A coating method for circuit board triple-proof paint and circuit board

Abstract

The present application provides a method for coating a circuit board with a conformal coating, the method comprising the following steps: cleaning the circuit board, activating the surface of the circuit board, coating the conformal coating, and curing the coating; the above processes enable the circuit board to obtain sufficient surface cleanliness and surface activity, while optimizing the formula composition and coating environment conditions of the conformal coating, improving the adhesion strength of the conformal coating to the circuit board, thereby improving the conformal performance, and avoiding the mottled surface phenomenon.

Description

Technical Field

[0001] The present invention relates to the field of circuit board protection technology, B05D1 / 00, and in particular to a circuit board three-proof coating method and a circuit board. Background Art

[0002] During processing, storage, transportation and use, the circuit board and the electronic components installed on it are easily affected by multiple factors such as temperature, moisture, salt, microorganisms, etc., causing corrosion, softening, mildew, deformation and other adverse phenomena. As highly sensitive equipment, the circuit board and the electronic components on it will reduce the reliability of the equipment. At present, the most commonly used method in the industry is to apply a coating with "three-proof" effect (moisture-proof, salt spray-proof, and mold-proof) on the circuit board with the electronic components installed. However, the traditional three-proof paint coating process has the following disadvantages: (1) long curing time and low production efficiency; (2) the selected solvents are mostly aromatic solvents, which are harmful to the human body; (3) the film is too thick, and the edge resolution and uniformity are uncontrollable; (4) in the ordinary coating process, the adhesion and density of the three-proof paint to the circuit board are poor, and a kind of mottled face phenomenon is easily produced. The "three-proof" effect is not enough, and the coating process is not reliable.

[0003] Chinese patent CN 113410141B discloses a method for reducing the bubble rate of three-proof paint coating on circuit chips. This method provides a pin structure on the chip and a static exhaust outflow treatment process, so that the three-proof paint sprayed continuously has a very low packaging bubble rate, thereby improving the reliability of the circuit board; however, this process requires the addition of a structure for opening precise channels on the chip, which is not conducive to improving production efficiency. In addition, it cannot solve the problem of poor adhesion of the three-proof paint. Chinese patent CN103589320B discloses a method for using three-proof paint for printed circuit board protection, which reduces the surface energy of the three-proof paint by silicone acrylate resin and polyurethane acrylate resin and improves the self-cleaning performance, but it does not optimize the coating process of the three-proof paint to enhance the three-proof performance. Summary of the invention

[0004] In order to solve the above technical problems, the present invention first provides a method for coating a circuit board with a conformal coating, the method comprising the following steps: cleaning the circuit board, activating the surface of the circuit board, coating the conformal coating, and curing the coating.

[0005] Furthermore, the method comprises the following steps:

[0006] S1. Circuit board cleaning: put the circuit board into the cleaning agent for ultrasonic cleaning for 5-10 minutes, and then rinse with ultrapure water;

[0007] S2. PCB surface activation: Use plasma to activate the surface of the PCB and remove residual glue;

[0008] S3, conformal coating: apply conformal coating to the circuit board;

[0009] S4. Coating curing: Place the coated circuit board in an oven at 50-70℃ and bake for 10-30 minutes to cure. The dry film thickness of the conformal coating is 5-30μm.

[0010] Furthermore, in the circuit board cleaning step, an environmentally friendly phosphorus-free and halogen-free cleaning agent is used; the formula raw materials of the cleaning agent include, by weight: 8-12 parts of alcohol ether solvents, 12-18 parts of tetrahydrofuran methanol, 10-15 parts of surfactants, 2-10 parts of saponifiers, 6-9 parts of sodium borate, 3-7 parts of triethanolamine oleate, 1-2 parts of sustained-release agents, and 80-120 parts of water.

[0011] Furthermore, the alcohol ether solvent is selected from at least one of ethylene glycol methyl ether, ethylene glycol ethyl ether, ethylene glycol butyl ether, dipropylene glycol propyl ether, dipropylene glycol ethyl ether, dipropylene glycol butyl ether, and diethylene glycol hexyl ether; preferably dipropylene glycol butyl ether.

[0012] Furthermore, the surfactant includes a nonionic surfactant and an anionic surfactant, and the weight ratio of the nonionic surfactant and anionic surfactant is (1-6):1.

[0013] Furthermore, the nonionic surfactant is selected from at least one of lauryl alcohol polyoxyethylene ether, alkylphenol polyoxyethyl ether, pentaerythritol fatty acid ester, polyoxyethylene fatty acid ester polyoxyethylene fatty alcohol ether, and fatty acid monoglyceride.

[0014] Furthermore, the anionic surfactant is selected from at least one of alkyl sulfonates, alkylbenzene sulfonates, alkyl sulfates, and succinate sulfonates.

[0015] In a preferred embodiment, the nonionic surfactant is lauryl alcohol polyoxyethylene ether, and the anionic surfactant is alkyl sulfate, more preferably pentadecyl sulfate.

[0016] This application uses a cleaning agent to clean the residual flux and other dust, oil, etc. on the surface of the circuit board to prevent the residue of the above substances from reducing the adhesion strength of the three-proof paint on the board, and effectively inhibit the cracking of the three-proof paint film after drying. The cleaning agent uses dipropylene glycol butyl ether and tetrahydrofuran methanol as part of the solvent, which effectively improves the cleaning agent's solubility of oily substances, especially rosin in the flux, but the residual density of the flux is relatively high. How to effectively decompose the connection state of the rosin resin from the inside is a key part of improving cleaning efficiency and cleaning crystallinity. On this basis, this application uses a specific surfactant, among which lauryl alcohol polyoxyethylene ether has rich polar groups and non-polar alkyl groups of suitable chain length, which makes it have good wettability and permeability. Pentadecyl sulfate can assist the former to further improve the wetting, penetration and emulsification ability of the surfactant, and it also has a better effect of reducing the surface tension of water. By controlling the relative content of the two, the two with special molecular structures produce a specific cross-linking state, improve their penetration, emulsification and removal effects on flux residues, improve the surface crystallinity of the circuit board, and thus improve the comprehensive performance of the three-proof paint. When the content of lauryl alcohol polyoxyethylene ether is too little, the foam production will increase, and if the content is too high, the difficulty of cleaning will increase.

[0017] Further preferably, the weight ratio of the nonionic surfactant to the anionic surfactant is (2-5):1.

[0018] Furthermore, the saponifying agent is selected from at least one of aminomethyl propanol, isopropanolamine, dimethylethanolamine and ethylenediamine.

[0019] Preferably, the saponifying agent is aminomethyl propanol.

[0020] Furthermore, the sustained-release agent is selected from at least one of methyl benzotriazole, mercaptobenzothiazole, 5-aminotetrazole, and tall oil hydroxyethyl imidazoline.

[0021] Preferably, the sustained-release agent is tall oil hydroxyethyl imidazoline.

[0022] Furthermore, the circuit board surface activation in step S2 is specifically as follows: first, pre-treat with a clean gas at a pressure of 0.1-0.15 torr and a total flow rate of 2.2-3.0 L / min for 10-20 min; then, activate the circuit board with a mixed nitrogen and hydrogen at a pressure of 225-245 torr and a total flow rate of 3.2-5.3 L / min for 5-30 min.

[0023] Furthermore, the volume ratio of nitrogen to hydrogen is 1:(1-2).

[0024] Furthermore, the cleaning gas comprises a mixed gas of oxygen, argon and carbon tetrachloride, in which the volume fraction of oxygen is 40-50%, the volume fraction of carbon tetrachloride is 8-10%, and the rest is argon.

[0025] Furthermore, the activation treatment time in S2 is 10-15 minutes.

[0026] Furthermore, the formula raw materials of the conformal coating in S3 include: by weight, 40-60 parts of polyurethane acrylate, 15-25 parts of modified alkyd resin, 30-45 parts of isopropyl alcohol, 15-20 parts of ethyl acetate, 15-20 parts of butyl acetate, 1-6 parts of defoaming agent, 1-5 parts of coupling agent, and 5-10 parts of drying agent.

[0027] Furthermore, the modified alkyd resin is a rosin-modified alkyd resin.

[0028] The present application adopts polyurethane-acrylate resin and rosin-modified alkyd resin to provide film-forming properties for the three-proof paint. When the polyurethane-acrylate resin is used alone, the prepared three-proof paint system has high fluidity and insufficient surface energy due to insufficient cross-linking between the molecular chains of the polyurethane-acrylate resin, and is prone to the phenomenon of mottled face. The present application compounded with rosin-modified alkyd resin increases the surface energy of the system and enhances the adhesion to the substrate; in addition, the intermolecular effect generated by the polar groups of the polyurethane-acrylate resin and the rosin-modified alkyd resin, and the network connection effect generated by the cross-linking of the molecular chains of the two enhances the film-forming density and film hardness of the three-proof paint, reduces the penetration and erosion of water molecules and salt spray molecules to the paint film, significantly improves the three-proof performance, and increases the curing rate of the three-proof paint; however, it is necessary to strictly control the relative amounts of the two so that the two can produce a suitable cross-linking degree, cross-linking curing speed and flow performance under the action of the coupling agent to prevent the paint film from having bubbles, pinholes, overflowing glue and other undesirable phenomena.

[0029] Preferably, the weight ratio of the polyurethane acrylate to the modified alkyd resin is (45-55):(20-23).

[0030] In a preferred embodiment, the weight ratio of the polyurethane acrylate to the modified alkyd resin is 50:22.

[0031] Furthermore, the functionality of the polyurethane acrylate is 3-6, and the viscosity at 25° C. is 10,000-20,000 cps.

[0032] This application introduces a specific content of rosin-modified alkyd resin to enhance the surface energy of the three-proof paint and increase its adhesion to the substrate, but the above will also increase the affinity of the paint film for water molecules. In order to solve the above problems, the inventor further stipulates that the polyurethane-acrylate resin is multifunctional, increasing its reactive sites with the rosin-modified alkyd resin and reducing the affinity of the polar groups in the system for water, so as to balance the contradictions faced by the surface energy; when the functionality is 4, the adhesion and hydrophobicity of the three-proof paint can reach appropriate values. Viscosity can also reflect the size of its molecular weight, further limit its viscosity, make it have appropriate flow properties and dispersibility, further regulate the flow effect of the three-proof paint, and avoid the problems of overflow caused by the three-proof paint coating being too thick, long coating drying time, and difficult coating operation.

[0033] Preferably, the polyurethane acrylate has a functionality of 4 and a viscosity of 10,000-15,000 cps at 25°C.

[0034] Furthermore, the coupling agent is a titanate coupling agent, such as isopropyl tri(dioctyl pyrophosphate) titanate, model KR-38S.

[0035] Furthermore, the drying agent is selected from at least one of a cobalt-based drying agent, a calcium-based drying agent, and a zinc-based drying agent.

[0036] Preferably, the drier is a cobalt-based drier, specifically cobalt isooctanoate.

[0037] Furthermore, the present invention does not particularly limit the defoaming agent, and silicone defoaming agents such as BYK-088 may be selected.

[0038] Furthermore, in step S3, the conformal coating is applied by at least one of spraying, dipping, and screen printing, preferably spraying or screen printing.

[0039] Furthermore, in the coating step of S3, the process environment conditions are as follows: temperature: 15-28° C.; humidity 60-70% RH, so as to suppress the whitening phenomenon of the paint film.

[0040] Furthermore, in S4, the baking temperature is 55-60° C., and the dry film thickness is 5-20 μm.

[0041] Secondly, the present invention also provides a circuit board, the surface of which is coated with a three-proof paint by the above coating method.

[0042] Beneficial Effects

[0043] 1. This application uses a special surface cleaning and plasma activation process in the coating process of the three-conformal paint, so that the circuit board obtains sufficient surface cleanliness and surface activity, improves the adhesion strength of the three-conformal paint to the circuit board, and thus improves the three-conformal performance;

[0044] 2. This application optimizes the cleaning agent used in the surface cleaning process, and uses a combination of decontamination ingredients with different activities to effectively improve the penetration, emulsification and removal effects of residual pollutants on the surface of the circuit board, and ensure that the removed stains will not re-adhere to the surface of the circuit board, thereby improving the cleaning efficiency and providing a basis for the smooth coating and long-lasting adhesion of the three-conformal paint;

[0045] 3. This application adopts a specific plasma activation process to further improve the surface crystallinity and surface energy of the circuit board and enhance its bonding with the conformal coating film;

[0046] 4. This application optimizes the formula components of the three-proof paint to make the three-proof paint have higher hardness and appropriate cross-linking strength, ensure that the paint film has a suitable flow state, does not produce shrinkage, cracking, or mottled phenomena, and at the same time improves the three-proof performance and curing efficiency of the paint film;

[0047] 5. The three-proof paint used in this application is applied in a specific environment to further prevent the three-proof paint from being stained from the construction process. DETAILED DESCRIPTION

[0048] Example

[0049] Example 1

[0050] This embodiment provides a method for coating a circuit board with conformal coating, the method comprising the following steps:

[0051] S1. Circuit board cleaning: put the circuit board into the cleaning agent for ultrasonic cleaning for 8 minutes, and then rinse with ultrapure water;

[0052] The formula raw materials of the cleaning agent include: 10 parts of dipropylene glycol butyl ether, 15 parts of tetrahydrofuran methanol, 13 parts of surfactant, 6 parts of aminomethyl propanol, 7 parts of sodium borate, 4 parts of triethanolamine oleate, 1.6 parts of tall oil hydroxyethyl imidazoline, and 110 parts of water; the surfactant is lauryl alcohol polyoxyethylene ether and pentadecyl sulfate, with a weight ratio of 4:1.

[0053] S2. Activation of the circuit board surface: first pre-treat with a clean gas at a pressure of 0.12 torr and a total flow rate of 2.5 L / min for 15 minutes; then use a mixture of nitrogen and hydrogen (volume ratio of 1:1) to activate the circuit board for 10 minutes at a pressure of 230 torr and a total flow rate of 4.0 L / min; the clean gas is a mixed gas of oxygen, argon and carbon tetrachloride, in which the volume fraction of oxygen is 45%, the volume fraction of carbon tetrachloride is 9%, and the rest is argon.

[0054] S3, conformal coating: The conformal coating is applied to the circuit board by spraying at 25°C and 68% RH.

[0055] The formula raw materials of the three-proof paint include: by weight, 50 parts of polyurethane acrylate, 22 parts of rosin-modified alkyd resin, 36 parts of isopropyl alcohol, 18 parts of ethyl acetate, 18 parts of butyl acetate, 2 parts of defoamer (BYK-088), 3 parts of coupling agent (KR-38S), and 6 parts of drier (cobalt isooctanoate); the functionality of the polyurethane acrylate is 4, and the viscosity at 25°C is 10000-15000cps, purchased from Nanjing Jiazhong Chemical Technology Co., Ltd., JZ-303; the rosin-modified alkyd resin is purchased from Jiangyin Liren Chemical Co., Ltd.

[0056] S4. Coating curing: Place the coated circuit board in an oven at 60°C and bake for 20 minutes to cure. The dry film thickness of the conformal coating is 10μm.

[0057] Example 2

[0058] This embodiment provides a method for coating a circuit board with conformal coating, the method comprising the following steps:

[0059] S1. Circuit board cleaning: Place the circuit board in a cleaning agent and ultrasonically clean it for 10 minutes, then rinse it with ultrapure water;

[0060] The formula raw materials of the cleaning agent include: 8 parts of dipropylene glycol butyl ether, 12 parts of tetrahydrofuran methanol, 10 parts of surfactant, 2 parts of aminomethyl propanol, 6 parts of sodium borate, 3 parts of triethanolamine oleate, 1 part of tall oil hydroxyethyl imidazoline, and 80 parts of water; the surfactant is lauryl alcohol polyoxyethylene ether and pentadecyl sulfate, and the weight ratio is 2:1.

[0061] S2. Activation of the circuit board surface: first pre-treat with a clean gas at a pressure of 0.15 torr and a total flow rate of 3.0 L / min for 10 minutes; then activate the circuit board with a mixed nitrogen and hydrogen (volume ratio of 1:1.5) at a pressure of 225 torr and a total flow rate of 3.2 L / min for 10 minutes; the clean gas is a mixed gas of oxygen, argon and carbon tetrachloride, in which the volume fraction of oxygen is 50%, the volume fraction of carbon tetrachloride is 10%, and the rest is argon.

[0062] S3, conformal coating: The conformal coating is applied to the circuit board by spraying at 20°C and 70% RH.

[0063] The formula raw materials of the three-proof paint include: by weight, 45 parts of polyurethane acrylate, 23 parts of rosin-modified alkyd resin, 45 parts of isopropyl alcohol, 20 parts of ethyl acetate, 15 parts of butyl acetate, 2 parts of defoamer (BYK-088), 5 parts of coupling agent (KR-38S), and 5 parts of drier (cobalt isooctanoate); the functionality of the polyurethane acrylate is 4, and the viscosity at 25°C is 10000-15000cps, purchased from Nanjing Jiazhong Chemical Technology Co., Ltd., JZ-303; the rosin-modified alkyd resin is purchased from Jiangyin Liren Chemical Co., Ltd.

[0064] S4. Coating curing: Place the coated circuit board in an oven at 55°C for 30 minutes to cure. The dry film thickness of the conformal coating is 15μm.

[0065] Example 3

[0066] This embodiment provides a method for coating a circuit board with conformal coating, the method comprising the following steps:

[0067] S1. Circuit board cleaning: Place the circuit board in a cleaning agent and ultrasonically clean it for 5 minutes, then rinse it with ultrapure water;

[0068] The formula raw materials of the cleaning agent include: 12 parts of dipropylene glycol butyl ether, 18 parts of tetrahydrofuran methanol, 15 parts of surfactant, 10 parts of aminomethyl propanol, 9 parts of sodium borate, 7 parts of triethanolamine oleate, 2 parts of tall oil hydroxyethyl imidazoline, and 120 parts of water; the surfactant is lauryl alcohol polyoxyethylene ether and pentadecyl sulfate, and the weight ratio is 5:1.

[0069] S2. Activation of the circuit board surface: first pre-treat with clean gas at a pressure of 0.15 torr and a total flow rate of 3.0 L / min for 10 minutes; then activate the circuit board with a mixture of nitrogen and hydrogen (volume ratio of 1:2) at a pressure of 245 torr and a total flow rate of 5.3 L / min for 15 minutes; the clean gas is a mixed gas of oxygen, argon and carbon tetrachloride, in which the volume fraction of oxygen is 40%, the volume fraction of carbon tetrachloride is 8%, and the rest is argon.

[0070] S3, conformal coating: The conformal coating is applied to the circuit board by spraying at 28°C and 60% RH.

[0071] The formula raw materials of the three-proof paint include: by weight, 55 parts of polyurethane acrylate, 20 parts of rosin-modified alkyd resin, 45 parts of isopropyl alcohol, 15 parts of ethyl acetate, 20 parts of butyl acetate, 5 parts of defoamer (BYK-088), 1 part of coupling agent (KR-38S), and 10 parts of drier (cobalt isooctanoate); the functionality of the polyurethane acrylate is 4, and the viscosity at 25°C is 10000-15000cps, which is purchased from Nanjing Jiazhong Chemical Technology Co., Ltd., JZ-303; the rosin-modified alkyd resin is purchased from Jiangyin Liren Chemical Co., Ltd.

[0072] S4. Coating curing: Place the coated circuit board in an oven at 60°C for 10 minutes to cure. The dry film thickness of the conformal coating is 20μm.

[0073] Comparative Example 1

[0074] The method is basically the same as Example 1, except that in the cleaning agent, lauryl alcohol polyoxyethylene ether is replaced by polyoxyethylene polyoxypropylene ether; the cleaning agent is purchased from Nantong Petroleum Runfeng Petrochemical Co., Ltd., F-6.

[0075] Comparative Example 2

[0076] The method is basically the same as Example 1, except that in the cleaning agent, the weight ratio of lauryl alcohol polyoxyethylene ether to pentadecyl sulfate is 1:2.

[0077] Comparative Example 3

[0078] It is basically the same as Example 1, except that: the functionality of the polyurethane acrylate is 2, the viscosity at 25°C is 10000-15000 cps, and it is purchased from Nanjing Jiazhong Chemical Technology Co., Ltd., JZ-302.

[0079] Comparative Example 4

[0080] It is basically the same as Example 1, except that: the formula raw materials of the three-proof paint include: by weight, 40 parts of polyurethane acrylate, 40 parts of rosin-modified alkyd resin, 36 parts of isopropyl alcohol, 18 parts of ethyl acetate, 18 parts of butyl acetate, 2 parts of defoamer (BYK-088), 3 parts of coupling agent (KR-38S), and 6 parts of drying agent (cobalt isooctanoate).

[0081] Comparative Example 5

[0082] The invention is basically the same as Example 1, except that: the formula raw materials of the three-proof paint include: by weight, 77 parts of polyurethane acrylate, 36 parts of isopropyl alcohol, 18 parts of ethyl acetate, 18 parts of butyl acetate, 2 parts of defoaming agent (BYK-088), 3 parts of coupling agent (KR-38S), and 6 parts of drying agent (cobalt isooctanoate).

[0083] Comparative Example 6

[0084] It is basically the same as Example 1, except that: S2 is: using clean gas at a pressure of 0.12 torr and a total flow rate of 2.5 L / min to pretreat the circuit board for 15 minutes.

[0085] Comparative Example 7

[0086] It is basically the same as Example 1, except that the coating environment conditions of S3 are 23° C. and 85% RH.

[0087] Performance testing method:

[0088] The test results of the conformal coating performance of the embodiment are shown in Table 1, wherein the hardness of the conformal coating is tested in accordance with the standard of GB / T 6739-2006, the adhesion is tested in accordance with the standard of GB / T 9286-1998, the salt spray resistance test is tested in accordance with GB / T2423.17-2008, and the moisture test is tested in accordance with IEC60068-2-30 and IPC-TM-650 2.6.3.7.

[0089] Performance test results:

[0090] Table 1

[0091]

[0092] Result analysis:

[0093] The three-proof paints coated by the methods of Examples 1-3 all showed high curing efficiency, adhesion and three-proof performance, and there was no smearing phenomenon; Comparative Example 1 used other non-ionic surfactants, and the penetration, emulsification and cleaning effects on the residues on the surface of the circuit board were poorer than those of Example 1, which may be because a specific surfactant combination can have suitable surface properties, and increasing the removal effect further improves the adhesion of the paint film; In Comparative Example 2, sodium dodecyl sulfate is excessive, which is equivalent to a low content of lauryl alcohol polyoxyethylene ether, and the penetration and emulsification effects of the detergent are insufficient, and the generation of a large number of bubbles also affects the surface cleaning effect. Only the surfactant effect under a specific content range is more excellent; In Comparative Example 3, the functionality of polyurethane acrylate is low, and its cross-linking reaction sites with rosin-modified alkyd resin are reduced, so that there are more rosin-modified alkyd resins The polar groups in the paint film are arranged on the surface of the paint film, which increases the affinity of the paint film for polar molecules such as water molecules, thereby reducing the three-proof performance; in Comparative Example 4, the excessive content of rosin-modified alkyd resin will cause its cross-linking and curing effect with polyurethane acrylate to be too strong, and the paint film dries too fast, which will cause the air wrapped in the paint film to be unable to escape in a short time, thereby producing poor performance such as pinholes, and the escape of these gases will significantly reduce the compactness of the paint film in later tests; in Comparative Example 5, no rosin-modified alkyd resin was added, and the comprehensive performance of the paint film was not as good as that of Examples 1-3; in Comparative Example 6, the step of activating the circuit board surface was reduced, the activation energy of the circuit board surface was low, and the binding force with the three-proof paint was not strong; in Comparative Example 7, the humidity of the coating environment was too high, the initial surface curing efficiency of the three-proof paint decreased, and the difference between the surface and internal curing rates was too large, which easily caused the mottled face phenomenon.

Claims

1. A method for coating a circuit board with conformal coating, characterized in that: The coating method comprises the following steps: S1. Circuit board cleaning: put the circuit board into the cleaning agent for ultrasonic cleaning for 5-10 minutes, and then rinse with ultrapure water; S2. PCB surface activation: Use plasma to activate the surface of the PCB and remove residual glue; S3, conformal coating: apply conformal coating to the circuit board; S4, coating curing: Place the coated circuit board in an oven at 50-70°C and bake for 10-30 minutes to cure. The dry film thickness of the three-proof paint is 5-30μm; in the circuit board cleaning step, the formula raw materials of the cleaning agent include, by weight: 8-12 parts of alcohol ether solvent, 12-18 parts of tetrahydrofuran methanol, 10-15 parts of surfactant, 2-10 parts of saponifier, 6-9 parts of sodium borate, 3-7 parts of triethanolamine oleate, 1-2 parts of sustained-release agent, and 80-120 parts of water; the surfactant includes a nonionic surfactant and an anionic surfactant, and the weight ratio of the two is (1-6): 1; the nonionic surfactant is lauryl alcohol polyoxyethylene ether, the anionic surfactant is alkyl sulfate, and the alkyl sulfate is pentadecyl sulfate; the circuit board surface activation step S2 is specifically: first use a clean gas at 0.1-0.15tor r pressure, 2.2-3.0L / min total flow rate for 10-20min; then the circuit board is activated and treated with mixed nitrogen and hydrogen at a pressure of 225-245torr and a total flow rate of 3.2-5.3L / min for 5-30min; the formula raw materials of the three-proof paint in S3 include: by weight, 40-60 parts of polyurethane acrylate, 15-25 parts of modified alkyd resin, 30-45 parts of isopropyl alcohol, 15-20 parts of ethyl acetate, 15-20 parts of butyl acetate, 1-6 parts of defoamer, 1-5 parts of coupling agent, and 5-10 parts of drying agent; the weight ratio of the polyurethane acrylate and the modified alkyd resin is (45-55): (20-23), the functionality of the polyurethane acrylate is 3-6, and the viscosity at 25°C is 10000-20000cps; the modified alkyd resin is rosin modified alkyd resin; the S3 In the coating step, the process environment conditions are temperature: 15-28°C; humidity 60-70% RH.

2. A circuit board, characterized in that: The surface of the circuit board is coated with conformal coating by the coating method according to claim 1.

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