A polyamic acid copolymer solution, a polyimide film, its preparation method, and its applications.

Abstract

This invention discloses a polyamic acid copolymer solution, a polyimide film, its preparation method, and its applications. The film is prepared by casting the polyamic acid copolymer solution, gradient thermal imidization, and biaxial stretching. The polyamic acid copolymer is obtained by solution polymerization of diamine monomers selected from group A, diamine monomers selected from group B, fluorodianhydrides, and amino-containing thermally unstable polymer nanoparticles in a non-proton strongly polar solvent. This invention utilizes a dual mechanism of reducing molecular polarizability through perfluorinated side groups and synergistically combining dielectric loss with the in-situ formation of uniform nanopores by thermally unstable nanoparticles. This enables the film to simultaneously possess the advantages of low dielectric constant, low dielectric loss, high elongation at break, and good dimensional stability, solving the problems of high dielectric constant, high dielectric loss, insufficient toughness, and high high-frequency signal transmission loss in traditional polyimide films. It can be widely used in 5G high-frequency communication, integrated circuits, wafer-level advanced packaging, flexible electronics, and aerospace.

Description

Technical Field

[0001] This invention relates to the field of polyimide film technology, and in particular to a polyimide film with low dielectric constant, low dielectric loss, high toughness and good dimensional stability for 5G high-frequency communication and advanced electronic packaging, and its preparation method. Background Technology

[0002] With the rapid development of 5G communication, high-frequency and high-speed electronic devices, and wafer-level advanced packaging technology, electronic devices are moving towards miniaturization, high frequency, and high speed. The parasitic capacitance, signal crosstalk, and transmission delay issues of the insulating dielectric layer are becoming increasingly prominent. Traditional polyimide films have a dielectric constant of 3.0~3.5, which is insufficient to meet the requirements of low signal loss in high-frequency and high-speed applications.

[0003] The main methods for reducing the dielectric constant of polyimide currently include: 1) Introducing fluorine-containing monomers can reduce electronic polarizability due to the strong electronegativity of fluorine atoms, but large fluorine groups can easily reduce reactivity, heat resistance and mechanical properties. 2) Adding inorganic nanoparticles can easily lead to poor compatibility, uneven dispersion, and decreased mechanical properties; 3) Adding pore-forming agents can lead to defects such as uneven pore formation, deterioration of mechanical strength, and poor dimensional stability.

[0004] Meanwhile, high-frequency applications not only require low dielectric constant (Dk) but also low dielectric loss (Df). Single fluorine modification or single-pore solutions cannot simultaneously achieve low Dk, low Df, high toughness, and dimensional stability. Therefore, developing a polyimide film that combines low dielectric constant, low dielectric loss, high toughness, and dimensional stability has become an urgent need in the 5G and advanced packaging fields. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a low-dielectric-loss polyimide film containing perfluorinated side groups and thermally induced micropores. The fluorinated side group structure has a strong electron-withdrawing effect, which can reduce molecular polarization. Furthermore, the fluorinated side group selected in this invention restricts the main chain movement, which can suppress dipole polarization and thus reduce losses. At the same time, it works synergistically with the thermally induced nanopores to achieve a balance between low Dk, low Df, high toughness, and dimensional stability.

[0006] The present invention adopts the following technical solution: A polyamic acid copolymer solution, wherein the film is obtained by polymerizing the polyamic acid copolymer solution from diamine, dianhydride and amino-containing thermally unstable polymer nanoparticles in an aprotic strongly polar solvent; Based on the total molar number of diamines, the diamines include diamine monomers selected from group A and diamine monomers selected from group B, wherein the diamine monomers of group A have a structural formula of one of the structural formulas shown in formulas I-1 to I-4; .

[0007] The diamine monomer of group B is at least one of 2,2'-di(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine (TFDB) and 2,3,4,5,6-pentafluoro-3,5-bis(4-aminophenoxy)-1,1-biphenyl (5FBODA), with the structural formula II-1 to II-2. The dianhydride component is at least one of (perfluoropropane-2,2-diyl)bis(4,1-phenylene)bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) (6FESDA) and hexafluorodianhydride (6FDA), with structural formulas IV-1 to IV-2 respectively. The components of the amino-containing thermally unstable polymer nanoparticles are at least one of the polymers with the structural formulas shown in III-1 to III-4; Furthermore, the solvent is at least one of N,N-dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP); the diamine comprises 70-80 mol% of a diamine monomer selected from group A and 20-30 mol% of a diamine monomer selected from group B; the molar ratio of the diamine to the dianhydride is 1:1.

[0008] Furthermore, the average particle size of the amino-containing thermally unstable polymer nanoparticles is 200–500 nm, the relative molecular mass is 10,000–20,000 g / mol, the thermal decomposition temperature range is 200–300 °C, and the amount added is 1–20% of the total molar mass of the diamine and dianhydride, preferably 3–6%. As an example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%.

[0009] A polyimide film is obtained by casting a polyamic acid copolymer solution as described above onto a support, followed by gradient thermal imidization and biaxial stretching. The film has a thickness of 23-27 μm, a tensile strength ≥235 MPa, an elongation at break ≥36%, a dielectric constant ≤2.63 at 5 GHz, and a dielectric loss ≤2.87 × 10⁻⁶. - ³.

[0010] Furthermore, a method for preparing the thin film as described above includes the following steps: (1) Under a nitrogen atmosphere, the diamine is dissolved in an aprotic strong polar solvent, and then dianhydride is added to carry out a polymerization reaction to obtain a polyamic acid resin solution. (2) Add amino-containing thermally unstable polymer nanoparticles to the polyamic acid resin solution, disperse them evenly, and then carry out a copolymerization reaction to obtain a polyamic acid copolymer solution; (3) The polyamic acid copolymer solution is cast onto a support to form a film, thereby obtaining a self-supporting film; (4) The self-supporting film is thermally imidized under a gradient temperature of 100 to 420°C, and then stretched longitudinally and laterally. After being naturally cooled to room temperature, it is immersed in deionized water, automatically demolded, and then vacuum dried at 120°C for 12 hours to obtain a flat film.

[0011] Furthermore, in step (1), the viscosity of the polyamic acid resin solution is 50-120 Pa·s, the reaction temperature is 0-50℃, and the reaction time is 6-12 h.

[0012] Furthermore, in step (2), the viscosity of the polyamic acid copolymer solution is 80-120 Pa·s, the reaction temperature is 30-60℃, and the reaction time is 2-5 h; the method for uniform dispersion is to stir and disperse at 2500-3000 r / min for 2-3 h.

[0013] Furthermore, in step (3), the casting temperature is 150-200℃, the casting speed is 1-6m / min, and the support used is one or more of mirror steel strip and mirror stainless steel strip.

[0014] Further, in step (4), the gradient heating thermal imidization step is as follows: heating sequentially at 100-120℃ for 10-60 min, at 140-160℃ for 10-60 min, at 180-220℃ for 10-60 min, at 240-260℃ for 10-60 min, at 280-320℃ for 10-120 min, at 340-360℃ for 10-120 min, and at 380-420℃ for 10-120 min; the longitudinal stretching ratio is 1.0-1.15, and the transverse stretching ratio is 1.0-1.25.

[0015] Application of a polyimide film prepared by any of the methods described above in 5G high-frequency communication and advanced electronic packaging.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) By introducing the non-coplanar super-twisted diamine monomer of group A, the structural regularity of the polyimide molecular chain can be effectively destroyed, the symmetric structure can be reduced, thereby reducing the molecular chain packing density, increasing the intermolecular distance, weakening the intermolecular forces, increasing the free volume of the system, which is macroscopically manifested as a decrease in dielectric constant, achieving Dk≤2.6; (2) By introducing nanoparticles formed from low molecular weight, thermally unstable polymers to participate in the polymerization reaction, ABA-type or grafted polyamic acid copolymers are formed, making them a whole. This avoids the incompatibility and uneven dispersion problems that easily occur when inorganic particles are dispersed in the matrix, thus making the mechanical properties of the film more stable. The low molecular weight polymer is a thermally unstable component, which can decompose and form uniformly dispersed nanopores at 200-300℃. This temperature range is below the glass transition temperature (Tg) of the polyimide molecular chain, and the decomposition process will not have a significant impact on the relative molecular mass and physical and mechanical properties of polyimide. These uniformly dispersed nanopores are equivalent to introducing air into the interior of the polyimide film, which can significantly reduce the dielectric constant of the film and further reduce Dk. (3) Ultra-low dielectric loss: The fluorinated phenyl side group of the diamine monomer of group B can suppress dipole polarization, reduce Df, and significantly reduce the transmission loss of high frequency signals. (4) High toughness: The synergistic effect of fluorine structure and flexible ether bond makes the film breakage elongation ≥35%, which solves the problem of poor toughness of traditional low dielectric polyimide; (5) Process compatibility: The mature casting-biaxial stretching process can be used to achieve large-scale industrial production. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 Microscopic morphology of thermally induced micropores in thin films. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0020] Example 1 Preparation of polyamic acid copolymers: (1) The diamine monomers with structural formulas I-1 and II-1 were dissolved in N-methylpyrrolidone, I-1:II-1=7:3, and then dianhydride (perfluoropropane-2,2-diyl)bis(4,1-phenylene)bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylic acid ester)IV-1 was added. The polymerization reaction was carried out at 45°C for 12 h to obtain a polyamic acid resin solution with a viscosity of 50-120 Pa·s; wherein the total molar ratio of diamine monomers to dianhydride used in the reaction was 1:1. (2) Diamino-terminated polystyrene nanoparticles with an average particle size of 200 nm were added to a polyamic acid resin solution and dispersed by stirring at 2800 r / min for 2.5 h. Then, a copolymerization reaction was carried out at 50 °C for 4 h to obtain a polyamic acid copolymer solution with a viscosity of 80–120 Pa·s. The molar mass of the added diamino-terminated polystyrene nanoparticles was 3% of the sum of the molar masses of the diamine monomer and the dianhydride. The structural formula of the diamino-terminated polystyrene is shown in Formula III-1, and the molecular weight is 10000–12000 g / mol. The microstructure of the thermally induced micropores in the film is shown in the attached figure. Figure 1 As shown.

[0021] Film formation: (3) The polyamic acid copolymer solution prepared above is cast into a film on a mirror steel strip at a casting temperature of 150-180℃ and a casting speed of 3.5m / min to obtain a self-supporting film. (4) The self-supporting film is subjected to thermal imidization under a gradient temperature of 100-420℃. The specific steps are as follows: heating at 100℃ for 30 min, heating at 150℃ for 30 min, heating at 200℃ for 30 min, heating at 250℃ for 30 min, heating at 300℃ for 30 min, heating at 350℃ for 30 min, and heating at 400℃ for 30 min in sequence, and performing longitudinal stretching and transverse stretching. The longitudinal stretching ratio is 1.0 and the transverse stretching ratio is 1.15. Then the obtained self-supporting film is naturally cooled to room temperature, immersed in deionized water, automatically demolded, and vacuum dried at 120℃ to obtain a flat film.

[0022] Example 2 The only difference between Example 2 and Example 1 is the amount of diamino-terminated polystyrene nanoparticles used: the molar mass of the diamino-terminated polystyrene nanoparticles is 6% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0023] Everything else is the same as in Example 1.

[0024] Example 3 The only difference between Example 3 and Example 1 is that a diamine monomer with the structural formula shown in Formula I-2 is used instead of a diamine monomer with the structural formula shown in Formula I-1 (I-2:II-1=7:3).

[0025] Everything else is the same as in Example 1.

[0026] Example 4 The only difference between Example 4 and Example 1 is that a diamine monomer with the structural formula shown in Formula I-2 is used instead of a diamine monomer with the structural formula shown in Formula I-1 (I-2:II-1=7:3); and the amount of diamine-terminated polystyrene nanoparticles is different: the molar mass of the diamine-terminated polystyrene nanoparticles is 6% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0027] Everything else is the same as in Example 1.

[0028] Example 5 The only difference between Example 5 and Example 1 is that diamino-terminated polymethyl methacrylate nanoparticles III-3 are used instead of diamino-terminated polystyrene nanoparticles III-1.

[0029] Everything else is the same as in Example 1.

[0030] Example 6 The only difference between Example 6 and Example 1 is that diamino-terminated polymethyl methacrylate nanoparticles III-3 are used instead of diamino-terminated polystyrene nanoparticles III-1; and the amount of diamino-terminated polymethyl methacrylate nanoparticles used is different: the molar mass of the diamino-terminated polymethyl methacrylate nanoparticles is 6% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0031] Everything else is the same as in Example 1.

[0032] Example 7 The only difference between Example 7 and Example 1 is that the diamine monomer II-1 is replaced with the diamine monomer II-2 (I-1:II-2=7:3).

[0033] Everything else is the same as in Example 1.

[0034] Example 8 The only difference between Example 8 and Example 1 is that the ratio of the added diamine monomer is changed: I-1:II-1=8:2.

[0035] Everything else is the same as in Example 1.

[0036] Example 9 The only difference between Example 9 and Example 1 is that a diamine monomer with the structural formula shown in Formula I-3 is used instead of a diamine monomer with the structural formula shown in Formula I-1 (I-3:II-1=7:3).

[0037] Everything else is the same as in Example 1.

[0038] Example 10 The only difference between Example 10 and Example 1 is that a diamine monomer with the structural formula shown in Formula I-4 is used instead of a diamine monomer with the structural formula shown in Formula I-1 (I-4:II-1=7:3).

[0039] Everything else is the same as in Example 1.

[0040] Example 11 The only difference between Example 11 and Example 1 is that the dianhydride monomer with the structural formula shown in Formula IV-2 is used instead of the dianhydride monomer with the structural formula shown in Formula IV-1.

[0041] Everything else is the same as in Example 1.

[0042] Example 12 The only difference between Example 12 and Example 1 is that monoamino-terminated polystyrene nanoparticles III-2 are used instead of diamino-terminated polystyrene nanoparticles III-1.

[0043] Everything else is the same as in Example 1.

[0044] Example 13 The only difference between Example 13 and Example 1 is that monoamino-terminated polymethyl methacrylate nanoparticles III-4 are used instead of diamino-terminated polystyrene nanoparticles III-1.

[0045] Everything else is the same as in Example 1.

[0046] Example 14 The only difference between Example 14 and Example 1 is that the ratio of the added diamine monomer is changed: I-1:II-1=9:1.

[0047] Everything else is the same as in Example 1.

[0048] Example 15 The only difference between Example 15 and Example 1 is that the ratio of the added diamine monomer is changed: I-1:II-1=6:4.

[0049] Everything else is the same as in Example 1.

[0050] Example 16 The only difference between Example 16 and Example 1 is the amount of diamino-terminated polystyrene nanoparticles used: the molar mass of the diamino-terminated polystyrene nanoparticles is 1% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0051] Everything else is the same as in Example 1.

[0052] Example 17 The only difference between Example 17 and Example 1 is the amount of diamino-terminated polystyrene nanoparticles used: the molar mass of the diamino-terminated polystyrene nanoparticles is 10% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0053] Everything else is the same as in Example 1.

[0054] Example 18 The only difference between Example 18 and Example 1 is the amount of diamino-terminated polystyrene nanoparticles used: the molar mass of the diamino-terminated polystyrene nanoparticles is 20% of the sum of the molar masses of the diamine monomer and the dianhydride.

[0055] Everything else is the same as in Example 1.

[0056] Comparative Example 1 The typical polyimide film is as follows: Preparation of polyamic acid resin solution: (1) Dissolve 4,4-diaminodiphenyl ether in N-methylpyrrolidone, then add pyromellitic dianhydride, and polymerize at 45°C for 12 h to obtain a polyamic acid resin solution with a viscosity of 50-120 Pa·s; wherein the molar ratio of 4,4-diaminodiphenyl ether to pyromellitic dianhydride is 1:1.

[0057] Everything else is the same as in Example 1.

[0058] Comparative Example 2 The polyimide film without the addition of thermally unstable polymer nanoparticles is as follows: Preparation of polyamic acid resin solution: (1) The diamine monomers with structural formulas as shown in Formula I-1 and II-2 are dissolved in N-methylpyrrolidone, with I-1:II-2=7:3. Then, pyromellitic dianhydride is added, and the polymerization reaction is carried out at 45°C for 12 h to obtain a polyamic acid resin solution with a viscosity of 50-120 Pa·s. The total amount of added diamine monomers and the molar ratio of pyromellitic dianhydride are 1:1. Everything else is the same as in Example 1.

[0059] Comparative Example 3 The polyimide film with only thermally unstable polymer nanoparticles added is as follows: Preparation of polyamic acid resin solution: (1) Dissolve 4,4-diaminodiphenyl ether in N-methylpyrrolidone, then add pyromellitic dianhydride, and polymerize at 45°C for 12 h to obtain a polyamic acid resin solution with a viscosity of 50-120 Pa·s; wherein the molar ratio of 4,4-diaminodiphenyl ether to pyromellitic dianhydride is 1:1.

[0060] (2) Diamino-terminated polystyrene nanoparticles with an average particle size of 200 nm were added to a polyamic acid resin solution and stirred and dispersed at 2800 r / min for 2.5 h. Then, the copolymerization reaction was carried out at 50 °C for 4 h to obtain a polyamic acid copolymer solution with a viscosity of 80-120 Pa·s. The molar mass of the diamino-terminated polystyrene nanoparticles was 3% of the sum of the molar masses of 4,4'-diaminodiphenyl ether and pyromellitic dianhydride. The structural formula of the diamino-terminated polystyrene is shown in Formula III-1, and the molecular weight is 10000-12000 g / mol.

[0061] Everything else is the same as in Example 1.

[0062] Comparative Example 4 Polyimide film without the addition of group A diamine monomer: The diamine monomer with the structural formula shown in II-1 is dissolved in N-methylpyrrolidone.

[0063] Everything else is the same as in Example 1.

[0064] Comparative Example 5 Polyimide film without the addition of group B diamine monomer: The diamine monomer with the structural formula shown in Formula I-1 is dissolved in N-methylpyrrolidone.

[0065] Everything else is the same as in Example 1.

[0066] The compositions of Examples 1-18 and Comparative Examples 1-5 are shown in Table 1.

[0067] Table 1 The performance of the polyimide films prepared in Examples 1-18 and Comparative Examples 1-5 was tested, and the results are shown in Table 2: Table 2 From the above results, it can be seen that As can be seen from the test results of Examples 1-13 and Examples 14-15, when the amount of diamine monomer added in Group B is 20-30 mol% and the amount of diamine monomer added in Group A is 70-80 mol%, the intermolecular forces can be enhanced, and the elongation at break can be significantly improved while maintaining thermal performance. If there is too much diamine monomer in Group B, the dielectric constant and dielectric loss will increase due to the increased density of the strongly electron-withdrawing polar fluorine groups of fluorinated benzene. If there is too little diamine monomer in Group B, the steric hindrance of the diamine monomer in Group A will be too strong, which will hinder the movement and slippage of chain segments, resulting in a comprehensive decrease in the strength and elongation of the film.

[0068] As can be seen from the test results of Examples 1-13 and Examples 16-18, when the amount of nanoparticles added is 1%~20%, and more preferably 3%~6%, the pore-forming effect is sufficient and appropriate. The number of micropores generated by the decomposition of nanoparticles is moderate, which can effectively reduce the dielectric constant of the system and meet the low dielectric performance technical requirements of 5G and high-frequency electronic packaging. At the same time, the number of micropores is reasonable, without too many interconnected pores, and the mechanical properties remain stable and excellent. The tensile strength and elongation at break are maintained at a good level, the film has good toughness and is not easy to crack, and can be smoothly cast and biaxially stretched. Therefore, the optimal range for the addition of amino-containing thermally unstable polymer nanoparticles is 3%~6%, which can form uniform closed pores, reduce the dielectric constant to 2.0~2.6, and also have strength, elongation, heat resistance, and dimensional stability.

[0069] In summary, this invention achieves the advantages of low dielectric constant, low dielectric loss, high elongation at break, and good dimensional stability in a film through the synergistic effect of perfluorinated side groups reducing molecular polarizability and dielectric loss and thermally unstable nanoparticles forming uniform nanopores in situ. This solves the problems of high dielectric constant, high dielectric loss, insufficient toughness, and high high-frequency signal transmission loss in traditional polyimide films.

[0070] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A polyamic acid copolymer solution, characterized in that, The solution was obtained by polymerizing diamine, dianhydride and amino-containing thermally unstable polymer nanoparticles in a solvent; The diamine includes a diamine monomer selected from group A and a diamine monomer selected from group B, wherein the diamine monomer of group A has a structural formula of one of the structural formulas shown in formulas I-1 to I-4; 。 The diamine monomer of group B is at least one of 2,2'-bis(trifluoromethyl)-(1,1'-diphenyl)-4,4'-diamine and 2,3,4,5,6-pentafluoro-3,5-bis(4-aminophenoxy)-1,1-biphenyl; The dianhydride component is at least one of (perfluoropropane-2,2-diyl)bis(4,1-phenylene)bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) and hexafluorodianhydride. The amino-containing thermally unstable polymer nanoparticles are composed of at least one polymer with the structural formulas shown in III-1 to III-4: 。 2. The polyamic acid copolymer solution according to claim 1, characterized in that, The diamine comprises, by total molar number, 70-80 mol% of diamine monomers selected from group A and 20-30 mol% of diamine monomers selected from group B.

3. The polyamic acid copolymer solution according to claim 1, characterized in that, The amino-containing thermally unstable polymer nanoparticles have an average particle size of 200–500 nm, a relative molecular mass of 10,000–20,000 g / mol, and a thermal decomposition temperature range of 200–300 °C.

4. The polyamic acid copolymer solution according to claim 1, characterized in that, The amount of the amino-containing thermally unstable polymer nanoparticles added is 1-20% of the total molar mass of the diamine and dianhydride, preferably 3-6%.

5. A polyimide film, characterized in that, The film is obtained by casting the polyamic acid copolymer solution according to any one of claims 1-4 onto a support, followed by gradient thermal imidization and biaxial stretching.

6. The polyimide film according to claim 5, characterized in that, The polyimide film has a thickness of 23~27μm, a tensile strength ≥235 MPa, an elongation at break ≥36%, a dielectric constant ≤2.63 at 5GHz, and a dielectric loss ≤2.87×10⁻⁶. - ³.

7. The method for preparing the polyimide film according to claim 5 or 6, characterized in that, include: (1) Under a nitrogen atmosphere, the diamine is dissolved in an aprotic strong polar solvent, and then dianhydride is added to carry out a polymerization reaction to obtain a polyamic acid resin solution. (2) Add amino-containing thermally unstable polymer nanoparticles to the polyamic acid resin solution, disperse them evenly, and then carry out a copolymerization reaction to obtain a polyamic acid copolymer solution; (3) The polyamic acid copolymer solution is cast onto a support to form a film, thereby obtaining a self-supporting film; (4) The self-supporting film is thermally imidized under gradient heating conditions, and then stretched longitudinally and laterally. It is then naturally cooled to room temperature, immersed in deionized water, automatically demolded, and vacuum dried to obtain a flat film.

8. The method for preparing the polyimide film according to claim 7, characterized in that, In step (1), the viscosity of the polyamic acid resin solution is 50-120 Pa·s, the reaction temperature is 0-50℃, and the reaction time is 6-12 h; In step (2), the viscosity of the polyamic acid copolymer solution is 80-120 Pa·s, the reaction temperature is 30-60℃, and the reaction time is 2-5 h; the method for uniform dispersion is to stir and disperse at 2500-3000 r / min for 2-3 h. In step (3), the casting temperature is 150-200℃, the casting speed is 1-6 m / min, and the support used is one or more of mirror steel strip and mirror stainless steel strip.

9. The method for preparing the polyimide film according to claim 7, characterized in that, In step (4), the gradient heating thermal imidization step is as follows: heating sequentially at 100-120℃ for 10-60 min, at 140-160℃ for 10-60 min, at 180-220℃ for 10-60 min, at 240-260℃ for 10-60 min, at 280-320℃ for 10-120 min, at 340-360℃ for 10-120 min, and at 380-420℃ for 10-120 min; the longitudinal stretching ratio is 1.0-1.15, and the transverse stretching ratio is 1.0-1.

25.

10. The application of the polyimide film according to claim 5 or 6, or the polyimide film prepared by the preparation method of any one of claims 7-9, in 5G high-frequency communication and advanced electronic packaging.

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