Ester-containing polyimide resin, polyimide adhesive and flexible copper-clad plate

Abstract

The invention relates to ester-containing polyimide resin, a polyimide adhesive and a flexible copper-clad plate. The ester-containing polyimide resin is carboxyl-terminated polyimide resin, and the structural general formula of the ester-containing polyimide resin is as follows: n is the degree of polymerization and is more than or equal to 5 and less than or equal to 40; ar1 is a combination of residues of ester-containing aromatic tetracarboxylic dianhydride and residues of other aromatic tetracarboxylic dianhydride; ar2 is selected from one of a residue of 1, 2, 4-cyclohexane trimethyl anhydride or a residue of trimellitic anhydride; and B is a combination of residues of ester-containing aromatic diamine and residues of other aromatic diamines. The polyimide adhesive comprises a resin composition and a solvent, wherein the resin composition comprises the following components in parts by weight: 80-95 parts of ester-containing polyimide resin, 5-20 parts of ester-containing epoxy resin and 0-25 parts of a flame retardant. Compared with the prior art, the polyimide adhesive prepared by the invention has the characteristics of low dielectric constant, low water absorption and low dielectric loss, and can be effectively applied to high-frequency and high-speed circuit boards.

Description

Technical Field

[0001] This invention belongs to the field of polyimide adhesive technology, and in particular relates to an ester-containing polyimide resin, a polyimide adhesive, and a flexible copper-clad laminate. Background Technology

[0002] Since 2020, high-frequency and high-speed 5G communication technology has been developing globally. Industrial equipment, electronic equipment, and consumer electronics products related to communication have increasingly higher requirements for signal transmission performance. The dielectric constant and dielectric loss of the dielectrics in the printed circuit boards / flexible printed circuit boards (PCBs / FPCs) and integrated circuits (ICs) used in these products are also being continuously reduced in order to mitigate the negative impacts of RC delay, crosstalk, and power loss on high-frequency and high-speed signal transmission.

[0003] To adapt to this change, the insulating and bonding materials used in PCB / FPC, such as epoxy prepregs, polyimide film substrates, epoxy resin adhesives, acrylate adhesives, and polyurethane adhesives, need to have lower dielectric constants and lower dielectric losses. However, epoxy resin adhesives, acrylate adhesives, and polyurethane adhesives have high dielectric constants, high dielectric losses, and poor heat resistance, making them difficult to apply. Therefore, in the field of 5G communication technology, polyimide adhesives with low dielectric constants, low dielectric losses, and high heat resistance have received extensive research and attention.

[0004] Polyimide is a high-performance engineering plastic. Due to its inherent heat resistance, resistance to thermal oxidation, chemical stability, and excellent mechanical properties, it can withstand thermal shock and has a long-term operating temperature range from -50℃ to 250℃, making it widely used in industrial products. Besides using polyimide film as the insulating base film for FPCs, polyimide can also be processed into adhesives for use in PCBs / FPCs. Although the dielectric constant and dielectric loss of polyimide adhesives are lower than those of epoxy resin adhesives, acrylic adhesives, and polyurethane adhesives, the dielectric constant of conventional polyimide adhesives is still 3.3-3.5, and the dielectric loss is still 0.005-0.007, which does not yet meet the performance requirements of 5G communication technology. Therefore, polyimide adhesives and the polyimide resins used in them still need to be improved to further reduce the dielectric constant and dielectric loss.

[0005] The dielectric constant of polyimide is related to the molecular polarizability and density of its polymer structure. Generally, methods to reduce the dielectric constant of polyimide resin include: in-film pore creation, fluoropolymer doping modification, aliphatic monomer modification, and fluorine-containing monomer modification. In-film pore creation requires pore sizes in the submicron range or below 100 nm, which is difficult to manufacture and can lead to reduced mechanical and insulating properties, increasing the possibility of electrical breakdown. Doping polyimide resin with fluoropolymers can reduce the dielectric constant through blending; however, fluoropolymers and polyimide resins have poor compatibility, easily leading to phase separation and a significant reduction in mechanical properties. Adding aliphatic segments to the polyimide molecular chain can interrupt charge transfer effects and reduce the dielectric constant; however, the introduction of aliphatic chains inevitably reduces heat resistance. Adding fluorinated groups to the polyimide molecular chain reduces the dipole moment of the polyimide molecular chain segments through the strong electron-withdrawing ability of fluorine atoms, and increases the inter-chain gaps through the repulsion between the fluorinated groups and other molecular chain segments, thereby reducing the dielectric constant; however, there is still room for further reduction in the dielectric constant of polyimide resin.

[0006] The dielectric loss of polyimide is related not only to the molecular polarizability and density of its polymer structure, but also directly to its water absorption rate. Researchers have shown a direct linear relationship between the water absorption rate and dielectric loss of insulating materials; the more water absorbed, the greater the dielectric loss. Polyimide is no exception. Although thoroughly drying insulating materials can reduce absorbed moisture and thus lower dielectric loss, during use, the material's contact with and absorption of moisture from the environment still increases dielectric loss. Therefore, the core issue in reducing the dielectric loss of polyimide resin is reducing its water absorption rate.

[0007] In summary, with the development of current high-frequency and high-speed communication technologies, the requirements for the dielectric constant, dielectric loss, heat resistance, and weather resistance of insulating and adhesive materials used in circuit boards are becoming increasingly stringent. How to improve the performance of polyimide adhesives, especially by reducing their dielectric constant and dielectric loss, to make them suitable for high-frequency and high-speed circuit board applications has become an urgent technical problem to be solved. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects of the prior art by providing an ester-containing polyimide resin, a polyimide adhesive, and a flexible copper-clad laminate, so as to obtain a polyimide adhesive with low dielectric constant, low water absorption rate, and low dielectric loss, and further use the polyimide adhesive to prepare a flexible copper-clad laminate for application in high-frequency and high-speed circuit boards.

[0009] The objective of this invention can be achieved through the following technical solutions:

[0010] This invention provides an ester-containing polyimide resin, wherein the ester-containing polyimide resin is a carboxyl-terminated polyimide resin, and its general structural formula is as follows:

[0011]

[0012] Where n is the degree of aggregation, 5≤n≤40;

[0013] Ar1 is a combination of residues of an ester-containing aromatic tetracarboxylic dianhydride and residues of other aromatic tetracarboxylic dianhydrides, wherein the ester-containing aromatic tetracarboxylic dianhydride includes one of (4-phthalic anhydride)formyloxy-4-phthalate, p-phenylene-bisphenyltriptate dianhydride, bis[(3,4-dianhydride)phenyl]terephthalate or p-biphenylene-bisphenyltriptate dianhydride;

[0014]

[0015] The Ar2 is selected from one of the residues of 1,2,4-cyclohexanetricarboxylic anhydride or trimellitic anhydride;

[0016] B is a combination of residues of an ester-containing aromatic diamine and residues of other aromatic diamines, wherein the ester-containing aromatic diamine includes one of p-aminobenzoic acid p-phenyl ester, di-p-aminobenzoic acid ester, or [4-(4-aminobenzoyl)oxyphenyl]4-aminobenzoate.

[0017]

[0018]

[0019] Furthermore, in the Ar1, the residues of ester-containing aromatic tetracarboxylic dianhydride account for 50-80% of the total residues of all tetracarboxylic dianhydrides.

[0020] Further, the other aromatic tetracarboxylic dianhydrides include one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-terephthalodioxydiphthalic anhydride, bisphenol A type diether dianhydride, or 4,4'-(hexafluoroisopropyl)diphthalic dianhydride.

[0021] Furthermore, in B, the residues of the ester-containing aromatic diamine account for 50-80% of the total residues of all diamines.

[0022] Further, the other aromatic diamines include one or more of p-phenylenediamine, m-phenylenediamine, benzidine, 4,4'-diaminobenzoylaniline, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ketone, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 1,3-bis(4'-aminophenoxy)benzene, or 2,2'-bis[4-(4-aminophenoxy)phenyl]propane.

[0023] Furthermore, the ester-containing polyimide resin is prepared by the following method:

[0024] S11: Dissolve diamine compound N1 in a polar solvent, then add dianhydride compound G1, wherein the molar ratio of dianhydride compound G1 to diamine compound N1 is (0.83-0.98):1.0;

[0025] S12: Add triacid anhydride G2 to the above system as a capping agent so that the molar ratio of the total anhydride groups in G1 and G2 to the amino groups in N1 is 1:1.

[0026] S13: React under a nitrogen atmosphere for 4-48 hours at a reaction temperature of 5-45℃ to obtain a polyamic acid solution with a solid content of 10-25%.

[0027] S14: Add 10-20% by weight of an azeotropic dehydrating agent to the polyamic acid solution and react at 180-200℃ for 12-24 hours;

[0028] S15: The reaction solution is added to an aqueous methanol solution, and the mixture is precipitated, filtered, and dried to obtain the ester-containing polyimide resin.

[0029] Further, in step S11, the polar solvent includes one of N,N-dimethylacetamide, N-methylpyrrolidone, m-cresol, and γ-butyrolactone; N1 is a combination of an ester-containing aromatic diamine and other diamines; and G1 is a combination of an ester-containing aromatic tetracarboxylic dianhydride and other aromatic tetracarboxylic dianhydrides.

[0030] Further, in step S12, G2 is one of 1,2,4-cyclohexanetricarboxylic anhydride or trimellitic anhydride.

[0031] Furthermore, in step S14, the azeotropic water-carrying agent is either toluene or xylene.

[0032] The present invention also provides a polyimide adhesive comprising a resin composition and a solvent, wherein the resin composition has a solid content of 8-15 wt%.

[0033] Further, the resin composition includes the above-mentioned ester-containing polyimide resin, specifically comprising the following components by weight: 80-95 parts of ester-containing polyimide resin, 5-20 parts of ester-containing epoxy resin, and 0-25 parts of flame retardant, wherein the sum of the weight parts of the ester-containing polyimide resin and the ester-containing epoxy resin is 100 parts.

[0034] Furthermore, the ester-containing epoxy resin, as a crosslinking agent, includes one of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexylcarboxylate, 1,4-cyclohexanediethanol bis(3,4-epoxycyclohexanecarboxylate), or 4-[2-(2-epoxyethyl)ethoxy]benzoic acid 4-[2-(2-epoxyethyl)ethoxy]phenyl ester.

[0035]

[0036] Furthermore, the flame retardant includes one of aluminum hydroxide, flame retardant SPB-100, flame retardant SPB-703H, flame retardant OP-930, or flame retardant OP-935.

[0037] Further, the solvent includes one of γ-butyrolactone, butanone, xylene, N-methylpyrrolidone, N,N-dimethylacetamide, or m-cresol.

[0038] Furthermore, the polyimide adhesive is prepared by the following method:

[0039] S21: In a nitrogen atmosphere, disperse the ester-containing polyimide resin in a solvent and heat it at 100-150℃ for 2-4 hours to promote dissolution;

[0040] S22: After cooling, add the corresponding weight parts of ester-containing epoxy resin and flame retardant, and react at 30-35℃ for 1-4 hours to obtain polyimide adhesive.

[0041] Furthermore, the polyimide adhesive, after curing, has a dielectric constant of 2.7-3.0, a dielectric loss of 0.001-0.003, a thermal decomposition temperature (Td5%) exceeding 320°C, a bonding strength with copper foil exceeding 12 N / cm, and a bonding strength with insulating film exceeding 8 N / cm.

[0042] The present invention also provides a flexible copper-clad laminate made using the above-mentioned polyimide adhesive, which is specifically prepared by the following method:

[0043] S31: The polyimide adhesive is uniformly coated on the insulating film, and then the solvent in the polyimide adhesive is removed at 80-160℃ to obtain an uncured polyimide adhesive film.

[0044] S32: Copper foil is laminated onto the surface of the film layer, and after lamination, a semi-finished product of single-sided flexible copper-clad laminate is obtained;

[0045] S33: The semi-finished single-sided flexible copper clad laminate is subjected to gradient heating heat treatment to finally obtain a cured single-sided flexible copper clad laminate.

[0046] S34: Repeat the above steps on the back side of the single-sided flexible copper clad laminate to finally obtain a double-sided flexible copper clad laminate.

[0047] Furthermore, the flexible copper-clad laminate is applied in the field of high-frequency and high-speed communication.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] (1) The ester-containing polyimide resin of the present invention introduces ester functional groups into the molecular chain segments of polyimide resin, which can enhance the hydrophobicity of polyimide molecular chains, effectively reduce the water absorption of polyimide resin, reduce the dielectric loss Df of polyimide adhesive, and at the same time, the introduction of ester groups can also reduce the dipole moment of polyimide molecular chains and reduce the dielectric constant Dk, ultimately obtaining an ester-containing polyimide resin with low dielectric constant and low dielectric loss.

[0050] (2) The present invention uses an ester-containing polyimide resin with a low degree of polymerization to improve the solubility of polyimide resin in solvents, thus solving the problem that polyimide resin is difficult to dissolve in conventional solvents; and uses epoxy resin as a crosslinking agent to extend the polyimide resin molecular chain through the crosslinking reaction between epoxy resin and the terminal carboxyl group of polyimide resin, thereby improving properties such as heat resistance and peel strength.

[0051] (3) By precisely controlling the ratio of ester-containing epoxy resin and ester-containing polyimide resin, the present invention reduces the negative impact of epoxy resin on properties such as Dk, Df, and water absorption rate, making the polyimide adhesive of the present invention easy to process and prepare, and the properties such as Dk, Df, and water absorption rate can all achieve the expected goals.

[0052] (4) This invention uses a low dielectric constant polyimide adhesive to produce flexible copper-clad laminate products through conventional coating, bonding, and curing processes. Before preparing the polyimide adhesive solution, the polyimide resin used is fully imidized, which does not affect subsequent coating, bonding, and curing processes. When preparing the polyimide adhesive solution, the polyimide resin is dissolved and dispersed in an optimized combined solvent, and is thoroughly mixed with the epoxy resin without phase separation issues. The processing is simple and can be referenced from the application processes of epoxy adhesives, acrylate adhesives, polyurethane adhesives, etc., to complete the production of flexible copper-clad laminate products, enabling effective application in the large-scale preparation of high-frequency and high-speed circuit boards. Detailed Implementation

[0053] The present invention will now be described in detail with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. The following embodiments are implemented based on the technical solutions of the present invention, providing detailed implementation methods and specific operating procedures; however, the scope of protection of the present invention is not limited to the following embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0054] Unless otherwise specified, the reagents, methods, instruments, and equipment used in this invention are conventional in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0055] The raw material information used in the following examples and comparative examples is shown in the table below:

[0056] Table 1 Summary of Raw Material Information

[0057]

[0058]

[0059] The specific preparation methods of the ester-containing polyimide resin, polyimide adhesive, and flexible copper-clad laminate in Examples 1-30 are as follows:

[0060] (1) Preparation of ester-containing polyimide resin:

[0061] Under a nitrogen atmosphere, the quantitative amounts of diamine compounds N1 listed in Table 2 are dissolved in a strongly polar solvent S1 in a container with stirring, and the solution temperature is controlled at 5-45℃. The quantitative amounts of dianhydride compounds G1 listed in Table 2 are added to the solution in three portions, each time accounting for 60%, 30%, and 10% of the total weight of dianhydride compounds G1, respectively. Then, the quantitative amounts of triacid anhydride compounds G2 listed in Table 2 are added as a capping agent. The solution temperature is controlled at 5-45℃, and the reaction is carried out under a nitrogen atmosphere for 4-48 hours to obtain a polyamic acid solution with a solid content of 10-25%. The quantitative amounts of azeotropic dehydrating agent S2 listed in Table 2 are added to the solution, and the solution is heated to 180-200℃ and reacted for 12-24 hours to remove the water generated in the reaction. After the reaction is complete, the solution is cooled to room temperature and added to a methanol-water solution composed of methanol and water in a 1:1 volume ratio, where the volume of the methanol-water solution is more than three times the volume of the polyimide resin solution. The resulting precipitate was filtered, washed with deionized water, and dried in a vacuum oven at 80°C for 12 hours to obtain a slightly yellow ester-containing polyimide resin powder.

[0062] (2) Preparation of polyimide adhesive:

[0063] Under a nitrogen atmosphere, the ester-containing polyimide resin obtained in step (1) is dispersed in the quantitative solvent S3 listed in Table 2, and heated at 100-150℃ for 2 hours to promote the dissolution of the ester-containing polyimide resin. After cooling, the quantitative ester-containing epoxy resin H1 and the quantitative flame retardant R1 listed in Table 2 are added to the solution, and the mixture is stirred and reacted at 30-50℃ for 1-4 hours to obtain a solution-like polyimide adhesive with a low dielectric constant.

[0064] (3) Preparation of flexible copper-clad laminates:

[0065] Using a coating machine, the polyimide adhesive solution obtained in step (2) is uniformly coated onto one surface of a polyimide film (PI film) or other insulating film. It is then placed in an oven at 80-160°C to thoroughly remove the solvent, resulting in an uncured polyimide adhesive film. The film thickness is approximately 10 μm, and the solvent content in the film is less than 1%. Copper foil is then laminated onto the film surface, and the resulting composite material yields a semi-finished single-sided flexible copper-clad laminate. The semi-finished single-sided flexible copper-clad laminate is placed in a programmable temperature oven and treated at 60°C, 80°C, 100°C, 120°C, and 140°C for 1 hour each, followed by treatment at 160-180°C for 2-8 hours to allow the polyimide adhesive to cure uniformly and slowly, resulting in a single-sided flexible copper-clad laminate.

[0066] A polyimide adhesive solution is uniformly coated onto the other surface of the insulating film of a single-sided flexible copper-clad laminate using a coating machine. The laminate is then placed in an oven at 80-160℃ to thoroughly remove the solvent, resulting in an uncured polyimide adhesive film layer. The film layer thickness is approximately 10μm, and the solvent content is less than 1%. Copper foil is then laminated onto the film layer surface, and the resulting composite material yields a semi-finished double-sided flexible copper-clad laminate. This semi-finished double-sided flexible copper-clad laminate is placed in a programmable temperature oven and treated at 60℃, 80℃, 100℃, 120℃, and 140℃ for 1 hour each, followed by treatment at 160-180℃ for 2-8 hours to allow the polyimide adhesive to cure uniformly and slowly, thus obtaining the double-sided flexible copper-clad laminate.

[0067] (4) Preparation of polyimide adhesive film samples for subsequent performance testing:

[0068] The polyimide adhesive solution obtained in step (2) is coated onto the surface of a carrier such as glass or copper foil, or poured into a suitable mold. It is then evaporated at 80-180℃ for 5-60 minutes to remove all solvents. The evaporation temperature and time are finely adjusted to ensure a good appearance of the adhesive. Afterward, it is heat-treated at 160-180℃ for 2-8 hours to complete the curing of the polyimide adhesive. The polyimide adhesive film sample is obtained by removing it from the glass carrier surface, etching away the copper foil carrier, or removing it from the mold.

[0069] The ester-containing polyimide resin, polyimide adhesive, and flexible copper-clad laminate in Comparative Examples 1-6 are prepared using the same methods as in the above embodiments, except that the types and quantities of raw materials used are different. Specific steps are as described in Examples 1-30.

[0070] Based on the types and amounts of raw materials shown in Table 2 and the parameter information of the samples shown in Table 3, 30 sets of polyimide adhesives and 6 sets of comparative examples and corresponding flexible copper clad laminate samples were obtained.

[0071] Table 2. Raw material mixing amounts for Examples 1-30 and Comparative Examples 1-6

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] Table 3. Parameters of polyimide adhesives in Examples 1-30 and Comparative Examples 1-6

[0079]

[0080]

[0081] The present invention conducted the following performance tests on the polyimide adhesive film samples obtained in Examples 1-30 and Comparative Examples 1-6:

[0082] (1) Dielectric constant (Dk) and dielectric loss (Df) test. The polyimide adhesive film sample was placed in a 150℃ circulating oven for 2 hours. After drying, the sample was allowed to stand in a desiccator for 2 hours before testing. According to IEC61189-2-721-2015 standard, the Dk and Df values ​​of the polyimide adhesive film sample were measured using a Dextronix E5080B network analyzer with a 10GHz resonant cavity fixture and the Split-Support Dielectric Resonant Cavity (SPDR) method.

[0083] (2) Water Absorption Test. The polyimide adhesive film sample was placed in deionized water and left to stand at room temperature for 24 hours. After removal, the surface was wiped dry with absorbent paper, and the weight was recorded as W0. The sample was then placed in a 150℃ circulating oven for 2 hours. After drying, the sample was cooled in a desiccator for 0.5 hours, and its weight was immediately measured and recorded as W1. The water absorption rate of the polyimide adhesive was calculated using the formula "Water Absorption Rate = (W1 - W0) / W0".

[0084] (3) Thermal decomposition temperature (Td) 5％ The thermal decomposition temperature of the polyimide film was tested using TA's Q50. The test conditions were as follows: starting at 30°C, the temperature was increased to 800°C at a rate of 10°C / min, followed by natural cooling. The temperature at which 5% weight loss occurred was recorded as the thermal decomposition temperature of the polyimide film.

[0085] (4) Peel Strength Test. A universal tensile testing machine was used to test the peel strength of the flexible copper-clad laminate (CCL) samples. The copper foil layer on the CCL sample was etched into a rectangular shape with a width of 3 mm and a length of at least 20 cm. One end was peeled off from the polyimide insulation layer for approximately 5 cm. Using a flat clamp, the CCL sample was fixed to the clamp, and the peeled copper strip was fixed to a movable clamp. The clamp force range was 20 N, and the tensile rate was 50.8 mm / min. During tensile testing, the copper strip was kept at a 90° angle to the plane of the CCL sample. A straight and stable section on the curve was selected, and the peel strength of the flexible CCL sample was calculated using the formula "peel strength = peel force / copper strip width".

[0086] (5) Etching method for flexible copper-clad laminate. Preparation of etching solution: Prepare an acidic CuCl2 etching solution with pH=1. Immerse the flexible copper-clad laminate sample in the etching solution until the copper foil is etched away.

[0087] The performance test results of Examples 1-30 and Comparative Examples 1-6 are shown in Table 4.

[0088] Table 4 Performance tests of Examples 1-30 and Comparative Examples 1-6

[0089]

[0090]

[0091] Based on the above performance test results, we can conclude that:

[0092] (1) The polyimide adhesives in Examples 1-30 all exhibit satisfactory properties. Their Dk ranges from 2.7 to 3.0, Df ranges from 0.001 to 0.003, water absorption is less than 0.7%, and Td... 5％At temperatures above 320°C, the peel strength with PI film is above 8 N / cm, and the peel strength with copper foil is above 12 N / cm. Therefore, the polyimide adhesives obtained in Examples 1-30 are suitable for applications in high-frequency, high-speed communication circuit board products.

[0093] (2) In Examples 1-5, the same ester-containing polyimide resin was used to prepare the polyimide adhesive. As the weight ratio of epoxy resin increased, the Dk of the polyimide adhesive increased slowly, as did the water absorption rate and Df. Although the Dk and Df properties still met the expected targets, the addition of epoxy resin was detrimental to properties such as Dk, Df, and water absorption rate. The same conclusion was reached in Examples 14-18; increasing the amount of epoxy resin increased Dk, Df, and water absorption rate. When the amount of epoxy resin was excessive, such as in Comparative Examples 5-6, the core indicators such as Dk, Df, and water absorption rate were significantly higher, and the heat resistance was also reduced, making it difficult to apply in high-frequency, high-speed communication circuit boards. Epoxy resin, as a crosslinking agent, links the polyimide resin molecular chains together through a crosslinking reaction with the terminal carboxyl groups, improving mechanical properties and enhancing the peel strength with PI films or copper foils, making it indispensable in polyimide adhesives. The present invention optimizes the addition ratio of epoxy resin: in the polyimide adhesive, epoxy resin is 5-20 parts by weight, ester-containing polyimide resin is 80-95 parts by weight, and the sum of the two is 100 parts by weight, thereby achieving a balanced improvement in the above-mentioned properties.

[0094] (3) In Examples 1, 6-9, the same ester-containing polyimide resin was used, but different types and proportions of ester-containing epoxy resins were used, without changing the weight percentage of the epoxy resin. The Dk, Df, and water absorption rates of the polyimide adhesive were not affected, and the peel strength between the polyimide adhesive and the PI film or copper foil did not change significantly. This indicates that different types of ester-containing epoxy resins, as long as the weight percentage is consistent, do not significantly affect the performance of the polyimide adhesive. The same rule and conclusion can be drawn in Examples 14, 19-20. This also suggests that other ester-containing epoxy resins can also be used in the polyimide adhesive of this invention, provided the addition ratio is maintained.

[0095] (4) In Examples 1, 10-13, ester-containing polyimide resins with different degrees of polymerization (5-40) were prepared by varying the ratio of dianhydride to diamine and the amount of trianic anhydride used as the end-capping agent. The results showed that increasing the degree of polymerization had little effect on properties such as Dk, Df, and water absorption, but it did increase the thermal decomposition temperature of the polyimide adhesive. This was attributed to the elongation of the molecular chains and the increase in molecular weight after increasing the degree of polymerization. Although crosslinking of epoxy resin also elongates the molecular chains of polyimide resin, its effect is not as significant as the intrinsic molecular chain elongation of polyimide resin. Simultaneously, the peel strength between the polyimide adhesive and PI film or copper foil was also improved.

[0096] However, experiments also revealed that a degree of polymerization exceeding 40 leads to difficulty in dissolving the polyimide resin in solvents, making it impossible to prepare the polyimide adhesive solution. Examples 14, 21-24, also investigated the effect of the degree of polymerization on the properties of the polyimide adhesive. Similarly, increasing the degree of polymerization increases the heat resistance of the polyimide adhesive and the peel strength to PI films or copper foils, while properties such as DK, Df, and water absorption remain unchanged. The degree of polymerization can indeed effectively improve the performance of polyimide adhesives. However, considering that ester-containing polyimide resins are difficult to dissolve in solvents when the degree of polymerization is too high, after experimental optimization, the degree of polymerization of ester-containing polyimide resins needs to be controlled below 40.

[0097] (5) In Examples 25-30, ester-containing polyimide resins were prepared using different dianhydride and diamine monomers. When the proportion of ester-containing dianhydride monomers in the dianhydride exceeded 50% and the proportion of ester-containing diamine monomers in the diamine exceeded 50%, the performance indicators of the polyimide adhesive, such as Dk, Df, and water absorption, were all good. As the proportion of ester-containing dianhydride and ester-containing diamine monomers increased, Dk decreased, Df decreased, water absorption decreased, and the core performance continuously improved. On the other hand, the type of ester-containing monomer did not have a significant impact on the performance of the polyimide adhesive. In Comparative Examples 1-4, the molar ratio of ester-containing dianhydride and ester-containing diamine in the polymer monomers was less than 50% and continuously decreased, while the core indicators such as Dk, Df, and water absorption continuously increased, which was no longer suitable for use in high-frequency and high-speed circuit boards. When ester-containing dianhydride and ester-containing diamine were not used, Dk reached 3.44-3.65 and Df reached 0.0068-0.0078, far exceeding the requirements for use. This suggests that other ester-containing dianhydrides or ester-containing diamine monomers, when maintained in a suitable molar ratio (greater than 50%) in the polyimide polymerization formulation, can also be applied to the polyimide adhesives of this invention.

[0098] In summary, the polyimide adhesive of this invention mainly consists of a self-made ester-containing polyimide resin and a commercially available ester-containing epoxy resin. By introducing hydrophobic ester groups into the molecular chain of the polyimide resin, the hydrophobicity of the molecular chain segments is enhanced, thereby reducing the water absorption rate and dielectric loss of the polyimide adhesive. Simultaneously, the use of the ester-containing epoxy resin reduces the negative impact of the epoxy resin on the performance of the polyimide adhesive, resulting in a final polyimide adhesive with low dielectric constant, low water absorption rate, and low dielectric loss, enabling its effective application in high-frequency, high-speed circuit boards.

[0099] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A polyimide-containing resin, characterized in that, The ester-containing polyimide resin is a carboxyl-terminated polyimide resin, and its general structural formula is as follows: Where n is the degree of aggregation, 5≤n≤40; Ar1 is a combination of residues of an ester-containing aromatic tetracarboxylic dianhydride and residues of other aromatic tetracarboxylic dianhydrides, wherein the ester-containing aromatic tetracarboxylic dianhydride includes one of (4-phthalic anhydride)formyloxy-4-phthalate, p-phenylene-bisphenyltriptate dianhydride, bis[(3,4-dianhydride)phenyl]terephthalate or p-biphenylene-bisphenyltriptate dianhydride; The Ar2 is selected from one of the residues of 1,2,4-cyclohexanetricarboxylic anhydride or trimellitic anhydride; B is a combination of residues of an ester-containing aromatic diamine and residues of other aromatic diamines, wherein the ester-containing aromatic diamine includes one of p-aminobenzoic acid p-phenyl ester, di-p-aminobenzoic acid ester, or [4-(4-aminobenzoyl)oxyphenyl]4-aminobenzoate.

2. The ester-containing polyimide resin according to claim 1, characterized in that, In Ar1, the residues of ester-containing aromatic tetracarboxylic dianhydrides account for 50-80% of the total residues of all tetracarboxylic dianhydrides; the other aromatic tetracarboxylic dianhydrides include one of pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-terephthalodioxydiphthalic anhydride, bisphenol A type diether dianhydride, or 4,4'-(hexafluoroisopropyl)diphthalic dianhydride; In B, the residues of the ester-containing aromatic diamine account for 50-80% of the total residues of all diamines; the other aromatic diamines include one of p-phenylenediamine, m-phenylenediamine, benzidine, 4,4'-diaminobenzoylaniline, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodibenzophenone, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, 1,3-bis(4'-aminophenoxy)benzene, or 2,2'-bis[4-(4-aminophenoxy)phenyl]propane.

3. The ester-containing polyimide resin according to claim 1, characterized in that, The ester-containing polyimide resin was prepared by the following method: S11: Dissolve diamine compound N1 in a polar solvent, then add dianhydride compound G1, wherein the molar ratio of dianhydride compound G1 to diamine compound N1 is (0.83-0.98):1.0; S12: Add triacid anhydride G2 to the above system as a capping agent so that the molar ratio of the total anhydride groups in G1 and G2 to the amino groups in N1 is 1:

1. S13: React under a nitrogen atmosphere for 4-48 hours at a reaction temperature of 5-45℃ to obtain a polyamic acid solution with a solid content of 10-25%. S14: Add 10-20% by weight of an azeotropic dehydrating agent to the polyamic acid solution and react at 180-200℃ for 12-24 hours; S15: The reaction solution is added to an aqueous methanol solution, and the mixture is precipitated, filtered, and dried to obtain the ester-containing polyimide resin.

4. The ester-containing polyimide resin according to claim 3, characterized in that, In step S11, the polar solvent includes one of N,N-dimethylacetamide, N-methylpyrrolidone, m-cresol, and γ-butyrolactone; N1 is a combination of an ester-containing aromatic diamine and other diamines; and G1 is a combination of an ester-containing aromatic tetracarboxylic dianhydride and other aromatic tetracarboxylic dianhydrides. In step S12, G2 is one of 1,2,4-cyclohexanetricarboxylic anhydride or trimellitic anhydride; In step S14, the azeotropic water-carrying agent is either toluene or xylene.

5. A polyimide adhesive, characterized in that, It includes a resin composition and a solvent, wherein the solid content of the resin composition is 8-15 wt%; The resin composition comprises the ester-containing polyimide resin according to any one of claims 1-4, specifically comprising the following components by weight: 80-95 parts of ester-containing polyimide resin, 5-20 parts of ester-containing epoxy resin, and 0-25 parts of flame retardant, wherein the sum of the weight parts of the ester-containing polyimide resin and the ester-containing epoxy resin is 100 parts.

6. The polyimide adhesive according to claim 5, characterized in that, The ester-containing epoxy resin, used as a crosslinking agent, includes one of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexylcarboxylate, 1,4-cyclohexanediethanol bis(3,4-epoxycyclohexanecarboxylate), or 4-[2-(2-epoxyethyl)ethoxy]benzoic acid 4-[2-(2-epoxyethyl)ethoxy]phenyl ester.

7. The polyimide adhesive according to claim 5, characterized in that, The flame retardant includes one of aluminum hydroxide, flame retardant SPB-100, flame retardant SPB-703H, flame retardant OP-930, or flame retardant OP-935; The solvent includes one of γ-butyrolactone, butanone, xylene, N-methylpyrrolidone, N,N-dimethylacetamide, or m-cresol.

8. The polyimide adhesive according to claim 5, characterized in that, The polyimide adhesive is prepared by the following method: S21: In a nitrogen atmosphere, disperse the ester-containing polyimide resin in a solvent and heat it at 100-150℃ for 2-4 hours to promote dissolution; S22: After cooling, add the corresponding weight parts of ester-containing epoxy resin and flame retardant, and react at 30-35℃ for 1-4 hours to obtain polyimide adhesive.

9. A polyimide adhesive according to claim 5, characterized in that, The polyimide adhesive, after curing, has a dielectric constant of 2.7-3.0, a dielectric loss of 0.001-0.003, and a thermal decomposition temperature Td. 5％ At temperatures exceeding 320℃, the bonding strength with copper foil reaches over 12 N / cm, and the bonding strength with insulating film reaches over 8 N / cm.

10. A flexible copper-clad laminate, characterized in that, The polyimide adhesive according to claim 5 is specifically prepared by the following method: S31: The polyimide adhesive is uniformly coated on the insulating film, and then the solvent in the polyimide adhesive is removed at 80-160℃ to obtain an uncured polyimide adhesive film. S32: Copper foil is laminated onto the surface of the film layer, and after lamination, a semi-finished product of single-sided flexible copper-clad laminate is obtained; S33: The semi-finished single-sided flexible copper clad laminate is subjected to gradient heating heat treatment to finally obtain a cured single-sided flexible copper clad laminate. S34: Repeat the above steps on the back side of the single-sided flexible copper clad laminate to finally obtain a double-sided flexible copper clad laminate.

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