JP2006077158A - Polyimidebenzoxazole film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a film composed of an organic material having slight temperature dependence and all of heat resistance, responsiveness to high frequency and flexibility at higher levels. <P>SOLUTION: The polyimidebenzoxazole film is a film consisting essentially of a polyimidebenzoxazole obtained by reacting aromatic diamines having a benzoxazole structure with aromatic tetracarboxylic anhydrides and is characterized as having the ratio ε<SB>65</SB>/ε<SB>D</SB>of the dielectric constant ε<SB>65</SB>at 100 GHz of the film subjected to humidity conditioning under constant-temperature and constant-humidity conditions of 20°C and 65%RH for 94 h and measured by a cavity resonance perturbation method to the dielectric constant ε<SB>D</SB>at 100 GHz of the film vacuum dried under conditions of 120°C for 24 h and measured by the cavity resonance perturbation method within the range of 1.00-1.10. The moisture content evaporated during heating at 500°C for 10 s is ≤5,000 ppm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polyimide benzoxazole film, and particularly to a polyimide benzoxazole film having excellent electrical characteristics with little humidity dependency, which is suitable as a base material for high frequency electronic components.

  Conventionally, ceramic has been used as a base material for electronic components such as information communication equipment (broadcast equipment, mobile radio equipment, portable communication equipment, etc.), radar, high-speed information processing devices, and the like. A base material made of ceramic has heat resistance, and can cope with a recent increase in the frequency band of information communication equipment (reaching the GHz band). However, ceramics are not flexible and cannot be thinned, so the fields that can be used are limited.

  For this reason, studies have been made to use a film made of an organic material as a base material for an electronic component, and a film made of polyimide and a film made of polytetrafluoroethylene have been proposed. A film made of polyimide is excellent in heat resistance and has the advantage that the film can be thin because it is tough, but there are concerns about problems such as a decrease in signal strength and a delay in signal transmission in application to high-frequency signals. . A film made of polytetrafluoroethylene can handle high frequencies, but it has a low elastic modulus, so the film cannot be made thin, the adhesion to metal conductors and resistors on the surface is poor, and the linear expansion coefficient is low. The problem is that it is not suitable for the production of a circuit having a fine wiring due to a large dimensional change due to a temperature change, and the usable fields are limited. As described above, a film for a substrate having both heat resistance, high frequency compatibility and flexibility has not been obtained.

Moreover, the polyimide benzoxazole film which consists of a polyimide which has a benzoxazole ring in the principal chain is proposed as a polyimide film which raised the elasticity modulus (refer patent document 1). A printed wiring board using the polyimide benzoxazole film as a dielectric layer has also been proposed (see Patent Document 2 and Patent Document 3).
JP-A-6-56992 Japanese National Patent Publication No. 11-504369 Japanese National Patent Publication No. 11-505184

However, the use of a base material made of a polyimide benzoxazole film, which has been reported in the past, is inferior to the use of a base material made of a ceramic in dealing with ultra-high frequencies reaching the millimeter wave region. Specifically, the environment is particularly dependent on humidity, the transmission efficiency of high-frequency signals is low, and the response speed is slow (the rise of the pulse signal is poor), so it is difficult to speed up the operation of the circuit using the polyimide benzoxazole film. .
An object of the present invention is to provide a polyimide benzoxazole film that maintains heat resistance, high-frequency compatibility, and flexibility at a higher level, and has low electrical properties, particularly humidity dependency.

As a result of intensive studies, the present inventors have reacted aromatic diamines having a benzoxazole structure with aromatic tetracarboxylic anhydrides in order to obtain a polyimide benzoxazole film so that specific electrical characteristics are exhibited. The amount of moisture that volatilizes while heating at 500 ° C. for 10 seconds, such as by holding the polyimide benzoxazole precursor film obtained at the time of ring-closing imidization at a temperature of at least 400 ° C. for at least 5 minutes. Amount) is a polyimide benzoxazole film having a high imidization rate of 5000 ppm or less with respect to the film before preliminary drying, and the ratio of the relative permittivity between dry and wet is within a certain range. Polyimide benzoxazol with excellent electrical characteristics that can lower the rate than before Film is obtained, which is heat-resistant, high-frequency response, and found that both flexibility, and completed the present invention.
That is, the present invention is a film mainly composed of polyimide benzoxazole obtained by reacting an aromatic diamine having a benzoxazole structure and an aromatic tetracarboxylic acid anhydride, and is a film at 120 ° C. for 24 hours. Measured by cavity resonance perturbation method of film conditioned for 94 hours under constant temperature and humidity condition of 20 ° C. and 65% RH with respect to relative permittivity ε _D of 100 GHz measured by cavity resonance perturbation method of vacuum dried film under conditions A polyimide benzoxazole film characterized in that a ratio ε ₆₅ / ε _D of a relative dielectric constant ε ₆₅ of 100 GHz is in the range of 1.00 to 1.10, and the film is heated at 500 ° C. for 10 seconds. The polyimide benzoxazol described above, wherein the amount of water volatilized in between is 5000 ppm or less with respect to the film before preliminary drying. Film.

  Compared to conventionally known polyimide films, the film of the present invention has a specific dielectric constant ratio between dry and wet, and the amount of water that volatilizes during heating for 10 seconds at 500 ° C. This is a polyimide benzoxazole film with a high imidization ratio of 5000 ppm or less with respect to the film before preliminary drying, excellent electrical characteristics, lower loss with respect to high frequency, and faster response speed (pulse signal The circuit using this polyimide benzoxazole film can be expected to operate with low humidity dependence. At the same time, since it has higher rigidity, strength, and heat resistance than conventional polyimide films, it is suitable for use in high frequency electronic devices and other electronic devices. It is particularly useful as a base material for flexible electronic circuit boards.

The polyimide benzoxazole film of the present invention is a film mainly composed of polyimide benzoxazole obtained by reacting an aromatic diamine having a benzoxazole structure and an aromatic tetracarboxylic anhydride, Cavity resonance perturbation of a film conditioned for 94 hours under constant temperature and humidity conditions of 20 ° C. and 65% RH for a relative permittivity ε _D of 100 GHz measured by a cavity resonance perturbation method of a film dried in a vacuum for 24 hours The polyimide benzoxazole film is characterized in that the ratio ε ₆₅ / ε _D of the relative permittivity ε ₆₅ of 100 GHz measured by the method is in the range of 1.00 to 1.10. A preferred embodiment is a polyimide benzoxazole film having a high imidization ratio in which the amount of water volatilized while heating the film at 500 ° C. for 10 seconds is 5000 ppm or less with respect to the film before preliminary drying.
The expression as a main component here consists of a polymer in which polyimide benzoxazole obtained by reacting an aromatic diamine having a benzoxazole structure with an aromatic tetracarboxylic acid anhydride is 70 mol% or more in the total polymer components. This means that the polymer contains 90% by mass or more in the film.

The polyimide benzoxazole film of the present invention (hereinafter also referred to as PIBO film) is mainly composed of polyimide benzoxazole obtained by polycondensation of an aromatic diamine having a benzoxazole structure and an aromatic tetracarboxylic acid anhydride. Is a substantially flat (long) film that is vacuum-dried at 120 ° C. for 24 hours, and a constant temperature of 20 ° C. and 65% RH for a relative dielectric constant ε _D of 100 GHz measured by a cavity resonance perturbation method. A ratio ε ₆₅ / ε _D of a relative permittivity ε ₆₅ of 100 GHz measured by a cavity resonance perturbation method of a film conditioned for 94 hours under a constant humidity condition is in a range of 1.00 to 1.10. When the film is pre-dried by holding at 170 ° C. for 7 minutes in an inert gas atmosphere and then heated at 500 ° C. for 10 seconds. The amount (ppm) of moisture that volatilizes during heating at 500 ° C. for 10 seconds with respect to the polyimide benzoxazole film before preliminary drying (hereinafter referred to as “during heating at 500 ° C. for 10 seconds”). The amount of volatile water or the amount of volatile water at a high temperature may be abbreviated to 5000 ppm or less, and the amount of volatile water at a high temperature is preferably 4000 ppm or less, more preferably 3000 ppm. It is as follows.
The relative dielectric constant of the film is defined with respect to the signal entering the film surface direction. From the viewpoint of high frequency compatibility, the film preferably has a low relative dielectric constant and has a specific range. The relative dielectric constant ε of the film of the present invention measured by the cavity resonance perturbation method is measured by the cavity resonance perturbation method of a film conditioned for 94 hours under a constant temperature and humidity condition of 20 ° C. and 65% RH at 1 to 200 GHz. ₆₅ is preferably 3 or less, more preferably 2.75 to 3.0, and still more preferably 2.75 to 2.9.
The lower limit of the relative dielectric constant is preferably as small as possible from the viewpoint of high-frequency characteristics. However, if the relative dielectric constant of the PIBO film is not more than a predetermined range, other physical properties, particularly mechanical characteristics, are significantly deteriorated, which is not preferable in practice.
The film of the present invention preferably has a smaller dielectric loss tangent at 1 to 200 GHz, preferably 0.01 or less, more preferably 0.003 or less, and still more preferably 0.001 or less. Although the lower limit of the dielectric loss tangent is not particularly defined, 0.0001 is exemplified from the viewpoint of film production.

When the amount of volatile water at a high temperature is larger than 5000 ppm, the swelling of the polyimide benzoxazole film due to the generation of voids or the like becomes too large, and contact failure due to peeling tends to occur.
“The amount of volatile water at a high temperature” is preferably as small as possible, but considering the ease of production, cost, etc., it is sufficient that the defect does not substantially occur. As the lower limit, specifically, 10 ppm.
In the measurement of “volatile water content at high temperature”, first, polyimide benzoxazole film is preliminarily dried by holding at 170 ° C. for 7 minutes in an inert gas atmosphere. It is for removing. It is considered that the water generated after the preliminary drying by this step is derived from the water generated by the condensation of the remaining amic acid. Here, examples of the inert gas include helium, argon, neon, and the like.

The amount of volatile water at a high temperature in the present invention is measured by a GCMS method using a Curie point type thermal decomposition apparatus. Specifically, the amount of volatile water at high temperature is measured by the following method. In other words, a polyimide benzoxazole film sample is precisely weighed with 8 mg as a guide (weighing value is A (mg)) in foil that has been heat-dried in advance (for example, pyrofoil manufactured by Nihon Analytical Industrial Co., Ltd.), and a curie point type. Set the heat retention temperature in a pyrolysis apparatus (for example, made by HP, JHS-3, etc.) to 170 ° C., introduce a sample foil, purge with an inert gas (preferably helium) for 7 minutes, and in the film Removes moisture adsorbed on the surface. Then, immediately after heating at 500 ° C. for 10 seconds by an oscillation operation, the area of the water ion peak of m / z = 18 detected by volatilization from the film while heating at 500 ° C. for 10 seconds is obtained, and absolute calibration is performed. The amount of volatile water (B (μg)) is obtained by a linear method, and the amount of volatile water at a high temperature is calculated by the following equation.
Volatile water content at high temperature (ppm) = B (μg) / (A (mg) × 1000)
The calibration curve is prepared, for example, by preparing a standard solution obtained by adding a certain amount of water to a solvent (eg, methanol) dehydrated with anhydrous sodium sulfate or the like, and using a water ion peak area of m / z = 18 by GCMS. Can be used. The calibration curve at this time can be expressed by, for example, the formula: generated water content B (μg) = ax + b (where, a represents a slope, b represents an intercept, and x represents a peak area). The generated water content B can be obtained by substituting the area value of the water ion peak.

<Aromatic diamines>
Specific examples of the aromatic diamine having a benzoxazole structure used in the present invention include the following.

  Among these, amino (aminophenyl) benzoxazole isomers are preferable from the viewpoint of ease of synthesis. Here, “each isomer” refers to each isomer in which two amino groups of amino (aminophenyl) benzoxazole are determined according to the coordination position (eg, the above “formula 1” to “formula 4”). Each compound described in the above. These diamines may be used alone or in combination of two or more.

  In this invention, if it is less than 30 mol% of all the diamines, you may use together the diamine which does not have the benzoxazole structure illustrated below 1 type, or 2 or more types. Examples of such diamines include 4,4′-bis (3-aminophenoxy) biphenyl, bis [4- (3-aminophenoxy) phenyl] ketone, and bis [4- (3-aminophenoxy) phenyl]. Sulfide, bis [4- (3-aminophenoxy) phenyl] sulfone, 2,2-bis [4- (3-aminophenoxy) phenyl] propane, 2,2-bis [4- (3-aminophenoxy) phenyl] -1,1,1,3,3,3-hexafluoropropane, m-phenylenediamine, o-phenylenediamine, p-phenylenediamine, m-aminobenzylamine, p-aminobenzylamine,

3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfoxide, 3,4'-diaminodiphenyl sulfoxide 4,4′-diaminodiphenyl sulfoxide, 3,3′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl sulfone, 3,3′-diaminobenzophenone, 3,4′- Diaminobenzophenone, 4,4′-diaminobenzophenone, 3,3′-diaminodiphenylmethane, 3,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylmethane, bis [4- (4-aminophenoxy) phenyl] methane, 1 , 1-Bi [4- (4-aminophenoxy) phenyl] ethane, 1,2-bis [4- (4-aminophenoxy) phenyl] ethane, 1,1-bis [4- (4-aminophenoxy) phenyl] propane, 1 , 2-bis [4- (4-aminophenoxy) phenyl] propane, 1,3-bis [4- (4-aminophenoxy) phenyl] propane, 2,2-bis [4- (4-aminophenoxy) phenyl ] Propane, 1,1-bis [4- (4-aminophenoxy) phenyl] butane, 1,3-bis [4- (4-aminophenoxy) phenyl] butane, 1,4-bis [4- (4- Aminophenoxy) phenyl] butane, 2,2-bis [4- (4-aminophenoxy) phenyl] butane, 2,3-bis [4- (4-aminophenoxy) phenyl] butane,

2- [4- (4-Aminophenoxy) phenyl] -2- [4- (4-aminophenoxy) -3-methylphenyl] propane, 2,2-bis [4- (4-aminophenoxy) -3- Methylphenyl] propane, 2- [4- (4-aminophenoxy) phenyl] -2- [4- (4-aminophenoxy) -3,5-dimethylphenyl] propane, 2,2-bis [4- (4 -Aminophenoxy) -3,5-dimethylphenyl] propane, 2,2-bis [4- (4-aminophenoxy) phenyl] -1,1,1,3,3,3-hexafluoropropane, 1,4 -Bis (3-aminophenoxy) benzene, 1,3-bis (3-aminophenoxy) benzene, 1,4-bis (4-aminophenoxy) benzene, 4,4'-bis (4-aminophenoxy) bif Nyl, bis [4- (4-aminophenoxy) phenyl] ketone, bis [4- (4-aminophenoxy) phenyl] sulfide, bis [4- (4-aminophenoxy) phenyl] sulfoxide, bis [4- (4 -Aminophenoxy) phenyl] sulfone, bis [4- (3-aminophenoxy) phenyl] ether, bis [4- (4-aminophenoxy) phenyl] ether, 1,3-bis [4- (4-aminophenoxy) Benzoyl] benzene, 1,3-bis [4- (3-aminophenoxy) benzoyl] benzene, 1,4-bis [4- (3-aminophenoxy) benzoyl] benzene, 4,4′-bis [(3- Aminophenoxy) benzoyl] benzene, 1,1-bis [4- (3-aminophenoxy) phenyl] propane, 1,3-bis [ -(3-aminophenoxy) phenyl] propane, 3,4'-diaminodiphenyl sulfide, 2,2-bis [3- (3-aminophenoxy) phenyl] -1,1,1,3,3,3-hexa Fluoropropane,

Bis [4- (3-aminophenoxy) phenyl] methane, 1,1-bis [4- (3-aminophenoxy) phenyl] ethane, 1,2-bis [4- (3-aminophenoxy) phenyl] ethane, Bis [4- (3-aminophenoxy) phenyl] sulfoxide, 4,4′-bis [3- (4-aminophenoxy) benzoyl] diphenyl ether, 4,4′-bis [3- (3-aminophenoxy) benzoyl] Diphenyl ether, 4,4′-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] benzophenone, 4,4′-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] Diphenylsulfone, bis [4- {4- (4-aminophenoxy) phenoxy} phenyl] sulfone, 1,4-bis [4- (4-aminophene) Xyl) phenoxy-α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-aminophenoxy) phenoxy-α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-amino) -6-trifluoromethylphenoxy) -α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-amino-6-fluorophenoxy) -α, α-dimethylbenzyl] benzene, 1,3- Bis [4- (4-amino-6-methylphenoxy) -α, α-dimethylbenzyl] benzene, 1,3-bis [4- (4-amino-6-cyanophenoxy) -α, α-dimethylbenzyl] Benzene, 3,3′-diamino-4,4′-diphenoxybenzophenone, 4,4′-diamino-5,5′-diphenoxybenzophenone, 3,4′-diamino-4,5′-di Phenoxybenzophenone, 3,3'-diamino-4-phenoxybenzophenone, 4,4'-diamino-5-phenoxybenzophenone, 3,4'-diamino-4-phenoxybenzophenone, 3,4'-diamino-5'-phenoxy Benzophenone, 3,3′-diamino-4,4′-dibiphenoxybenzophenone, 4,4′-diamino-5,5′-dibiphenoxybenzophenone, 3,4′-diamino-4,5′-dibiphenoxybenzophenone, 3,3′-diamino-4-biphenoxybenzophenone, 4,4′-diamino-5-biphenoxybenzophenone, 3,4′-diamino-4-biphenoxybenzophenone, 3,4′-diamino-5′-bi Phenoxybenzophenone, 1,3-bis (3-amino-4-phenoxybenzoy ) Benzene, 1,4-bis (3-amino-4-phenoxybenzoyl) benzene, 1,3-bis (4-amino-5-phenoxybenzoyl) benzene, 1,4-bis (4-amino-5-phenoxy) Benzoyl) benzene, 1,3-bis (3-amino-4-biphenoxybenzoyl) benzene, 1,4-bis (3-amino-4-biphenoxybenzoyl) benzene, 1,3-bis (4-amino-) 5-biphenoxybenzoyl) benzene, 1,4-bis (4-amino-5-biphenoxybenzoyl) benzene, 2,6-bis [4- (4-amino-α, α-dimethylbenzyl) phenoxy] benzonitrile And some or all of the hydrogen atoms on the aromatic ring in the aromatic diamine are halogen atoms, alkyl groups having 1 to 3 carbon atoms or alkoxyl groups. Cyano group, or a part or all of the hydrogen atoms of the alkyl group or an alkoxyl group such as an aromatic diamine substituted with substituted halogenated alkyl group or an alkoxyl group having 1 to 3 carbon atoms with a halogen atom,.

<Aromatic tetracarboxylic acid anhydrides>
The tetracarboxylic acid anhydrides used in the present invention are aromatic tetracarboxylic acid anhydrides. Specific examples of the aromatic tetracarboxylic acid anhydrides include the following.

These tetracarboxylic dianhydrides may be used alone or in combination of two or more.
In the present invention, one or two or more non-aromatic tetracarboxylic dianhydrides exemplified below may be used in combination as long as they are less than 30 mol% of the total tetracarboxylic dianhydrides. Examples of such tetracarboxylic acid anhydrides include butane-1,2,3,4-tetracarboxylic dianhydride, pentane-1,2,4,5-tetracarboxylic dianhydride, and cyclobutanetetracarboxylic acid. Acid dianhydride, cyclopentane-1,2,3,4-tetracarboxylic dianhydride, cyclohexane-1,2,4,5-tetracarboxylic dianhydride, cyclohex-1-ene-2,3 5,6-tetracarboxylic dianhydride, 3-ethylcyclohex-1-ene-3- (1,2), 5,6-tetracarboxylic dianhydride, 1-methyl-3-ethylcyclohexane-3 -(1,2), 5,6-tetracarboxylic dianhydride, 1-methyl-3-ethylcyclohex-1-ene-3- (1,2), 5,6-tetracarboxylic dianhydride 1-ethylcyclohexane -(1,2), 3,4-tetracarboxylic dianhydride, 1-propylcyclohexane-1- (2,3), 3,4-tetracarboxylic dianhydride, 1,3-dipropylcyclohexane- 1- (2,3), 3- (2,3) -tetracarboxylic dianhydride,

  Dicyclohexyl-3,4,3 ′, 4′-tetracarboxylic dianhydride, bicyclo [2.2.1] heptane-2,3,5,6-tetracarboxylic dianhydride, 1-propylcyclohexane-1 -(2,3), 3,4-tetracarboxylic dianhydride, 1,3-dipropylcyclohexane-1- (2,3), 3- (2,3) -tetracarboxylic dianhydride, dicyclohexyl -3,4,3 ', 4'-tetracarboxylic dianhydride, bicyclo [2.2.1] heptane-2,3,5,6-tetracarboxylic dianhydride, bicyclo [2.2.2 ] Octane-2,3,5,6-tetracarboxylic dianhydride, bicyclo [2.2.2] oct-7-ene-2,3,5,6-tetracarboxylic dianhydride and the like. . These tetracarboxylic dianhydrides may be used alone or in combination of two or more.

  In the polyimide benzoxazole film of the present invention, a so-called lubricant for forming fine irregularities on the film surface can be added. As the lubricant, organic and inorganic fine particles having an average particle diameter of about 0.01 to 5 μm can be used. The addition amount of the lubricant is about 0.01% to 3% with respect to the weight of the film. In the present invention, it is preferable to use inorganic particles having high heat resistance, and it is also preferable to use metal oxide particles such as alumina, silica and titanium oxide.

In the polyimide benzoxazole film of the present invention, first, (a) diamines and tetracarboxylic anhydrides are condensed in a solvent to obtain a polyamic acid solution (hereinafter also referred to as step (a)), and then. (B) A polyamic acid solution (also referred to as a green film) having a self-supporting property is obtained by applying a polyamic acid solution on a support and drying it so that the amount of residual solvent is 50% by mass or less ( Hereinafter, it is also referred to as a step (b).) Next, (c) the obtained green film is peeled off from the support and heat-treated with a temperature profile that stays in a temperature region of 400 ° C. or higher for 5 minutes or longer (hereinafter referred to as the step (b) c))).
Hereinafter, the manufacturing method of the polyimide benzoxazole film of the present invention (hereinafter simply referred to as the manufacturing method of the present invention) will be described in detail.

<Process (a)>
The solvent used when polymerizing diamines and aromatic tetracarboxylic acid anhydrides to obtain polyamic acid is not particularly limited as long as it dissolves both the raw material monomer and the polyamic acid to be produced. , Polar organic solvents are preferable, for example, N-methyl-2-pyrrolidone (also referred to as NMP), N-acetyl-2-pyrrolidone, N, N-dimethylformamide, N, N-diethylformamide, N, N-dimethylacetamide (Also referred to as DMAc), dimethyl sulfoxide, hexamethylphosphoric amide, ethyl cellosolve acetate, diethylene glycol dimethyl ether, sulfolane, halogenated phenols and the like. These solvents can be used alone or in combination. The amount of the solvent used may be an amount sufficient to dissolve the monomer as a raw material. As a specific amount used, the mass of the monomer in the solution in which the monomer is dissolved is usually 5 to 40% by mass, The amount is preferably 10 to 30% by mass.

  Conventionally known conditions may be applied for the polymerization reaction for obtaining the polyamic acid (hereinafter also simply referred to as “polymerization reaction”). As a specific example, in a temperature range of 0 to 80 ° C., 10 Stirring and / or mixing continuously for min to 80 hours. If necessary, the polymerization reaction may be divided or the temperature may be increased or decreased. In this case, the order of adding both monomers is not particularly limited, but it is preferable to add aromatic tetracarboxylic acid anhydrides to the solution of aromatic diamines.

The polyamic acid of the present invention preferably has a reduced viscosity (ηsp / C) of 2.0 dl / g or more when measured with a solution having a concentration of 0.2 g / dl in N-methylpyrrolidinone at 30 ° C. . By making these relatively high reduced viscosities, physical properties and chemical properties such as tensile breaking strength, tensile elastic modulus, tensile breaking elongation, and thermal expansion coefficient are excellent. More preferably, the polyamic acid has a reduced viscosity of about 2.5 dl / g or higher, particularly preferably a reduced viscosity of about 3.0 dl / g or higher, particularly preferably a reduced viscosity of about 4.0 dl / g or higher.
The concentration of the polyamic acid in the polyamic acid solution obtained by the polymerization reaction is preferably 5 to 40% by mass, more preferably 10 to 30% by mass, and the viscosity of the solution is measured with a Brookfield viscometer (25 ° C.). From the viewpoint of the stability of liquid feeding, it is preferably 10 to 2000 Pa · s, and more preferably 100 to 1000 Pa · s.

  Vacuum defoaming during the polymerization reaction is effective for producing a high-quality polyamic acid organic solvent solution. Moreover, you may perform polymerization by adding a small amount of terminal blockers to aromatic diamines before a polymerization reaction. Examples of the end-capping agent include compounds having a carbon-carbon double bond such as maleic anhydride. The amount of maleic anhydride used is preferably 0.001 to 1.0 mol per mol of aromatic diamine.

<Step (b)>
The support on which the polyamic acid solution is applied needs only to have smoothness and rigidity sufficient to form the polyamic acid solution into a film, and a drum or belt whose surface is made of metal, plastic, glass, porcelain, etc. And the like. Among them, the surface of the support is preferably a metal, more preferably stainless steel that does not rust and has excellent corrosion resistance. The surface of the support may be plated with metal such as Cr, Ni, or Sn. The surface of the support can be mirror-finished or processed into a satin finish as required. Application of the polyamic acid solution to the support includes, but is not limited to, casting from a slit base, extrusion through an extruder, squeegee coating, reverse coating, die coating, applicator coating, wire bar coating, etc. Conventionally known solution coating means can be appropriately used.

Examples of the support instead of the above include metal plates (foil, film, plate). That is, as a manufacturing method when using a polyimide benzoxazole film, which will be described later, for manufacturing a base substrate for a printed wiring board, a metal plate to be a metal layer of the base substrate is used as a support and a polyamic acid solution is applied thereto. is there. The means for applying the polyamic acid solution in this case is not particularly limited, and examples thereof include known application means such as a comma coater, knife coater, roll coater, reverse coater, die coater, gravure coater, and wire bar.
In the step (b), the residual solvent amount (% by mass) with respect to the total mass of the obtained green film is preferably 50% by mass or less, more preferably 40% by mass or less, and still more preferably 35% by mass. It is as follows. When the residual solvent amount is larger than 50% by mass, the handling property is inferior, which is not preferable. The lower limit of the residual solvent amount is not particularly limited, but is preferably 25% by mass or more, and more preferably 30% by mass or more in order to prevent film breakage in the imidation reaction step. The amount of the residual solvent can be measured (calculated) by TGA (thermogravimetric analysis) or gel permeation chromatograph (GPC).

  Examples of drying conditions for obtaining a green film (a film of a polyamic acid solution having a self-supporting property) in which the residual solvent amount relative to the total mass of the film is in a predetermined range include N, N-dimethylacetamide and N-methylpyrrolidone. When used as a solvent, the drying temperature is preferably 70 to 130 ° C, more preferably 75 to 125 ° C, and still more preferably 80 to 120 ° C. When the drying temperature is higher than 130 ° C., the molecular weight is lowered and the green film tends to be brittle. In addition, imidization partially proceeds during the production of the green film, and it becomes difficult to obtain desired physical properties during the imidization process. On the other hand, when the temperature is lower than 70 ° C., the drying time becomes long, the molecular weight tends to decrease, and the handling property tends to be poor due to insufficient drying. The drying time is preferably 10 to 90 minutes, more preferably 15 to 80 minutes, although it depends on the drying temperature. When the drying time is longer than 90 minutes, the molecular weight is lowered and the film tends to be brittle. When the drying time is shorter than 10 minutes, the drying property is insufficient and the handling property tends to be poor. Further, in order to improve the drying efficiency or suppress the generation of bubbles at the time of drying, the temperature may be raised stepwise in the range of 70 to 130 ° C. for drying. A conventionally known drying apparatus that satisfies such conditions can be applied, and examples thereof include hot air, hot nitrogen, far infrared rays, and high frequency induction heating.

<Step (c)>
A polyimide benzoxazole film having a volatile water content at a high temperature of 10,000 ppm or less can be obtained by imidizing the green film obtained in the step (b) under predetermined conditions.
As a specific method of imidization, a conventionally known imidation reaction can be appropriately used. For example, a method (so-called thermal ring closure method) in which an imidization reaction proceeds by subjecting to a heat treatment after performing a stretching treatment if necessary using a polyamic acid solution containing no ring-closing catalyst or a dehydrating agent, or a polyamic acid solution There may be mentioned a chemical ring closing method in which a ring closing catalyst and a dehydrating agent are contained in the catalyst and an imidization reaction is performed by the action of the ring closing catalyst and the dehydrating agent.

The ratio of the relative dielectric constant ε ₆₅ of 100 GHz measured by the cavity resonant perturbation method at 20 ° C. and 65% RH to the relative dielectric constant ε _D of 100 GHz measured by the cavity resonant perturbation method during drying is 1.00 to 1. In order to obtain a polyimide benzoxazole film that is a polyimide benzoxazole film that is in the range of 10 and has a low amount of volatile water at high temperature, in both the thermal ring closure method and the chemical ring closure method, in a temperature region of 400 ° C. or more, It is necessary to perform heat treatment with a temperature profile that stays for 5 minutes or more, preferably 20 minutes or more in the temperature range of 400 ° C. to 430 ° C., more preferably 10 minutes or more in the temperature range of 430 to 460 ° C., and more preferably 460 to 500 It is a temperature profile which retains for 5 minutes or more in the temperature range of ° C. By heat-treating the green film with such a temperature profile, the imidization reaction can proceed so that the amount of volatile water at a high temperature falls below a predetermined range.
The upper limit of the temperature and time of the temperature profile is not particularly limited, but in order to avoid the decomposition of the film, the temperature is 600 ° C. or less, preferably 550 ° C. or less. If considering the productivity, the time is 60 minutes. Hereinafter, it is preferably 40 minutes or less, more preferably 30 minutes or less.

  In the chemical ring closure method, after the polyamic acid solution is applied to a support, the imidization reaction is partially advanced to form a self-supporting film, and then imidization can be performed completely by heating. In this case, the condition for causing the imidization reaction to partially proceed is a heat treatment for 3 to 20 minutes at 100 to 200 ° C., and then the heat treatment may be performed with the above temperature profile.

  The timing for adding the ring-closing catalyst to the polyamic acid solution is not particularly limited, and may be added in advance before the polymerization reaction for obtaining the polyamic acid. Specific examples of the ring-closing catalyst include aliphatic tertiary amines such as trimethylamine and triethylamine, and heterocyclic tertiary amines such as isoquinoline, pyridine, and betapicoline. Among them, heterocyclic tertiary amines are mentioned. At least one amine selected from is preferred. Although the usage-amount of a ring-closing catalyst with respect to 1 mol of polyamic acids does not have limitation in particular, Preferably it is 0.5-8 mol.

  The timing of adding the dehydrating agent to the polyamic acid solution is not particularly limited, and may be added in advance before the polymerization reaction for obtaining the polyamic acid. Specific examples of the dehydrating agent include aliphatic carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, butyric anhydride, and aromatic carboxylic acid anhydrides such as benzoic anhydride. Among them, acetic anhydride, benzoic anhydride, etc. Acids or mixtures thereof are preferred. Moreover, the usage-amount of the dehydrating agent with respect to 1 mol of polyamic acids is not particularly limited, but is preferably 0.1 to 4 mol. When a dehydrating agent is used, a gelation retarder such as acetylacetone may be used in combination.

  Although the thickness of a polyimide benzoxazole film is not specifically limited, When considering using it for the base substrate for printed wiring boards mentioned later, it is 5-150 micrometers normally, Preferably it is 8-50 micrometers. This thickness can be easily controlled by the amount of the polyamic acid solution applied to the support and the concentration of the polyamic acid solution.

Next, a printed wiring board base substrate using the above-described polyimide benzoxazole film will be described as an example.
Here, the “base substrate for a printed wiring board” is a substantially flat substrate having a structure in which a metal layer is laminated on at least one surface of an insulating plate. The metal layer to be laminated may be a metal layer for a circuit that is intended to form a circuit by processing such as etching or the like. It may be a metal layer used for the purpose.
As the use of “base substrate for printed wiring board”, FPC, TAB carrier tape, and the like are preferable because the characteristics of the polyimide benzoxazole film of the present invention that the swelling and peeling in a high temperature environment are small can be utilized.

The metal laminated on at least one surface of the polyimide benzoxazole film is not particularly limited, and is preferably copper, aluminum, stainless steel, or the like. The lamination means is not particularly limited, and the following means are exemplified.
-Means for attaching a metal plate to the polyimide benzoxazole film using an adhesive,
-Means for forming a film by applying a polyamic acid solution to a metal plate as a support and imidizing as described above,
-Means for forming a metal layer on the polyimide benzoxazole film using vacuum coating techniques such as vapor deposition, sputtering, ion plating,
-Means for forming a metal layer on a polyimide benzoxazole film by a wet plating method such as electroless plating or electroplating.
A metal layer can be laminated on at least one surface of the polyimide benzoxazole film by using these means alone or in combination.

  The thickness of the metal layer is not particularly limited, but when the metal layer is used for circuit (conductive), the thickness of the metal layer is preferably 1 to 35 μm, more preferably 3 to 18 μm. is there. When using the polyimide benzoxazole film which bonded the metal layer as a heat dissipation board, the thickness of a metal layer becomes like this. Preferably it is 50-3000 micrometers. The surface roughness of the surface of the metal layer bonded to the polyimide is not particularly limited, but the center line average roughness (hereinafter referred to as Ra) and ten points in JIS B 0601 (definition and display of surface roughness). A value represented by average roughness (hereinafter referred to as Rz) is preferably 0.1 μm or less for Ra and 1.00 μm or less for Rz because the effect of improving the adhesion between the film and the metal layer is great. . Of these, those satisfying these conditions are preferred. Although Ra and Rz are preferably as small as possible, the lower limit of Ra is 0.0001 μm and the lower limit of Rz is 0.001 μm because of ease of acquisition and processing.

  An inorganic coating film such as a single metal or a metal oxide may be formed on the surface of the metal layer used in the present invention. Further, the surface of the metal layer may be subjected to a treatment with a coupling agent (aminosilane, epoxysilane, etc.), a sand plast treatment, a holing treatment, a corona treatment, a plasma treatment, an etching treatment or the like. Similarly, the surface of the polyimide benzoxazole film may be subjected to honing treatment, corona treatment, plasma treatment, etching treatment, and the like.

  Hereinafter, the effectiveness of the present invention will be described with reference to examples, but the present invention is not limited thereto. In addition, the evaluation method of the physical property in the following examples is as follows.

1. Reduced viscosity of polyamic acid (ηsp / C)
A solution dissolved in N, N-dimethylacetamide or N-methyl-2-pyrrolidone so that the polymer concentration was 0.2 g / dl was measured at 30 ° C. with an Ubbelohde type viscosity tube.

2. Method for measuring residual solvent amount Using a TGA apparatus (TG-DTA2000S manufactured by MAC Science), the precursor film was heated from room temperature to 400 ° C. at 10 ° C./min in a nitrogen stream, and 30 ° C. at 400 ° C. The weight loss by heating after holding for a minute was measured, and the weight reduction rate was defined as the amount of residual solvent (% by mass) assuming that all the weight loss was volatilized by the residual solvent.

3. Film thickness of the polyimide benzoxazole film The thickness of the film was measured using a micrometer (Minetron 1245D, manufactured by Finelfu).

4). Tensile modulus, tensile breaking strength and tensile breaking elongation of polyimide benzoxazole film Cut the dried film into strips of length 100mm and width 10mm in the longitudinal direction (MD direction) and width direction (TD direction), respectively Using a tensile tester (manufactured by Shimadzu Autograph (R) model name AG-5000A) and measuring in the MD direction and the TD direction under conditions of a tensile speed of 50 mm / min and a distance between chucks of 40 mm, tensile elasticity The rate, tensile strength at break and tensile elongation at break were determined.

5. Linear expansion coefficient (CTE) of polyimide benzoxazole film
About the polyimide benzoxazole film to be measured, the stretch rate in the MD direction and the TD direction is measured under the following conditions, and the stretch rate / temperature at intervals of 30 ° C. to 45 ° C., 45 ° C. to 60 ° C.,. Was measured up to 300 ° C., and the average value of all measured values was calculated as CTE. The meanings of the MD direction and the TD direction are the same as in the measurement of “3.” above.
Device name: TMA4000S manufactured by MAC Science
Sample length; 20mm
Sample width: 2 mm
Temperature rise start temperature: 25 ° C
Temperature rising end temperature: 400 ° C
Temperature increase rate: 5 ° C / min
Atmosphere: Argon

6). The melting point and glass transition temperature of the polyimide benzoxazole film were subjected to DSC measurement under the following conditions, and the melting point (melting peak temperature Tpm) and glass transition point (Tmg) were determined under the following measurement conditions in accordance with JIS K7121.
Device name: DSC3100S manufactured by MAC Science
Pan ： Aluminum pan (non-airtight)
Sample weight; 4mg
Temperature rising start temperature: 30 ° C
Temperature increase rate: 20 ° C / min
Atmosphere: Argon

7). Thermal decomposition temperature of polyimide benzoxazole film The thermal decomposition temperature was defined by 5% weight loss by TGA measurement (thermobalance measurement) of a fully dried sample under the following conditions.
Device name: TG-DTA2000S manufactured by MAC Science
Pan ： Aluminum pan (non-airtight)
Sample weight: 10mg
Temperature rising start temperature: 30 ° C
Temperature increase rate: 20 ° C / min
Atmosphere: Argon

8). Volatile water content of polyimide benzoxazole film at high temperature The volatile water content was determined from the GCMS method using a Curie point thermal decomposition apparatus.
A sample (guideline 8 mg) is precisely weighed in a pyrofoil for use at 500 ° C. manufactured by Nihon Analytical Industries in advance (weighing value is A (mg)), and the temperature inside the thermal decomposition apparatus is set to 170 ° C. The sample foil was introduced and purged with helium for 7 minutes to preliminarily remove the water adsorbed in the film. Immediately thereafter, heating was performed at 500 ° C. for 10 seconds by an oscillation operation. A water ion peak at m / z = 18 was detected by GCMS for the water volatilized from the film during the heating at 500 ° C. for 10 seconds. This peak area was determined, and the amount of generated water B (μg) was determined by an absolute calibration curve method. A calibration curve was prepared by preparing a standard solution to which a predetermined amount of water was added in two or more levels using dehydrated methanol with anhydrous sodium sulfate as a preparation reagent, and using a peak area of m / z = 18 by GCMS. At this time, the calibration curve was set as y = ax + b (a: slope, b: intercept, y: generated water amount B (μg), x: peak area). The amount of volatile water at a high temperature relative to the polyimide benzoxazole film was calculated by the following formula.
Volatile water content at high temperature (ppm) = B (μg) / {A (mg) × 1000}
Pyrolysis GCMS conditions Equipment: HP5971A (HPMS GCMS), JHS-3 (Nippon Analytical Industrial Pyrolysis Equipment)
Column: PORAPLOT-Q (manufactured by GL Sciences Inc.), φ0.32 mm × 10 m
Column temperature: 60 ° C constant Flow rate: He 0.7 ml / min, split introduction

9. Solder heat resistance and contact failure rate of polyimide benzoxazole film <Preparation of adhesive>
As an adhesive, (A) polyamide resin (acid content: dimer acid, amine component: hexamethylenediamine, acid value 1.0, amine value 0), (B) epoxy resin I: 4,4′-bis (2, 3-epoxypropoxy) -3,3 ′, 5,5′-tetramethylbiphenyl (epoxy equivalent: 190) (C) epoxy resin II. Bisphenol A type epoxy resin (epoxy equivalent: 186), (D) phenol resin resol phenol "CKM-1282" (manufactured by Showa Polymer Co., Ltd.), (E) additive 2-heptadecylimidazole, 50.0: It mix | blended in the ratio (mass ratio) of 8.0: 12.0: 29.5: 0.5, and obtained the adhesive agent.

<Manufacture of build-up multilayer wiring boards>
A four-layer printed wiring board of FR4 was used as a core layer, and a build-up multilayer printed wiring board shown in FIG. 1 was prototyped. The copper foil thickness on the core layer surface is 25 μm. First, an adhesive solution was applied to one side of a polyimide film and dried at 80 ° C. for 40 minutes. The dry film thickness of the adhesive was 25 μm. Next, polyimide films coated with an adhesive on both surfaces of the core substrate were stacked and temporarily pressure-bonded with a vacuum laminator, and then pressed with a hot plate press heated to 150 ° C. to an actual load of 20 kgf / cm ² for 30 minutes. A YAG laser was used for drilling. The via diameter is 150 μm. After drilling, desmear treatment, washing with water, plasma treatment of the entire surface of the substrate, conditioning, catalyst application, activation, electroless copper plating of 0.8μm thickness in formalin reducing bath, then copper sulfate plating The thickness of the copper foil at the stage where electric thickening plating and via fill plating were performed in a bath and the surface was buffed to ensure flatness was 15 μm. Pattern formation was performed by laminating, exposing and developing a dry film resist having a thickness of 25 μm, etching with a cupric chloride solution, and then removing the resist and washing with diluted sulfuric acid to form a conductor pattern with a thin line width of 70 μm.

<Solder heat resistance>
The second buildup layer was formed in the same manner as the first buildup layer except that the adhesive applied to the polyimide film was 15 μm. Through the above steps, a total of 8 multilayer printed wiring boards having two build-up layers on each side were obtained.
The obtained multilayer printed wiring board was immersed in a tin-copper-silver-based lead-free solder bath heated to 280 ° C. for 10 seconds, and the presence or absence of peeling or swelling was observed visually.
Next, the ETC (R) temperature cycle test device (manufactured by Enomoto Kasei Co., Ltd.) is charged, and the heating / cooling cycle test is performed by repeatedly heating and cooling between a low temperature of −50 ° C. and a high temperature of 150 ° C. every 30 minutes. The sample was immersed in a tin-copper-silver-based lead-free solder bath heated to 280 ° C. for 10 seconds after the test, and the presence or absence of peeling or swelling was visually observed.

<Contact failure rate>
A 7 mm × 7 mm semiconductor chip was mounted on the obtained multilayer printed wiring board (25.4 mm × 25.4 mm) by face-down bonding. The number of contacts is 256.
A cycle test in which the package is loaded into an ETAC (R) temperature cycle test apparatus (manufactured by Enomoto Kasei Co., Ltd.) and repeatedly cooled and heated between a low temperature of −50 ° C. and a high temperature of 150 ° C. every 30 minutes is performed. Time done. A continuity test was conducted after the test to determine the defect rate at the connection point.

10. Relative permittivity of polyimide benzoxazole film (preparation of test piece)
The polyimide benzoxazole film was piled up to the required thickness, and a pressure of 300 kgf / cm ² was applied and pressed to prepare a 1.6 mm × 1.5 mm × 75 mm prismatic test piece.
Using this prismatic test piece, a dry sample and a humidity control sample were prepared as follows. The dried sample was obtained by vacuum drying a prismatic test piece under the conditions of 120 ° C. and 24 hrs. The humidity control sample was obtained by conditioning the prismatic test piece for 94 hours under constant temperature and humidity conditions of 20 ° C. and 65% RH. Dielectric property measurement is performed immediately upon completion of drying or humidity control, or stored in an aluminum bag in a nitrogen atmosphere until the dielectric property measurement is performed, thereby maintaining the film state during drying and humidity conditioning. .
(Measurement of specimen)
Using the N5250A millimeter wave PNA series network analyzer manufactured by Agilent Technologies, the relative dielectric constant at 100 GHz was measured for the above sample by the cavity resonance perturbation method.

11. Density of polyimide benzoxazole film A polyimide benzoxazole film was cut into a size of 5 mm x 5 mm and subjected to density measurement. The cut sample was put into a density gradient tube prepared with an aqueous calcium nitrate solution, and the density was measured from the float position-density calibration curve previously added and the sample position after 5 hours. The liquid temperature of the density gradient tube is 30 ° C.

Example 1
<Polymerization Example 1>
The inside of a reaction vessel equipped with a nitrogen introduction tube, a thermometer, and a stirring rod was purged with nitrogen, and then 450 parts by mass of 5-amino-2- (p-aminophenyl) benzoxazole (Chemical Formula 1) and 5-amino-2- ( 50 parts by mass of m-aminophenyl) benzoxazole (Chemical Formula 3) was charged. Next, after 5100 parts by mass of N, N-dimethylacetamide was added and completely dissolved, 485 parts by mass of pyromellitic dianhydride was added and stirred at a reaction temperature of 25 ° C. for 40 hours. An acid solution A was obtained. Ηsp / C of this product was 4.0.
<Film Production Example 1>
Subsequently, this polyamic acid solution was coated on a stainless steel belt with a squeegee / belt gap of 650 μm and dried at 90 ° C. for 60 minutes. The polyamic acid film that became self-supporting after drying was peeled off from the stainless steel belt to obtain a green film having a thickness of 40 μm. The residual solvent amount of the green film at this time was 35 mass%. The obtained green film was passed through a continuous drying furnace and heat-treated at 170 ° C. for 3 minutes, then heated to 450 ° C. at a rate of about 70 ° C./minute, and heat-treated at 450 ° C. for 10 minutes. After cooling to room temperature over a period of 25 minutes, a brown polyimide benzoxazole film (film 1) having a thickness of 25 μm was obtained. The characteristic value of the obtained polyimide benzoxazole film was evaluated. Moreover, a buildup multilayer wiring board was manufactured using the obtained polyimide benzoxazole film, and solder heat resistance and contact failure rate were evaluated. The results are shown in Table 1.

(Example 2)
A polyimide benzoxazole film and a build-up multilayer wiring board were obtained and evaluated in the same manner as in Example 1 except that the temperature profile of the heat treatment was as shown in Table 1. The results are shown in Table 1.
(Comparative Examples 1 and 2)
<Polymerization example 2>
The inside of a reaction vessel equipped with a nitrogen introduction tube, a thermometer, and a stirring rod was purged with nitrogen, and then 450 parts by mass of 5-amino-2- (p-aminophenyl) benzoxazole (Chemical Formula 1) and 5-amino-2- ( 50 parts by mass of m-aminophenyl) benzoxazole (Chemical Formula 3) was charged. Next, after 5100 parts by mass of N-methylpyrrolidone was added and completely dissolved, 485 parts by mass of pyromellitic dianhydride was added and stirred at a reaction temperature of 25 ° C. for 40 hours. B was obtained. Ηsp / C of this product was 4.1.
Hereinafter, a polyimide benzoxazole film and a build-up multilayer wiring board were obtained in the same manner as in Example 1 except that the obtained polyamic acid solution was used and the temperature profile of the heat treatment was as shown in Table 1, and the same was obtained. Was evaluated. The results are shown in Table 1.

  Solder heat resistance and contact failure of the polyimide benzoxazole films of Examples 1 and 2 having a volatile water content of 10000 ppm or less at high temperatures were good, but in Comparative Examples 1 and 2, soldering was performed after the heating / cooling cycle test. In the heat test, a leak occurred and the contact failure rate was poor.

The polyimide benzoxazole film of the present invention has a relative dielectric constant ε ₆₅ measured by a cavity resonance perturbation method of 20 ° C. and 65% RH with respect to a dielectric constant ε _D of 100 GHz measured by a cavity resonance perturbation method when dried. Is a range of 1.00 to 1.10, and the amount of volatile moisture at a high temperature is extremely small. Therefore, when it is used for various electronic component laminates as a base film, its performance depends on fluctuations in the environment, particularly humidity. Without occurrence, swelling and peeling can be prevented when used at a high temperature.
Therefore, the polyimide benzoxazole film of the present invention is useful as a base film for use in the production of copper-clad substrates for flexible printed wiring (FPC) and tape automated bonding (TAB) used at extremely high temperatures. .

Claims (2)

A film mainly composed of polyimide benzoxazole obtained by reacting an aromatic diamine having a benzoxazole structure with an aromatic tetracarboxylic acid anhydride, and dried under vacuum at 120 ° C. for 24 hours. Ratio of 100 GHz measured by cavity resonance perturbation method of a film conditioned for 94 hours under constant temperature and humidity conditions of 20 ° C. and 65% RH with respect to a relative dielectric constant ε _D of 100 GHz measured by cavity resonance perturbation method of the film A polyimide benzoxazole film characterized in that the ratio ε ₆₅ / ε _{D of} dielectric constant ε ₆₅ is in the range of 1.00 to 1.10.   2. The polyimide according to claim 1, wherein the film after preliminarily dried by purging with helium at 170 [deg.] C. for 7 minutes has a volatile water content of 5000 ppm or less at a high temperature which volatilizes while immediately heating at 500 [deg.] C. for 10 seconds. Benzoxazole film.