A method for manufacturing a flexible circuit board

Abstract

The application discloses a preparation method of a flexible circuit board and relates to the field of circuit board production and manufacturing. The flexible circuit board is used in some parts that need to be bent or folded in electronic equipment, and flexibility is a main index of the flexible circuit board. In a high-temperature, low-temperature or high-humidity scene, the performance of the flexible circuit board is influenced, brittle breaking or short circuit may occur, and therefore the application provides a flexible circuit board prepared by using a modified copper foil with enhanced adhesion and oxidation resistance, a substrate with high heat dissipation performance and a specially-made modified solder resist, and the flexible circuit board has the characteristics of strong bonding force, good heat dissipation performance and good flexibility.

Description

Technical Field

[0001] The present invention relates to the field of circuit board manufacturing, and in particular to a method for preparing a flexible circuit board. Background Art

[0002] In the current era of rapid development of electronic information, research on circuit board production and manufacturing technology is indispensable. Circuit boards may encounter harsh environments in certain application scenarios, such as high temperature, high humidity, frequent vibration, etc. Therefore, special materials and processes are needed to improve their environmental resistance. Currently, the industry lacks improvements in circuit boards' resistance to high temperature, low temperature, and moisture.

[0003] Flexible printed circuit boards (FPCs) are printed circuits made from a flexible insulating substrate. They offer the following advantages: they can be bent, wound, and folded freely, arranged to fit any spatial layout, and moved and expanded in three dimensions, integrating component assembly and wire connections. This significantly reduces the size and weight of electronic products, meeting the needs of the trend toward high-density, miniaturization, and high reliability. However, they also have disadvantages such as poor aging resistance, easy damage from improper handling, weak copper foil bonding, and poor heat dissipation. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a method for preparing a flexible circuit board with strong bonding force, good heat dissipation performance and good flexibility.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A method for preparing a flexible circuit board comprises pressing a copper foil and a flexible substrate into shape using a hot press, and then performing photolithography, cleaning, drilling, and welding.

[0007] The preparation method of the flexible substrate is as follows:

[0008] 1) Bake the boron nitride fiber in an oven at 90°C for 5 hours, cool it, immerse it in the modified solution, stir it at 30 rpm for 20-30 minutes, filter out the boron nitride fiber, and bake it in an oven at 90°C for 4 hours to obtain the modified boron nitride fiber;

[0009] 2) adding 5-7 parts of modified boron nitride fiber to 40-50 parts of polyamic acid solution, stirring and dispersing at a rate of 30 r / min for 1-1.5 hours to obtain a modified polyamic acid solution;

[0010] 3) The modified polyamic acid solution is evenly poured onto a flat glass plate, baked in an oven at 85°C for 4-5 hours, then baked at 120, 180, 260, and 350°C for 1 hour each, cooled to room temperature, taken out, and then placed in deionized water for natural peeling. After drying, the flexible substrate is obtained.

[0011] The modified solution is prepared by adding 2 parts of 2-benzimidazolidinone and 1 part of 2-aminotoluene-4-sulfonic acid into 2 parts of distilled water at 60° C. and stirring uniformly.

[0012] Among them, copper foil includes the following processing:

[0013] i. Prepare copper etching solution:

[0014] 1-3 parts of concentrated sulfuric acid solution are slowly added dropwise to 60 parts of distilled water while stirring, and then 1-2 parts of hydrogen peroxide and 0.2-0.4 parts of 1-phenyl-5-mercaptotetrazole are added, and the mixture is stirred evenly to obtain a first-level copper etching solution; 1-3 parts of concentrated sulfuric acid solution are slowly added dropwise to 60 parts of distilled water while stirring, and then 0.5-1 parts of hydrogen peroxide, 0.4-0.6 parts of 1-phenyl-5-mercaptotetrazole and 0.5-1.5 parts of N-methyl-p-toluidine are added, and the mixture is stirred evenly to obtain a second-level copper etching solution;

[0015] ii. Etching copper foil in etching solution:

[0016] Immerse the copper foil in the first-level copper etching solution, control the system temperature to 50-60℃, pour out the first-level copper etching solution after soaking for 2-3 hours, then add the second-level copper etching solution to completely immerse the copper foil, and continue soaking for 3 hours;

[0017] iii. Cleaning and drying:

[0018] After the reaction is completed, pour out the secondary copper etching solution, then rinse the copper foil with distilled water 2-3 times, and finally place it in a dryer to dry to obtain the modified copper foil.

[0019] Among them, the circuit board after welding also includes a solder resist coating, and the specific method is as follows:

[0020] I. Spraying:

[0021] Place the circuit board in a surface dust remover for 5 minutes, then place it in an ultrasonic spraying device, select a spray gun with a diameter of 1.5-2.0mm, adjust the spray gun pressure to 0.3-0.5MPa, and the conveyor speed to 4.5m / min. Add solder resist and spray evenly 1-2 times.

[0022] II. Pre-drying:

[0023] Pre-dry the circuit board after spraying the solder resist at 70-80°C for 10-20 minutes;

[0024] III. Light curing:

[0025] The pre-dried circuit board is placed under a UV lamp for light curing for 30-40 minutes to complete the coating.

[0026] The solder resist preparation method is as follows:

[0027] a. Add 5-7 parts of pure acrylic resin and 10-14 parts of epoxy resin to 40-50 parts of solvent and heat to 80-90°C, stirring and mixing at 30r / min speed for 0.8-1h;

[0028] b Then add 1-3 parts of highly transparent thickener, 0.1-0.3 parts of silicone defoamer and 0.8-1.5 parts of transparent phthalocyanine green pigment to the above system, and continue stirring and mixing at a speed of 30r / min for 2-3h;

[0029] c. Then cool the system to 50-60°C, add 0.5-1 parts of photoinitiator, 0.5-0.8 parts of 2,3-dibromo-1,4-butenediol, and 0.2-0.5 parts of dibutyl glutarate, stir evenly, and continue to react at a constant temperature for 4-6 hours. After cooling, the modified solder resist is obtained;

[0030] The solvent is a mixture of dibasic acid ester and ethyl acetate in a ratio of 1:1; and the photoinitiator is diacylphosphine oxide 819.

[0031] The characteristics of the present invention are:

[0032] 1) Enhance the bonding strength and oxidation resistance of copper foil. Generally, chemical corrosion is used to modify the surface of copper foil, but conventional chemical corrosion is difficult to control the degree of corrosion and is difficult to meet the requirements. The composite copper etching solution of the present invention, under the acidic medium provided by dilute sulfuric acid, hydrogen peroxide oxidizes copper to cupric oxide, and then 1-phenyl-5-mercaptotetrazole forms a protective film with metal ions or oxides on the copper surface, thereby preventing the copper surface from being further oxidized and corroded. However, using 1-phenyl-5-mercaptotetrazole solution alone produces an uneven coating. Therefore, the present invention composites N-methyl-p-toluidine to make the protective film formed dense and smooth. This protective film has good stability and durability, can prevent excessive corrosion of the copper etching solution and protect the copper material surface from being oxidized over a long period of time.

[0033] 2) The flexible substrate prepared from the polyamic acid solution has good flexibility, but its heat dissipation performance is relatively low. The heat dissipation performance can be enhanced by adding boron nitride fiber as a filler. However, direct addition of boron nitride fiber is prone to agglomeration and reduces the flexibility of the flexible substrate. 2-Benzimidazolinone can improve the mechanical strength of boron nitride fiber and improve agglomeration, but it will affect its dispersion in the resin. If a conventional dispersant is added, it will affect the thermal stability of the boron nitride fiber and the adsorption effect of 2-Benzimidazolinone. Therefore, the present invention compounds 2-aminotoluene. -4-sulfonic acid can not only make the boron nitride fiber better dispersed in the polyamic acid solution without affecting the effect of 2-benzimidazolidinone, but also reduce the surface tension of 2-benzimidazolidinone, so that 2-benzimidazolidinone can be better adsorbed on the surface of the boron nitride fiber. In addition, 2-aminotoluene-4-sulfonic acid can also enhance the chemical stability and antioxidant properties of the boron nitride fiber. The boron nitride fiber modified by the combination of the two has better interfacial compatibility and toughness. When dispersed in the polyamic acid solution, it can improve its heat dissipation performance while having better flexibility.

[0034] 3) The circuit board prepared by the present invention uses a special process to improve the adhesion of the copper foil and the thermal conductivity of the substrate. It was found that the solder mask on the surface of the circuit board is prone to bubbling when the circuit board temperature rises. This may be because conventional solder mask is unevenly distributed and has low heat resistance, resulting in trace amounts of air under the film causing bubbling after heating. Therefore, the present invention prepares a solder mask modified with 2,3-dibromo-1,4-butenediol and dibutyl glutarate. While ensuring insulation, it does not cause bubbling when the temperature rises. In addition, 2,3-dibromo-1,4-butenediol has a dibromoethylene group, so it can undergo a photochemical cross-linking reaction with double bonds or other functional groups in the resin, thereby forming a three-dimensional network structure of the resin, improving the flexibility and heat resistance of the solder mask, while dibutyl glutarate enhances the permeability of the resin, allowing it to be evenly distributed on the circuit board surface. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below with reference to the embodiments.

[0036] Unless otherwise specified, the raw materials and chemical reagents used in the examples of the present invention were obtained through conventional commercial channels.

[0037] Example 1

[0038] This embodiment includes the following steps:

[0039] A method for preparing a flexible circuit board comprises the following steps:

[0040] 1) Preparation of modified copper foil:

[0041] 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 150 ml of hydrogen peroxide and 30 g of 1-phenyl-5-mercaptotetrazole, and the mixture was stirred evenly to obtain a first-level copper etching solution; 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 75 ml of hydrogen peroxide, 50 g of 1-phenyl-5-mercaptotetrazole, and 100 g of N-methyl-p-toluidine, and the mixture was stirred evenly to obtain a second-level copper etching solution; a copper foil was immersed in the first-level copper etching solution, the system temperature was controlled to be 55° C., and after soaking for 2.5 hours, the first-level copper etching solution was poured out, and then the second-level copper etching solution was added to completely immerse the copper foil, and the soaking was continued for 3 hours; after the reaction was completed, the second-level copper etching solution was poured out, and the copper foil was rinsed with distilled water three times, and finally dried in a desiccator to obtain a modified copper foil;

[0042] 2) Preparation of flexible substrate:

[0043] 200 g of 2-benzimidazolidinone and 100 g of 2-aminotoluene-4-sulfonic acid were added to 2 L of 60 ° C distilled water and stirred evenly to prepare a modified solution, then the boron nitride fiber was placed in a 90 ° C oven and baked for 5 hours, cooled and immersed in the modified solution, stirred at a rate of 30 r / min for 25 minutes, and then the boron nitride fiber was filtered out and placed in a 90 ° C oven and baked for 4 hours to obtain a modified boron nitride fiber; 600 g of modified boron nitride fiber was added to 45 L of polyamic acid solution, and stirred and dispersed at a rate of 30 r / min for 1.3 hours to obtain a modified polyamic acid solution; the modified polyamic acid solution was evenly poured onto a flat glass, baked in an oven at 85 ° C for 4.5 hours, and then baked at 120, 180, 260, and 350 ° C for 1 hour each, cooled to room temperature, taken out, and then placed in deionized water for natural peeling, and dried to obtain a flexible substrate;

[0044] 3) Preparation of circuit boards:

[0045] The copper foil and substrate are pressed into shape by a hot press, and then photoetched, cleaned, drilled and welded to obtain a circuit board;

[0046] 4) Preparation and coating of solder resist:

[0047] 6L pure acrylic resin and 12L epoxy resin were added to 45L dibasic acid ester and ethyl acetate 1:1 mixed solvent and heated to 85°C, stirred and mixed at 30r / min for 0.9h; then 20ml high transparent thickener, 20ml silicone defoamer and 12g transparent phthalocyanine green pigment were added, and the mixture was stirred and mixed at 30r / min for 2.5h; then the temperature was lowered to 55°C, and 80g diacylphosphine oxide 819, 70g 2,3-dibromo-1,4-butenediol and 35g dipentanediol were added. Butyl ester, stir evenly and continue to react at a constant temperature for 5 hours, and then cool to obtain a modified solder resist; place the circuit board in a surface dust remover to remove dust for 5 minutes, and then place it in an ultrasonic spraying device, select a spray gun with a diameter of 2.0mm, adjust the spray gun pressure to 0.4MPa, and the conveyor belt speed to 4.5m / min, and add solder resist, and spray it evenly twice. The circuit board after the solder resist spraying is pre-dried at 85℃ for 15 minutes; the pre-dried circuit board is placed under a UV lamp for light curing for 35 minutes to obtain a flexible circuit board.

[0048] Example 2

[0049] This embodiment includes the following steps:

[0050] A method for preparing a flexible circuit board comprises the following steps:

[0051] 1) Preparation of modified copper foil:

[0052] 100 ml of concentrated sulfuric acid solution is slowly added dropwise to 6 L of distilled water while stirring, and then 100 ml of hydrogen peroxide and 20 g of 1-phenyl-5-mercaptotetrazole are added, and the mixture is stirred evenly to obtain a first-level copper etching solution; 100 ml of concentrated sulfuric acid solution is slowly added dropwise to 6 L of distilled water while stirring, and then 50 ml of hydrogen peroxide, 40 g of 1-phenyl-5-mercaptotetrazole and 50 g of N-methyl-p-toluidine are added, and the mixture is stirred evenly to obtain a second-level copper etching solution; a copper foil is immersed in the first-level copper etching solution, the system temperature is controlled to 50° C., and after soaking for 2 hours, the first-level copper etching solution is poured out, and then the second-level copper etching solution is added to completely immerse the copper foil, and the soaking is continued for 3 hours; after the reaction is completed, the second-level copper etching solution is poured out, and the copper foil is rinsed with distilled water twice, and finally dried in a desiccator to obtain a modified copper foil;

[0053] 2) Preparation of flexible substrate:

[0054] 200 g of 2-benzimidazolidinone and 100 g of 2-aminotoluene-4-sulfonic acid were added to 2 L of 60 ° C distilled water and stirred evenly to prepare a modified solution, then the boron nitride fiber was placed in a 90 ° C oven and baked for 5 hours, cooled and immersed in the modified solution, stirred at a rate of 30 r / min for 25 minutes, and then the boron nitride fiber was filtered out and placed in a 90 ° C oven and baked for 4 hours to obtain a modified boron nitride fiber; 600 g of modified boron nitride fiber was added to 45 L of polyamic acid solution, and stirred and dispersed at a rate of 30 r / min for 1.3 hours to obtain a modified polyamic acid solution; the modified polyamic acid solution was evenly poured onto a flat glass, baked in an oven at 85 ° C for 4.5 hours, and then baked at 120, 180, 260, and 350 ° C for 1 hour each, cooled to room temperature, taken out, and then placed in deionized water for natural peeling, and dried to obtain a flexible substrate;

[0055] 3) Preparation of circuit boards:

[0056] The copper foil and substrate are pressed into shape by a hot press, and then photoetched, cleaned, drilled and welded to obtain a circuit board;

[0057] 4) Preparation and coating of solder resist:

[0058] 5L pure acrylic resin and 10L epoxy resin were added to 40L dibasic acid ester and ethyl acetate 1:1 mixed solvent and heated to 80℃, stirred and mixed at 30r / min for 0.8h; then 10ml high transparent thickener, 10ml silicone defoamer and 8g transparent phthalocyanine green pigment were added, and the mixture was stirred and mixed at 30r / min for 2h; then the temperature was lowered to 50℃, 50g diacylphosphine oxide 819, 50g 2,3-dibromo-1,4-butenediol and 20g dibutyl glutarate were added. Ester, stir evenly and continue to react at a constant temperature for 4 hours, and then cool to obtain a modified solder resist; place the circuit board in a surface dust remover to remove dust for 5 minutes, and then place it in an ultrasonic spraying device, select a spray gun with a diameter of 2.0mm, adjust the spray gun pressure to 0.3MPa, and the conveyor belt speed to 4.5m / min, and add solder resist, and spray it evenly once; pre-dry the circuit board after spraying the solder resist at 85℃ for 15min; place the pre-dried circuit board under a UV lamp for light curing for 35min to obtain a flexible circuit board.

[0059] Example 3

[0060] This embodiment includes the following steps:

[0061] A method for preparing a flexible circuit board comprises the following steps:

[0062] 1) Preparation of modified copper foil:

[0063] 300 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 200 ml of hydrogen peroxide and 40 g of 1-phenyl-5-mercaptotetrazole, and the mixture was stirred evenly to obtain a first-level copper etching solution; 300 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 100 ml of hydrogen peroxide, 60 g of 1-phenyl-5-mercaptotetrazole, and 150 g of N-methyl-p-toluidine, and the mixture was stirred evenly to obtain a second-level copper etching solution; a copper foil was immersed in the first-level copper etching solution, the system temperature was controlled to be 60° C., and after immersing for 3 hours, the first-level copper etching solution was poured out, and the second-level copper etching solution was then added to completely immerse the copper foil, and the immersion was continued for 3 hours; after the reaction was completed, the second-level copper etching solution was poured out, and the copper foil was rinsed with distilled water 3 times, and finally dried in a desiccator to obtain a modified copper foil;

[0064] 2) Preparation of flexible substrate:

[0065] 200 g of 2-benzimidazolidinone and 100 g of 2-aminotoluene-4-sulfonic acid were added to 2 L of 60 ° C distilled water and stirred evenly to prepare a modified solution, then the boron nitride fiber was placed in a 90 ° C oven and baked for 5 hours, cooled and immersed in the modified solution, stirred at a rate of 30 r / min for 25 minutes, and then the boron nitride fiber was filtered out and placed in a 90 ° C oven and baked for 4 hours to obtain a modified boron nitride fiber; 600 g of modified boron nitride fiber was added to 45 L of polyamic acid solution, and stirred and dispersed at a rate of 30 r / min for 1.3 hours to obtain a modified polyamic acid solution; the modified polyamic acid solution was evenly poured onto a flat glass, baked in an oven at 85 ° C for 4.5 hours, and then baked at 120, 180, 260, and 350 ° C for 1 hour each, cooled to room temperature, taken out, and then placed in deionized water for natural peeling, and dried to obtain a flexible substrate;

[0066] 3) Preparation of circuit boards:

[0067] The copper foil and substrate are pressed into shape by a hot press, and then photoetched, cleaned, drilled and welded to obtain a circuit board;

[0068] 4) Preparation and coating of solder resist:

[0069] 7L pure acrylic resin and 14L epoxy resin were added to 50L of a solvent prepared by mixing dibasic acid ester and ethyl acetate in a ratio of 1:1 and heated to 90°C. The mixture was stirred at 30r / min for 1h. Then 30ml of a highly transparent thickener, 30ml of an organosilicon defoamer and 15g of a transparent phthalocyanine green pigment were added. The mixture was stirred at 30r / min for 3h. The mixture was then cooled to 60°C and 100g of diacylphosphine oxide 819, 80g of 2,3-dibromo-1,4-butenediol and 50g of dibutyl glutarate were added. Ester, stir evenly and continue to react at a constant temperature for 6 hours, and then cool to obtain a modified solder resist; place the circuit board in a surface dust remover to remove dust for 5 minutes, and then place it in an ultrasonic spraying device, select a spray gun with a diameter of 1.5mm, adjust the spray gun pressure to 0.5MPa, and the conveyor belt speed to 4.5m / min, and add solder resist, and spray it evenly twice. The circuit board after the solder resist spraying is pre-dried at 85℃ for 15 minutes; the pre-dried circuit board is placed under a UV lamp for light curing for 35 minutes to obtain a flexible circuit board.

[0070] Comparative Example 1

[0071] The difference between this comparative example and Example 1 is that the copper foil is not modified.

[0072] Comparative Example 2

[0073] The difference between this comparative example and Example 1 is:

[0074] Step 1) Preparation of modified copper foil:

[0075] 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, and then 150 ml of hydrogen peroxide was added and stirred to obtain a first-level copper etching solution; 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, and then 75 ml of hydrogen peroxide and 150 g of N-methyl-p-toluidine were added and stirred to obtain a second-level copper etching solution; a copper foil was immersed in the first-level copper etching solution, the system temperature was controlled to be 55° C., and after soaking for 2.5 hours, the first-level copper etching solution was poured out, and then the second-level copper etching solution was added to completely immerse the copper foil, and the soaking was continued for 3 hours; after the reaction was completed, the second-level copper etching solution was poured out, and the copper foil was rinsed with distilled water 3 times, and finally dried in a desiccator to obtain a modified copper foil;

[0076] The rest is the same as in Example 1.

[0077] Comparative Example 3

[0078] The difference between this comparative example and Example 1 is:

[0079] Step 1) Preparation of modified copper foil:

[0080] 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, and then 150 ml of hydrogen peroxide and 30 g of 1-phenyl-5-mercaptotetrazole were added, and the mixture was stirred evenly to obtain a first-level copper etching solution; 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, and then 75 ml of hydrogen peroxide and 150 g of 1-phenyl-5-mercaptotetrazole were added, and the mixture was stirred evenly to obtain a second-level copper etching solution; a copper foil was immersed in the first-level copper etching solution, the system temperature was controlled to be 55° C., and after soaking for 2.5 hours, the first-level copper etching solution was poured out, and then the second-level copper etching solution was added to completely immerse the copper foil, and the soaking was continued for 3 hours; after the reaction was completed, the second-level copper etching solution was poured out, and the copper foil was rinsed with distilled water three times, and finally dried in a desiccator to obtain a modified copper foil;

[0081] The rest is the same as in Example 1.

[0082] Comparative Example 4

[0083] The difference between this comparative example and Example 1 is:

[0084] Step 2) Preparation of flexible substrate:

[0085] 600 g of boron nitride fiber was added to 45 L of polyamic acid solution, and the mixture was stirred and dispersed at a rate of 30 r / min for 1.3 h to obtain a modified polyamic acid solution; the modified polyamic acid solution was evenly poured onto a flat glass plate, baked in an oven at 85° C. for 4.5 h, and then baked at 120° C., 180° C., 260° C., and 350° C. for 1 h each, cooled to room temperature, removed from the plate, and then placed in deionized water for natural peeling. After drying, a flexible substrate was obtained;

[0086] The rest is the same as in Example 1.

[0087] Comparative Example 5

[0088] The difference between this comparative example and Example 1 is:

[0089] Step 2) Preparation of flexible substrate:

[0090] 300 g of 2-aminotoluene-4-sulfonic acid was added to 2 L of 60° C. distilled water and stirred evenly to obtain a modified solution. The boron nitride fiber was then placed in an oven at 90° C. and baked for 5 h. After cooling, the fiber was immersed in the modified solution and stirred at a rate of 30 r / min for 25 min. The boron nitride fiber was then filtered out and placed in an oven at 90° C. and baked for 4 h to obtain a modified boron nitride fiber. 600 g of the modified boron nitride fiber was added to 45 L of polyamic acid solution, and the modified polyamic acid solution was obtained by stirring and dispersing the fiber at a rate of 30 r / min for 1.3 h. The modified polyamic acid solution was evenly poured onto a flat glass plate, baked in an oven at 85° C. for 4.5 h, and then baked at 120, 180, 260, and 350° C. for 1 h each. The fiber was taken out after cooling to room temperature, and then naturally peeled in deionized water. After drying, a flexible substrate was obtained.

[0091] The rest is the same as in Example 1.

[0092] Comparative Example 6

[0093] The difference between this comparative example and Example 1 is:

[0094] Step 2) Preparation of flexible substrate:

[0095] 300 g of 2-benzimidazolidinone was added to 2 L of 60° C. distilled water and stirred evenly to prepare a modified solution, then the boron nitride fiber was placed in a 90° C. oven and baked for 5 h. After cooling, it was immersed in the modified solution and stirred at a rate of 30 r / min for 25 min. Then, the boron nitride fiber was filtered out and placed in a 90° C. oven and baked for 4 h to obtain a modified boron nitride fiber; 600 g of the modified boron nitride fiber was added to 45 L of polyamic acid solution, and the modified polyamic acid solution was obtained by stirring and dispersing at a rate of 30 r / min for 1.3 h. The modified polyamic acid solution was evenly poured onto a flat glass, baked in an oven at 85° C. for 4.5 h, and then baked at 120, 180, 260, and 350° C. for 1 h each. After cooling to room temperature, it was taken out and then placed in deionized water for natural peeling. After drying, a flexible substrate was obtained.

[0096] The rest is the same as in Example 1.

[0097] Comparative Example 7

[0098] The difference between this comparative example and Example 1 is:

[0099] Step 4) Prepare and coat solder resist:

[0100] 6L pure acrylic resin and 12L epoxy resin were added to 45L dibasic acid ester and ethyl acetate 1:1 compound solvent and heated to 85°C, stirred and mixed at 30r / min for 0.9h; then 20ml high transparent thickener, 20ml silicone defoamer and 12g transparent phthalocyanine green pigment were added, and the mixture was stirred and mixed at 30r / min for 2.5h; then the temperature was lowered to 55°C, 80g diacylphosphine oxide 819 and 105g dibutyl glutarate were added, stirred and mixed evenly, and then The reaction was continued at a constant temperature for 5 hours, and the modified solder resist was obtained after cooling; the circuit board was placed in a surface dust remover for 5 minutes, and then placed in an ultrasonic spraying device, a spray gun with a diameter of 2.0 mm was selected, the spray gun pressure was adjusted to 0.4 MPa, the conveyor speed was 4.5 m / min, and the solder resist was added and evenly sprayed twice; the circuit board after the solder resist spraying was pre-dried at 85° C. for 15 minutes; the pre-dried circuit board was placed under a UV lamp for light curing for 35 minutes to obtain a flexible circuit board;

[0101] The rest is the same as in Example 1.

[0102] Comparative Example 8

[0103] The difference between this comparative example and Example 1 is:

[0104] Step 4) Prepare and coat solder resist:

[0105] 6L pure acrylic resin and 12L epoxy resin were added to 45L dibasic acid ester and ethyl acetate 1:1 compound solvent and heated to 85°C, stirred and mixed at 30r / min for 0.9h; then 20ml high transparent thickener, 20ml silicone defoamer and 12g transparent phthalocyanine green pigment were added, and the mixture was stirred and mixed at 30r / min for 2.5h; then the temperature was lowered to 55°C, 80g diacylphosphine oxide 819 and 105g 2,3-dibromo-1,4-butenediol were added, and the mixture was stirred and mixed at 30r / min for 2.5h; After uniformity, the reaction was continued at a constant temperature for 5 hours, and the modified solder resist was obtained after cooling; the circuit board was placed in a surface dust remover for 5 minutes, and then placed in an ultrasonic spraying device, a spray gun with a diameter of 2.0 mm was selected, the spray gun pressure was adjusted to 0.4 MPa, the conveyor speed was 4.5 m / min, and the solder resist was added and evenly sprayed twice; the circuit board after the solder resist spraying was pre-dried at 85° C. for 15 minutes; the pre-dried circuit board was placed under a UV lamp for light curing for 35 minutes to obtain a flexible circuit board;

[0106] The rest is the same as in Example 1.

[0107] Comparative Example 9

[0108] The difference between this comparative example and Example 1 is that in step 4), the solder resist covering the circuit board is replaced with SM-120 solder resist.

[0109] experiment:

[0110] Multiple tests were performed on the above groups of circuit boards, and the test results are shown in Table 1;

[0111] 1) Test of the bonding strength between the copper foil and the substrate: Standard parts prepared according to the preparation processes of Examples 1 to 3 and Comparative Examples 1 to 3 and the GB / T13557 standard were placed on a peel tester to test their peel strength;

[0112] 2) Copper foil oxidation resistance test: The copper foil materials used in Examples 1 to 3 and Comparative Examples 1 to 3 were cut into 10*10 mm samples, baked in an oven at 250°C for 15 minutes, and then taken out and cooled. If there was any discoloration or brittleness, it was considered unqualified;

[0113] 3) Determination of thermal conductivity of substrate: The substrate materials used in Examples 1 to 3 and Comparative Examples 4 to 6 were placed on a SZ-DRE-2C thermal conductivity tester to measure their thermal conductivity;

[0114] 4) Bending resistance test: According to the preparation process of Examples 1 to 3 and Comparative Examples 4 to 6, the bending resistance test method of SJ 20604-1996 was used for testing. After the specified number of bending cycles, if performance degradation or unacceptable delamination occurred, it was considered unqualified.

[0115] 5) Solder mask heat resistance test: The circuit boards prepared in Examples 1 to 3 and Comparative Examples 7 to 9 were placed in an oven at 105° C. for 15 minutes and then taken out to observe whether the solder mask had bubbling. If no bubbling occurred, the circuit boards were qualified.

[0116] Table 1

[0117]

[0118]

Claims

1. A method for preparing a flexible circuit board, comprising: forming a copper foil and a flexible substrate by hot pressing, followed by photolithography, cleaning, drilling, and welding, wherein: The preparation method of the flexible substrate is as follows: 1) Bake the boron nitride fiber in a 90°C oven for 5 hours, cool it, immerse it in the modified solution, stir it at 30 r / min for 20-30 minutes, then filter out the boron nitride fiber and bake it in a 90°C oven for 4 hours to obtain the modified boron nitride fiber; 2) Add 600 g of modified boron nitride fiber to 45 L of polyamic acid solution, and stir and disperse at a rate of 30 r / min for 1-1.5 h to obtain a modified polyamic acid solution; 3) The modified polyamic acid solution is evenly poured onto a flat glass plate, baked in an oven at 85°C for 4-5 hours, then baked at 120°C, 180°C, 260°C, and 350°C for 1 hour each. After cooling to room temperature, the plate is taken out and placed in deionized water for natural peeling. After drying, the flexible substrate is obtained. The modified solution is prepared by adding 200 g of 2-benzimidazolidinone and 100 g of 2-aminotoluene-4-sulfonic acid into 2 L of 60° C. distilled water and stirring uniformly.

2. The method for preparing a flexible circuit board according to claim 1, wherein: The copper foil includes the following processing: i. Preparation of copper etching solution: 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 150 ml of hydrogen peroxide and 30 g of 1-phenyl-5-mercaptotetrazole, and the mixture was stirred to obtain a first-level copper etching solution; 200 ml of concentrated sulfuric acid solution was slowly added dropwise to 6 L of distilled water while stirring, followed by the addition of 75 ml of hydrogen peroxide, 50 g of 1-phenyl-5-mercaptotetrazole, and 100 g of N-methyl-p-toluidine, and the mixture was stirred to obtain a second-level copper etching solution; ii. Etching copper foil in etching solution: Immerse the copper foil in the first-level copper etching solution, control the system temperature to 50-60℃, pour out the first-level copper etching solution after soaking for 2-3 hours, then add the second-level copper etching solution to completely immerse the copper foil, and continue soaking for 3 hours; iii. Cleaning and drying: After the reaction is completed, pour out the secondary copper etching solution, then rinse the copper foil with distilled water 2-3 times, and finally place it in a dryer to dry to obtain the modified copper foil.

3. The method for preparing a flexible circuit board according to claim 1, wherein: The circuit board after soldering also includes a solder resist coating, the specific method is as follows: I. Spraying: Place the circuit board in a surface dust remover for 5 minutes, then place it in an ultrasonic spraying device, select a spray gun with a diameter of 1.5-2.0mm, adjust the spray gun pressure to 0.3-0.5MPa, and the conveyor speed to 4.5m / min. Add solder resist and spray evenly 1-2 times. II. Pre-drying: Pre-dry the circuit board after spraying the solder resist at 70-80°C for 10-20 minutes; III. Light curing: The pre-dried circuit board is placed under a UV lamp for light curing for 30-40 minutes to complete the coating.

4. The method for preparing a flexible circuit board according to claim 3, wherein: The solder resist preparation method is as follows: a. Add 6L of pure acrylic resin and 12L of epoxy resin to 45L of solvent and heat to 80-90°C, stirring and mixing at 30r / min for 0.8-1h; b Then add 20ml of highly transparent thickener, 20ml of silicone defoamer and 12g of transparent phthalocyanine green pigment to the above system, and continue stirring and mixing at a speed of 30r / min for 2-3h; c. The system was then cooled to 50-60°C, and 80g of a photoinitiator, 70g of 2,3-dibromo-1,4-butenediol, and 35g of dibutyl glutarate were added. The mixture was stirred and then allowed to react at a constant temperature for 4-6 hours. After cooling, the modified solder resist was obtained. The solvent is a mixture of divalent acid ester and ethyl acetate in a ratio of 1:1; and the photoinitiator is diacylphosphine oxide 819.

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