Flame-retardant flexible circuit board and preparation method thereof

Abstract

The present invention discloses a flame-retardant flexible circuit board and a preparation method thereof, relating to the technical field of flexible circuit boards. The method comprises the following steps: mixing a diamine monomer and a dianhydride monomer in an organic solvent, stirring the mixture for reaction, adding a filler to obtain a polyamic acid solution; casting the mixture into a film, drying the mixture to form a polyamic acid film; thermal imidization to obtain a polyimide film; adding a crosslinking agent to the polyamic acid solution to form a crosslinking solution; coating the crosslinking solution on both surfaces of the polyimide film, drying the solution to form a polyamic acid coating film; thermal imidization again to form a polyimide coating to obtain a base film; and providing a conductive circuit to obtain a circuit board. By adding a crosslinking agent to the polyamic acid solution, the present invention effectively increases the crosslinking density of the coating, forms a network structure, and improves the stability of the polyimide, thereby improving the mechanical properties, heat resistance, and dielectric properties of the prepared base film.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible circuit boards, in particular to a flame-retardant flexible circuit board and a preparation method thereof. Background Art

[0002] Flexible printed circuit boards (FPCs), also known as flexible printed circuit boards (FPCs), are printed circuit boards made from a flexible insulating substrate. They meet the demands of smaller, higher-density designs, reducing assembly steps and enhancing reliability. They can be bent, rolled, and folded freely, and can withstand millions of dynamic flexes without damaging the conductive circuits. They meet the demands of electronic products moving towards higher density, miniaturization, and increased reliability. Flexible insulating substrates are primarily made of polyimide or polyester. Polyester film is flammable, while polyimide film offers relatively good flame retardancy, but there is still room for improvement. Furthermore, they are difficult to mold and can expand and deform at high temperatures, leading to performance degradation. To reduce the risk of fire in devices and protect user and device safety, we propose a flame-retardant flexible circuit board and its preparation method. Summary of the Invention

[0003] The object of the present invention is to provide a flame retardant flexible circuit board and a preparation method thereof, so as to solve the problems raised in the above background technology.

[0004] In order to solve the above technical problems, the present invention provides the following technical solutions: a flame-retardant flexible circuit board, comprising a base film and a conductive circuit on its surface, wherein the base film comprises a polyimide film and a polyimide coating provided on the upper and lower surfaces of the polyimide film;

[0005] Furthermore, the polyimide film is obtained by casting a polyamic acid solution into a film and then imidizing it;

[0006] The polyamic acid solution includes the following components: diamine monomer, dianhydride monomer and filler.

[0007] Furthermore, the polyimide coating is obtained by coating a cross-linking solution on the surface of a polyimide film and then imidizing it;

[0008] The cross-linking solution includes the following components: a diamine monomer, a dianhydride monomer, a cross-linking agent and a filler.

[0009] Furthermore, the filler includes but is not limited to a mixture of one or more of aluminum oxide, silicon oxide, aluminum nitride, polysiloxane, montmorillonite, mica powder, and borosilicate.

[0010] Furthermore, the dianhydride monomer is a mixture of one or more of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-triphenyl ether tetracarboxylic dianhydride, biphenyl diether dianhydride, 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, 4,4'-terephthaloyl diphthalic anhydride, bisphenol A diether dianhydride, and pyromellitic dianhydride.

[0011] Furthermore, the diamine monomer is 4,4'-diaminodiphenyl ether, 1,4-phenylenediamine, 4,4'-diaminobibenzyl, 4,4'-diaminobiphenyl, 1,4-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone, 5(6)-1-(4-aminophenyl)-1,3,3-trimethylindane (CAS: 54628-90-9), 4,4'-bis(4-aminophenoxy)diphenyl sulfone, 4,4'-bis(3-aminophenoxy)benzophenone, 9,9- A mixture of one or more of bis[4-(4-aminophenoxy)phenyl]fluorene, bis[4-(3-aminophenoxy)phenyl]phenylphosphine oxide, 1,4-phenylenebis[[4-(4-aminophenoxy)phenyl]methanone], 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline], 1,3-bis(4-aminophenyl)adamantane, 1,3-bis(fluoro-aminophenyl)adamantane, 2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, and 3,7-diamino-2,8-dimethyldibenzothiophene sulfone.

[0012] Furthermore, the cross-linking agent is one of 1,3,5-tris(4-aminophenoxy)benzene and melamine, or a mixture of the two.

[0013] A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0014] Step 1: Mixing a diamine monomer and a dianhydride monomer in an organic solvent, stirring the mixture for reaction, and adding a filler to obtain a polyamic acid solution; casting the mixture into a film, drying the film, and forming a polyamic acid film; and thermal imidization to obtain a polyimide film;

[0015] Step 2: adding a crosslinking agent to the polyamic acid solution to form a crosslinking solution; coating the crosslinking solution on both surfaces of the polyimide film, drying, to form a polyamic acid coating; thermal imidization to form a polyimide coating to obtain a base film;

[0016] Step 3: Set up the conductive circuit to obtain the circuit board.

[0017] Furthermore, the polyamic acid solution is prepared by the following process:

[0018] At a temperature of 3 to 7°C, diamine monomer and organic solvent are mixed, and under the protection of nitrogen atmosphere, 93% to 97% by mass of dianhydride monomer is added, and the reaction is stirred for 60 to 90 minutes; the remaining dianhydride monomer is added, and the stirring reaction is continued for 4.5 to 5.0 hours to obtain a polyamic acid solution.

[0019] Furthermore, the molar ratio of the dianhydride monomer to the diamine monomer is (1.0-1.5):1.

[0020] Furthermore, the organic solvent is one of dimethyl sulfoxide and dimethylformamide, or a mixture of the two.

[0021] Furthermore, the drying process conditions are: temperature 110-125° C., time 10-12 minutes, to remove the organic solvent.

[0022] Furthermore, the process conditions of thermal imidization are: heating to 280-300°C and keeping warm for 30-40 minutes; starting at 30-50°C, heating by 50°C for 30-40 minutes, with a heating rate of 4-5°C / min; and slowly cooling to room temperature after thermal imidization.

[0023] Furthermore, the thickness of the polyimide film is 5 to 60 μm;

[0024] The cross-linking solution is coated on one side of the polyimide film to a thickness of 20 to 85 μm.

[0025] Furthermore, the amount of the cross-linking agent is 10% to 50% of the amount of the diamine monomer.

[0026] Furthermore, the solid content of the polyamic acid solution is 60% to 70%.

[0027] In the above technical solution, a dianhydride monomer and a diamine monomer are mixed in an organic solvent, causing the anhydride groups and amino groups to react to form an amic acid structure. This polyamic acid solution, which exhibits good processing properties, is then cast into a film, dried, and thermally imidized to form a polyimide film. The polyamic acid solution is then coated on both surfaces of the polyimide film, dried, and cured to form a polyimide coating, forming the base film for the circuit board. Finally, conductive circuits are formed on the base film surface through processes such as copper plating, development, etching, and gold deposition, thereby forming a flexible circuit board.

[0028] Dianhydride monomers can be selected from pyrophthalic anhydride, biphenyl anhydride, ether phthalic anhydride, phenylsulfone anhydride, and bisphenol A ether anhydride, which have high rigidity. The prepared polyimide has better mechanical properties and heat resistance, which contributes to the improvement of the flame retardant ability of the circuit board. Fluorine-containing dianhydride, by introducing fluorine atoms, has a lower molar polarizability that can reduce the dielectric constant and dielectric loss of the polyimide, improve the insulation performance and corrosion resistance of the base film, and can enhance its thermal stability and oxidation resistance, improve the heat resistance stability of the base film. And the fluorine atom has a higher electronegativity, which can destroy the electron cloud conjugation with the color-producing functional structure in the polyimide molecular structure, so that the polyimide shows good light transmittance.

[0029] In the above technical solution, the precursor raw material for the polyimide coating is a mixture of a polyamic acid solution and a crosslinking agent. The crosslinking agent is a polyamino compound. By adjusting the amounts of dianhydride monomer, diamine monomer, and crosslinking agent, the dianhydride monomer reacts with the diamine monomer and crosslinking agent, effectively increasing the crosslink density of the resulting polyimide (coating), forming a network structure and improving the stability of the polyimide. This significantly improves the mechanical properties, heat resistance, and dielectric properties of the resulting polyimide coating.

[0030] Further, the amount of filler is 0.5 to 2.0 wt%;

[0031] The filler is silica, with a particle size of 2.5 to 20 μm;

[0032] Silica is modified by coupling, and the coupling agent is selected from 4-perfluorotriethoxysilane and M-aminophenyltrimethoxysilane. The specific process is as follows:

[0033] Disperse silica in an alcohol aqueous solution, add ammonia water to adjust the pH to 7.5-9.0, add a coupling agent, stir and react for 6-8 hours; filter and dry to obtain a modified filler;

[0034] The amount of the coupling agent is 1% to 10% of the mass of the silica; the alcohol aqueous solution is an ethanol aqueous solution with a concentration of 50 to 90 v%, and the ratio of silica to alcohol aqueous solution is (20 to 40) g / 100 mL.

[0035] In the above technical solution, the addition of silica to the polyamic acid solution significantly improves the mechanical, thermal, and dielectric properties of polyimide films and coatings. As a filler, silica undergoes coupling modification, which improves its dispersibility in the polyimide and its interfacial bonding strength, enhances the polyimide's flexibility, and reduces its thermal expansion coefficient, contributing to further improvements in the base film's mechanical properties, light transmittance, and heat and flame resistance.

[0036] Furthermore, when the polyamic acid contains a reactive amine group, the crosslinking agent may be 2,5-dichloro-p-xylene, 1,3,5-trichlorobenzene, or a polychlorinated compound, which is prepared by the following process:

[0037] S1. Mix N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine in dichloromethane, raise the temperature to 40-60° C., and reflux for 14-15 hours; and distill under reduced pressure and elute to obtain trimaleimide cyclotriphosphazene;

[0038] S2. Mixing allyl DOPO and chlorine-containing maleimide in o-dichlorobenzene, heating to 150-180° C., stirring and reacting for 8-10 hours; precipitating, and drying to obtain a DOPO derivative;

[0039] S3. Trimaleimide cyclotriphosphazene and DOPO derivative are mixed in diphenyl ether, and the mixture is heated to 170-200° C. under nitrogen atmosphere, and reacted for 60-150 min. Methanol is added for precipitation, and the mixture is vacuum dried to obtain a cross-linking agent.

[0040] Furthermore, in S1, the molar ratio of N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine is (3.3-3.6):1:(3.3-3.6):(3.3-3.6);

[0041] The ratio of N-(3,4-dihydroxyphenyl)maleimide to dichloromethane is (8-10) g / 100 mL.

[0042] Furthermore, in S2, the molar ratio of propenyl DOPO (CAS No. 311342-65-1) and chlorine-containing maleimide is 1:1;

[0043] The chlorine-containing maleimide is a mixture of one or more of 1-(4-chlorophenyl)-pyrrole-2,5-dione, N-(2-chloropyridin-3-yl)maleamide, and N-(3-chloro-2-tolyl)maleimide;

[0044] Iron phosphate is added as a catalyst in the reaction system, with the amount being 3% to 5% of the total mass of the reactants;

[0045] The ratio of propenyl DOPO to o-dichlorobenzene is (3-8) g / 100 mL.

[0046] Furthermore, in S3, the mass ratio of trimaleimidocyclotriphosphazene and DOPO derivative is 10:(20.6-21.2);

[0047] Nickel perchlorate is added to the reaction system as a catalyst, with the amount being 3% to 6% of the total mass of the reactants;

[0048] The ratio of trimaleimidocyclotriphosphazene to diphenyl ether is (3-5) g / 100 mL.

[0049] In the above technical solution, the cross-linking agent can be selected to contain a polychlorinated compound, so that it reacts with the amino groups in the polyamic acid during the thermal imidization process to achieve cross-linking. Compared with the use of amine cross-linking agents, the dianhydride monomer and the diamine monomer have better reactivity, which helps to increase the molecular weight of the polyimide, and the resulting coating has better mechanical properties and heat resistance and flame retardancy.

[0050] The crosslinker is prepared by mixing N-(3,4-dihydroxyphenyl)maleimide and hexachlorocyclotriphosphazene. In the presence of carbon tetrachloride and triethylamine, the catechol in the reactants reacts with two chlorine groups to form a benzodioxaphosphine structure, resulting in a trifunctional maleimide compound, designated as trimaleimidocyclotriphosphazene. Propylene DOPO is then mixed with the chlorine-containing maleimide at high temperature. The allyl group in the propenyl DOPO reacts with the double bond in the maleimide through an Alder-ene reaction, yielding a DOPO compound with alkenyl and chlorine groups, designated as a DOPO derivative. The trimaleimide cyclotriphosphazene and DOPO derivative obtained above are mixed, and the maleimide groups in the reactants undergo addition reaction with alkenyl groups, resulting in a DA reaction. This yields a trifunctional chlorine-containing compound, or crosslinker. When used in the preparation of polyimide, this crosslinker introduces a twisted and non-coplanar structure into the polyimide system, resulting in a deformed and distorted spatial configuration of the macromolecular backbone. This effectively inhibits long-distance charge transfer along the molecular chain, thereby effectively improving the mechanical and thermal properties of the resulting polyimide coating. Furthermore, the twisted and non-coplanar structure weakens the stacking effect of the molecular chains, suppressing intermolecular forces and the CTC effect, significantly improving the light transmittance of the resulting polyimide coating.

[0051] And from the above, it can be seen that the cross-linking agent contains structures such as cyclotriphosphazene, benzodioxaphosphine, DOPO, nitrogen heterocycle, cycloalkane, etc., which improve the heat resistance of the base film, inhibit combustion, and reduce the risk of combustion; and during combustion, the thermal decomposition will absorb a large amount of heat, reduce the ambient temperature, slow down the heating rate of the base film, and inhibit the reaching of the ignition temperature; the decomposition products form a carbonized layer on the surface of the base film, which serves as a protective layer, and cooperates with the non-combustible gas produced by decomposition to block the transfer of heat and combustible gas, prevent the combustion reaction process, and inhibit the spread of flames, thereby giving the base film good heat resistance and flame retardancy.

[0052] Furthermore, the diamine monomer may be the following components, specifically prepared by the following process:

[0053] Mixing allyl DOPO and 4-maleimidobenzoic acid in o-dichlorobenzene, heating to 150-180°C, stirring and reacting for 8-10 hours; precipitating, and drying to obtain a maleimide derivative;

[0054] Mix a maleimide derivative and 4-maleimidobenzoic acid in diphenyl ether, heat to 170-200°C under nitrogen atmosphere, and react for 60-150 minutes; add methanol for precipitation, and vacuum dry to obtain a diacid compound;

[0055] Tin chloride, polyphosphoric acid and 1,2,4-triaminobenzene hydrochloric acid are mixed, heated to 80-85°C under nitrogen atmosphere, and stirred for 150-200 minutes; a diacid compound is added, the temperature is raised to 200-210°C, and the reaction is stirred for 12-14 hours; the temperature is lowered to 75-85°C, precipitated with ice water, filtered, deacidified with sodium carbonate solution, the pH of the system is neutralized to 8, filtered, purified by chromatography, and eluted with ethyl acetate / ethanol to obtain a diamine monomer.

[0056] Furthermore, the molar ratio of propenyl DOPO and 4-maleimidobenzoic acid is 1:1;

[0057] Iron phosphate is added as a catalyst in the reaction system, with the amount being 3% to 5% of the total mass of the reactants;

[0058] The ratio of propenyl DOPO to o-dichlorobenzene is (3-8) g / 100 mL.

[0059] Furthermore, the mass ratio of 4-maleimidobenzoic acid and maleimide derivative is (5.0-5.5):10;

[0060] Nickel perchlorate is added to the reaction system as a catalyst, with the amount being 3% to 6% of the total mass of the reactants;

[0061] The ratio of maleimide derivative to diphenyl ether is (3-5) g / 100 mL.

[0062] Furthermore, the mass of 1,2,4-triaminobenzene hydrochloric acid and diacid compound is (5.5-6.9):10;

[0063] The dosage of tin chloride is 2.1% to 2.6% of the mass of 1,2,4-triaminobenzene hydrochloric acid;

[0064] The amount of polyphosphoric acid used is 9.0 to 9.7 times the mass of 1,2,4-triaminobenzene hydrochloric acid.

[0065] In the above technical solution, the diamine monomer can be a diamine compound containing imidazole groups, which increases reactivity. After the diamine groups react with the dianhydride monomer, during the thermal imidization process, the imidazole groups react with the chlorine groups in the crosslinker, achieving post-crosslinking of the polyimide coating, thereby improving the mechanical properties and heat and flame retardancy of the base film. Continuing the crosslinker preparation principle, at high temperature, acryl-DOPO reacts with 4-maleimidobenzoic acid to form an alder-ene reaction, yielding a maleimide derivative with an alkenyl group. This alder-ene reaction is then added to 4-maleimidobenzoic acid to produce a difunctional carboxyl compound, designated as a diacid compound. The carboxyl groups of this diamine react with o-phenylenediamine in 1,2,4-triaminobenzene hydrochloric acid to form a benzimidazole structure, thereby yielding a diamine monomer containing imidazole groups. This increases the rigidity and thermal stability of the resulting polyimide (coating), enhancing its mechanical properties and high-temperature flame retardancy.

[0066] In the above technical solution, the diamine monomers and dianhydride monomers in the polyimide film structural raw materials are preferably phenyl ether-type, fluorine-containing, copolymerized with two (or more) dianhydride / amine monomers, or the aforementioned self-made diamine monomers. By introducing a flexible structure, a non-coplanar structure, a highly electronegative structure, and a second anhydride or amine, the regularity of the polyimide molecular structure is disrupted, intermolecular forces are reduced, and its solubility in organic solvents is improved. When a cross-linking solution is applied to the surface of the polyimide film, the organic solvent in the cross-linking solution can dissolve the surface of the polyimide film to a certain extent, resulting in interfacial fusion between the polyimide film and the cross-linking solution. (If the diamine monomer in the polyimide film contains self-made components, it can also react with the self-made cross-linking agent in the cross-linking solution.) After thermal imidization, the bonding strength between the lamellar structures is significantly improved, which helps to eliminate the material and performance differences between the coating and the film, and improve the overall performance of the resulting base film. DETAILED DESCRIPTION

[0067] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0068] In the following specific embodiments, the thickness of the polyimide film is 35 μm;

[0069] The crosslinking solution is coated on one side of the polyimide film to a thickness of 20 μm; during the preparation process, after adding the crosslinking agent, an organic solvent is added to adjust the solid content to be consistent with the solid content of the polyamic acid solution;

[0070] The filler selected was silica, JL-SiO2-W06, with an average particle size of 3 μm, sourced from Ningbo Jinlei Nanomaterial Technology Co., Ltd.

[0071] Example 1: A method for preparing a flame-retardant flexible circuit board, comprising the following steps:

[0072] Step 1, 1.1. Silica was dispersed in a 50% alcohol aqueous solution, ammonia was added to adjust the pH to 7.5, 4-perfluorotriethoxysilane was added, and the mixture was stirred for 6 h; filtered and dried to obtain a modified filler; the ratio of silica, 4-perfluorotriethoxysilane, and alcohol aqueous solution was 40 g / 0.4 g / 100 mL;

[0073] 1.2. Mix allyl DOPO, 4-maleimidobenzoic acid, and 3% ferric phosphate in o-dichlorobenzene, heat to 150°C, and stir for 10 h. Precipitate and dry to obtain a maleimide derivative. The molar ratio of allyl DOPO to 4-maleimidobenzoic acid is 1:1, and the ratio of allyl DOPO to o-dichlorobenzene is 8 g / 100 mL.

[0074] A maleimide derivative, 4-maleimidobenzoic acid, and 3% nickel perchlorate were mixed in diphenyl ether, heated to 170°C under nitrogen atmosphere, and reacted for 150 minutes. Methanol was added for precipitation, and the mixture was dried under vacuum to obtain a diacid compound. The mass ratio of 4-maleimidobenzoic acid to maleimide derivative was 5.5:10, and the ratio of maleimide derivative to diphenyl ether was 5 g / 100 mL.

[0075] 1,2,4-triaminobenzene hydrochloric acid, 2.1% tin chloride, and 9.0 times polyphosphoric acid were mixed, heated to 80°C under nitrogen atmosphere, and stirred for 200 minutes; a diacid compound was added, the temperature was raised to 200°C, and the reaction was stirred for 14 hours; the temperature was lowered to 75°C, precipitated with ice water, filtered, and deacidified with sodium carbonate solution. The pH of the system was neutralized to 8, filtered, and purified by chromatography, eluting with ethyl acetate / ethanol to obtain a diamine monomer; the mass ratio of 1,2,4-triaminobenzene hydrochloric acid to the diacid compound was 5.9:10;

[0076] 1.3. At 3°C, diamine monomer and dimethylformamide (an organic solvent) were mixed. Under nitrogen atmosphere, 93% by weight of dianhydride monomer was added and stirred for 90 minutes. The remaining dianhydride monomer was added and the stirring reaction was continued for 5 hours. 0.5 wt% of modified filler was added to obtain a polyamic acid solution with a solid content of 60%. The dianhydride monomer was a mixture of 2,3,3',4'-biphenyltetracarboxylic dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride in a molar ratio of 9:1. The mass ratio of dianhydride monomer to diamine monomer was 3.5:10. The solution was cast and dried to form a polyamic acid film. The film was then thermally imidized to obtain a polyimide film.

[0077] Step 2, 2.1. Mix N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine in dichloromethane, heat to 40°C, and reflux for 15 hours; then distill under reduced pressure and elute to obtain trimaleimidocyclotriphosphazene; the molar ratio of N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine is 3.3:1:3.3:3.3; and the ratio of N-(3,4-dihydroxyphenyl)maleimide to dichloromethane is 10 g / 100 mL;

[0078] Propylene DOPO, chloromaleimide, and 3% ferric phosphate were mixed in o-dichlorobenzene, heated to 150°C, and stirred for 10 hours; precipitated and dried to obtain a DOPO derivative; the molar ratio of propenyl DOPO to chloromaleimide was 1:1; the chloromaleimide was 1-(4-chlorophenyl)-pyrrole-2,5-dione; and the ratio of propenyl DOPO to o-dichlorobenzene was 8 g / 100 mL;

[0079] Trismaleimide cyclotriphosphazene, a DOPO derivative, and 3% nickel perchlorate were mixed in diphenyl ether and heated to 170°C under nitrogen atmosphere for 150 minutes. Methanol was added for precipitation and vacuum drying to obtain a crosslinker. The mass ratio of trimaleimide cyclotriphosphazene to DOPO derivative was 10:20.6, and the ratio of trimaleimide cyclotriphosphazene to diphenyl ether was 5 g / 100 mL.

[0080] 2.2. Adding a crosslinking agent to the polyamic acid solution in an amount of 23% by mass of the diamine monomer to form a crosslinking solution; coating the crosslinking solution on both surfaces of the polyimide film and drying to form a polyamic acid coating; thermal imidization to form a polyimide coating to obtain a base film;

[0081] The drying process is as follows: temperature 125°C, duration 10 minutes; the thermal imidization process is as follows: heating to 280°C, keeping warm for 40 minutes; starting at 30°C, heating by 50°C, keeping warm for 30 minutes, with a heating rate of 4°C / min; after thermal imidization, slowly cooling to room temperature;

[0082] Step 3: Set up the conductive circuit to obtain the circuit board.

[0083] Example 2: A method for preparing a flame-retardant flexible circuit board, comprising the following steps:

[0084] Step 1, 1.1. Silica was dispersed in a 70% alcohol aqueous solution, ammonia was added to adjust the pH to 8.2, M-aminophenyltrimethoxysilane was added, and the reaction was stirred for 7 hours; filtered and dried to obtain a modified filler; the ratio of silica, M-aminophenyltrimethoxysilane, and alcohol aqueous solution was 30g / 1.5g / 100mL;

[0085] 1.2. Combine allyl DOPO, 4-maleimidobenzoic acid, and 4% ferric phosphate in o-dichlorobenzene, heat to 165°C, and stir for 9 hours. Precipitate and dry to obtain a maleimide derivative. The molar ratio of allyl DOPO to 4-maleimidobenzoic acid is 1:1, and the ratio of allyl DOPO to o-dichlorobenzene is 5 g / 100 mL.

[0086] A maleimide derivative, 4-maleimidobenzoic acid, and 4.5% nickel perchlorate were mixed in diphenyl ether, heated to 185°C under nitrogen atmosphere, and reacted for 100 minutes. Methanol was added for precipitation, and the mixture was dried under vacuum to obtain a diacid compound. The mass ratio of 4-maleimidobenzoic acid to maleimide derivative was 5.2:10, and the ratio of maleimide derivative to diphenyl ether was 4 g / 100 mL.

[0087] 1,2,4-triaminobenzene hydrochloric acid, 2.3% tin chloride, and 9.3 times polyphosphoric acid were mixed, heated to 82°C under nitrogen atmosphere, and stirred for 180 minutes; the diacid compound was added, the temperature was raised to 205°C, and the reaction was stirred for 13 hours; the temperature was lowered to 80°C, precipitated with ice water, filtered, and deacidified with sodium carbonate solution. The pH of the system was neutralized to 8, filtered, and purified by chromatography, eluting with ethyl acetate / ethanol to obtain a diamine monomer; the mass ratio of 1,2,4-triaminobenzene hydrochloric acid to the diacid compound was 6.6:10;

[0088] 1.3. At 5°C, a diamine monomer and an organic solvent, dimethylformamide, were mixed. Under a nitrogen atmosphere, 95% by weight of a dianhydride monomer was added and stirred for 75 minutes. The remaining dianhydride monomer was added and the stirring reaction continued for 4.8 hours. 1.2% by weight of a modified filler was added to obtain a polyamic acid solution with a solid content of 65%. The dianhydride monomer was a mixture of biphenyl diether dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride in a molar ratio of 8:2. The mass ratio of the dianhydride monomer to the diamine monomer was 5.3:10. The solution was cast and dried to form a polyamic acid film. The film was then thermally imidized to obtain a polyimide film.

[0089] Step 2, 2.1. Mix N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine in dichloromethane, raise the temperature to 50°C, and reflux for 14.5 hours. Then, distill under reduced pressure and elute to obtain trimaleimidocyclotriphosphazene. The molar ratio of N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine is 3.5:1:3.5:3.5; and the ratio of N-(3,4-dihydroxyphenyl)maleimide to dichloromethane is 9 g / 100 mL.

[0090] Propylene DOPO, chloromaleimide, and 4% ferric phosphate were mixed in o-dichlorobenzene, heated to 165°C, and stirred for 9 hours; precipitated and dried to obtain a DOPO derivative; the molar ratio of propenyl DOPO to chloromaleimide was 1:1; the chloromaleimide was N-(2-chloropyridin-3-yl)maleamide; and the ratio of propenyl DOPO to o-dichlorobenzene was 5 g / 100 mL;

[0091] Trismaleimide cyclotriphosphazene, a DOPO derivative, and 4.5% nickel perchlorate were mixed in diphenyl ether and heated to 185°C under nitrogen atmosphere for 100 minutes. Methanol was added for precipitation and vacuum drying to obtain a crosslinker. The mass ratio of trimaleimide cyclotriphosphazene to DOPO derivative was 10:20.9, and the ratio of trimaleimide cyclotriphosphazene to diphenyl ether was 4 g / 100 mL.

[0092] 2.2. Adding a crosslinking agent to the polyamic acid solution in an amount of 35% by mass of the diamine monomer to form a crosslinking solution; coating the crosslinking solution on both surfaces of the polyimide film and drying to form a polyamic acid coating; thermal imidization to form a polyimide coating to obtain a base film;

[0093] The drying process is as follows: temperature 120°C, duration 11 minutes; the thermal imidization process is as follows: heating to 290°C, holding for 35 minutes; starting at 40°C, heating by 50°C, holding for 35 minutes, heating rate 4.5°C / min; after thermal imidization, slowly cooling to room temperature;

[0094] Step 3: Set up the conductive circuit to obtain the circuit board.

[0095] Example 3: A method for preparing a flame-retardant flexible circuit board, comprising the following steps:

[0096] Step 1, 1.1. Silica was dispersed in an alcohol aqueous solution, ammonia was added to adjust the pH to 9.0, 4-perfluorotriethoxysilane was added, and the mixture was stirred for 8 h; filtered and dried to obtain a modified filler; the ratio of silica, 4-perfluorotriethoxysilane, and alcohol aqueous solution was 20 g / 2 g / 100 mL;

[0097] 1.2. Combine allyl DOPO, 4-maleimidobenzoic acid, and 5% ferric phosphate in o-dichlorobenzene, heat to 180°C, and stir for 8 h. Precipitate and dry to obtain a maleimide derivative. The molar ratio of allyl DOPO to 4-maleimidobenzoic acid is 1:1, and the ratio of allyl DOPO to o-dichlorobenzene is 3 g / 100 mL.

[0098] A maleimide derivative, 4-maleimidobenzoic acid, and 6% nickel perchlorate were mixed in diphenyl ether, heated to 200°C under nitrogen atmosphere, and reacted for 60 minutes. Methanol was added for precipitation, and the mixture was dried under vacuum to obtain a diacid compound. The mass ratio of 4-maleimidobenzoic acid to maleimide derivative was 5.5:10, and the ratio of maleimide derivative to diphenyl ether was 3 g / 100 mL.

[0099] 1,2,4-triaminobenzene hydrochloric acid, 2.6% tin chloride, and 9.7 times polyphosphoric acid were mixed and heated to 85°C under nitrogen atmosphere and stirred for 150 minutes. The diacid compound was added and the temperature was raised to 210°C and stirred for 12 hours. The temperature was then lowered to 85°C and precipitated with ice water. The mixture was filtered and deacidified with sodium carbonate solution. The pH of the system was neutralized to 8, filtered, and purified by chromatography. The diamine monomer was eluted with ethyl acetate / ethanol. The mass ratio of 1,2,4-triaminobenzene hydrochloric acid to the diacid compound was 7.3:10.

[0100] 1.3. A diamine monomer and an organic solvent, dimethylformamide, were mixed at 7°C. Under a nitrogen atmosphere, 97% by weight of a dianhydride monomer was added and stirred for 60 minutes. The remaining dianhydride monomer was added and the stirring reaction continued for 4.5 hours. 2.0 wt% of a modified filler was added to obtain a polyamic acid solution with a solid content of 70%. The dianhydride monomer was a mixture of 2,3,3',4'-diphenylethertetracarboxylic dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride in a molar ratio of 7:3. The mass ratio of the dianhydride monomer to the diamine monomer was 3.7:10. The film was cast and dried to form a polyamic acid film. The film was then thermally imidized to obtain a polyimide film.

[0101] Step 2, 2.1. Mix N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine in dichloromethane, heat to 60°C, and reflux for 14 hours; then distill under reduced pressure and elute to obtain trimaleimidocyclotriphosphazene; the molar ratio of N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine is 3.6:1:3.6:3.6; the ratio of N-(3,4-dihydroxyphenyl)maleimide to dichloromethane is 8 g / 100 mL;

[0102] Propylene DOPO, chloromaleimide, and 5% ferric phosphate were mixed in o-dichlorobenzene, heated to 180°C, and stirred for 8 hours; precipitated and dried to obtain a DOPO derivative; the molar ratio of propenyl DOPO to chloromaleimide was 1:1; the chloromaleimide was N-(3-chloro-2-methylphenyl)maleimide; and the ratio of propenyl DOPO to o-dichlorobenzene was 3 g / 100 mL;

[0103] Trismaleimide cyclotriphosphazene, a DOPO derivative, and 6% nickel perchlorate were mixed in diphenyl ether and heated to 200°C under nitrogen atmosphere for 60 minutes. Methanol was added for precipitation and vacuum drying to obtain a crosslinker. The mass ratio of trimaleimide cyclotriphosphazene to DOPO derivative was 10:21.2, and the ratio of trimaleimide cyclotriphosphazene to diphenyl ether was 3 g / 100 mL.

[0104] 2.2. Adding a crosslinking agent to the polyamic acid solution in an amount of 46% by mass of the diamine monomer to form a crosslinking solution; coating the crosslinking solution on both surfaces of the polyimide film and drying to form a polyamic acid coating; thermal imidization to form a polyimide coating to obtain a base film;

[0105] The drying process is as follows: temperature 110°C, duration 12 minutes; the thermal imidization process is as follows: heating to 300°C, holding for 40 minutes; starting at 50°C, heating by 50°C for 40 minutes at a heating rate of 5°C / min; after thermal imidization, slowly cooling to room temperature;

[0106] Step 3: Set up the conductive circuit to obtain the circuit board.

[0107] Comparative Example 1: A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0108] Step 1: 1,2,4-triaminobenzene hydrochloric acid, 2.6% tin chloride, and 9.7 times polyphosphoric acid are mixed, heated to 85°C under nitrogen atmosphere, and stirred for 150 minutes; terephthalic acid is added, the temperature is raised to 210°C, and the reaction is stirred for 12 hours; the temperature is lowered to 85°C, precipitated with ice water, filtered, deacidified with sodium carbonate solution, the pH of the system is neutralized to 8, filtered, purified by chromatography, and eluted with ethyl acetate / ethanol to obtain a diamine monomer; the molar ratio of 1,2,4-triaminobenzene hydrochloric acid to terephthalic acid is 2.4:1;

[0109] At a temperature of 3°C, a diamine monomer and an organic solvent, dimethylformamide, were mixed. Under nitrogen atmosphere, 93% by weight of a dianhydride monomer was added, and the mixture was stirred and reacted for 90 minutes. The remaining dianhydride monomer was added, and the mixture was stirred and reacted for 5.0 hours. 0.5 wt% of silicon dioxide was added to obtain a polyamic acid solution with a solid content of 60%. The dianhydride monomer was a mixture of 2,3,3',4'-biphenyltetracarboxylic dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride in a molar ratio of 9:1. The mass ratio of the dianhydride monomer to the diamine monomer was 7.5:10. The mixture was cast into a film, dried, and formed into a polyamic acid film. Thermal imidization was performed to obtain a polyimide film.

[0110] Steps 2 and 3 are the same as those in Example 1 to obtain a circuit board.

[0111] Comparative Example 2: A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0112] Step 1: 1,2,4-triaminobenzene hydrochloric acid, 2.6% tin chloride, and 9.7 times polyphosphoric acid are mixed, heated to 85°C under nitrogen atmosphere, and stirred for 150 minutes; terephthalic acid is added, the temperature is raised to 210°C, and the reaction is stirred for 12 hours; the temperature is lowered to 85°C, precipitated with ice water, filtered, deacidified with sodium carbonate solution, the pH of the system is neutralized to 8, filtered, purified by chromatography, and eluted with ethyl acetate / ethanol to obtain a diamine monomer; the molar ratio of 1,2,4-triaminobenzene hydrochloric acid to terephthalic acid is 2.4:1;

[0113] At a temperature of 3°C, a diamine monomer and an organic solvent, dimethylformamide, were mixed. Under nitrogen atmosphere, 93% by weight of a dianhydride monomer was added, and the mixture was stirred and reacted for 90 minutes. The remaining dianhydride monomer was added, and the mixture was stirred and reacted for 5.0 hours. 0.5 wt% of silicon dioxide was added to obtain a polyamic acid solution with a solid content of 60%. The dianhydride monomer was a mixture of 2,3,3',4'-biphenyltetracarboxylic dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride in a molar ratio of 9:1. The mass ratio of the dianhydride monomer to the diamine monomer was 7.5:10. The mixture was cast into a film, dried, and formed into a polyamic acid film. Thermal imidization was performed to obtain a polyimide film.

[0114] Steps 2 and 3 are the same as those in Example 1, except that the cross-linking agent is 2,5-dichloro-p-xylene, and the amount used is 20% of the amount of the diamine monomer to obtain a circuit board.

[0115] Comparative Example 3: A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0116] Step 1: at a temperature of 3°C, a diamine monomer 4,4'-diaminodiphenyl ether and an organic solvent dimethylformamide are mixed, and under a nitrogen atmosphere, 93% by weight of a dianhydride monomer is added, and the reaction is stirred for 90 minutes; the remaining dianhydride monomer is added, and the reaction is continued with stirring for 5.0 hours, and 0.5wt% of silicon dioxide is added to obtain a polyamic acid solution with a solid content of 60%; the dianhydride monomer is a mixture of 2,3,3',4'-biphenyltetracarboxylic dianhydride and 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, and the molar ratio is 9:1; the molar ratio of the dianhydride monomer to the diamine monomer is 1:1.2; the film is cast and dried to form a polyamic acid film; and thermal imidization is performed to obtain a polyimide film;

[0117] Steps 2 and 3 are the same as in Example 1, except that the amount of the cross-linking agent is 20% of the amount of the diamine monomer to obtain a circuit board.

[0118] Comparative Example 4: A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0119] Steps 1, 2, and 3 were the same as those in Comparative Example 3, except that the dianhydride monomer was 2,3,3',4'-biphenyltetracarboxylic dianhydride; the molar ratio of the dianhydride monomer to the diamine monomer was 1.2:1; and the cross-linking agent was 1,3,5-tris(4-aminophenoxy)benzene, used in an amount that was 20% of the amount of the diamine monomer, to obtain a circuit board.

[0120] Comparative Example 5: A method for preparing a flame-retardant flexible circuit board, comprising the following processes:

[0121] Steps 1 and 3 are the same as those in Comparative Example 4; the dianhydride monomer is 4,4'-terephthalamide diphthalic anhydride, and the molar ratio of the dianhydride monomer to the diamine monomer is 1:1. In step 2, a polyamic acid solution is coated on both surfaces of the polyimide film and dried to form a polyamic acid coating film; thermal imidization is performed to form a polyimide coating to obtain a base film, thereby obtaining a circuit board.

[0122] Experiment: The base film plates obtained in Examples 1-3 and Comparative Examples 1-5 were used to prepare samples, and their properties were tested and the test results were recorded:

[0123] Mechanical properties test: Based on GB / T 1040 as the reference standard, a universal testing machine was used to test the tensile properties of the base film sample at a tensile rate of 50 mm / min.

[0124] Dielectric performance test: Use an impedance analyzer to measure the dielectric constant of the base film sample at a frequency of 10MHz;

[0125] High temperature flame retardant performance test: Use static mechanical analyzer to test the thermal expansion coefficient of the sample in a nitrogen atmosphere. The heat treatment temperature is 250℃ and the heat treatment time is 2h.

[0126] Based on GB / T 2409 as the reference standard, a vertical burning test was conducted on the base film sample using a vertical burning tester to determine its flame retardant grade (UL-94);

[0127] Light transmittance test: A UV-visible spectrometer was used to detect the light transmittance of the coating sample at 400 nm. The thickness of the polyimide coating was 20 μm.

[0128]

[0129] According to the data in the above table, we can clearly draw the following conclusions:

[0130] The basement membranes obtained in Examples 1-3 were compared with those obtained in Comparative Examples 1-5. The test results show that:

[0131] Compared to the comparative examples, the base films obtained in Examples 1-3 exhibited higher tensile strength, dielectric constant, flame retardancy, and light transmittance, and lower thermal expansion coefficients. This demonstrates that the present invention improves the mechanical properties, dielectric properties, high-temperature flame retardancy, and light transmittance of the base films.

[0132] Compared to Example 1, the diamine monomer used in the polyimide film and polyimide coating of Comparative Example 1 differs in preparation process. In Comparative Example 2, the diamine monomer used in both the polyimide film and polyimide coating is 4,4'-diaminodiphenyl ether. The diamine monomer used in Comparative Example 3 is the same as in Comparative Example 1, with 2,5-dichloro-p-xylene used as the crosslinker. Compared to Comparative Example 2, the crosslinker used in Comparative Example 4 is 1,3,5-tris(4-aminophenoxy)benzene; no crosslinker is used in Comparative Example 5. The base films obtained in Comparative Examples 1-5 exhibited significant decreases in flame retardancy, with deterioration in tensile strength, dielectric constant, thermal expansion coefficient, and light transmittance. This indicates that the present invention's design of the base film preparation process and its components can promote comprehensive improvements in mechanical properties, dielectric properties, high-temperature flame retardancy, and light transmittance.

[0133] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A method for preparing a flame-retardant flexible circuit board, characterized in that: Including the following processes: Step 1: Mixing a diamine monomer and a dianhydride monomer in an organic solvent, stirring to react, and adding a filler to obtain a polyamic acid solution; Casting film and drying to form a polyamic acid film; Thermal imidization to obtain a polyimide film; Step 2: adding a crosslinking agent to the polyamic acid solution to form a crosslinking solution; A cross-linking solution is applied to both surfaces of the polyimide film and dried to form a polyamic acid coating film; Thermal imidization to form a polyimide coating to obtain a base film; Step 3: Set up a conductive circuit to obtain a circuit board; The cross-linking agent is prepared by the following process: N-(3,4-dihydroxyphenyl)maleimide, hexachlorocyclotriphosphazene, carbon tetrachloride, and triethylamine are mixed in dichloromethane, heated to 40-60°C, and refluxed for 14-15 hours to obtain trimaleimide cyclotriphosphazene; Mixing allyl DOPO and chlorine-containing maleimide in o-dichlorobenzene, heating to 150-180°C, and stirring for 8-10 hours to obtain a DOPO derivative; The trimaleimido cyclotriphosphazene and DOPO derivative are mixed in diphenyl ether, heated to 170-200° C. under nitrogen atmosphere, and reacted for 60-150 minutes to obtain a crosslinking agent; The diamine monomer is prepared by the following process: Mixing allyl DOPO and 4-maleimidobenzoic acid in o-dichlorobenzene, heating to 150-180°C, and stirring for 8-10 hours to obtain a maleimide derivative; Mixing a maleimide derivative and 4-maleimidobenzoic acid in diphenyl ether, heating to 170-200°C under a nitrogen atmosphere, and reacting for 60-150 minutes to obtain a diacid compound; Mix tin chloride, polyphosphoric acid and 1,2,4-triaminobenzene hydrochloric acid, heat to 80-85°C under nitrogen atmosphere, and stir for 150-200 minutes; add diacid compound, raise the temperature to 200-210°C, and stir and react for 12-14 hours to obtain diamine monomer.

2. The method for preparing a flame-retardant flexible circuit board according to claim 1, wherein: The dianhydride monomer is a mixture of one or more of 2,3,3',4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, 3,3',4,4'-triphenyl ether tetracarboxylic dianhydride, biphenyl diether dianhydride, 2,2'-difluoromethyl-4,4',5,5'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride, 4,4'-terephthaloyl diphthalic anhydride, bisphenol A diether dianhydride, and pyromellitic dianhydride.

3. The method for preparing a flame-retardant flexible circuit board according to claim 2, wherein: The molar ratio of the dianhydride monomer to the diamine monomer is (1.0-1.5):1; The amount of the cross-linking agent is 10% to 50% of the amount of the diamine monomer.

4. The method for preparing a flame-retardant flexible circuit board according to claim 1, wherein: The process conditions of the thermal imidization are: heating to 280-300° C. and keeping the temperature for 30-40 minutes.

5. The method for preparing a flame-retardant flexible circuit board according to claim 1, wherein: The amount of the filler is 0.5-2.0 wt%; the filler is silicon dioxide, the silicon dioxide is coupled modified, and the coupling modifier is selected from 4-perfluorotriethoxysilane and M-aminophenyltrimethoxysilane.

6. A flame retardant flexible circuit board prepared according to the preparation method according to any one of claims 1 to 5.

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