the average particle size and total amount are the same as in the case of silica particles 14.

[0040] The polyimide layer 11 and the polyimide layer 13 preferably contain thermoplastic polyimide as the main component of the resin component, and the polyimide layer 12 preferably contains non-thermoplastic polyimide as the main component of the resin component. Here, "main component of the resin component" means a component that makes up more than 50% by weight of the resin component, preferably 70% by weight or more, more preferably 80% by weight or more, and most preferably 90-100% by weight of the resin component. In addition, in the present invention, "thermoplastic polyimide" generally refers to polyimide whose glass transition temperature (Tg) can be clearly confirmed, but in the present invention, the storage modulus at 30°C measured using a dynamic viscoelasticity analyzer (DMA) is 1.0 × 10⁻⁶. 8 The Pa is greater than or equal to the storage modulus of elasticity at 300°C, and the storage modulus of elasticity at 300°C is 3.0 × 10⁻⁶. 7 This refers to polyimides with a storage modulus of less than Pa. Furthermore, "non-thermoplastic polyimides" generally refer to polyimides that do not soften or become adhesive when heated. However, in this invention, the storage modulus at 30°C measured using a dynamic viscoelasticity analyzer (DMA) is 1.0 × 10⁻⁶. 9 The Pa is greater than or equal to the storage modulus of elasticity at 300°C, and the storage modulus of elasticity at 300°C is 3.0 × 10⁻⁶. 8 This refers to polyimides with a Pa rating of 1.5 or higher.

[0041] In the example configuration shown in Figure 1, the polyimide layers 11 and 13 may each be composed of the same or different types of thermoplastic polyimides. The thermoplastic polyimides used in the polyimide layers 11 and 13 are thermoplastic polyimides obtained by reacting a tetracarboxylic anhydride component with a diamine component. Monomers commonly used in the synthesis of thermoplastic polyimides can be used as the tetracarboxylic anhydride component and diamine component that serve as raw materials for the thermoplastic polyimide.

[0042] The polyimide layers 11 and 13 have an average coefficient of thermal expansion in the in-plane direction greater than 30 ppm / K, preferably in the range of greater than 30 ppm / K and 100 ppm / K or less, and more preferably in the range of greater than 30 ppm / K and 80 ppm / K or less. The polyimide layers 11 and 13 may contain optional components, such as flame retardants, as appropriate.

[0043] The non-thermoplastic polyimide used in the polyimide layer 12 is a non-thermoplastic polyimide obtained by reacting a tetracarboxylic anhydride component with a diamine component. As the tetracarboxylic anhydride component and diamine component used as raw materials for the non-thermoplastic polyimide, monomers commonly used in the synthesis of non-thermoplastic polyimides can be used.

[0044] The average in-plane thermal expansion coefficient of the polyimide layer 12 is preferably in the range of 1 ppm / K to 30 ppm / K, more preferably in the range of 1 ppm / K to 25 ppm / K, and more preferably in the range of 15 ppm / K to 25 ppm / K. The polyimide layer 12 may be appropriately blended with optional components, such as fillers and flame retardants.

[0045] <Layer thickness> The overall thickness of the polyimide film 10 is not particularly limited, but from the viewpoint of ensuring electrical insulation while accommodating miniaturization, the upper limit of the thickness is preferably 20 μm or less, more preferably 16 μm or less, and even more preferably 12 μm or less. The lower limit of the thickness is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more. If the overall thickness of the polyimide film 10 exceeds the upper limit, it becomes difficult to accommodate miniaturization, and if it is below the lower limit, the electrical insulation as an insulating resin layer may be insufficient or the handling properties may deteriorate.

[0046] The thickness of the polyimide layers 11 and 13 is not particularly limited, but the upper limit of the thickness is preferably 2.5 μm or less, more preferably 2 μm or less, and even more preferably 1.5 μm or less. The lower limit of the thickness is preferably 0.5 μm or more, more preferably 0.75 μm or more, and even more preferably 1 μm or more. If the thickness of the polyimide layers 11 and 13 exceeds the upper limit, it becomes difficult to accommodate miniaturization and dimensional stability tends to deteriorate. If it is below the lower limit, sufficient adhesion may not be guaranteed. The thicknesses of polyimide layer 11 and polyimide layer 13 may be the same or different.

[0047] In the polyimide film 10, the thickness of the polyimide layer 12 is not particularly limited, but from the viewpoint of ensuring the function as a base resin layer while accommodating miniaturization and ensuring transportability during film manufacturing, the upper limit of the thickness is preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 9 μm or less. The lower limit of the thickness is preferably 2 μm or more, more preferably 2.5 μm or more, and even more preferably 3 μm or more. If the thickness of the polyimide layer 12 exceeds the upper limit, it becomes difficult to accommodate miniaturization, and if it is below the lower limit, electrical insulation and handling properties become insufficient, and it becomes difficult to guarantee the mechanical properties of the polyimide film 10.

[0048] [Manufacturing of polyimide film] The polyimide film of the present invention is obtained through the following steps i) and ii); Step i) A step of preparing a polyimide film containing one or more polyimide layers, And, Step ii) A step of treating the surface of the polyimide film with a plasma of a gas containing fluorine, It can be manufactured by carrying out a method that includes the following.

[0049] The polyimide film prepared in step i) is, for example, (1) A method of applying a polyamic acid solution to any support substrate, drying it, repeating this as needed, then heat-treating the entire mixture to imide it, and finally peeling it off the support substrate to form a film. (2) A method of applying a polyamic acid solution to any support substrate, drying it, repeating this as needed, peeling the polyamic acid gel film from the support substrate, and imidizing it by heat treatment to form a film. (3) A method of applying a polyamic acid solution or a soluble polyimide solution to any support substrate by multilayer extrusion, drying it, imidizing it as needed, and peeling it off the support substrate to form a film. (4) A method of applying a soluble polyimide solution to any support substrate, drying it, and peeling it off the support substrate to form a film. It can be manufactured by methods such as the above. Furthermore, by using metal foil as the support substrate and leaving it without peeling it off, the metal-clad laminate described later can be manufactured.

[0050] The plasma treatment in step ii) is not particularly limited as long as fluorine atoms can be diffused so that the fluorine atom concentration on the modified surface MS is within the desired range, and can be carried out according to conventional methods. For example, the fluorine-containing gas is not particularly limited, and can be CF4, CH2F2, CH3F, etc. In addition, an inert gas may be used in combination as needed. The pressure of the plasma treatment may be atmospheric pressure or vacuum conditions. The electromagnetic waves used to generate the plasma may be high frequency or microwaves.

[0051] The depth in the thickness direction (depth of the modified layer) and the fluorine atom concentration on the modified surface (MS) can be controlled by the plasma treatment in step ii) to which fluorine atoms can be diffused into the polyimide film. In particular, it is preferable to use these as variable factors, as it is easy to control the depth of the modified layer and the fluorine atom concentration in the polyimide film within an appropriate range by changing one or more conditions selected from the treatment time, output, or plasma speed while keeping other plasma treatment conditions fixed. Specifically, by employing a certain type of plasma treatment and changing one or more conditions selected from treatment time, power output, or plasma speed, the depth of the modified layer and the fluorine atom concentration in the polyimide film under each change condition can be confirmed in preliminary experiments. Therefore, the treatment time, power output, or plasma speed can be adjusted so that these values ​​become the desired values. Consequently, there are no particular restrictions regarding whether it is atmospheric pressure plasma or vacuum plasma, high-frequency plasma or microwave plasma, what type of gas is used as the fluorine-containing gas, or what type of plasma treatment apparatus and device configuration is adopted.

[0052] [Metal-clad laminate] Figure 2 shows a cross-sectional configuration of a metal-clad laminate 100 according to a preferred embodiment of the present invention. The metal-clad laminate 100 is a single-sided metal-clad laminate in which a metal layer 30 is laminated on one side of an insulating resin layer 20. The metal-clad laminate 100 comprises an insulating resin layer 20 and a metal layer 30 laminated on one side of the insulating resin layer 20, wherein the insulating resin layer 20 contains the polyimide film of the present invention, and the modified surface MS of the polyimide film is exposed on the side opposite to the metal layer 30.

[0053] There are no particular restrictions on the material of the metal layer 30, and examples include copper, stainless steel, iron, nickel, beryllium, aluminum, zinc, indium, silver, gold, tin, zirconium, tantalum, titanium, lead, magnesium, manganese, and alloys thereof. Among these, copper or copper alloys are particularly preferred. The copper foil may be rolled copper foil or electrolytic copper foil, and commercially available copper foil can be used. The material of the circuit wiring layer in the circuit board of the present invention, which will be described later, is the same as that of the metal layer 30. Furthermore, the metal foil may be subjected to surface treatments such as siding, aluminum alkoxide, aluminum chelate, or silane coupling agents, for purposes such as rust prevention or improved adhesion.

[0054] The thickness of the metal layer 30 is not particularly limited, but for example, when using copper foil, the upper limit of the thickness is preferably 18 μm or less, more preferably 15 μm or less, and even more preferably 12 μm or less, in order to accommodate miniaturization. From the viewpoint of production stability and handling, the lower limit of the thickness of the copper foil is preferably 3 μm or more, more preferably 6 μm or more, and even more preferably 9 μm or more. For extremely thin copper foil with a thickness of about 3 μm, it is preferable to use copper foil with a carrier. In this case, the peelable carrier may be made of metal or resin, and its thickness may be in the range of 5 to 100 μm, and there may be a release layer between the carrier and the copper foil. For example, copper foil with a carrier is described in Japanese Patent No. 4762742, and the description in said publication is incorporated herein by reference.

[0055] In the metal-clad laminate 100, the structure of the insulating resin layer 20 is the same as that of the polyimide film 10.

[0056] [Manufacturing of metal-clad laminates] Although not shown in the figures, the metal-clad laminate 100 can be manufactured by, for example, repeatedly applying and drying a polyamic acid solution or a soluble polyimide solution onto a metal foil, followed by imidization as needed, or by heat-pressing the polyimide film of the present invention with the metal foil.

[0057] The metal-clad laminate 100 obtained in this embodiment can be used to manufacture circuit boards such as single-sided FPCs by processing the metal layer 30 with wiring circuits, for example.

[0058] [Circuit board] A circuit board according to one embodiment of the present invention can be manufactured by processing the metal layer 30 of a metal-clad laminate 100 into a pattern by a conventional method to form a circuit wiring layer. For example, referring to Figure 3, the circuit board 200 of this embodiment comprises an insulating resin layer 20 and a circuit wiring layer 40 laminated on one side of the insulating resin layer 20 and formed by circuit processing of the metal layer 30. The insulating resin layer 20 contains the polyimide film of the present invention, and the modified surface MS of the polyimide film is exposed on the side opposite to the circuit wiring layer 40. The circuit board 200 of this embodiment can be preferably applied as, for example, an FPC, a rigid-flex circuit board, etc.

[0059] The circuit board of the present invention can also be applied to multilayer circuit boards. Figure 4 shows an example of a cross-sectional configuration of a multilayer circuit board 300 to which the circuit board of the present invention is applied. The multilayer circuit board 300 is constructed by laminating multiple circuit boards, including a circuit board 201 having a circuit wiring layer 41 and a circuit board 202 having a circuit wiring layer 42, via a bonding sheet BS. The bonding sheet BS is partially interposed between the circuit boards 201 and 202, bonding them together. Both the circuit boards 201 and 202 are single-sided circuit boards, and in the region where the bonding sheet BS is absent, the surface of the circuit board 201 facing the insulating resin layer 21 and the surface of the circuit board 202 facing the insulating resin layer 22 face each other and are spaced apart from each other. Using the bonding sheet BS as a spacer, an air gap AG is formed between the insulating resin layer 21 of the circuit board 201 and the insulating resin layer 22 of the circuit board 202.

[0060] The multilayer circuit board 300 may be such that the gap portion AG is the bending portion and the bending operation is repeatedly performed in the stacking direction (up and down direction in Figure 4) near the line BB in Figure 4. In the multilayer circuit board 300, circuit boards 201 and 202 may have the same configuration or different configurations, but either one or both of circuit boards 201 and 202 may be the circuit boards of the present invention. In the example shown in Figure 4, both the insulating resin layer 21 of circuit board 201 and the insulating resin layer 22 of circuit board 202 have the modified surface MS. In other words, both circuit boards 201 and 202 are the circuit boards of the present invention. In the multilayer circuit board 300, even if bending operations are repeated, the occurrence of adhesion (sticking) is effectively suppressed because the opposing insulating resin layers 21 and 22 have the modified surface MS.

[0061] [Electronic Devices / Electronic Equipment] The electronic devices and electronic equipment according to embodiments of the present invention are equipped with the circuit board of the present invention. Examples of electronic devices of the present invention include display devices such as liquid crystal displays, organic EL displays, and electronic paper, organic EL lighting, solar cells, touch panels, camera modules, inverters, converters, and their components. Examples of electronic equipment include HDDs, DVDs, mobile phones, smartphones, tablet terminals, electronic control units (ECUs) and power control units (PCUs) of automobiles. The circuit board is preferably used in these electronic devices and electronic equipment as components such as wiring for movable parts, cables, and connectors. [Examples]

[0062] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way by these examples. In the following examples, unless otherwise specified, various measurements and evaluations are performed as described below.

[0063] [Measurement of fluorine atom concentration] Measurements were performed using an X-ray photoelectron spectroscopy analyzer (KRATOS ULTRA2; manufactured by Shimadzu Corporation) under the following conditions, and the data was analyzed using the accompanying analysis software (ESCApe). (XPS measurement conditions) (1) Wide measurement • X-ray source: AlKa rays, X-ray intensity 300W Measurement range: 300 μm x 700 μm • Charging neutralization mechanism: ON (Filament Current: 0.45A, Filament Bias: 1V, Charge Balance: 4V) • Charge correction: Correction to ensure CH / CC is between 284.8 and 285.0 eV. • Number of scans: 2 • Obtained spectra with binding energies of 1200 to 0 eV. We analyzed what elements are present at a depth of 2-4 nm from the surface. (2) Narrow measurement Based on the qualitative elemental analysis results obtained by wide-area measurements, quantitative analysis and chemical structure analysis of each element were performed by analyzing the energy range in which specific elements appear with high resolution. By applying background correction to the peaks of each element, the peak area was calculated, and by applying the relative sensitivity coefficient for each element to the obtained peak area, the content of each element at the measurement site was calculated in terms of atomic concentration (atm%). • X-ray source: AlKa rays, X-ray intensity 300W Measurement range: 300 μm x 700 μm • Charging neutralization mechanism: ON (Filament Current: 0.45A, Filament Bias: 1V, Charge Balance: 4V) • Charge correction: Correction to ensure CH / CC is between 284.8 and 285.0 eV. • Scanned peaks: F1s, O1s, N1s, C1s • Scan conditions: F1s…695~675eV, 4 scans, background correction 692~684eV O1s…543~523eV, 4 scans, background correction 537~529eV N1s…410~390eV, 4 scans, background correction 404~397eV C1s…297~275eV, 4 scans, background correction 296~281eV *The background correction range varies by approximately ±0.5eV depending on the peak position. (3) Measurement of fluorine atom concentration in the depth direction Using a gas cluster ion beam (GCIB), the fluorine atom concentration in the depth direction was quantitatively analyzed by XPS narrow beam measurement while etching the modified surface of a polyimide film in the depth direction. GCIB etching was performed on a 1 mm diameter area of ​​the modified surface under conditions of an acceleration voltage of 5 keV and an Ar flow rate of 3000 mL / min. Under these etching conditions, etching to a depth of approximately 3.3 nm per minute was possible.

[0064] [Measurement of average particle size and volume content of silica particles] The average particle size of the silica particles before compounding was measured using a laser diffraction particle size distribution analyzer (Malvern, product name: Master Sizer 3000) with water as the dispersion medium and a particle refractive index of 1.54, employing a laser diffraction / scattering measurement method. The volume content was calculated based on the average volume obtained from the average particle size of the silica particles and the amount of silica particles added to the polyamic acid resin solution.

[0065] [Method for evaluating adhesions] Two pieces of the metal-clad laminate to be evaluated, cut to a size of 125 mm x 175 mm, were prepared, and the insulating resin layers of each were overlapped. Next, they were heat-pressed at a temperature of 150°C and a pressure of 3 MPa for 45 minutes. After heat pressing, the samples were examined, and those where the bonded surfaces did not peel apart and were integrated were judged as "adhered," while those where the bonded surfaces peeled apart naturally and were not adhered were judged as "not adhered."

[0066] [Measuring contact angle] The contact angle was measured using the following method. 1. Sample preparation: Clean the solid surface to be measured and dry it if necessary. 2. Droplet formation: A certain amount of water droplets are placed on a solid surface using a syringe or similar device. 3. Image acquisition: Observe the solid surface on which the water droplet is placed from the side and acquire an image with a camera. 4. Image analysis: The acquired images are analyzed using the θ / 2 method, and the contact angle is calculated from the contour of the water droplet.

[0067] [Arithmetic mean height (Sa), ratio of the developed area of ​​the interface (Sdr)] The surface roughness of a sample was measured using a Keyence laser microscope (VK-X3000) in white light microscope mode (10x objective lens), and surface roughness was measured for the entire measurement area. The arithmetic mean height (Sa) and interface area ratio (Sdr) were calculated for the entire measurement field based on the ISO 25178 standard (surface roughness). (Filter type: Gaussian, S-filter: 2, F-operation: waviness removal, L-filter: 0.025)

[0068] [Measurement of Storage Modulus] The storage modulus was determined by cutting a resin film into 5mm x 20mm pieces and heating them in an oven at 120°C for 2 hours and then at 170°C for 3 hours. The resulting samples were measured using a dynamic viscoelasticity analyzer (DMA: UBM Corporation, trade name: E4000F) by gradually heating them from 30°C to 400°C at a heating rate of 4°C / min, at a frequency of 11Hz. The storage modulus measured in this way at 30°C was 1.0 × 10⁻⁶. 9 The Pa is greater than or equal to the storage modulus of elasticity at 300°C, and the storage modulus of elasticity at 300°C is 3.0 × 10⁻⁶. 8 If the storage modulus is Pa or higher, it is classified as a non-thermoplastic polyimide, and the storage modulus at 30°C is 1.0 × 10⁻⁶. 8 The Pa is greater than or equal to the storage modulus of elasticity at 300°C, and the storage modulus of elasticity at 300°C is 3.0 × 10⁻⁶. 7 Polyimides with a Pa of less than 1 Pa were defined as thermoplastic polyimides.

[0069] [Measurement of glass transition temperature] The dynamic viscoelasticity of a polyimide film (10 mm × 22.6 mm) obtained by etching away copper foil was measured when heated from 20°C to 500°C at a rate of 5°C / min using DMA, and the glass transition temperature Tg (tanδ maximum value) was determined.

[0070] [Measurement of thermal expansion coefficient] A polyimide film obtained by etching copper foil was heated to 250°C using a Seiko Instruments thermomechanical analyzer, held at that temperature for 10 minutes, and then cooled at a rate of 5°C / minute to determine the average thermal expansion coefficient (linear thermal expansion coefficient) from 240°C to 100°C.

[0071] [Viscosity measurement] Viscosity was measured at 25°C using an E-type viscometer (Brookfield, product name: DV-II+Pro). The rotation speed was set so that the torque was between 10% and 90%, and the value was read after 2 minutes had elapsed since the start of measurement, when the viscosity had stabilized.

[0072] Synthesis Example 1 (without filler): N,N-dimethylacetamide was added to a reaction vessel equipped with a thermocouple and a stirrer, and capable of introducing nitrogen, so that the final solid content concentration was 20% by weight. 100 moles of 4,4'-diaminodiphenyl ether were dissolved in this reaction vessel while stirring. Next, 100 moles of 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added. Stirring was then continued for 3 hours to obtain polyamic acid resin solution a with a solution viscosity of 2,960 mPa·s. The solution viscosity is the value obtained by applying the viscosity only at 25°C using an E-type viscometer (the same applies hereafter). The polyimide obtained from this polyamic acid resin solution a is a thermoplastic polyimide, with a glass transition temperature of 312°C and a coefficient of thermal expansion of 45 ppm / K.

[0073] Synthesis Example 2 (Filler content 1 vol%): N,N-dimethylacetamide was added to a reaction vessel equipped with a thermocouple and a stirrer, and capable of introducing nitrogen, so that the final solid content concentration was 20% by weight. 0.60 g of spherical filler (silica, average particle size 1.0 μm, Admatex, "SE4050"; the same applies hereafter) was added to this reaction vessel and dispersed using an ultrasonic disperser for 3 hours. 100 moles of 4,4'-diaminodiphenyl ether were dissolved in this solution while stirring in the vessel. Next, 100 moles of 3,3',4,4'-benzophenonetetracarboxylic dianhydride were added. Stirring was then continued for 3 hours to obtain a polyamic acid resin solution b with a solution viscosity of 3,160 mPa·s.

[0074] Synthesis Example 3 (Filler content 10% by volume): Polyamic acid resin solution c was obtained in the same manner as in Synthesis Example 2, except that 6.60 g of spherical filler was used. The viscosity of polyamic acid resin solution c was 3,500 mPa·s.

[0075] Synthesis Example 4 N,N-dimethylacetamide was added to a reaction vessel equipped with a thermocouple and a stirrer, and capable of introducing nitrogen, so that the final solid content concentration was 20% by weight. 100 moles of 4,4'-diamino-2,2'dimethylbiphenyl were dissolved in this reaction vessel while stirring. Next, 20 moles of 3,3'-4,4'-biphenyltetracarboxylic dianhydride and 80 moles of pyromellitic dianhydride were added. Stirring was then continued for 3 hours to obtain a polyamic acid resin solution d with a solution viscosity of 21,000 mPa·s. The polyimide obtained from this polyamic acid resin solution d was a non-thermoplastic polyimide with a glass transition temperature of 360°C and a coefficient of thermal expansion of 15 ppm / K.

[0076] Synthesis Example 5: N,N-dimethylacetamide was added to a reaction vessel equipped with a thermocouple and a stirrer, and capable of introducing nitrogen, so that the final solid content concentration was 15% by weight. 60 moles of 1,3-bis(3-aminophenoxy)benzene and 40 moles of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl were added to this reaction vessel and dissolved while stirring. Next, 100 moles of 4,4'-oxydiphthalic anhydride were added, with an acid / amine ratio of 1.010. The mixture was then heated at 40°C for 2 hours to dissolve. Furthermore, the solution was stirred at room temperature for 1 day to carry out the polymerization reaction, yielding a polyamic acid resin solution e with a solution viscosity of 5,500 mPa·s. The polyimide obtained from this polyamic acid resin solution e was a thermoplastic polyimide with a glass transition temperature of 270°C and a thermal expansion coefficient of 59 ppm / K.

[0077] (Example 1) A long, 12 μm thick electrolytic copper foil was uniformly coated on one side with polyamic acid resin solution a, prepared in Synthesis Example 1, so that the cured thickness would be 2 μm (first layer). The foil was then heated and dried at 130°C to remove the solvent. Next, polyamic acid resin solution d, prepared in Synthesis Example 4, was uniformly coated on this coated side so that the cured thickness would be 8 μm (second layer). The foil was then heated and dried at 130°C to remove the solvent. Furthermore, polyamic acid resin solution b, prepared in Synthesis Example 2, was uniformly coated on this coated side so that the cured thickness would be 2 μm (third layer). The foil was then heated and dried at 130°C to remove the solvent. This long laminate was heat-treated in a continuous curing furnace set to gradually increase the temperature from 130°C to 300°C for a total of approximately 10 minutes to obtain a single-sided flexible copper-clad laminate E1 with a total polyimide layer thickness of 12 μm.

[0078] A modified surface was obtained by irradiating the exposed third layer of the polyimide layer of a single-sided flexible copper-clad laminate E1 with a plasma containing fluorine under the conditions described below, thereby performing a modification treatment.

[0079] The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the modified surface of single-sided flexible copper-clad laminate E1P. Furthermore, the presence or absence of adhesion was evaluated by bonding the modified surfaces of two single-sided flexible copper-clad laminate E1P sheets together. The results are shown in Table 1.

[0080] (Example 2) Single-sided flexible copper-clad laminates E2 and E2P were prepared in the same manner as in Example 1, except that the third layer was formed with a polyamic acid resin solution c prepared in Synthesis Example 3. The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the modified surface, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0081] (Example 3) A single-sided flexible copper-clad laminate E3P was prepared in the same manner as in Example 1, except that the plasma treatment conditions were changed, using the single-sided flexible copper-clad laminate E1 prepared in Example 1. The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the modified surface, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0082] (Comparative Example 1) A single-sided flexible copper-clad laminate C1P was obtained in the same manner as in Example 1, except that the plasma treatment conditions were changed as shown in Table 1. The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the modified surface of the single-sided flexible copper-clad laminate C1P, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0083] (Comparative Example 2) A single-sided flexible copper-clad laminate C2P was obtained in the same manner as in Example 1, except that the plasma treatment conditions were changed as shown in Table 1. The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the modified surface of the single-sided flexible copper-clad laminate C2P, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0084] (Comparative Example 3) In the single-sided flexible copper-clad laminate E1 fabricated in Example 1, the fluorine atom concentration, contact angle, Sa, and Sdr were measured on the exposed surface of the third layer, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0085] (Comparative Example 4) A single-sided flexible copper-clad laminate C4 was fabricated in the same manner as in Example 1, except that the third layer was formed using a polyamic acid resin solution e prepared in Synthesis Example 5. Plasma treatment was not performed on the single-sided flexible copper-clad laminate C4. The fluorine atom concentration, contact angle, Sa, and Sdr were measured on the exposed surface of the third layer of this single-sided flexible copper-clad laminate C4, and the presence or absence of adhesion was evaluated. The results are shown in Table 1.

[0086] <Plasma treatment conditions> A single-sided flexible copper-clad laminate was placed on the sample stage (lower electrode) of a plasma processing apparatus having a pair of parallel plate electrodes inside a chamber, with the copper layer side in contact with the sample. Next, a fluorine-containing gas was introduced into the discharge space inside the chamber, and a predetermined voltage was applied between the electrodes to generate a fluorine-containing plasma in the discharge space, thereby plasma-treating the surface of the insulating resin layer. Using Example 2 as a reference, the conditions were modified so that the output was -30% in Example 1, the output was +30% in Example 3, the plasma speed was 10 times greater and the output was +30% in Comparative Example 1, and the plasma speed was 15 times greater in Comparative Example 2.

[0087] [Table 1]

[0088] Table 1 shows that in Examples 1-3, plasma treatment was performed so that the fluorine atom concentration on the modified surface was within the range of 25-50 atm%, and adhesion between insulating resin layers was suppressed. On the other hand, adhesion was observed in Comparative Examples 1 and 2, where plasma treatment was performed but the fluorine atom concentration was too low, and in Comparative Example 3, where no plasma treatment was performed. Furthermore, Comparative Example 4 confirmed that adhesion occurs even when monomer-derived fluorine atoms are present. No correlation was found between contact angle, surface roughness, and the presence or absence of adhesion.

[0089] (Example test) Fluorine atom concentration in the depth direction: Test samples T1 to T5 of polyimide films with different fluorine atom concentrations on the modified surface were prepared in the same manner as in Example 1, except that the plasma treatment conditions were changed. For each test sample, the fluorine atom concentration in the depth direction was quantitatively analyzed by XPS narrow-wave measurement while etching with GCIB. The results are shown in Table 2. In Table 2, the numbers in the upper row represent the fluorine atom concentration, and the numbers in parentheses in the lower row represent the ratio when the fluorine atom concentration on the modified surface is set to 100%.

[0090] [Table 2]

[0091] Table 2 shows that when the fluorine atom concentration on the modified surface is 30 atm% or less, the majority of fluorine atoms are located within a depth range of 3 nm from the modified surface. From these results, it is estimated that when the fluorine atom concentration on the modified surface is 30 atm% or less, the maximum thickness of the modified layer is at most 10 nm, and it is presumed that there are virtually no fluorine atoms due to the modification in areas deeper than the modified layer.

[0092] Although embodiments of the present invention have been described in detail above for illustrative purposes, the present invention is not limited to the above embodiments and various modifications are possible. [Explanation of Symbols]

[0093] 10…Polyimide film, 11,12,13…Polyimide layer, 14…Silica particles, 20,21,22…Insulating resin layer, 30…Metal layer, 40,41,42…Circuit wiring layer, 100…Metal-clad laminate, 200,201,202…Circuit board, 300…Multilayer circuit board, MS…Modified surface, BS…Bonding sheet, AG…Void area

Claims

1. A polyimide film comprising a single or multiple polyimide layers, The polyimide film has a modified layer containing fluorine atoms, The modified layer has a modified surface exposed to the outside, and the concentration of fluorine atoms on the modified surface, measured by X-ray photoelectron spectroscopy, is in the range of 25 to 50 atm%, making it a polyimide film.

2. The polyimide film according to claim 1, wherein the polyimide film comprises a polyimide layer having the modified layer, and the polyimide layer having the modified layer contains inorganic filler particles.

3. The polyimide film according to claim 2, wherein the average particle size of the inorganic filler particles is in the range of 0.3 to 1.5 μm, and the content of the inorganic filler particles in the polyimide layer having the modified layer is in the range of 1 to 10 volume percent.

4. The polyimide film according to claim 2, wherein the polyimide film has a structure in which a first thermoplastic polyimide layer having the modified layer, a non-thermoplastic polyimide layer, and a second thermoplastic polyimide layer are laminated in this order.

5. The polyimide film according to claim 1, wherein the maximum thickness of the modified layer is 30 nm or less in the depth direction from the surface.

6. A method for producing a polyimide film according to any one of claims 1 to 5, Steps i) and ii) below; Step i) A step of preparing a polyimide film containing one or more polyimide layers, And, Step ii) A step of treating the surface of the polyimide film with a plasma of a gas containing fluorine, A method for producing a polyimide film, characterized by containing [the specified ingredient].

7. A metal-clad laminate comprising an insulating resin layer and a metal layer laminated on one side of the insulating resin layer, A metal-clad laminate characterized in that the insulating resin layer includes a polyimide film according to any one of claims 1 to 5, and the modified layer in the polyimide film is exposed on the side opposite to the metal layer.

8. A circuit board comprising an insulating resin layer and a circuit wiring layer laminated on one side of the insulating resin layer, A circuit board characterized in that the insulating resin layer includes a polyimide film according to any one of claims 1 to 5, and the modified layer of the polyimide film is exposed on the side opposite to the circuit wiring layer.

9. A multilayer circuit board in which multiple circuit boards are stacked, It comprises at least a first circuit board, a second circuit board, and an adhesive layer partially interposed between the first circuit board and the second circuit board to bond them together. Both the first circuit board and the second circuit board are single-sided circuit boards having an insulating resin layer and a circuit wiring layer laminated on one side of the insulating resin layer. In the region where the adhesive layer is absent, the insulating resin layer side of the first circuit board and the insulating resin layer side of the second circuit board face each other and are spaced apart from each other. A multilayer circuit board characterized in that one or both of the first circuit board and the second circuit board are the circuit boards described in claim 8.

10. The multilayer circuit board according to claim 9, wherein a gap is formed between the two insulating resin layers with the adhesive layer acting as a spacer, and the gap is used as a bending portion to repeatedly perform bending operations.

11. An electronic device characterized by comprising the circuit board described in claim 8.

12. An electronic device characterized by comprising the circuit board described in claim 8.

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