JP7813170B2 - High dielectric constant and low dielectric loss tangent polyimide film, multilayer polyimide film, flexible metal clad laminate, and flexible printed circuit board - Google Patents

Description

The present invention relates to a polyimide film, a multilayer polyimide film, a flexible metal-clad laminate, and a flexible printed circuit board with a high dielectric constant and low dielectric loss tangent that can be suitably used in high-frequency circuit boards.

Polyimide film is widely used as an electronic circuit board material due to its excellent mechanical strength, heat resistance, electrical insulation, and chemical resistance. For example, flexible copper-clad laminates (hereinafter also referred to as FCCLs) are manufactured by laminating copper foil on at least one side of polyimide film as a substrate material, and flexible printed circuit boards (hereinafter also referred to as FPCs) are manufactured by further fabricating circuits using polyimide film as a substrate material, and these are used in a variety of electronic devices.

In recent years, the frequency of electrical signals propagating through circuits has increased in conjunction with the high-speed signal transmission of electronic devices, resulting in a growing demand for electronic circuit board materials with low dielectric constants and low dielectric dissipation factors. This is because lowering the dielectric constant and dielectric dissipation factor is effective in suppressing transmission loss of electrical signals. In recent years, as we are in the dawn of the IoT society, the trend toward higher frequencies is continuing, and there is a demand for circuit board materials that can suppress transmission loss, even in the range of 10 GHz and above.

At the same time, with the trend toward lighter, thinner, and more compact designs, there is also a demand for materials with a high dielectric constant that can shorten the signal propagation wavelength. Ceramics have been used as such materials with a high dielectric constant and low dielectric dissipation factor, but with the need for even lower transmission loss and the need for even lower dielectric dissipation factors, organic materials are also seen as promising.

No. 5011697

Various methods have been proposed to achieve a high dielectric constant and a low dielectric dissipation factor. For example, as in Patent Document 1, there has been much research into adding inorganic fillers with high dielectric constants to polyimide resins. However, while this results in good film properties such as heat resistance, it does not result in a low dielectric dissipation factor.

In view of the above problems, the object of the present invention is to provide a polyimide film that is heat resistant, has a high dielectric constant, and a low dielectric loss tangent.

In light of the above current situation, the inventors conducted extensive research and discovered that the above objectives can be achieved by the following configuration.

1) A polyimide film characterized by a dielectric dissipation factor of 0.006 or less at 10 GHz and a dielectric constant of 4 or more.

2) The polyimide film described in 1), characterized in that the polyimide film is composed of a condensation reaction product of an acid dianhydride and a diamine compound, the condensation reaction product having a dielectric dissipation factor of 0.004 or less at 10 GHz, and an inorganic filler having a dielectric constant of 5 or more at 10 GHz.

3) The polyimide film according to 1) or 2), characterized in that the polyimide dianhydride contains one or more acid dianhydrides selected from 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, pyromellitic dianhydride, and ester group-containing tetracarboxylic dianhydrides, and the diamine compound contains paraphenylenediamine and one or more diamine compounds selected from 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene residue, 1,4-bis(4-aminophenoxy)benzene residue, and 4,4'-diamino-2,2'-dimethylbiphenyl.

4) A polyimide film according to any one of 1) to 3), characterized in that it contains 75 mol% to 95 mol% of paraphenylenediamine, when the total diamine compounds are taken as 100 mol%.

5) A polyimide film according to any one of 1) to 4), characterized in that it contains 10 mol% to 80 mol% of 3,3',4,4'-biphenyltetracarboxylic dianhydride, when the total acid dianhydride is taken as 100 mol%.

6) A polyimide film according to any one of 1) to 5), characterized in that the acid dianhydride contains 4,4'-oxydiphthalic anhydride.

7) A polyimide film according to any one of 1) to 5), characterized in that the acid dianhydride contains pyromellitic dianhydride.

8) A polyimide film according to any one of 1) to 7), characterized in that the acid dianhydride contains 3,3',4,4'-benzophenonetetracarboxylic dianhydride.

9) A polyimide film according to any one of 1) to 8), characterized in that the inorganic filler forms a perovskite-type crystal structure.

10) The polyimide film according to any one of 1) to 9), wherein the A-site element of the inorganic filler that forms the perovskite (ABO3) crystal structure is Ca.

11) A multilayer polyimide film comprising a thermoplastic polyimide layer laminated on at least one side of the polyimide film described in any one of 1) to 10).

12) A flexible metal-clad laminate in which a metal layer is provided on the multilayer polyimide film described in 11).

13) A flexible printed circuit board having a circuit formed on the metal layer of the flexible metal clad laminate described in 12).

The present invention makes it possible to provide a polyimide film that is heat resistant, has a high dielectric constant, and a low dielectric loss tangent.

The polyimide film of the present invention is characterized by a dielectric dissipation factor of 0.006 or less and a dielectric constant of 4 or more at 10 GHz at a temperature of 23°C and humidity of 50%.

The dielectric constant and dielectric loss tangent of the present invention were measured at 10 GHz using a 2 mm x 100 mm sample cut out of a material using a network analyzer (Keysight Corporation, E5071C) and the cavity resonator perturbation method. This measurement was performed in accordance with ASTM D2520 (JIS C2565).

The polyimide film of the present invention is composed of a condensation reaction product (polyimide) of an acid dianhydride and a diamine compound, which has a dielectric dissipation factor of 0.004 or less at 10 GHz, and an inorganic filler with a dielectric constant of 5 or more at 10 GHz.

This article describes the condensation reaction product (polyimide) of the present invention between an acid dianhydride and a diamine compound, and an inorganic filler with a dielectric constant of 5 or more at 10 GHz.

(Condensation reaction product of acid dianhydride and diamine compound)
The condensation reaction product of the acid dianhydride and the diamine compound of the present invention preferably contains, as the acid dianhydride, one or more acid dianhydrides selected from 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter sometimes referred to as BPDA), 4,4'-oxydiphthalic anhydride (hereinafter sometimes referred to as ODPA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (hereinafter sometimes referred to as BTDA), pyromellitic dianhydride (hereinafter sometimes referred to as PMDA), and ester group-containing tetracarboxylic dianhydrides, because this has the effect of lowering the dielectric tangent.

BPDA is an essential component, and the BPDA content, assuming that the total acid dianhydride is 100 mol%, is preferably 10 mol% or more and 80 mol% or less, more preferably 15 mol% or more and 75 mol% or less, and even more preferably 20 mol% or more and 70 mol% or less.

The ester group-containing tetracarboxylic acid dianhydride is not particularly limited as long as it contains an ester group within its structure. Examples include p-phenylene bis(trimellitic acid monoester anhydride) (hereinafter sometimes referred to as "TMHQ"), ethylene bis(trimellitic acid monoester anhydride), bisphenol A bis(trimellitic acid monoester anhydride), p-biphenylene bis(trimellitic acid monoester anhydride) (hereinafter sometimes referred to as "BP-TME"), 4-[4-(1,3-dioxoisobenzofuran-5-ylcarbonyloxy)-2,3,5-trimethylphenyl]-2,3,6-trimethylphenyl 1,3-dioxoisobenzofuran-5-carboxylate, 4-{[4-(1,3-dioxoisobenzofuran-5-ylcarbonyloxy)phenyl]cyclohexyl}phenyl 1,3-dioxoisobenzofuran-5-carboxylate, and derivatives thereof. Of these ester group-containing tetracarboxylic dianhydrides, TMHQ and BP-TME tend to exhibit low dielectric tangents and are particularly preferred.

Inclusion of an ester group-containing tetracarboxylic acid dianhydride as the tetracarboxylic acid dianhydride component is preferred because it tends to reduce the dielectric dissipation factor of the resulting polyamic acid or polyimide. When an ester group-containing tetracarboxylic acid dianhydride is included, the content is preferably 5 mol% to 70 mol%, more preferably 5 mol% to 60 mol%, and even more preferably 10 mol% to 50 mol%, based on 100 mol% of the total acid dianhydrides. In particular, to obtain a polyimide with a reduced dielectric dissipation factor, the total content of BPDA and ester group-containing tetracarboxylic acid dianhydride is preferably 75 mol% or less, more preferably 70 mol% or less, based on the total tetracarboxylic acid dianhydride components constituting the polyimide precursor and polyimide.

In addition, it is preferable that the diamine compound contains paraphenylenediamine and one or more diamine compounds selected from 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene residue, 1,4-bis(4-aminophenoxy)benzene residue, and 4,4'-diamino-2,2'-dimethylbiphenyl, as this has the effect of lowering the dielectric tangent.

Furthermore, when the total diamine compounds are taken as 100 mol%, it is preferable to contain 75 mol% to 95 mol%, and more preferably 80 mol% to 95 mol%, of paraphenylenediamine, as this tends to improve heat resistance, particularly shape retention at 380°C.

Containing ODPA as a tetracarboxylic dianhydride component is preferred because it tends to reduce the dielectric dissipation factor of the resulting polyamic acid or polyimide. When ODPA is included, the content is preferably 20 mol% to 60 mol%, more preferably 25 mol% to 60 mol%, and even more preferably 25 mol% to 55 mol%, based on 100 mol% of the total acid dianhydride.

Including PMDA as a tetracarboxylic dianhydride component is preferred because it tends to improve the molecular weight of the resulting polyamic acid or polyimide. When PMDA is used as a tetracarboxylic dianhydride component, in order to obtain a polyimide that has improved heat resistance and a reduced dielectric tangent, the PMDA content of the total tetracarboxylic dianhydride components constituting the polyimide precursor and polyimide is preferably 15 mol% or less, more preferably 12 mol% or less, and even more preferably 10 mol% or less.

It is also preferable to contain BTDA as the tetracarboxylic dianhydride component. If ODDA is contained, the content is preferably 5 mol% to 40 mol%, more preferably 5 mol% to 35 mol%, and even more preferably 10 mol% to 30 mol%, assuming that the total acid dianhydrides are 100 mol%.

In order to obtain a polyimide that can further reduce the dielectric loss tangent, the substance amount ratio obtained by dividing the total substance amount of all tetradicarboxylic dianhydrides constituting the polyimide precursor and polyimide by the total substance amount of all diamines constituting the polyimide precursor and polyimide is preferably 0.95 or more and 1.05 or less, more preferably 0.97 or more and 1.03 or less, and even more preferably 0.99 or more and 1.01 or less.
The polyimide of the present invention may contain components (additives) other than the components constituting the polyimide precursor and polyimide. Examples of additives that can be used include dyes, surfactants, leveling agents, plasticizers, silicones, and sensitizers. The content of the additives in the polyimide is, for example, 30% by weight or less, preferably 20% by weight or less, more preferably 10% by weight or less, and may even be 0% by weight, based on the total amount of the polyimide.

(Inorganic filler with a dielectric constant of 5 or more at 10 GHz)
The inorganic filler of the present invention having a dielectric constant of 5 or more at 10 GHz is not particularly limited, but preferably has a perovskite (ABO3) crystal structure.

The inorganic filler having a perovskite-type crystal structure of the present invention is not particularly limited, but examples include the following materials:

It is preferred that the A-site element is at least one selected from the group consisting of Ba, Ca, Mg and Sr, and the B-site element is at least one selected from the group consisting of Ti and Zr. Specific preferred compositions include BaTiO ₃ , CaTiO ₃ , SrTiO ₃ , Ba _x Ca _1-x TiO ₃ (wherein x is 0<x<1), Ba _x Sr _1-x ZrO ₃ (wherein x is 0<x<1), BaTi _x Zr _1-x O ₃ (wherein x is 0<x<1), Ba _x Ca _1-x Ti _y Zr _1-y O ₃ (wherein x is 0<x<1, y is 0<y<1), and Ba _1-x-y Ca _x Mg _y Ti _z Zr _1-z O ₃ (wherein x is 0<x<1, y is 0<y<1, z is 0<z<1, 0<x+y<1) and the like. These perovskite complex oxides may be used singly or in combination of two or more.

Also, although the reason is unclear, it is most preferable for the A-site element to be Ca in order to achieve a low dielectric tangent.

The amount of inorganic filler added in the present invention is preferably 10% by volume or more, and more preferably 70% by volume or less. If the amount of inorganic filler added in the present invention is 10% by volume or less, the dielectric constant may not be 4 or more, and if the amount added is 70% by volume or more, there is a concern that the mechanical strength and insulating properties may decrease.

(Method of Including Inorganic Filler in Polyimide Film)
The method for incorporating inorganic fillers into polyimide films of the present invention will now be described. Polyamic acid containing inorganic fillers is synthesized by mixing an inorganic filler with a polyamic acid solution, which is a precursor of polyimide, and then the polyamic acid is converted into polyimide to obtain an inorganic filler-containing polyimide film.

The method for mixing the polyamic acid solution and inorganic filler can be any known method and is not particularly limited. For example, the polyamic acid solution can be dispersed in a solvent before synthesis, and the polyamic acid can be synthesized in the inorganic filler dispersion, or the synthesized polyamic acid solution can be mixed with the inorganic filler dispersion that has been dispersed in a solvent beforehand. To obtain a polyamic acid solution in which the inorganic filler is uniformly dispersed, it is preferable to mix the synthesized polyamic acid solution with the inorganic filler dispersion.

Methods for imidizing polyamic acid, a precursor to polyimide, can be broadly divided into thermal imidization and chemical imidization. Thermal imidization is a method in which a polyamic acid solution is cast onto a support as a film-forming dope without using a dehydrating ring-closing agent, and imidization is promoted simply by heating. Chemical imidization, on the other hand, is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization accelerator is used as a film-forming dope to promote imidization. Either method is acceptable, but chemical imidization offers superior productivity.

As the dehydration ring-closing agent, an acid anhydride such as acetic anhydride can be suitably used. As the catalyst, a tertiary amine such as an aliphatic tertiary amine, an aromatic tertiary amine, or a heterocyclic tertiary amine can be suitably used. Among the tertiary amines, pyridine compounds having alkyl groups at the β- and/or γ-positions, such as β-picoline, γ-picoline, and 3,5-dimethylpyridine, pyridine, and isoquinoline are particularly preferred.
Suitable supports for casting the film-forming dope include glass plates, aluminum foils, endless stainless steel belts, stainless steel drums, etc. Heating conditions are set depending on the thickness of the final film to be obtained and the production rate, and the film is partially imidized or dried, and then peeled off from the support to obtain a polyamic acid film (hereinafter referred to as a gel film).

The edges of the gel film are fixed to prevent shrinkage during curing, and the film is then dried. Water, residual solvent, and imidization accelerator are removed from the gel film, and the remaining amide acid is completely imidized to obtain a polyimide-containing film. Heating conditions can be set appropriately depending on the final film thickness and production speed.

The polyimide film of the present invention can be laminated with copper foil via an adhesive to form a flexible copper-clad laminate. Furthermore, a multilayer polyimide film can be laminated with copper foil as one layer of a multilayer polyimide film to form a flexible copper-clad laminate. The multilayer polyimide film will now be described.

(multilayer polyimide film)
The multilayer polyimide film is a multilayer polyimide film having a non-thermoplastic polyimide film and at least one thermoplastic polyimide layer, and a specific example is a multilayer polyimide film having a thermoplastic polyimide layer on one or both sides of a non-thermoplastic polyimide film.

The polyimide film of the present invention can be used as either a non-thermoplastic polyimide layer or a thermoplastic polyimide layer, depending on its properties.

Flexible printed wiring boards are produced by laminating a metal foil layer to an insulating film layer, such as the aforementioned multilayer polyimide film, to create a flexible metal-clad laminate, and then forming a circuit pattern. Epoxy resins and acrylic resins have traditionally been used for the adhesive layer, but these have poor heat resistance and are therefore limited in their applications. However, two-layer flexible printed wiring boards that use thermoplastic polyimide as the adhesive layer offer excellent heat resistance and flexibility, and demand is expected to grow further.

In the present invention, a method for producing a multilayer polyimide film includes the following steps: i) reacting a diamine with an acid dianhydride in an organic solvent to obtain a polyamic acid solution; ii) casting a film-forming dope containing the polyamic acid solution onto a support; iii) heating the film on the support and then peeling the gel film from the support; and iv) further heating to imidize the remaining polyamic acid and drying the film to produce a non-thermoplastic polyimide film, which is then used as a non-thermoplastic polyimide layer and an adhesive layer is provided on at least one side of the film by coating or the like.

In addition, in step ii) above, a co-extrusion die having multiple flow paths is used to cast and coat a solution containing a polyimide resin precursor for forming the non-thermoplastic polyimide layer and a solution containing a polyimide resin precursor for forming the adhesive layer onto a support, thereby simultaneously forming multiple resin layers (co-extrusion casting and coating method).

As will be described in detail in the section on flexible metal-clad laminates below, there is also a method (metal foil casting method) in which multiple polyamic acid solutions are sequentially cast onto a metal foil and then imidized to directly form a layer equivalent to a multilayer polyimide film on the metal foil.

ii) The steps from this point onwards can be broadly divided into thermal imidization and chemical imidization. Thermal imidization is a method in which a polyamic acid solution is cast onto a support as a film-forming dope without using a dehydrating ring-closing agent, and imidization is promoted simply by heating. Chemical imidization, on the other hand, is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization accelerator is used as a film-forming dope to promote imidization. Either thermal imidization or chemical imidization can be used, but chemical imidization offers superior productivity.

Acid anhydrides, such as acetic anhydride, are suitable dehydration ring-closing agents. Tertiary amines, such as aliphatic tertiary amines, aromatic tertiary amines, and heterocyclic tertiary amines, are suitable catalysts. Among tertiary amines, pyridine compounds with alkyl groups at the β- and/or γ-positions, such as β-picoline, γ-picoline, and 3,5-dimethylpyridine, as well as pyridine and isoquinoline, are particularly preferred.

In steps iii) and beyond, suitable supports for casting the film-forming dope include glass plates, aluminum foil, endless stainless steel belts, and stainless steel drums. Heating conditions are set according to the final film thickness and production speed, and the film is partially imidized and/or dried, after which it is peeled off from the support to obtain a polyamic acid film (hereinafter also referred to as a gel film).

In the subsequent steps of iv) the edges of the gel film are fixed to prevent shrinkage during curing and then dried to remove water, residual solvent, and any imidization accelerator remaining in the film. The remaining polyamic acid is then completely imidized to obtain a polyimide film. In addition to fixing the edges of the gel film, it may also be stretched in the conveying direction or perpendicular to the conveying direction.

Because imidization requires extremely high temperatures, when forming a resin layer other than polyimide, it is preferable to use the latter method to prevent thermal decomposition. When forming a thermoplastic polyimide film by coating, a thermoplastic polyimide precursor may be applied and then imidized, or a thermoplastic polyimide solution may be applied and dried. Furthermore, a thermoplastic polyimide film may be obtained by casting a polyimide solution and then cooling it, instead of casting a polyamic acid solution onto a support in the above-mentioned process.

(Flexible Metal Laminate)
A flexible metal laminate may be produced by forming a layer produced by a dry film-forming method on at least one surface of the polyimide film of the present invention, and then forming a metal layer directly on the polyimide film by forming an electroless plating layer in a subsequent step. Examples of methods for forming a metal layer by dry plating include vacuum deposition, sputtering, ion plating, and CVD.

Other methods include the following:
a) After obtaining a multilayer polyimide film as described above, a metal foil is laminated onto the multilayer polyimide film by heating and pressing to obtain a flexible metal foil laminate.
b) A method in which a solution containing polyamic acid in an organic solvent is cast onto a metal foil, and the solvent is removed by heating, followed by imidization to obtain a flexible metal foil laminate.

In method a), a metal foil is laminated to the resulting multilayer polyimide film under heat and pressure (lamination), thereby obtaining the flexible metal foil laminate of the present invention. The method and conditions for laminating the metal foil may be appropriately selected from those conventionally known.

In method b), the means for casting the organic solvent solution containing polyamic acid onto the metal foil is not particularly limited, and conventionally known means such as a die coater, comma coater (registered trademark), reverse coater, or knife coater can be used. Conventional known means can also be used for heating to remove the solvent and perform imidization, such as a hot air oven or far-infrared oven.

When the polyimide film of the present invention is to be provided with multiple layers of other polyimide or layers of resins other than polyimide, the above-mentioned casting and heating steps can be repeated multiple times, or multiple cast layers can be formed by co-extrusion or continuous casting and then heated all at once.

In method b), the flexible metal foil laminate of the present invention is obtained as soon as imidization is completed. When metal foil layers are provided on both sides of the resin layer, the metal foil can be bonded to the opposite surface of the resin layer by applying heat and pressure.

The metal foil that can be used in the present invention is not particularly limited, but when the flexible metal-clad laminate of the present invention is used for electronic or electrical equipment applications, examples include foils made of copper or copper alloys, stainless steel or its alloys, nickel or nickel alloys (including 42 alloy), and aluminum or aluminum alloys. Copper foils such as rolled copper foil and electrolytic copper foil are commonly used in general flexible laminates, and they can also be preferably used in the present invention. The surface of these metal foils may be coated with an anti-rust layer, heat-resistant layer, or adhesive layer. The thickness of the metal foil is also not particularly limited, as long as it is thick enough to perform its functions appropriately for the application.

(flexible printed circuit board)
A flexible printed circuit board obtained by etching the metal layer of the flexible metal clad laminate according to the present invention is a circuit board that can be miniaturized, has low transmission loss, and is suitable for high frequency applications.

The present invention will be explained in detail below using examples, but the present invention is not limited to these examples. The dielectric constant and dielectric loss tangent were evaluated as follows:

<Measurement of dielectric constant (Dk) and dielectric loss tangent (Df)>
The dielectric constant and dielectric loss tangent were measured using a network analyzer 8719C manufactured by Hewlett-Packard and a cavity resonator vibration method dielectric constant measuring device CP511 manufactured by Kanto Electronics Application Development Co., Ltd. Samples were cut into 2 mm x 100 mm pieces and measured after 24 hours of humidity conditioning in a 23°C/50% RH environment (normal condition). The measurements were performed at 10 GHz.

(Synthesis Example 1)
In a 500 ml glass flask, 164.2 g of N,N-dimethylformamide (DMF), 3.0 g of 1,3-bis(4-aminophenoxy)benzene (TPE-R), and 6.4 g of p-phenylenediamine (p-PDA) were dissolved. To the resulting solution, 12.2 g of s-biphenyltetracarboxylic dianhydride (s-BPDA) and 7.9 g of 4,4'-oxyphthalic dianhydride (ODPA) were added, and the solution in the flask was stirred for 30 minutes to dissolve the components. A separately prepared DMF solution of pyromellitic dianhydride (PMDA) (0.5 g PMDA/5.8 g DMF) was then carefully added to the resulting solution, and the addition was stopped when the viscosity reached approximately 1500 poise. The liquid in the flask was then stirred for 1 hour to obtain a polyamic acid solution having a solids concentration of about 15% by weight and a rotational viscosity at 23° C. of 1500 to 2000 poise.

(Synthesis Example 2)
DMF was added to a reaction vessel and stirred under a nitrogen atmosphere. 100 parts by weight of 2,2'-bis(trifluoromethyl)benzidine (TFMB), 30 parts by weight of p-phenylenebis(trimellitic acid monoester anhydride) (TMHQ), and 70 parts by weight of 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) were sequentially added and stirred for 5 hours under a nitrogen atmosphere to obtain a polyamic acid solution with a solids concentration of 18%. Pyridine was added to the polyamic acid solution as an imidization catalyst, and after complete dispersion, acetic anhydride was added. The solution was stirred at 120°C for 2 hours and then cooled to room temperature. Isopropyl alcohol (IPA) was added while stirring the solution to precipitate polyimide. The solution was then subjected to suction filtration and washed with IPA four times, after which it was dried for 12 hours in a vacuum oven set at 120°C to obtain a polyimide resin.

(Reference example 1)
To 55 g of the polyamic acid solution obtained in Synthesis Example 1, 27.5 g of a curing agent consisting of 5.15 g acetic anhydride, 6.01 g isoquinoline, and 16.34 g DMF was added, stirred, and degassed at a temperature below 0°C. The mixture was then cast onto aluminum foil using a comma coater. The resulting resin film was dried at 110°C for 133 seconds, after which the gel film was peeled off the aluminum foil and secured to a metal frame, taking care not to shrink the gel film. The gel film secured to the metal frame was then heated in a preheated hot air circulating oven at 250°C for 15 seconds and 350°C for 79 seconds, and then cut from the frame to obtain a 17 μm-thick polyimide film.

(Reference example 2)
A polyimide solution was prepared by dissolving the polyimide resin obtained in Synthesis Example 2 in methylene chloride to a solids concentration of 12%. Next, this polyimide solution was applied to a glass plate under the following conditions, and a polyimide film was obtained by primary drying on a hot plate and secondary drying in a metal frame at 200°C for 30 minutes.
Coating conditions: Clearance 300 μm, speed 10 mm/sec

Example 1
A DMF dispersion was prepared by dispersing 18.79 g of inorganic filler (calcium titanate (CT-03)) in 10.0 g of DMF. 28.79 g of the resulting inorganic filler dispersion was added to 55 g of the polyamic acid solution synthesized in Synthesis Example 1 and mixed. 17.5 g of a curing agent consisting of 5.15 g of acetic anhydride, 6.01 g of isoquinoline, and 6.34 g of DMF was added to the resulting inorganic filler-containing polyamic acid solution, stirred and degassed at a temperature below 0°C, and then cast onto aluminum foil using a comma coater. The subsequent procedures were the same as in Reference Example 1 to obtain a polyimide film. The type of inorganic filler, the amount added, the dielectric constant, and the dielectric loss tangent are listed in Table 1. In Table 1, the ratio of Synthesis Example 1:CT-03 = 60:40 refers to vol%.

(Examples 2 to 4 and Comparative Examples 1 to 3)
A polyimide film was obtained in the same manner as in Example 1, except that the type and content of the inorganic filler in Example 1 were changed. The type, amount added, dielectric constant, and dielectric loss tangent of the inorganic filler are shown in Table 1.

(Comparative Examples 4 and 5)
As in Reference Example 2, a polyimide solution was prepared by dissolving the polyimide resin obtained in Synthesis Example 2 and a predetermined amount of inorganic filler: calcium titanate (CT-03) in methylene chloride to a solids concentration of 12%. A polyimide film was then obtained in the same manner as in Reference Example 2. The type, amount added, dielectric constant, and dielectric loss tangent of the inorganic filler are shown in Table 1.

<Inorganic filler used>
・CT-03 Calcium titanate manufactured by Sakai Chemical Industry Co., Ltd. ・CZ-03 Calcium zirconate manufactured by Sakai Chemical Industry Co., Ltd. ・BT-03 Barium titanate manufactured by Sakai Chemical Industry Co., Ltd. ・TA-300K Titanium oxide manufactured by Fuji Titanium Industry Co., Ltd.

Claims (11)

The dielectric loss tangent at 10 GHz is 0.006 or less and the dielectric constant is 4 or more,
a condensation reaction product of an acid dianhydride and a diamine compound, the condensation reaction product having a dielectric loss tangent of 0.004 or less at 10 GHz, and an inorganic filler having a dielectric constant of 5 or more at 10 GHz,
the acid dianhydride comprises 3,3',4,4'-biphenyltetracarboxylic dianhydride and one or more selected from 4,4'-oxydiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, pyromellitic dianhydride and ester group-containing tetracarboxylic dianhydride;
The diamine compound includes paraphenylenediamine and at least one selected from 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, and 4,4'-diamino-2,2'-dimethylbiphenyl . 2. The polyimide film according to claim 1 , wherein the content of paraphenylenediamine is 75 mol % or more and 95 mol % or less when the total amount of all diamine compounds is taken as 100 mol %. 3. The polyimide film according to claim 1 , wherein the content of 3,3',4,4'-biphenyltetracarboxylic dianhydride is 10 mol % or more and 80 mol % or less, when the total acid dianhydride is 100 mol %. 4. The polyimide film according to claim 1 , wherein the acid dianhydride includes 4,4'-oxydiphthalic anhydride. 5. The polyimide film according to claim 1 , wherein the acid dianhydride includes pyromellitic dianhydride. 6. The polyimide film according to claim 1 , wherein the acid dianhydride comprises 3,3',4,4'-benzophenonetetracarboxylic dianhydride. 7. The polyimide film according to claim 1, wherein the inorganic filler forms a perovskite (ABO3) crystal structure. 8. The polyimide film according to claim 7 , wherein the A-site element of the inorganic filler that forms the perovskite (ABO3) crystal structure is Ca. A multilayer polyimide film, comprising the polyimide film according to any one of claims 1 to 8 and a thermoplastic polyimide layer laminated on at least one surface of the polyimide film. A flexible metal clad laminate comprising the multilayer polyimide film according to claim 9 and a metal layer provided thereon. A flexible printed circuit board comprising a circuit formed on the metal layer of the flexible metal clad laminate according to claim 10 .