Preparation method of UV-cured organosilicon conformal coating for flexible circuit board
By combining modified silicone resin with photoinitiators, reactive diluents, and polymerization inhibitors, a UV-curable silicone conformal coating is formed, which solves the cracking problem of flexible circuit boards when bent at high frequencies, improves flexibility and curing speed, and is suitable for LED light source production.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-08
- Publication Date
- 2026-04-07
AI Technical Summary
Existing conformal coatings have high hardness after curing on flexible circuit boards, which makes them prone to cracking and delamination when subjected to high-frequency and high-intensity bending. Furthermore, oxygen inhibition slows down the curing speed, affecting flexibility and adhesion.
The UV-curable silicone conformal coating uses a combination of modified silicone resin, photoinitiator, reactive diluent, and polymerization inhibitor to form a three-dimensional network structure, which improves flexibility and adhesion, reduces oxygen inhibition, and enhances curing speed.
This technology enables flexible circuit boards to remain intact and unbroken during repeated bending, improving photocuring efficiency and surface curing effects, meeting the production needs of LED light sources, and reducing environmental pollution.
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Abstract
Description
Technical Field
[0001] This invention relates to a UV-curable silicone conformal coating, specifically a UV-curable silicone conformal coating with excellent flexibility. Background Technology
[0002] In recent years, with the rapid development of industries such as intelligent robots, new energy vehicles, and foldable phones, flexible printed circuits (FPCs) have maintained a high growth rate every year. Flexible circuit board technology is a key enabling technology for the miniaturization, lightweighting, and high reliability of modern electronic devices. With the rapid development of industries such as 5G, AIoT, wearables, and automotive electronics, the technological innovation and application boundaries of FPCs will continue to expand, playing an increasingly important role in future electronic products.
[0003] However, in practical applications, flexible circuit boards operate in complex environments and are susceptible to factors such as humidity, mold, salt spray, high dust levels, and repeated bending. Conformal coatings are applied to the surface of the circuit board, and after curing, form a dense protective film, providing multiple layers of protection including moisture resistance, stain resistance, salt spray resistance, and insulation. This improves the performance, stability, safety, and lifespan of the circuit board. However, most conformal coatings are quite hard after curing, posing significant challenges when applied to flexible circuit boards. For example, high-frequency, high-intensity bending can easily cause cracking and delamination. Summary of the Invention
[0004] To address the issues of oxygen inhibition leading to surface oil seepage after UV curing and poor adhesion, flexibility, and bending resistance after curing due to the addition of large amounts of solvent for dilution in acrylic-modified silicone conformal coatings, this invention provides a method for preparing a UV-curable silicone conformal coating for flexible circuit boards. This conformal coating is cured by ultraviolet light and exhibits both good hardness and flexibility after curing. Even after repeated bending, it adheres well to the insulating substrate, achieving a satisfactory protective effect.
[0005] In addition, this silicone material can reduce the inhibitory effect of oxygen in the air on the double bonds of acrylate on the coating surface, effectively improve the curing speed of the material and improve the surface curing effect, which can meet the production needs of LED light sources, reduce the environmental pollution and human health hazards of UV mercury lamps, and reduce energy consumption.
[0006] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing a modified organosilicon resin, comprising the following steps: (1) Mix hydroxyl-terminated silicone oil, silane coupling agent containing acrylic groups, phenolic polymerization inhibitor and alkaline catalyst, heat and react for a period of time, add acidic neutralizer to neutralize, continue to heat and vacuum for a period of time to obtain acrylate modified organosilicon resin. (2) Mix the acrylate-modified silicone resin from step (1), a photoinitiator containing hydroxyl groups, an active diluent containing acrylic groups, and a phenolic polymerization inhibitor to obtain a UV-curable silicone conformal coating. The preparation method of this invention comprises the following raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil to the silane coupling agent containing acrylic acid groups is 0.3 to 1.2.
[0007] The alkaline catalyst is an alcoholic solution of potassium hydroxide, which catalyzes the O-alkylation reaction between the hydroxyl groups of hydroxyl silicone oil and the alkoxy groups in the silane coupling agent to synthesize an organosilicon resin with a three-dimensional network structure. After the catalytic reaction, the viscosity of the organosilicon resin ranges from 800 to 3400 cp, and it exhibits good miscibility with the photoinitiator. The formulated conformal coating has high fluidity and mechanical properties that meet the requirements for flexible circuit boards. The alcohol is methanol, and the mass concentration of the potassium hydroxide alcoholic solution is 1-5 wt%. The lower potassium hydroxide concentration helps to suppress excessively fast reaction rates and overpolymerization. The amount of alkaline catalyst added is 0.02-0.2 times the total mass of the silane coupling agent containing acrylic acid groups.
[0008] The phenolic polymerization inhibitor is 0.01-0.05 times the total mass of the silane coupling agent with acrylic acid groups.
[0009] The hydroxyl-containing photoinitiator accounts for 0.3 to 1% of the total mass of the raw materials.
[0010] The reactive diluent containing acrylic groups accounts for 10-20% of the total mass of the raw materials.
[0011] The viscosity of the hydroxyl-terminated silicone oil at 25°C is 200–1500 mPa·s.
[0012] The silane coupling agent containing acrylic acid groups is any one or more of γ-methacryloxypropyltrimethoxysilane, 3-acryloxypropylmethyldimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, and 3-acryloxypropylmethyldiethoxysilane.
[0013] The photoinitiator is selected from any one or more of 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, and ethyl 2,4,6-trimethylbenzoylphenylphosphonate, preferably ethyl 2,4,6-trimethylbenzoylphenylphosphonate. The phenolic polymerization inhibitor is selected from any one or more of hydroquinone, 4-methoxyphenol, 2,6-di-tert-butyl-p-cresol, 2,5-di-tert-butyl-p-quinone, and 2-tert-butyl-p-quinone, preferably hydroquinone and 4-methoxyphenol.
[0014] The reaction described in step 1) is carried out at a temperature of 45–65°C for 2–4 hours. Step 1) involves heating and vacuuming at a pressure of -(0.07~0.1)MPa, a temperature of 55~65℃, and a time of 0.5~1h.
[0015] The reaction described in step 2) is carried out at a temperature of 55–65°C for 0.1–0.5 h.
[0016] A UV-curable silicone conformal coating for flexible circuit boards is formulated with raw materials in the following mass ratio: The phenolic polymerization inhibitor is 0.01-0.02% of the modified organosilicon resin by mass. The photoinitiator is 0.3-0.7% of the modified organosilicon resin by mass.
[0017] The acrylic-containing reactive diluent accounts for 5-20% of the mass of the modified silicone resin.
[0018] The polymerization inhibitor is 0.05-1% of the mass of EM-1 modified silicone resin.
[0019] The method involves mixing and stirring modified silicone resin, photoinitiator, and reactive diluent in a sealed, light-proof container at a temperature not exceeding 40°C to obtain a UV-curable silicone conformal coating for circuit boards.
[0020] Compared with the prior art, the beneficial effects of the technical solution of the present invention are as follows: This invention relates to a UV-curable silicone conformal coating material for flexible circuit boards. It exhibits good flowability, ease of processing, high transparency, and excellent flexibility and adhesion, remaining detached and unbroken from the substrate even after 0.5 to 3.5 million bending cycles. Furthermore, this silicone material reduces the inhibitory effect of oxygen in the air on the double bonds of acrylates, improving photocuring efficiency and surface curing effect, thus meeting the production requirements of LED light sources. Detailed Implementation
[0021] The technical solution of the present invention will be further described in detail below with reference to specific implementation examples.
[0022] A UV-curable silicone conformal coating for flexible circuit boards is made from acrylate-modified silicone resin with the addition of a photoinitiator, reactive diluent, and polymerization inhibitor. Unless otherwise specified, the raw materials and reagents in the following examples are commercially available products or prepared by known methods.
[0023] Example 1 The steps for preparing modified organosilicon resin EM-1 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 200 mPa·s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:0.3.
[0024] The catalyst containing potassium hydroxide in methanol solution (potassium hydroxide mass fraction of 5% wt) is 0.02 times the mass of the silane coupling agent with acrylic acid groups, and is added in step 1).
[0025] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. The hydroxyl-containing photoinitiator (1-hydroxycyclohexylphenyl ketone) is 0.5% of the mass of EM-1.
[0026] The acrylic-containing reactive diluent (isoborneol acrylate) is 10% of the mass of EM-1.
[0027] The polymerization inhibitor (4-methoxyphenol) is 1% of the mass of EM-1.
[0028] 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 200mPa.s), 17.53g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 0.88g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring and heat to 45°C until the polymerization inhibitor dissolves. Finally, add 0.3506g of potassium hydroxide methanol solution catalyst and react under nitrogen purging until the viscosity of the reactants is about 800cp. The reaction time is about 4 hours.
[0029] Add 0.6246 g of phosphate methanol solution to the reactants and allow the neutralization reaction to proceed for 0.5 h.
[0030] The reaction product was heated to 55°C, evacuated to a pressure of -0.08 MPa, and stirred for 1 hour to obtain acrylate-modified silicone resin EM-1.
[0031] 2) Add 500g of acrylate-modified silicone resin EM-1 to a three-necked flask, then add 5g of 1-hydroxycyclohexylphenyl ketone, 50g of isoborneol acrylate and 0.25g of polymerization inhibitor and stir evenly in the dark to obtain UV-curable silicone conformal coating EMUV-1.
[0032] Example 2 The steps for preparing modified organosilicon resin EM-2 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 500 mPa·s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:0.6.
[0033] The catalyst containing potassium hydroxide in methanol solution (potassium hydroxide mass fraction of 5% wt) is 0.02 times the mass of the silane coupling agent with acrylic acid groups, and is added in step 1).
[0034] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. The hydroxyl-containing photoinitiator (1-hydroxycyclohexylphenyl ketone) is 0.3% of the mass of EM-2.
[0035] The acrylic-containing reactive diluent (isoborneol acrylate) is 5% of the mass of EM-2.
[0036] The polymerization inhibitor (4-methoxyphenol) is 0.05% of the mass of EM-2.
[0037] 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 500mPa.s), 13.15g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 0.66g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring and heat to 45℃ until the polymerization inhibitor dissolves. Finally, add 0.2630g of potassium hydroxide methanol solution catalyst and react under nitrogen purging until the viscosity of the reactants reaches approximately 1000cp. The reaction time is approximately 2.5 hours.
[0038] Add 0.4685 g of phosphate methanol solution to the reactants and allow the neutralization reaction to proceed for 0.5 h.
[0039] The reaction product was heated to 55°C, and then evacuated to a pressure of -0.08 MPa and stirred for 1 hour to obtain acrylate-modified silicone resin EM-2.
[0040] 2) Add 500g of acrylate-modified silicone resin EM-1 to a three-necked flask, then add 1.5g of 1-hydroxycyclohexylphenyl ketone, 25g of isoborneol acrylate and 0.25g of polymerization inhibitor and stir evenly in the dark to obtain UV-curable silicone conformal coating EMUV-2.
[0041] Example 3 The steps for preparing modified organosilicon resin EM-3 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 750 mPa.s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:0.4.
[0042] The catalyst containing potassium hydroxide in methanol solution (potassium hydroxide mass fraction of 5% wt) is 0.02 times the mass of the silane coupling agent with acrylic acid groups, and is added in step 1).
[0043] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. The hydroxyl-containing photoinitiator (1-hydroxycyclohexylphenyl ketone) is 0.7% of the mass of EM-3.
[0044] The acrylic-containing reactive diluent (isoborneol acrylate) is 20% of the mass of EM-3.
[0045] The polymerization inhibitor (4-methoxyphenol) is 0.05% of the mass of EM-3.
[0046] 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 750mPa.s), 5.84g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 0.29g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring and heat to 45℃ until the polymerization inhibitor dissolves. Finally, add 0.1168g of potassium hydroxide methanol solution catalyst and react under nitrogen purging until the viscosity of the reactants reaches approximately 1400cp. The reaction time is approximately 2 hours.
[0047] Add 0.2081 g of phosphate methanol solution to the reactants and allow the neutralization reaction to proceed for 0.5 h.
[0048] The reaction product was heated to 55°C, evacuated to a pressure of -0.08 MPa, and stirred for 1 hour to obtain acrylate-modified silicone resin EM-3.
[0049] 2) Add 500g of acrylate-modified silicone resin EM-1 to a three-necked flask, then add 3.5g of 1-hydroxycyclohexylphenyl ketone, 100g of isoborneol acrylate and 0.25g of polymerization inhibitor and stir evenly in the dark to obtain UV-curable silicone conformal coating EMUV-3.
[0050] Example 4 The steps for preparing modified organosilicon resin EM-4 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 1500 mPa·s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:0.6.
[0051] The catalyst containing potassium hydroxide in methanol solution (potassium hydroxide mass fraction of 5% wt) is 0.02 times the mass of the silane coupling agent with acrylic acid groups, and is added in step 1).
[0052] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. The hydroxyl-containing photoinitiator (1-hydroxycyclohexylphenyl ketone) is 0.5% of the mass of EM-4.
[0053] The acrylic-containing reactive diluent (isoborneol acrylate) is 20% of the mass of EM-4.
[0054] The polymerization inhibitor (4-methoxyphenol) is 0.05% of the mass of EM-4.
[0055] 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 1500mPa.s), 4.38g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 0.219g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring and heat to 45℃ until the polymerization inhibitor dissolves. Finally, add 0.0876g of potassium hydroxide methanol solution catalyst and react until the viscosity of the reactants is about 3400cp. The reaction time is about 2 hours.
[0056] Add 0.1561 g of phosphate methanol solution to the reactants and allow the reaction to proceed for 0.5 h.
[0057] The reaction product was heated to 55°C, evacuated to a pressure of -0.08 MPa, and stirred for 1 hour to obtain acrylate-modified silicone resin EM-3.
[0058] 2) Add 500g of acrylate-modified silicone resin EM-1 to a three-necked flask, then add 2.5g of 1-hydroxycyclohexylphenyl ketone, 100g of isoborneol acrylate and 0.25g of polymerization inhibitor and stir evenly in the dark to obtain UV-curable silicone conformal coating EMUV-4.
[0059] Comparative Example 1 Modified organosilicon resin EMUV-5 was prepared and compared with the synthesis method of Example 2. The difference was that the catalyst in steps 1) and 2) of Example 2 was replaced with an organotin catalyst, and the amount added was 0.4685g.
[0060] Comparative Example 2 The modified organosilicon resin EMUV-6 was prepared and compared with the synthesis method of Example 3. The difference was that the molar ratio of the dihydroxyl-terminated hydroxyl silicone oil (viscosity 500 mPa·s) in step 1) of Example 3 to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) was 1:1.2. The amount of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) added was 11.68 g.
[0061] Comparative Example 3 The steps for preparing modified organosilicon resin EM-7 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 500 mPa·s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:1.2.
[0062] The catalyst containing potassium hydroxide in methanol solution (potassium hydroxide mass fraction of 5% wt) is 0.2 times the mass of the silane coupling agent with acrylic acid groups, added in step 1).
[0063] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 500mPa.s), 26.3g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 1.32g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring, heat to 45℃ until the polymerization inhibitor dissolves, and finally add 0.526g of potassium hydroxide methanol solution catalyst. React under nitrogen atmosphere for 2.5 hours.
[0064] Add 0.9370 g of phosphate methanol solution to the reactants and allow the neutralization reaction to proceed for 0.5 h.
[0065] 2) The reaction product was heated to 65°C, and the vacuum was drawn to a pressure of -0.08MPa and stirred for 1 hour to obtain semi-solid acrylate-modified silicone resin EM-7, which could not be formulated into liquid conformal coating.
[0066] Comparative Example 4 The steps for preparing modified organosilicon resin EM-7 are as follows: In the preparation of raw materials: The molar ratio of the hydroxyl-terminated hydroxyl silicone oil (viscosity 500 mPa·s) to the silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane) is 1:1.2.
[0067] The butanone oxime is 0.2 times the mass of the silane coupling agent with an acrylic group, added in step 1).
[0068] The phenolic polymerization inhibitor (4-methoxyphenol) is 0.02% of an acrylic silane coupling agent. 1) In a three-necked flask equipped with a stirrer, add 500g of dihydroxy-terminated hydroxyl silicone oil (viscosity 500mPa.s), 26.3g of silane coupling agent containing acrylic acid groups (γ-methacryloyloxypropyltrimethoxysilane), and 1.32g of phenolic polymerization inhibitor (4-methoxyphenol). Start stirring, heat to 45℃ until the polymerization inhibitor dissolves, and finally add 0.526g of butanone oxime catalyst. React under nitrogen atmosphere for 2.5 hours.
[0069] 2) The reaction product is heated to 65°C, vacuumed to a pressure of -0.08MPa and stirred for 1 hour to prepare a liquid conformal coating, which cannot be photocured.
[0070] The performance of the products in Examples 1-5 and Comparative Examples 1-2 of the present invention was compared, and the results are shown in Table 1 below: Table 1
[0071] As shown in Table 1, the UV-curable silicone conformal coating for flexible circuit boards, by using KOH as a catalyst and hydroxyl silicone oil of suitable viscosity as a raw material, can greatly improve the flexibility and bending resistance of the silicone conformal coating material. Furthermore, comparative examples and comparative cases revealed that using other catalysts or increasing the viscosity of the hydroxyl silicone oil leads to a significant decrease in the flexibility and bending resistance of the silicone conformal coating.
[0072] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A method for preparing a UV-curable silicone conformal coating for flexible circuit boards, characterized in that, Includes the following steps: (1) Mix hydroxyl-terminated silicone oil, silane coupling agent containing acrylic groups, phenolic polymerization inhibitor and alkaline catalyst, heat and react for a period of time, add acidic neutralizer to neutralize, continue to heat and vacuum for a period of time to obtain acrylate modified organosilicon resin. (2) Preparation of conformal coating: The acrylate-modified silicone resin obtained in step (1), the photoinitiator containing hydroxyl groups, the reactive diluent containing acrylic groups and the phenolic polymerization inhibitor are mixed evenly under the condition of not exceeding 40°C and avoiding light to obtain the UV-curable silicone conformal coating.
2. The preparation method according to claim 1, characterized in that, In step (1), the molar ratio of the dihydroxy-terminated hydroxyl silicone oil to the silane coupling agent containing acrylic acid groups is 0.3:1 to 1.2:
1.
3. The preparation method according to claim 1, characterized in that, In step (1), the viscosity of the dihydroxy-terminated hydroxyl silicone oil at 25°C is 200-1500 mPa·s; The alkaline catalyst is an alcoholic solution of potassium hydroxide, wherein the alcohol is methanol, and the mass concentration of the potassium hydroxide or the alcoholic solution of potassium hydroxide is 1-5 wt%; the amount of alkaline catalyst added is 0.02-0.2 times the total mass of the silane coupling agent containing acrylic acid groups.
4. The preparation method according to claim 1, characterized in that, In step (1), the silane coupling agent containing acrylic acid groups is selected from one or more of γ-methacryloxypropyltrimethoxysilane, 3-acryloxypropylmethyldimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane and 3-acryloxypropylmethyldiethoxysilane.
5. The preparation method according to claim 1, characterized in that, In step (1), the acrylate-modified silicone resin is prepared by mixing a hydroxyl-terminated silicone oil, a silane coupling agent containing acrylic acid groups, a phenolic polymerization inhibitor, and an alkaline catalyst, reacting at 45-65°C for 2-4 hours, adding an acidic neutralizing agent for neutralization, and then continuing the reaction at 55-65°C and a vacuum degree of -0.07 to -0.1 MPa for 0.5-1 hours to obtain the acrylate-modified silicone resin.
6. The preparation method according to claim 1, characterized in that, In step (1) and / or step (2), the phenolic polymerization inhibitor is selected from one or more of hydroquinone, 4-methoxyphenol, 2,6-di-tert-butyl-p-cresol, 2,5-di-tert-butyl-p-quinone and 2-tert-butyl-p-quinone.
7. The preparation method according to claim 1, characterized in that, In step (2), the photoinitiator containing hydroxyl groups is selected from one or more of 1-hydroxycyclohexylphenyl ketone, 2-hydroxy-2-methyl-1-phenyl-1-propanone and ethyl 2,4,6-trimethylbenzoylphenylphosphonate, and its mass is 0.3-0.7% of the mass of the acrylate-modified silicone resin obtained in step (1).
8. The preparation method according to claim 1, characterized in that, In step (2), the active diluent containing acrylic groups is isobornyl acrylate, and its mass is 5-20% of the mass of the acrylate-modified silicone resin obtained in step (1).
9. A UV-curable silicone conformal coating for flexible circuit boards, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.
10. The application of the UV-curable silicone conformal coating according to claim 9 in the preparation of a protective coating for flexible printed circuit boards, characterized in that, The conformal coating is applied to the surface of the flexible circuit board and cured by ultraviolet light to form a protective coating. The coating can withstand 0.5-3.5 million repeated bending cycles without peeling or breaking.