Polyamic acid, polyamic acid composition, polymide, polymide film, copper foil with resin, multilayer wiring board, coil structure, magnetic device, and insulated wire

Abstract

A polyamic acid according to the present invention is a product of a polyaddition reaction between (A) an acid dianhydride and (B) a diamine, said polyamic acid containing, with respect to all acid dianhydride components, a molar ratio of 0.3 or more of (A1) an acid dianhydride having an ether bond.

Description

Polyamic acid, polyamic acid compositions, polyimides, polyimide films, resin-coated copper foil, multilayer wiring boards, coil structures, magnetic devices, and insulated wires.

[0001] The present invention relates to polyamic acid, polyamic acid compositions, polyimides, polyimide films, resin-coated copper foil, multilayer wiring boards, coil structures, magnetic devices, and insulated wires.

[0002] Polyimides are widely used as insulating materials in electronic devices due to their excellent heat resistance, mechanical properties, and insulating properties. However, typical polyimides have high dielectric properties such as dielectric constant, which causes delays in signal transmission and hinders high-speed operation. To address this issue of high frequencies, there is a growing demand for polyimides that exhibit low dielectric properties. For example, Patent Document 1 discloses that a polyimide made using an aromatic diamine and a tetracarboxylic dianhydride having a specific structure exhibits a low dielectric constant and also has excellent heat resistance.

[0003] Furthermore, for example, Patent Document 2 discloses that the dielectric constant of polyimide can be further reduced by using an ester-type acidic dianhydride and a diamine component containing a dimer amine.

[0004] Japanese Patent Publication No. 10-152559 Japanese Patent Publication No. 2024-139564

[0005] On the other hand, while polyimides have excellent dielectric properties and dielectric strength, few materials achieve both low dielectric constant and AC dielectric strength. Furthermore, the polyimides described in Patent Documents 1 and 2 were unable to achieve both low dielectric constant and AC dielectric strength.

[0006] The present invention aims to provide a polyamic acid having a low dielectric constant and excellent AC withstand voltage, as well as polyamic acid compositions, polyimides, polyimide films, resin-coated copper foils, multilayer wiring boards, coil structures, magnetic devices, and insulated wires using the same.

[0007] The present invention provides the following polyamic acid, polyamic acid composition, polyimide, polyimide film, resin-coated copper foil, multilayer wiring board, coil structure, magnetic device, and insulated wire. [1] A polyamic acid which is a polyaddition reaction product of (A) an acid dianhydride and (B) a diamine, wherein (A1) an acid dianhydride having an ether bond is contained in a molar ratio of 0.3 or more relative to the total acid dianhydride component. [2] The polyamic acid according to [1], wherein the (B) diamine contains (B1) a dimer amine. [3] The polyamic acid according to [1] or [2], wherein the (B) diamine contains (B2) a diamine having an ether bond. [4] A polyamic acid composition which contains the polyamic acid according to any one of [1] to [3] and (C) an organic solvent. [5] A polyimide obtained by imidizing a polyamic acid according to any one of [1] to [3]. [6] A polyimide film comprising a polyimide obtained by imidizing a polyamic acid according to any one of [1] to [3]. [7] A resin-coated copper foil comprising the polyimide film according to [6] and a copper foil laminated on the polyimide film. [8] A multilayer wiring board comprising an insulating layer formed using the polyimide film according to [6]. [9] A coil structure comprising an insulating layer formed using the polyimide film according to [6].

[10] A magnetic device comprising an insulating layer formed using the polyimide film according to [6].

[11] An insulated wire comprising an insulating layer made of the polyimide according to [5] and a wire covered with the insulating layer.

[0008] According to one aspect of the present invention, a polyamic acid having a low dielectric constant and excellent AC withstand voltage, as well as a polyamic acid composition, polyimide, polyimide film, resin-coated copper foil, multilayer wiring board, coil structure, magnetic device, and insulated wire using the same can be provided.

[0009] [Polyamic Acid] The polyamic acid according to this embodiment is a polyamic acid obtained as a polyaddition reaction product of (A) an acid dianhydride and (B) a diamine. It contains (A1) an acid dianhydride having an ether bond in a molar ratio of 0.3 or more relative to the total acid dianhydride component.

[0010] (Component (A)) The polyamic acid according to this embodiment uses (A) acid dianhydride as one of its raw materials. Known acid dianhydrides can be used as appropriate. The acid dianhydride component must contain (A1) acid dianhydride having an ether bond. In addition, the polyamic acid according to this embodiment must contain (A1) acid dianhydride having an ether bond in a molar ratio of 0.3 or more relative to the total acid dianhydride component. By including (A1) acid dianhydride having an ether bond in a molar ratio above a certain level relative to the total acid dianhydride component, the dielectric constant of the resulting polyimide can be reduced and the AC withstand voltage can be improved. The molar ratio of (A1) acid dianhydride having an ether bond relative to the total acid dianhydride component is preferably 0.4 or more, and more preferably 0.5 to 0.8.

[0011] (A1) The acid dianhydride having an ether bond is an acid dianhydride having an ether bond (C-O-C) in one molecule. By having a polyamic acid have structural units derived from the (A1) acid dianhydride having an ether bond, the dielectric constant of the resulting polyimide can be reduced and the AC withstand voltage can be improved. The acid dianhydride has two acid anhydride groups in one molecule, and is preferably a tetracarboxylic dianhydride, and more preferably an aromatic tetracarboxylic dianhydride. There may be one or more ether bonds in the molecule, preferably one to three, and more preferably one or two. The acid dianhydride may be used alone, or two or more may be used in combination.

[0012] Such acidic dianhydrides preferably have the structure shown in the following formula (1). In formula (1), X １ is a single bond or linking group, X １ When it is a linking group, the linking group is -CH ２ -, -C ２ H ４-, -C(CH ３ ) ２ -, -(C ６ H ４ )-C(CH ３ ) ２ -(C ６ H ４ )-, and -(C ６ H ４ )-CH ２ -(C ６ H ４ )-, and the like. X ２ is a single bond when X １ is a single bond, and is -O- when X １ is a linking group.

[0013]

[0014] Preferable examples of the tetracarboxylic dianhydride represented by formula (1) include a compound represented by the following formula (1-1) and a compound represented by the following formula (1-2), and the compound represented by the following formula (1-1) (oxydiphthalic anhydride (ODPA)) is more preferable.

[0015]

[0016] The polyamic acid according to the present embodiment may contain a tetracarboxylic dianhydride component other than component (A1) (hereinafter also referred to as component (A2)). Examples of other tetracarboxylic dianhydride components include 2,2',3,3',5,5'-hexamethyl[1,1'-biphenyl]-4,4'-diyl bis(1,3-dioxo-1,3-dihydro-2-benzofuran-5-carboxylate) (TMPBP-TME), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA).

[0017] (Component (B)) The polyamic acid according to this embodiment uses (B) diamine as one of the raw materials. Any known diamine can be used as appropriate. The diamine component has structural units derived from a diamine, and it is preferable that one of the diamine components has structural units derived from (B1) dimer amine. Here, dimer amine is a cyclic or acyclic dimer acid obtained as a dimer of an unsaturated fatty acid in which two terminal carboxylic acid groups (-COOH) are primary aminomethyl groups (-CH ２ -NH ２ ) or amino group (-NH ２ This refers to an aliphatic diamine substituted with (B1). By having a polyamic acid with structural units derived from dimer amine, low dielectric properties can be imparted to the resulting polyimide. (B1) Dimer amine may be used alone or in combination of two or more types.

[0018] Dimer acids are dibasic acids obtained by the intermolecular polymerization reaction of unsaturated fatty acids. Aliphatic diamines derived from dimer acids are obtained by polymerizing unsaturated fatty acids such as oleic acid, linoleic acid, or linolenic acid to form dimer acids, which are then reduced and aminated. Such dimer amines are preferably diamine compounds obtained by substituting the terminal carboxylic acid group of a dibasic acid compound having 18 to 54 carbon atoms, preferably 22 to 44 carbon atoms, with a primary aminomethyl group or an amino group.

[0019] Commercially available dimeramines include "Versamin® 551" and "Versamin® 552" from Cognics Japan, and "PRIAMINE® 1073," "PRIAMINE® 1074," and "PRIAMINE® 1075" from Croda Japan.

[0020] In the polyamic acid according to this embodiment, it is preferable that (B1) dimeramine is contained in a molar ratio of 0.2 or more relative to the total diamine components. By including dimeramine in a molar ratio above a certain level relative to the total diamine, the resulting polyimide can be given a lower dielectric constant and a lower dielectric loss tangent. The molar ratio of (B1) dimeramine to the total diamine components is more preferably 0.3 to 0.9, even more preferably 0.4 to 0.8, and particularly preferably 0.5 to 0.75.

[0021] The polyamic acid according to this embodiment preferably contains a diamine having an ether linkage as another diamine component. Examples of the (B2) component include 4,4'-oxydianiline (4,4'-DPE), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), 1,4-bis(p-aminophenoxy)benzene, 4,4'-bis(p-aminophenoxy)biphenyl, diaminoanthraquinone, and 4,4'-bis(3-aminophenoxyphenyl)diphenylsulfone. These may be used alone or in combination of two or more.

[0022] In the polyamic acid according to this embodiment, it is preferable that the diamine having a (B2) ether bond is contained in a molar ratio of 0.1 or more relative to the total diamine components. By including the diamine having a (B2) ether bond in a molar ratio above a certain level relative to the total diamine, the AC withstand voltage of the resulting polyimide can be further improved. The molar ratio of the diamine having a (B2) ether bond relative to the total diamine components is more preferably 0.15 or more and 0.7 or less, even more preferably 0.2 or more and 0.6 or less, and particularly preferably 0.25 or more and 0.5 or less.

[0023] The polyamic acid according to this embodiment may also contain (B3) aromatic diamines other than component (B2) as other diamine components. Examples of (B3) aromatic diamines include p-phenylenediamine (PDA), m-phenylenediamine, 3,3'-bistrifluoromethyl-4,4'-diaminobiphenyl (TFMB), 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 1,2-bis(anilino)ethane, diaminodiphenyl sulfone, diaminobenzanilide, diaminobenzoate, diaminodiphenyl sulfide, 2,2-bis(p-aminophenyl)propane, 2,2-bis(p-aminophenyl)hexafluoropropane, 1,5-diaminonaphthalene, diaminotoluene, and diaminobenzotrifluoride. These may be used alone or in combination of two or more.

[0024] The molar ratio of diamine to acidic dianhydride [(B) / (A)] is not particularly limited, but is preferably 0.90 or more and 1.10 or less, more preferably 0.95 or more and 1.05 or less, even more preferably 0.97 or more and 1.03 or less, and particularly preferably 0.98 or more and 1.02 or less.

[0025] (Component (C)) The polyamic acid according to this embodiment can be synthesized by known general methods. For example, a polyamic acid composition (polyamic acid solution) can be obtained by reacting (A) an acidic dianhydride and (B) a diamine in (C) an organic solvent. The organic solvent used for polymerization of polyamic acid is not particularly limited as long as it can dissolve the acidic dianhydride and diamine as monomer components and also dissolve the polyamic acid produced by the polyaddition reaction. Examples of such organic solvents include urea-based solvents such as tetramethylurea and N,N-dimethylethylurea; sulfone-based solvents such as dimethyl sulfoxide, diphenyl sulfone, and tetramethyl sulfone; amide-based solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, and hexamethylphosphate triamide; ester-based solvents such as γ-butyrolactone; alkyl halide-based solvents such as chloroform and methylene chloride; aromatic hydrocarbon-based solvents such as benzene and toluene; phenol-based solvents such as phenol and cresol; ketone-based solvents such as cyclopentanone; and ether-based solvents such as tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, dimethyl ether, diethyl ether, and p-cresol methyl ether. These organic solvents may be used individually or in combination of two or more. From the viewpoint of enhancing the solubility and reactivity of polyamic acids, the organic solvent is preferably selected from the group consisting of amide solvents, ketone solvents, ester solvents, and ether solvents, with amide solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N,N-diethylacetamide, and N-methyl-2-pyrrolidone being more preferred.

[0026] The molecular weight of polyamic acid can be adjusted by adjusting the molar ratio of the total number of moles of the acid dianhydride component to the total number of moles of the diamine component. The molecular weight (weight-average molecular weight) of polyamic acid is not particularly limited, but from the viewpoint of solubility in organic solvents, it is more preferable to be between 10,000 and 100,000. The weight-average molecular weight of polyamic acid can be determined, for example, from the value converted to standard polystyrene by gel filtration chromatography (GPC) measurement.

[0027] The synthesis of polyamic acids by the polyaddition reaction of acidic dianhydrides and diamines is preferably carried out in an inert atmosphere such as argon or nitrogen. In an inert atmosphere, the polyaddition reaction proceeds by dissolving the acidic dianhydride and diamine in an organic solvent and mixing them. The order of addition of the acidic dianhydride and diamine is not particularly limited. For example, the diamine may be dissolved in an organic solvent or dispersed in a slurry to form a diamine solution, and the acidic dianhydride may be added to the diamine solution. The acidic dianhydride and diamine may be added directly to the organic solvent in a solid state, or they may be added separately dissolved in an organic solvent or dispersed in a slurry.

[0028] The temperature conditions for the polyaddition reaction are not particularly limited, but from the viewpoint of suppressing the decrease in molecular weight of the polyamic acid due to depolymerization, the reaction temperature is preferably 100°C or lower, and from the viewpoint of allowing the polyaddition reaction to proceed appropriately, the reaction temperature is more preferably 20°C to 80°C. The reaction time can be arbitrarily set in the range of 1 hour to 72 hours, and if necessary, it may be left overnight at room temperature.

[0029] When preparing the polyamic acid composition according to the present embodiment, the viscosity of the solution is preferably 500 mPa·s or more from the viewpoint of film-forming properties. Further, the concentration of polyamic acid in the polyamic acid composition according to the present embodiment is preferably 10% by mass or more, more preferably 15% by mass or more, and still more preferably 25% by mass or more. In particular, when the concentration of polyamic acid is 15% by mass or more, productivity can be improved when forming a polyimide coating film using the polyamic acid. Further, the upper limit of the concentration of polyamic acid is preferably 50% by mass or less, and more preferably 30% by mass or less, from the viewpoint of sufficiently dissolving the polyamic acid in an organic solvent.

[0030] In order to impart processing characteristics or various functions to polyamic acid and polyimide formed using the polyamic acid, various organic or inorganic low-molecular-weight or high-molecular-weight compounds may be blended into the polyamic acid composition. For example, the polyamic acid composition may contain a solvent-soluble polyimide resin, a flame retardant, a dye, a surfactant, a leveling agent, a plasticizer, fine particles, a sensitizer, a silane coupling agent, and the like. The fine particles may be either organic fine particles or inorganic fine particles, and may have a porous or hollow structure. Further, after the polyamic acid is polyimidized, some amino groups may be maleimidated.

[0031] [Polyimide and Polyimide Film] The polyimide according to the present embodiment is a polyimide obtained by imidizing the polyamic acid according to the foregoing present embodiment. The method for converting polyamic acid into polyimide is not particularly limited, and polyimide can be produced by dehydrating and ring-closing (imidizing) the polyamic acid obtained as described above. As the method for dehydration ring closure (imidization), known methods such as thermal imidization in which dehydration ring closure is performed by heating, or chemical imidization in which ring closure is chemically performed using a known dehydration ring closure catalyst can be adopted.

[0032] In the case of thermal imidation, the heating temperature is preferably 120°C to 350°C, and more preferably 150°C to 250°C. In the case of chemical imidation, for example, pyridine, triethylamine, or acetic anhydride can be used as the dehydration and ring-closing catalyst. In this case, the reaction temperature can be selected from any temperature between 20°C and 180°C, but it is preferably 150°C or lower. Imidation may be carried out in air, under reduced pressure, or in an inert gas such as nitrogen, but in order to obtain a highly transparent polyimide film, it is preferable to carry it out under reduced pressure or in an inert gas such as nitrogen.

[0033] The molecular weight (weight-average molecular weight) of the polyimide is not particularly limited, but it is preferably between 10,000 and 100,000 from the viewpoint of low dielectric properties, solubility in organic solvents, and film-forming properties of the resulting polyimide. The weight-average molecular weight of the polyimide can be determined, for example, from the value converted to standard polystyrene by gel filtration chromatography (GPC) measurement.

[0034] The polyimide film according to this embodiment contains the polyimide obtained as described above. The method for producing such a polyimide film is not particularly limited, and examples include coating the above-mentioned polyamic acid composition in a film-like manner onto a substrate (for example, a plastic film made of a resin such as polyethylene, polypropylene, urethane, polyester, polyethylene terephthalate (PET), or polycarbonate, a glass plate, a stainless steel plate, a copper plate including thin copper foil, or an aluminum plate), then drying and heating to remove the solvent and dehydrate and cyclize (imidize), or coating a solution in which soluble polyimide obtained by converting polyamic acid to polyimide is dissolved in an organic solvent onto a substrate in a film-like manner, and then drying and removing the solvent. The method of coating onto the substrate is not particularly limited, and conventionally known coating methods can be applied.

[0035] The thickness of the polyimide film is not particularly limited, and can be appropriately selected according to the application. The thickness of the polyimide film can be easily controlled by appropriately adjusting the solid content concentration of each component in the polyamic acid composition, the coating thickness, the viscosity, and the like.

[0036] [Use of Polyimide] The polyimide according to the present embodiment can be suitably used for copper foils with resin, multilayer wiring boards, coil structures, magnetic devices, insulated electric wires, and the like. Furthermore, the polyimide film according to the present embodiment is suitably used as a film for various members such as color filters, flexible displays, semiconductor components, or optical members. Since the polyimide film exhibits insulating properties, it is suitably applied as an insulating substrate for printed wiring boards. In addition, since the polyimide film is a thin and flexible film with insulating properties, it is also effectively applicable as a base film for flexible circuit boards called Flexible Printed Circuits (FPC).

[0037] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited in any way by these examples. The materials used in the examples and comparative examples are listed below. (Component A1) Acid dianhydride A: Oxydiphthalic anhydride (ODPA) (Component A2) Acid dianhydride B: 2,2',3,3',5,5'-Hexamethyl[1,1'-biphenyl]-4,4'-Diyl-bis(1,3-Dioxo-1,3-Dihydro-2-benzofuran-5-carboxylate), Product name "TMPBP-TME", manufactured by Honshu Chemical Industry Co., Ltd. Acid dianhydride C: 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) Acid dianhydride D: 3,3',4,4'-Biphenyltetracarboxylic acid dianhydride (BTDA) Acid dianhydride E: Pyromellitic acid dianhydride (PMDA) (Component B1) Diamine A: Dimer amine, Product name "PRIAMINE 1075", manufactured by Cargill Japan ((B2) component) Diamine B: 4,4'-oxydianiline, product name "4,4'-DPE", manufactured by Seika Co., Ltd. Diamine C: 2,2-bis[4-(4-aminophenoxy)phenyl]propane, product name "BAPP", manufactured by Seika Co., Ltd. ((C) component) Organic solvent A: N-methyl-2-pyrrolidone (NMP) Organic solvent B: Cyclopentanone (CPN)

[0038] [Example 1] 10.9 parts by mass of diamine A, 1.4 parts by mass of diamine B, and 75 parts by mass of organic solvent A were placed in a flask and the diamine was dissolved. Next, 4.2 parts by mass of acidic dianhydride A and 8.5 parts by mass of acidic dianhydride B were added, and the mixture was stirred at 80°C for 5 hours to prepare a polyamic acid composition.

[0039] [Examples 2-5] Polyamic acid compositions were obtained in the same manner as in Example 1, except that each material was blended according to the composition shown in Table 1.

[0040] [Comparative Examples 1 and 2] Polyamic acid compositions were obtained in the same manner as in Example 1, except that each material was blended according to the composition shown in Table 1.

[0041] [Evaluation of Polyimide] Polyimide was evaluated (film-forming ability, dielectric constant, AC withstand voltage, elastic modulus, fracture stress, elongation) using the following method. The results obtained are shown in Table 1. Table 1 also shows the molar ratio of component (A), the molar ratio of component (B), and the molar ratio of diamine to acidic dianhydride [(B) / (A)] in each example. (1) Film-forming ability First, test specimens were prepared as follows. That is, a polyamic acid composition was applied to a substrate (PET film, thickness: 38 μm, with release treatment) using a bar coater, and heat treatment was performed at 100°C for 10 minutes, followed by 190°C for 20 minutes. The coating film was isolated from the substrate to prepare test specimens of a predetermined size. The obtained test specimens (thickness: 20-30 μm, length: 50 mm, width: 50 mm) were observed and the film-forming ability was evaluated according to the following criteria. A: No cracks or chips, and it could be isolated. B: Some cracking or chipping occurred during isolation. (2) Except for the thickness and size of the dielectric constant test specimen, the specified test specimen was prepared in the same manner as the preparation of the film-forming test specimen in (1). The obtained test specimen (thickness: 25 μm, length: 20 mm, width: 20 mm) was measured using the impedance analyzer "4291B" manufactured by HEWLETT PACKARD, under the conditions of a measurement temperature of 25°C and a frequency of 1 MHz, and the dielectric constant at a frequency of 1 MHz was measured and evaluated according to the following criteria. A: The dielectric constant is 2.6 or more and less than 2.8. C: The dielectric constant is 2.8 or more and less than 3.2. D: The dielectric constant is 3.2 or more. (3) Except for the thickness of the AC withstand voltage test specimen, the specified test specimen was prepared in the same manner as the preparation of the film-forming test specimen in (1). For the obtained test specimens (thickness: 40-60 μm, length: 50 mm, width: 50 mm), the dielectric breakdown voltage was measured using a dielectric strength tester and air electrode fixture manufactured by Keisoku Gijutsu Kenkyusho Co., Ltd., under the conditions of a set voltage of 12 kV, a set current of 9999 μA, and a boost rate of 500 V / s. The AC dielectric strength was then evaluated from the dielectric breakdown voltage value converted to 50 μm according to the following criteria: A: Dielectric breakdown voltage converted to 50 μm is 10 kV / 50 μm or more. B: Dielectric breakdown voltage converted to 50 μm is 9 kV / 50 μm or more and less than 10 kV / 50 μm. C: Dielectric breakdown voltage converted to 50 μm is less than 9 kV / 50 μm.Except for the thickness of the test specimens, (4) modulus of elasticity, (5) fracture stress, and (6) elongation, the specified test specimens were prepared in the same manner as for (1) the preparation of film-forming test specimens. The obtained test specimens (thickness: 40-60 μm, length: 50 mm, width: 50 mm) were cut to the specified size, and the modulus of elasticity (GPa), fracture stress (MPa), and elongation (%) were measured using an Autograph manufactured by Shimazu Corporation at a tensile speed of 5 mm / min.

[0042]

[0043] As is clear from the results shown in Table 1, the polyimides of the present invention (Examples 1-5) were confirmed to have good results in all aspects, including film formation, dielectric constant, AC withstand voltage, elastic modulus, fracture stress, and elongation. Therefore, it was confirmed that the polyimides of the present invention have a low dielectric constant and excellent AC withstand voltage.

Claims

1. A polyamic acid obtained as a polyaddition reaction product of (A) an acid dianhydride and (B) a diamine, wherein the polyamic acid contains (A1) an acid dianhydride having an ether bond in a molar ratio of 0.3 or more relative to the total acid dianhydride component.

2. A polyamic acid according to claim 1, wherein the (B) diamine contains (B1) dimeramine.

3. A polyamic acid according to claim 1 or claim 2, wherein the (B) diamine contains (B2) a diamine having an ether linkage.

4. A polyamic acid composition comprising (C) an organic solvent and the polyamic acid described in claim 1 or claim 2.

5. A polyimide obtained by imidizing the polyamic acid described in claim 1 or claim 2.

6. A polyimide film comprising a polyimide obtained by imidizing the polyamic acid described in claim 1 or claim 2.

7. A resin-coated copper foil comprising a polyimide film according to claim 6 and a copper foil laminated on the polyimide film.

8. A multilayer wiring board comprising an insulating layer formed using the polyimide film described in claim 6.

9. A coil structure comprising an insulating layer formed using the polyimide film described in claim 6.

10. A magnetic device comprising an insulating layer formed using the polyimide film described in claim 6.

11. An insulated electric wire comprising an insulating layer made of polyimide as described in claim 5, and an electric wire covered with the insulating layer.

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