TWI771786B - Low dielectric polyimide film and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a low-dielectric polyimide film with reduced number of bubbles and a method for producing the polyimide film. The present invention provides a polyimide film obtained by imidizing a polyimide solution containing a dianhydride component and a diamine component, and containing a phosphorus (P) compound, wherein the dianhydride component includes biphenyl Tetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA), the aforementioned diamine components include diaminodiphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine .

Description

Low-dielectric polyimide film and method for producing the same

The present invention relates to a polyimide film, and more specifically, to a high-thickness polyimide film that has low dielectric properties and reduces the number of bubbles in the produced film and its production method.

Polyimide (POLYIMIDE; PI) is based on a rigid aromatic main chain and an imide ring with excellent chemical stability. It has the highest level of heat resistance, chemical resistance, electrical insulation, and chemical resistance among organic materials. , Weather-resistant polymer materials.

Polyimide films are attracting attention as materials for various electronic devices requiring the aforementioned properties.

At present, most of the polyimides are dissolved in organic solvents in the form of poly(amic acid), and they will not dissolve after becoming polyimides. Therefore, the processing of polyimides generally uses poly(amic acid) solutions. By drying this solution, after obtaining a desired film, a molded product, or a coating layer, it is heated and carried out by imidization.

On the other hand, thermal stress that occurs recently in the process of cooling a polyimide film and its laminate from the imidization temperature to room temperature often causes serious problems such as curling, film peeling, and cracking.

In particular, with the rapid development of high-density electronic circuits, the use of multilayer wiring boards and the like have become more serious problems due to thermal stress.

That is, this is because, due to thermal stress, even if film peeling or cracking is not achieved, the thermal stress remaining in the multilayer substrate significantly lowers the reliability of the device.

As a solution to reduce the influence of such thermal stress, low expansion of polyimide is being considered. However, polyimide that exhibits a low coefficient of thermal expansion generally has a rigid linear main chain structure, so Most of them have poor water vapor permeability, and there is a problem that foaming is likely to occur depending on the film forming conditions.

That is, because the molecules are arranged too densely, the water vapor permeability of the film is poor, and bubbles (bubbles, air, etc.) are often generated inside the film during the production process of the film.

The occurrence of such bubbles not only adversely affects the surface roughness of the produced polyimide film, but also reduces the electrical, optical, and mechanical properties of the polyimide film as a whole.

Therefore, there is a need for a solution capable of reducing the bubbles in the polyimide film while maintaining the original properties such as heat resistance of the polyimide exhibiting a low expansion coefficient, while having high elasticity and high heat resistance.

The matters described in the above prior art are used to help understanding of the background of the invention, and may include matters not known in the prior art to those of ordinary skill in the technical field.

[Prior Art Literature]

[Patent Literature]

Patent Document 1: Korean Granted Patent No. 10-1375276.

Patent Document 2: Korean Laid-Open Patent Publication No. 2016-0002402.

Therefore, the object of the present invention is to provide a high-thickness polyimide film with low dielectric properties.

However, the problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned can be clearly understood by practitioners from the following description.

One aspect of the present invention to achieve the above object provides a polyimide film obtained by imidizing a polyimide solution containing a dianhydride component and a diamine component , wherein the aforementioned dianhydride components include biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA), and the aforementioned diamine components include diamine diphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine (m-tolidine), based on the total content of the aforementioned diamine component of 100 mol %, the content of the aforementioned diamino diphenyl ether is 10 mol % or more and 30 mol % or less, and the aforementioned para- The content of phenylenediamine is 10 mol % or more and 30 mol % or less, the content of the meta-tolidine is 60 mol % or more and 80 mol % or less, and the polyimide film contains phosphorus (P) compound.

Based on the total content 100 mol % of the aforementioned dianhydride components, the content of the aforementioned biphenyltetracarboxylic dianhydride can be more than 30 mol % and less than 50 mol %, and the content of the aforementioned pyromellitic dianhydride can be 50 mol %. More than mol% and less than 70 mol%.

The said phosphorus type compound may contain more than 0.1 weight% and less than 3.5 weight% with respect to the solid content of the said dianhydride component and the said diamine component.

The aforementioned phosphorus compound may be selected from triphenyl phosphate, Trixylenyl phosphate, Tricresyl phosphate, Resorcinol diphehyl phosphate and One or more of the group consisting of ammonium polyphosphate.

The elastic modulus of the aforementioned polyimide film may be 4.5 GPa or more, and the thickness may be 70 μm or more.

In addition, the number of bubbles per 1 m ² of the polyimide film may be less than 50.

Another aspect of the present invention provides a method for manufacturing a polyimide film, comprising: (a) combining a dianhydride component comprising biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) with a method comprising: The first step in which diamine components of diaminodiphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine are polymerized in an organic solvent to produce polyamic acid; (b) The second step of adding an imidization catalyst and a phosphorus (P) compound to the aforementioned polyamic acid of the aforementioned first step and mixing; and (c) subjecting the aforementioned polyamic acid of the aforementioned second step to imidization The third step of amination; based on the total content of the aforementioned diamine component of 100 mol %, the content of the aforementioned diamine diphenyl ether may be more than 10 mol % and less than 30 mol %, and the aforementioned p-phenylenediamine The content can be more than 10 mol%, 30 mol% or less, the content of the aforementioned m-tolidine may be 60 mol% or more and 80 mol% or less.

Another aspect of the present invention provides a protective film and a carrier film comprising the aforementioned polyimide film.

The present invention adjusts the ratio and solid content of dianhydride and diamine components to provide a polyimide film containing phosphorus compounds, thereby providing a high-quality polyimide film with an elastic modulus of 4.5 GPa or more, a thickness of 70 μm or more, and low dielectric properties. Thickness Polyimide Film.

In addition, although the produced polyimide film was a relatively thick film with a film thickness of 70 μm or more, the number of bubbles in the film was observed to be less than 50 cells/m ² , and it was possible to obtain the absence of bubbles depending on the content of the phosphorus compound. A high-thickness film with excellent quality of observed bubbles.

This polyimide film not only has excellent mechanical properties with high elasticity, but also has low surface roughness, suppresses the formation of bubbles, and in particular improves the surface quality. Therefore, it can be used in fields requiring polyimide films with such diverse properties.

Terms and words used in this specification and the scope of the patent application should not be interpreted in the usual or dictionary sense, but should be based on "the concept that the inventor can appropriately define the terms in order to explain his own invention in the best way". principle, only to be interpreted as being in line with the meaning and concept of the technical idea of the present invention.

Therefore, the structure of the embodiment described in this specification is only the best embodiment of the present invention, and does not represent the technical idea of the present invention. Therefore, it should be understood that at the time of application of the present invention, there may be alternatives to replace them. Various equivalents and variants of .

In this specification, the expression of the singular includes the expression of the plural unless the difference is not clearly expressed in the text. In this specification, terms such as "comprising", "having" or "having" are to specify the existence of features, numbers, steps, constituent elements or their combinations, and should be understood as not precluded The presence or additional possibility of one or more other features or numbers, steps, constituent elements or combinations thereof.

In this specification, "dianhydride" is meant to include its precursors or derivatives, which technically may not be dianhydrides, but nonetheless react with diamines to form polyamides, which may Converted to polyimide again.

In this specification, when an amount, concentration, or other value or parameter is given by listing a range, a preferred range, or a preferred upper limit value and a preferred lower limit value, it should be understood that whether the range is disclosed independently or not All ranges formed by any pair of any upper range limit or preferred value and any lower range limit or preferred value are specifically disclosed.

When a range of values is referred to in this specification, unless recited differently, the range is meant to include its endpoint and all integers and fractions within the range. The scope of the invention is not meant to be limited to the specific values mentioned in defining the scope.

The polyimide film of one embodiment of the present invention is obtained by imidizing a polyimide solution containing a dianhydride component and a diamine component, wherein the dianhydride component includes biphenyltetracarboxylic dianhydride (BPDA) And pyromellitic dianhydride (PMDA), the aforementioned diamine components include diamine diphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine (m-tolidine), with the aforementioned diamine components The total content of 100 mol % is the benchmark, the content of the aforementioned diaminodiphenyl ether can be more than 10 mol % and less than 30 mol %, and the content of the aforementioned p-phenylenediamine can be more than 10 mol % and 30 mol %. % or less, the content of the m-tolidine can be more than 60 mol % and less than 80 mol %, and the polyimide film contains a phosphorus (P) compound.

P-phenylenediamine is a rigid monomer. With the increase of p-phenylenediamine (PPD) content, the synthesized polyimide has a more linear structure, which helps to improve the mechanical properties of polyimide such as elastic modulus.

When p-phenylenediamine is used below the aforementioned range based on the total amount of the diamine component, the elastic modulus of the high-thickness (film thickness of 70 μm or more) polyimide film decreases.

In addition, when using p-phenylenediamine beyond the above-mentioned range based on the total amount of the diamine component, especially when the solid content increases, gelation occurs due to secondary bonding, and it is difficult to produce high Thickness Polyimide Film.

On the other hand, in the high-thickness polyimide film containing p-phenylenediamine, bubbles frequently occur as the thickness increases.

The increase in the occurrence of bubbles is due to the fact that with the increase of p-phenylenediamine content, the synthesized polyimide chain has a more linear morphology. and evaporation of water becomes difficult.

The generation of air bubbles in the polyimide film corresponds to the poor quality that greatly affects the appearance and mechanical properties of the polyimide film. Even if the produced polyimide film is excellent in other characteristics, a polyimide that generates many air bubbles Films are also difficult to apply to actual products.

Therefore, a phosphorus-based compound having plasticizer properties that can impart a free volume to the strong interchain bonding of polyimide induced by p-phenylenediamine and increase the flexibility between polyimide chains is added.

It was confirmed that the number of bubbles formed in the polyimide thin film was greatly reduced by adding such a phosphorus compound.

According to another embodiment of the present invention, the polyimide film may include an inorganic filler. As the inorganic filler, for example, silicon dioxide (especially spherical silicon dioxide), titanium oxide, aluminum oxide, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, mica and the like can be used.

The particle size of the filler is not particularly limited, and may be determined according to the properties of the film to be modified and the type of filler to be added. In general, the average particle size is 0.05 to 100 μm, preferably 0.1 to 75 μm, more preferably 0.1 to 50 μm, particularly preferably 0.1 to 25 μm.

If the particle size is less than this range, it is difficult to express the effect of modification, and if it exceeds this range, the surface properties may be greatly damaged or the mechanical properties may be greatly reduced.

In addition, the addition amount of the filler is also not particularly limited, and may be determined according to the characteristics of the film to be modified, the particle size of the filler, and the like. Generally speaking, the addition amount of the filler is 0.01 to 100 parts by weight, preferably 0.01 to 90 parts by weight, more preferably 0.02 to 80 parts by weight, relative to 100 parts by weight of polyimide.

If the amount of filler added is less than this range, the modification effect of the filler will hardly be exhibited, and if it exceeds this range, the mechanical properties of the film may be greatly damaged. The method of adding the filler is not particularly limited, and any known method may be used.

The aforementioned inorganic filler is contained in the polyimide film, so that the surface of the polyimide film exhibits roughness, and imparts an aniti blocking property that prevents the polyimide film from sticking to each other during production or use.

Inorganic fillers are usually used as additives for polyimide films, but spherical silica particles, etc., are particularly excellent in anti-blocking properties.

For example, in the case of using spherical silica particles as the inorganic filler, when the average diameter of the spherical silica particles exceeds 1 μm, the surface roughness increases, and the surface roughness of the object in contact with the polyimide film is induced. Scratches and product defects occur, and when the average diameter of the spherical silica particles is less than 0.1 μm, the anti-blocking properties that prevent the film blocking phenomenon cannot be expressed.

Generally, if the spherical silica particles are used in an excess of an appropriate amount, the particles will agglomerate and bond on the film. Difficulty taking steps.

According to another embodiment of the present invention, the phosphorus-based compound having plasticizer properties for suppressing the formation of bubbles may contain more than 0.1 wt % of the solid content of the dianhydride component and the diamine component used in the synthesis of polyimide , less than 3.5 wt % or less, particularly preferably 0.5 to 3.0 wt %.

When the phosphorus-based compound is contained in an amount of 0.1% by weight or less, the effect of suppressing the formation of bubbles cannot be sufficiently exhibited, and when the phosphorus-based compound is contained in an amount of 3.5% by weight or more, the elastic modulus of the polyimide film decreases.

In addition, as the phosphorus compound used, it can be Triphenyl Phosphate (TPP), ammonium polyphosphate (ammonium polyphosphate), Trixylenyl Phosphate (TXP), Tricresyl Phosphate (Tricresyl Phosphate; TCP), resorcinol diphenyl phosphate and ammonium polyphosphate.

In particular, triphenyl phosphate (Triphenyl Phosphate; TPP) and polyphosphorus are preferably used One or more of ammonium polyphosphates, but not limited thereto, as long as they are phosphorus-based compounds with plasticizer properties that can impart free volume and increase the flexibility between polyimide chains Any compound can be used as a phosphorus compound for suppressing bubble formation.

The polyimide film of the aforementioned embodiment example of the present application is a high-thickness polyimide film having an elastic modulus of 4.5 GPa or more and a thickness of 70 μm or more and having high elastic properties.

The elastic modulus of the aforementioned polyimide film is adjusted with the content of p-phenylenediamine (PPD) and exhibits an excellent elastic modulus of 4.5 GPa or more. This excellent elastic modulus of the polyimide film can be applied to various aspects, But it is especially suitable for carrier film or protective film.

Moreover, the aforementioned polyimide films exhibit low surface roughness, which is directly related to the average diameter of the inorganic filler material. Generally, the larger the average diameter of the inorganic filler material, the larger the surface roughness. This increase in surface roughness causes scratches to be induced on the surface of the object in contact with the polyimide film, resulting in defective products.

Furthermore, as a high-thickness polyimide film having a thickness of 70 μm or more, the thickness of the polyimide film is preferably 75 μm or more.

The number of bubbles per 1 m ² of the polyimide film is less than 50, and the number of bubbles decreases as the content of the added phosphorus compound increases. By appropriately adjusting the content of the phosphorus-based compound, the number of bubbles can be minimized (the existence of bubbles was not confirmed by observation) while maintaining the elastic modulus and surface roughness suitable for product application.

Another embodiment of the present invention relates to a method for manufacturing a polyimide film, comprising: (a) adding dianhydride components including biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) The first step of producing polyamic acid by polymerizing in an organic solvent with diamine components including diaminodiphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine; ( b) a second step of adding an imidization catalyst and a phosphorus (P) compound to the aforementioned polyamic acid of the aforementioned first step and mixing; and (c) carrying out the aforementioned polyamide acid of the aforementioned second step The third step of imidization; based on the total content of the aforementioned diamine component 100 mol %, the content of the aforementioned diamine diphenyl ether is More than 10 mol % and less than 30 mol %, the content of the aforementioned p-phenylenediamine is more than 10 mol % and less than 30 mol %, and the content of the aforementioned m-tolidine is more than 60 mol % and 80 mol % the following.

A thermal imidization method or a chemical imidization method can be applied to the method of imidizing a polyamic acid, and a method of using a thermal imidization method and a chemical imidization method in combination can also be applied. Among them, the thermal imidization method is a method in which a chemical catalyst is excluded, and a heat source such as a hot air or an infrared dryer is used to induce an imidization reaction, and the chemical imidization method is a method using a dehydrating agent and an imidizing agent.

The produced polyimide film is suitable for a protective film or a carrier film, but is not limited to this, and can be used in various fields to which the properties of the produced polyimide film can be applied.

The functions and effects of the present invention will be described in more detail below by means of specific manufacturing examples and embodiments of the present invention. However, the manufacturing examples and the examples are presented only as examples of the invention, and the scope of the rights of the invention is not limited thereby.

[Production example: Production of polyimide film]

The polyimide film of the present invention can be produced by the following general methods known in the art. First, the above-mentioned dianhydride and diamine components are reacted in an organic solvent to obtain a polyamic acid solution.

At this time, the solvent is generally an amide solvent, and an aprotic polar solvent (Aprotic solvent) can be used, such as N,N'-dimethylformamide, N,N'-dimethylacetamide, N,N'-dimethylacetamide, N,N'-dimethylacetamide - methylpyrrolidone or a combination thereof.

The dianhydride and diamine components may be charged in powder, agglomerate, or solution form, and are preferably charged in solution form in order to adjust the polymerization viscosity after the reaction is performed in powder form at the initial stage of the reaction.

The obtained polyamic acid solution can be mixed with an imidization catalyst and a dehydrating agent, and coated on a support.

Examples of catalysts used include tertiary amines (eg, isoquinoline, β-picoline, pyridine, etc.), and examples of dehydrating agents include acid anhydrides, but are not limited thereto. In addition, as the support body used above, for example, a glass plate, an aluminum foil, a circulating stainless steel belt, a stainless steel tub, etc. can be used, but it is not limited thereto.

The film coated on the aforementioned support is gelled on the support by means of dry air and heat treatment.

The gelled thin film is separated from the support and subjected to heat treatment to complete drying and imidization.

The film after the above-mentioned heat treatment is heat-treated under a predetermined tension, so that the residual stress in the film that occurs in the film-forming process can be removed.

Specifically, 500 ml of NMP was injected while injecting nitrogen into a reactor equipped with a stirrer and a nitrogen injection/exhaust pipe, and the temperature of the reactor was set to 30° C., and biphenyltetracarboxylic dianhydride (BPDA), homogenized The pyromellitic dianhydride (PMDA), diaminodiphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine (m-tolidine) were added in the adjusted proportions to make the solution completely dissolved. Then, in a nitrogen atmosphere, the temperature of the reactor was raised to 40° C. and stirring was continued for 120 minutes, thereby producing a polyamic acid having a primary reaction viscosity of 1500 cP.

A solution of pyromellitic dianhydride (PMDA) was added to the thus-produced polyamic acid and stirred so that the final viscosity reached 100,000 to 120,000 cP.

In the final polyamic acid produced in this way, a catalyst and a dehydrating agent were added as phosphorus compounds, and the content of triphenyl phosphate (TPP) was adjusted and added together, and a high-thickness polyamic acid was produced by a coater. Amine film.

[Examples and Comparative Examples]

It was produced according to the production example described above, but as the dianhydride component, biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) were used in 40 mol % and 60 mol %, respectively, based on 100 mol % of the dianhydride component. , 100 mol% of the diamine component was reacted.

The ratio of diamine diphenyl ether (ODA), p-phenylenediamine (PPD) and m-tolidine (m-tolidine) as the diamine component, when the total content of diamine is 100 mol%, the diamine group Diphenyl ether, p-phenylenediamine and m-tolidine were used in 15 mol%, 15 mol% and 70 mol%, respectively.

As shown in Table 1 below, the content of triphenyl phosphate (Triphenyl Phosphate; TPP) in Examples 1 to 6 was 0.5 to 3% by weight based on the solid content of the dianhydride component and the diamine component to prepare The thickness of the fabricated polyimide films were all 75 μm.

In addition, the elastic modulus of the polyimide films produced in all Examples and Comparative Examples was measured 3 times according to ASTM D 882 using a Standard Instron testing apparatus, and an average value was taken.

The average number of bubbles is obtained by first photographing a polyimide film with a film defect analyzer equipped with an imaging device, and then directly visually checking the photographed defect image of the polyimide film to obtain the average number of bubbles.

A film having a predetermined width and length was used as a sample, and the number of bubbles was measured, and the measured number of bubbles was converted into the number of bubbles per 1 m ² .

According to the measurement results, in Examples 1 to 6 in which the content of triphenyl phosphate (TPP) was added at 0.5 wt % to 3.0 wt %, although the polyimide films were high-thickness films, they were different from those at all. Comparative Example 1 using triphenyl phosphate (TPP) (the number of bubbles: 63 to 68 per 1 m ² ) or Comparative Example 2 ( The number of bubbles: 60 to 68 per 1 m ² ), the number of bubbles is greatly reduced.

In particular, as the content of triphenyl phosphate (TPP) increased from 0.5 wt % to 3.0 wt %, the number of bubbles decreased from 20 to 30 per 1 m ² to 0.

In addition, when the content of Triphenyl Phosphate (TPP) contains 0.5 From % to 3.0% by weight (Examples 1 to 6), compared with Comparative Examples 1 and 2 (6.25GPa and 6.05GPa, respectively), the elastic modulus tended to be partially reduced, but the polyimide film had a The elastic modulus ranged from 4.76GPa to 5.87GPa, showing an elastic modulus of 4.5GPa or more, so it was confirmed that the manufactured polyimide film had no problem at all in terms of product application.

When the content of Triphenyl Phosphate (TPP) was used in excess of 3.5% by weight or more (Comparative Examples 3 to 4), as in Examples 5 and 6, although the number of bubbles was kept at 0, the elastic modulus was greatly increased. Decrease, the performance is less than 4.5Gpa.

This can be considered to be due to the plasticizer properties of Triphenyl Phosphate (TPP) where elasticity decreases with increasing Triphenyl Phosphate (TPP) content.

The embodiments of the polyimide film and the manufacturing method of the polyimide film of the present invention are only preferred embodiments for those skilled in the art to which the present invention pertains to easily implement the present invention, and are not limited to the foregoing embodiments. Therefore, the right scope of the present invention is not limited thereby.

Therefore, the true technical protection scope of the present invention should be determined according to the accompanying technical idea of the scope of the patent application. In addition, within the scope of the technical idea of the present invention, various substitutions, deformations and changes can be realized, which is self-evident to practitioners, and the parts that can be easily changed by practitioners are also included in the scope of rights of the present invention. It goes without saying.

Claims (10)

A polyimide film, wherein the polyimide film contains a phosphorus (P) compound, and the polyimide film is obtained by imidizing a polyimide solution containing a dianhydride component and a diamine component. Obtained, wherein, the aforementioned dianhydride components include biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA), and the aforementioned diamine components include diamine diphenyl ether (ODA), p-phenylenediamine (PPD) ) and m-tolidine, wherein the content of the diamine diphenyl ether is 10 mol % or more and 30 mol % or less based on 100 mol % of the total content of the diamine components , the content of the aforementioned p-phenylenediamine is more than 10 mol % and less than 30 mol %, the content of the aforementioned m-tolidine is more than 60 mol % and less than 80 mol %, and wherein, the aforementioned polyimide film The number of air bubbles per 1 m ² is less than 50. The polyimide film according to claim 1, wherein the content of the biphenyltetracarboxylic dianhydride is 30 mol % or more and 50 mol % based on the total content of the dianhydride components of 100 mol %. Hereinafter, the content of the aforementioned pyromellitic dianhydride is 50 mol % or more and 70 mol % or less. The polyimide film according to claim 1, wherein the phosphorus compound contains more than 0.1% by weight and less than 3.5% by weight of the solid content of the dianhydride component and the diamine component. The polyimide film according to claim 1, wherein the phosphorus compound is selected from the group consisting of Triphenyl phosphate, Trixylenyl phosphate, Tricresyl phosphate, One or more of the group consisting of resorcinol diphenyl phosphate and ammonium polyphosphate. The polyimide film according to claim 1, wherein the elastic modulus of the polyimide film is 4.5 GPa or more, and the thickness is 70 μm or more. The polyimide film according to any one of claims 1 to 5, wherein the polyimide film is used for a protective film or a carrier film. A method for producing a polyimide film, comprising the steps of: (a) combining a dianhydride component comprising biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA) with a dianhydride component comprising diaminodiphenyl ether (ODA), the diamine components of p-phenylenediamine (PPD) and m-tolidine (m-tolidine) are polymerized in an organic solvent to produce a first step of polyamide acid; (b) in the first step described above The second step of adding an imidization catalyst and a phosphorus (P) compound to the aforementioned polyamic acid and mixing; and (c) the third step of imidizing the aforementioned polyamic acid of the aforementioned second step Wherein, based on the total content of the aforementioned diamine component 100 mol %, the content of the aforementioned diamine diphenyl ether is more than 10 mol % and less than 30 mol %, and the content of the aforementioned p-phenylenediamine is 10 mol % % or more and 30 mol % or less, the content of the m-tolidine is 60 mol % or more and 80 mol % or less, and the number of bubbles per 1 m ² of the polyimide film is less than 50. The method for producing a polyimide film according to claim 7, wherein the phosphorus compound contains more than 0.1% by weight and less than 3.5% by weight of the solid content of the dianhydride component and the diamine component. The method for producing a polyimide film according to claim 7, wherein the phosphorus compound is selected from the group consisting of triphenyl phosphate, Trixylenyl phosphate, and Tricresyl phosphate. phosphate), resorcinol diphenyl phosphate (Resorcinol diphenyl phosphate) and ammonium polyphosphate (Ammonium polyphosphate) composed of one or more of the group. The method for producing a polyimide film according to claim 7, wherein the elastic modulus of the polyimide film is 4.5 GPa or more, and the thickness is 70 μm or more.