Cleaning process of PCB (Printed Circuit Board)

Abstract

The invention relates to the technical field of PCB cleaning production, in particular to a cleaning process of a PCB. According to the cleaning process, the problem that the cleaning effect on the PCB is poor in a traditional process is solved. By optimizing the cleaning process, the total inorganic ion residues on the surface of the PCB are remarkably reduced by adopting conductivity gradient control, supercritical carbon dioxide fluid cleaning and ethanol entrainer, chelating agent and other technologies; surface pollutants and oxide layers are systematically removed through the processes of alkaline degreasing, micro-etching activation, plasma cleaning and the like, and coating defects are avoided; the cleaning process is adjusted according to different stages, supercritical carbon dioxide, an entrainer and countercurrent rinsing are applied, and the surface cleanliness is greatly improved. And the cleaned PCB is low in total inorganic ion content and low in dirt amount, and is suitable for the field of electronics with high requirements on cleanliness.

Description

Technical Field

[0001] The present invention relates to the technical field of PCB board clean production, and specifically to a cleaning process for PCB boards. Background Art

[0002] A printed circuit board (PCB board), abbreviated as a printed board, is a basic component used to connect and support electronic components in electronic devices. Conductive paths are formed on an insulating substrate through copper foil lines to achieve electrical connections between electronic components. During the manufacturing and assembly processes of PCB boards, contaminants such as flux residues, solder balls, dust, and fingerprints often remain. These residues, especially flux residues, may cause short circuits, leakage, and corrosion in the circuit, seriously affecting the electrical performance and long-term reliability of the PCB; therefore, it is necessary to clean them.

[0003] Traditional cleaning methods include simple manual brushing, solvent wiping, and single spray or ultrasonic cleaning. However, as PCBs develop towards high density and fine pitch, the gaps at the bottom of components become smaller, and traditional single cleaning methods are difficult to thoroughly remove deeply hidden contaminants and may cause damage to sensitive components. Spray cleaning may have a shadow effect, and liquid immersion cleaning has limited effects on complex structures; physical methods, such as spraying, are prone to cleaning dead zones, and improper control of ultrasonic cleaning intensity may damage components or solder joints; single chemical cleaning is insufficient, with poor penetration effects and difficulty in removing all types of contaminants. At the same time, the contaminants on the surface of PCB boards are different at different process manufacturing stages, and targeted cleaning and removal are required.

[0004] Therefore, there is an urgent need for a new cleaning technology that can perform deep cleaning, is friendly to sensitive components, and has rapid online effect verification. For this purpose, a cleaning process for PCB boards is proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a cleaning process for PCB boards. The cleaning process of the present invention is applicable to the precision cleaning process of high-density, fine-pitch, and assembled printed circuit boards, combining water-based chemical cleaning, supercritical fluid-assisted chemical dissolution, high-efficiency rinsing, and drying, aiming to thoroughly remove flux residues, micro solder balls, fingerprints, ionic contaminants, etc., while protecting sensitive components and fragile structures. The entire process flow is divided into three main stages: water-based chemical treatment before PCB board assembly, supercritical carbon dioxide-assisted chemical main cleaning after PCB board assembly, water-based countercurrent rinsing, and vacuum drying post-treatment.

[0006] By optimizing the cleaning process, technologies such as conductivity gradient control, supercritical carbon dioxide fluid cleaning combined with ethanol entrainer, chelating agent, etc. are adopted to significantly reduce the total inorganic ion residues on the PCB board surface; by using processes such as alkaline degreasing, micro-etching activation, and plasma cleaning, surface contaminants and oxide layers are systematically removed to avoid plating defects; the cleaning process is adjusted according to different stages, and supercritical carbon dioxide, entrainer and countercurrent rinsing are used to greatly improve the surface cleanliness. The PCB board after cleaning has low total inorganic ion content and low dirt content, and is suitable for the electronic field with high cleanliness requirements.

[0007] To achieve the above object, the present invention provides the following technical solutions: The present invention provides a cleaning process for PCB boards. The cleaning process is as follows: Before assembly, during the inner layer manufacturing stage, it is immersed in an aqueous solution containing sodium hydroxide and non-ionic surfactant for cleaning, and after impregnation, it is transferred to acid etching, chelation and rinsing; before hole metallization, swelling, reduction neutralization and rinsing are carried out for cleaning; before electroplating, acid activation treatment and rinsing are carried out for activation cleaning; before solder mask, it is soaked, rinsed and assisted in cleaning with a water-based cleaning agent; before surface treatment, it is cleaned by using a solution containing dilute sulfuric acid and fluorocarbon surfactant, and then rinsed; the non-ionic surfactant is alkylphenol polyoxyethylene ether, Hubei Chushengwei Chemical Co., Ltd.; the fluorocarbon surfactant, Guangdong Wengjiang Chemical Reagent Co., Ltd.; the chelating agent is disodium ethylenediaminetetraacetate; before and after assembly, according to the cleaning process of the present invention, the resistance, capacitance, inductance, etc. of the final PCB board are not affected; Through alkaline degreasing, the alkalinity of sodium hydroxide is used to promote the saponification of grease, and the surfactant emulsifies the non-saponified oil stains and reduces the surface tension to help the cleaning solution penetrate and disperse the particulate matter; in the subsequent micro-etching step, the copper surface roughness (Ra) is controlled in the range of 0.2 - 0.5 μm to enhance the adhesion of the subsequent dry film or solder mask; during the chelation treatment and cleaning process, the chelating agent forms a stable complex with metal ions to prevent them from redepositing on the copper surface or entering the rinsing water, affecting the subsequent processes or polluting the rinsing tank.

[0008] Before electroplating, acid activation treatment and rinsing are carried out for activation cleaning; the acidic solution dissolves the trace oxides on the copper surface, and the organic sulfonic acid enhances the dissolution ability of organic substances; BTA adsorbs on the surface of the plated metal to form a protective film to prevent re-oxidation before entering the electroplating tank, and at the same time maintain the surface activity; then sufficient water rinsing is carried out to ensure that the residues of the activation solution are completely removed.

[0009] Cleaning before surface treatment: Rinse thoroughly with a large flow rate to remove loose surface contaminants and most soluble ions; Use dilute acid to remove trace oxides on the copper surface, and a fluorocarbon surfactant significantly reduces the surface tension (which can reduce the surface tension of water to below 20 mN / m), ensuring that the solution thoroughly wets the microscopic structure of the copper surface and enters the microscopic uneven areas. This improves the cleaning efficiency and is beneficial for the uniform growth of the subsequent OSP layer. Finally, perform a thorough rinse with ultrapure water to ensure the complete removal of acid and surfactant residues.

[0010] After assembly, perform alkaline degreasing, micro-etching, acid activation, and rinsing pretreatment; Add supercritical carbon dioxide containing ethanol during the main cleaning stage, release pressure and cool down after dynamic circulation in the reaction kettle; In the post-treatment stage, perform three-stage countercurrent rinsing, filtration, and drying to obtain the PCB board.

[0011] Preferably, during the inner layer manufacturing stage, a large amount of contaminants (such as grease, particulate matter, metal ions) and cleaning agent residues need to be removed and dissolved. After rinsing, the conductivity of the water is 150 - 500 μS / cm; During the cleaning before hole metallization, the acid / alkali and organic solvent components in the cleaning solution need to be removed, and contaminants need to be prevented from being reversely adsorbed onto the PCB surface due to concentration differences. After rinsing, the conductivity of the water is 60 - 140 μS / cm; During the activation cleaning before electroplating, the conductivity of the water after rinsing is 20 - 30 μS / cm; During the cleaning before surface treatment, as the cleaning process progresses, gradually reduce the conductivity, focus on removing residual micron-sized particles, organic chelates, and metal complexes, dilute residual ions through ion exchange, and at the same time avoid premature introduction of low-purity water causing secondary pollution. After rinsing, the conductivity of the water is 10 - 30 μS / cm; During the pretreatment of the assembled PCB finished board, the conductivity of the water after rinsing is 5 - 10 μS / cm; In the post-treatment stage, use water with a low resistivity to thoroughly remove nano-scale ionic impurities, prevent electrochemical migration or oxidation corrosion caused by trace surface residues, and the conductivity of the water before filtration is 0.5 - 1.5 μS / cm.

[0012] Preferably, the temperature of alkaline degreasing during the inner layer manufacturing stage cleaning is 40 - 55 °C; Acid etching and chelation are carried out simultaneously; The treatment temperature of acid etching and chelation is 25 - 35 °C.

[0013] Preferably, a dimethylformamide ethanol solution is used for swelling during the cleaning before hole metallization; The swelling temperature is 50 - 60 °C, and the time is 5 - 10 min; For reduction and neutralization, use an alkaline solution of potassium permanganate and sodium hydroxide to reduce at 60 - 70 °C for 4 - 8 min, and then soak and neutralize in a solution containing sulfuric acid and hydrogen peroxide.

[0014] Before hole metallization, the swelling agent penetrates and softens the epoxy resin smear generated by drilling during cleaning, making it easier to be decomposed by subsequent oxidants. The oxidation process after soaking is to oxidize and decompose the epoxy resin long chain through permanganate ions to generate soluble substances. Sulfuric acid neutralizes the residual lye, and hydrogen peroxide or a reducing agent reduces the residual brown-black insoluble manganese dioxide on the hole wall to colorless soluble manganese ions or other manganese compounds. The above residual substances are removed during the rinsing process.

[0015] Preferably, the raw materials for preparing the water-based cleaning agent for cleaning before solder mask include deionized water, treatment acid, sodium polyacrylate, and APG0810. The treatment acid is one of citric acid, hydrochloric acid, and oxalic acid. Sodium polyacrylate, Guangdong Wengjiang Chemical Reagent Co., Ltd.; APG0810, Sigma-Aldrich (Shanghai) Trading Co., Ltd.

[0016] More preferably, the treatment acid is citric acid.

[0017] Preferably, the auxiliary cleaning during cleaning before solder mask is to use a mixed plasma of argon and oxygen to clean at a total gas flow rate of 200 - 500 sccm.

[0018] The water-based cleaning agent used for cleaning before solder mask has good compatibility with the solder mask material and the substrate. Citric acid provides a weak acidic environment, which helps to dissolve trace inorganic substances and activate the surface, while being mild to copper and the solder mask material. Sodium polyacrylate acts as a dispersant to remove particulate matter and complex metal ions. The surfactant enhances the wetting and emulsifying ability. Subsequently, thorough rinsing with deionized water removes nanoscale organic residues, further activates the surface, and enhances the solder mask adhesion.

[0019] Preferably, the addition amount of ethanol in the main cleaning stage is 3 - 5 wt% of supercritical carbon dioxide; the pressure control for dynamic circulation in the reaction kettle is 12 - 18 MPa; the flow rate is 8 - 12 L / min; the pressure relief time is 0.8 - 1.5 h; the dynamic circulation time in the reaction kettle is 15 - 17 min.

[0020] The super-strong penetration of supercritical carbon dioxide makes up for the deficiencies of water-based or solvent cleaning in low-clearance situations; the ethanol entrainer enhances the dissolution of polar flux components; the adjustment of dynamic pressure and time improves the process adaptability and efficiency; after pressure reduction, carbon dioxide quickly gasifies and separates from the dissolved / suspended pollutants and ethanol, and carbon dioxide can be recycled. The treatment time is calculated according to the following formula: t = 15 + 0.5(n - 8) (n ≥ 8, n is the number of layers of the PCB board; unit: min); Preferably, the spray pressure for three-stage countercurrent rinsing in the post-treatment stage is 0.1 MPa - 0.3 MPa; the drying temperature is 60 - 80 °C.

[0021] Three - stage counter - current low - pressure spraying ensures the complete removal of trace entrained agents, dissolved pollutants, and ionic residues that may not have been completely carried out during the supercritical process, which may remain during the main cleaning stage; the vacuum environment reduces the boiling point of water and accelerates evaporation; infrared radiation directly heats the PCB to improve drying efficiency; controlling the drying temperature within the above range can not only accelerate water evaporation and improve drying efficiency, but also prevent thermal damage and deformation of materials, while inhibiting metal oxidation and pollution, thus preventing pollution to the PCB board. Nitrogen purging further removes moisture and provides an inert environment to avoid secondary oxidation on the PCB surface at high temperatures, ensuring that the surface moisture content is less than 50 ppm. The plating metals used can be substances such as copper, tin, nickel, and gold.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By optimizing the conductivity gradient control in multiple stages of the cleaning process, supercritical carbon dioxide fluid cleaning combined with ethanol entrained agent, and the application of chelating agents, the total inorganic ion residues on the PCB board surface are significantly reduced. By utilizing the high permeability and dissolution ability of supercritical carbon dioxide, polar and non - polar pollutants are removed deep into the microporous structure, and the ethanol entrained agent further enhances the dissolution efficiency; the chelating agent complexes metal ions to prevent their redeposition, ensuring the complete removal of ions during the rinsing process; by dynamically adjusting the cleaning parameters and controlling the conductivity in stages, the electrochemical corrosion problem caused by ion residues is effectively avoided, enabling the PCB board to maintain stable electrical performance in a complex circuit environment.

[0023] 2. Before PCB board assembly, through a precisely designed cleaning process, including key steps such as alkaline degreasing, micro - etching activation, and plasma cleaning, systematic removal of pollutants and oxide layers on the PCB surface is achieved. The alkaline degreasing and micro - etching processes work together to remove grease and organic residues, while controlling the roughness of the copper surface to enhance the adhesion of subsequent processes; plasma cleaning completely decomposes nano - scale organic substances through high - energy ion bombardment and activates the surface; for special areas such as pads and hole walls, through the application of weak acid treatment and fluorocarbon surfactants, the cleanliness and wettability of the micro - structure are ensured; thus effectively avoiding defects such as brittle plating, peeling, and pink circles, keeping the PCB surface uniform and flat during assembly and soldering processes, and meeting the process requirements of high - precision electronic devices.

[0024] 3. By adjusting the cleaning process before and after PCB board assembly, and reasonably optimizing the process according to the different stages of inner layer manufacturing, before hole metallization, before electroplating, before solder mask, and the main cleaning and post-treatment stages after assembly, the surface cleanliness of the PCB board has been significantly improved. In the main cleaning stage, the synergistic use of supercritical carbon dioxide and entraining agents, and countercurrent rinsing through staged dilution and directional water flow flushing gradually remove the residual entraining agents and particulate matters; ceramic membrane filtration accurately intercepts dissolved ions and submicron-level pollutants to prevent their recontamination of the surface; vacuum drying combined with nitrogen purging accelerates the volatilization of moisture, and inert gas protection avoids secondary oxidation at high temperatures. The synergistic effect of the above processes reduces the surface dirt amount to an extremely low level, meeting the industry cleanliness standard, and is especially suitable for fields with strict cleanliness requirements such as 5G communication and automotive electronics. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a flowchart of the cleaning process of the PCB board of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] Please refer to Figure 1 , the present invention provides a cleaning process for a PCB board, and the technical solution is as follows: Embodiments 1-5 S1 Cleaning before assembly The surface cleanliness of the PCB board before assembly is Grade 1. The cleanliness classification is as follows: The corresponding relationship between the surface dirt amount (µg / cm 2 ) and the surface cleanliness grade is as follows: ≥5µg / cm 2 - Grade 1, 2.5µg / cm 2 - Grade 2, 1.6µg / cm 2 - Grade 3, 1.25µg / cm 2 - Grade 4, 1µg / cm 2 - Grade 5, 0.75µg / cm 2 - Grade 6, 0.4µg / cm 2 - Grade 7, 0.25µg / cm 2 - Grade 8, 0.1µg / cm 2 - Grade 9, 0.01µg / cm 2 - Grade 10.

[0028] S11 Cleaning in the inner layer manufacturing stage Immerse the inner layer board in an aqueous solution containing 5 wt% sodium hydroxide and 0.5 wt% non-ionic surfactant alkylphenol polyoxyethylene ether, control the temperature at 40 - 55 °C, treat for 4 min, and maintain the solution pH value at 11; after taking it out, immerse it in a solution containing 6 wt% hydrogen peroxide, 5 wt% sulfuric acid, and 0.3 wt% disodium ethylenediaminetetraacetate chelating agent, at a temperature of 25 - 35 °C, treat for 3 - 4 min; then rinse until the conductivity of the water is 150 - 500 μS / cm; Cleaning before metallization of S12 holes Immerse in a swelling agent solution of dimethylformamide and ethanol, with the mass concentration of dimethylformamide being 15 wt%, soak at 50 - 60 °C for 5 - 10 min; after rinsing, transfer it to a strongly oxidizing alkaline solution containing 6 wt% potassium permanganate and 4% sodium hydroxide, raise the temperature to 60 - 70 °C, treat for 4 - 8 min; then soak in a solution containing 5 wt% sulfuric acid and 1.5 wt% hydrogen peroxide for 20 min, and then use rinsing water to rinse until the conductivity of the water is 60 - 140 μS / cm.

[0029] Activating cleaning before electroplating of S13 Treat in an acidic activating solution containing 5 - 10 wt% sulfuric acid and 3 wt% methanesulfonic acid, with the solution pH value < 2. Add 0.01% benzotriazole (BTA) as a corrosion inhibitor, treat for 2 min at room temperature of 26 °C, and then conduct sufficient water rinsing to make the conductivity of the water 20 - 30 μS / cm Cleaning before solder mask of S14 Use a water-based cleaning agent for immersion cleaning. The water-based cleaning agent consists of deionized water, 2 wt% citric acid, 0.5 wt% sodium polyacrylate, and 0.3 wt% APG0810, maintain the pH at 5, soak at room temperature for 5 min; then conduct sufficient deionized water rinsing; after cleaning, conduct Ar / O2 (4:1 ratio) mixed plasma-assisted cleaning, with the total gas flow rate of 200 - 500 sccm, power of 500 W, treatment time of 5 min, and chamber pressure of 50 mTorr; Cleaning before surface treatment (organic solderability protection OSP) of S15 First, use ultrapure water with a resistivity > 8 MΩ·cm to rinse thoroughly at a flow rate of 20 L / min; then conduct weak acid treatment, use a 1 wt% dilute sulfuric acid solution, add 0.05% fluorocarbon surfactant, treat at room temperature for 1 - 2 min, and finally conduct thorough ultrapure water rinsing to make the conductivity of the water 10 - 30 μS / cm.

[0030] Precision cleaning process for the finished board after S2 assembly Pretreatment stage of S21 The PCB is first treated in an alkaline degreasing tank for 90 s; the degreased PCB enters the micro-etching tank for 60 s; and finally, it is treated in the acid activation tank in the pretreatment stage for 30 s; it is rinsed until the conductivity of the water is 5 - 10 μS / cm; the solution in the alkaline degreasing tank is the same aqueous solution of 1.5 wt% sodium hydroxide and 0.2 wt% non-ionic surfactant alkylphenol polyoxyethylene ether; the solution in the micro-etching tank is a solution of 4 wt% hydrogen peroxide, 3 wt% sulfuric acid, and 0.3 wt% disodium ethylenediaminetetraacetate chelating agent; the solution in the acid activation tank is an acidic activation solution of 3 wt% sulfuric acid and 1 wt% methanesulfonic acid.

[0031] S22 Main cleaning stage Using electronic grade supercritical carbon dioxide with a purity ≥ 99.995%, the temperature of the reaction kettle is controlled at 50 °C, and the pressure is controlled at 12 - 18 MPa; 3 - 5 wt% ethanol is injected into the supercritical carbon dioxide as an entrainer, and the supercritical carbon dioxide / ethanol mixed fluid circulates dynamically in the reaction kettle at a flow rate of 8 - 12 L / min for 15 - 17 min. After the cleaning is completed, the pressure of the reaction kettle is reduced to 5 MPa within 0.8 - 1.5 h through a control valve, and the temperature is reduced to 25 °C simultaneously.

[0032] S23 Post-treatment stage (countercurrent rinsing + vacuum drying) Countercurrent rinsing: After the PCB is taken out of the main cleaning kettle, it immediately enters a three-stage countercurrent rinsing system; the rinsing medium is ultrapure water. Through three-stage countercurrent, spraying is carried out at 0.1 MPa - 0.3 MPa; the water in each rinsing tank flows at a flow rate of 15 L / min, and the spraying time for each stage is 1 min. Moreover, the "cleaner" water from the latter stage is supplied to the former stage, the "dirtiest" water is discharged from the first stage, and the cleanest fresh water enters the last stage to achieve high water conservation and high rinsing effect; the conductivity of the water in the last stage of rinsing is strictly controlled at 0.5 - 1.5 μS / cm; the rinsing water is filtered through a 0.1 μm ceramic membrane to remove particulate matter and dissolved ions and is partially recycled; the rinsed PCB enters the vacuum drying system. The system maintains a vacuum degree of -0.08 MPa, and at the same time, the surface temperature of the PCB is raised to 60 - 80 °C through infrared radiation. The entire drying process lasts for 8 - 10 min, and the drying process is supplemented with nitrogen purging (O2 < 50 ppm), and finally, a completely cleaned PCB board is obtained.

[0033] The specific processes of the examples are shown in Tables 1-1 to 1-3.

[0034] Table 1-1 Specific conditions of the cleaning process Table 1-2 Cleaning process before assembly Table 1-3 Cleaning process after assembly Comparative example Except for the following changed conditions, other cleaning processes remain unchanged, and other conditions are the same as those in Example 3.

[0035] Comparative example 1 In all cleaning process stages involving water rinsing, the conductivity of the water after rinsing is kept at 20 μS / cm.

[0036] Comparative example 2 In all cleaning process stages involving water rinsing, deionized water (0.1 μS / cm) is used for rinsing.

[0037] Comparative example 3 In S11, by extending the acid etching time to 8 min, the surface roughness is 1.0 μm.

[0038] Comparative example 4 In S11, treatment is carried out without adding a chelating agent.

[0039] Comparative example 5 In S11, the pH value is maintained at 8.

[0040] Comparative example 6 In S12, swelling is not carried out using dimethylformamide.

[0041] Comparative example 7 After swelling in S12, reduction neutralization is not carried out using a strongly oxidizing alkaline solution, and the rinsing step is not carried out either.

[0042] Comparative example 8 After treatment of S13 with an acidic activation solution, rinsing with water is not carried out.

[0043] Comparative example 9 In S13, the pH of the solution is adjusted to 4, and the concentration of sulfuric acid is correspondingly reduced to 3 wt%.

[0044] Comparative example 10 In S14, auxiliary cleaning is not carried out using a mixed plasma.

[0045] Comparative example 11 In S14, hydrochloric acid is used instead of citric acid.

[0046] Comparative example 12 In S14, oxalic acid is used instead of citric acid.

[0047] Comparative example 13 The pH of the treatment in S14 is 2.0.

[0048] Comparative example 14 In S15, a fluorocarbon surfactant is not added.

[0049] Comparative example 15 In S15, treatment is not carried out using a 1 wt% dilute sulfuric acid solution, and the treatment solution only contains 0.05% of a fluorocarbon surfactant, with the balance being water.

[0050] Comparative example 16 The S21 pretreatment process is not carried out, that is, alkaline degreasing treatment and acid activation treatment are not carried out.

[0051] Comparative Example 17 During the main cleaning stage of S22, no ethanol entrainer was added.

[0052] Comparative Example 18 S22 was not treated with supercritical carbon dioxide and ethanol.

[0053] Comparative Example 19 The pressure reduction time of the reaction kettle of S22 was 10 min.

[0054] Comparative Example 20 S23 used single-stage spraying, and the total spraying time was 5 min; and it was not treated by ceramic membrane filtration.

[0055] Comparative Example 21 The O2 content during the treatment of S23 was 200 ppm.

[0056] Experimental Example 1 The PCB boards obtained by the cleaning processes of the examples and Comparative Examples 1, 2, 11, and 16-20 were monitored for total inorganic ions. The total inorganic ion content was monitored by using ion chromatography. At the same time, the PCB board without the cleaning process was used as a blank control group. The final test results are shown in Table 2.

[0057] Table 2 Test results of total inorganic ion content For the PCB boards obtained by the cleaning process of the present invention, after the examples were cleaned, the total inorganic ion content was significantly reduced. The total inorganic ion content of Examples 1-5 was 4.3-5.5 µg / cm 2。During the entire preparation process, by adjusting the conductivity of the water used for rinsing and gradually reducing the conductivity in accordance with the process, the ion residue can be effectively reduced; the cleaning processes before and after PCB board assembly are optimized. In the cleaning process, the supercritical fluid entrainment ethanol treatment method is combined, so that the PCB board has a low total inorganic ion content. In Comparative Example 1, low-conductivity rinsing water was used for treatment in all stages, and the total inorganic ion content increased, which was not conducive to the removal of total inorganic ions; rinsing water with a relatively high conductivity can remove most visible impurities, oil stains and some ionic pollutants, and at the same time helps to dissolve acids, surfactants and other substances added in different stages. Low-conductivity rinsing water has a low ion content and can more effectively remove these residual fine ionic impurities through ion exchange and dissolution, while avoiding introducing new ionic pollutants itself, thus ensuring a high cleanliness and a low ion residue level on the surface of the PCB board, meeting its high-precision electrical performance requirements. High-conductivity and low-conductivity rinsing water are combined at different stages to achieve the removal of total inorganic ions; in Comparative Example 2, deionized water was used for rinsing, and good removal effect on total inorganic ions could not be achieved; in Comparative Example 11, hydrochloric acid introduced chloride ions, and the strong acidity caused corrosion of the copper surface, increasing the metal ion residue, resulting in a higher total inorganic ion content than that of the blank control group. In Comparative Example 16, alkaline degreasing and acid activation were missing, and grease and oxides were not removed, and subsequent cleaning could not make up for it, resulting in a decline in cleaning effect; in Comparative Example 17, the lack of ethanol led to insufficient solubility of supercritical carbon dioxide in polar soldering fluxes, increasing the residual pollutants; in Comparative Example 18, supercritical carbon dioxide and ethanol were not used for treatment, and pollutants and ions could not be effectively removed, but the cleaning effect was slightly better than that of the blank control group; in Comparative Example 19, the pressure reduction time was too fast, resulting in insufficient gasification of carbon dioxide, and the entrainer and pollutants remained on the surface of the PCB board. Therefore, slow pressure relief is required to achieve a good cleaning effect; in Comparative Example 20, single-stage spraying and no filtration were used, the countercurrent rinsing efficiency was low, and there was no ceramic membrane filtration, so ions and particulate matters were not intercepted, and the cleaning effect was significantly reduced.

[0058] Experimental Example 2 The appearance of the PCB boards after cleaning in the above examples and Comparative Examples 3, 4, 6 - 8, 10, 12, 14, 16 - 20 was inspected, and the final results are shown in Table 3.

[0059] Table 3 Quality Inspection Results The PCB boards obtained according to the cleaning process of the present invention have good surface states under the conditions of the examples. Under the conditions of Comparative Example 3, due to the excessive acid etching time, the copper surface was over-etched, its surface roughness increased, on the one hand, metal ions were released, on the other hand, the adhesion of dirt increased, and the metal and dirt were mixed into the coating, making it brittle and discolored; in Comparative Example 4, no chelating agent was added, and copper ions redeposited on the copper surface, interfering with the coating composition; in Comparative Example 6, no swelling agent was used, the epoxy resin slag was not completely removed, and the residual oxidant caused oxidation of the copper surface and pink circles; in Comparative Example 7, no alkali solution was used for neutralization, and the acidic environment corroded the copper surface, forming an oxide layer; in Comparative Example 8, no acidic activation was carried out, the copper surface oxide was not removed, and the oxide layer hindered the coating bonding, resulting in a change in the color of the PCB board; in Comparative Example 10, no plasma cleaning was used, and the organic residues adsorbed the activator, resulting in uneven coating and coating peeling, and the copper surface was rough; in Comparative Example 12, oxalic acid was used instead of citric acid, which chelated with the coating metal ions and was difficult to be completely removed. Oxalic acid has strong acidity and is difficult to rinse, and the residual acid solution corrodes the copper surface and interferes with the coating growth; in Comparative Example 14, no fluorocarbon surfactant was added, the cleaning solution failed to wet the microstructure, and the residual oxide caused coating defects; in Comparative Example 16, no alkaline degreasing and acid activation were carried out, the oil and oxide pollutants were not removed, interfering with the coating bonding, resulting in copper particles and coating peeling; in Comparative Example 17, no ethanol entrainer was used, supercritical carbon dioxide could not dissolve polar pollutants, and there were solder resist residues, hindering the coating deposition; in Comparative Example 18, no supercritical carbon dioxide cleaning was used and no entrainer was added, the pollutants and ions were not completely removed, directly resulting in coating defects and a significant decrease in the quality of the PCB board; in Comparative Example 19, the pressure reduction time was too short, the carbon dioxide gasification was insufficient, the residual entrainer interfered with the coating composition, and the residual ethanol and pollutants decreased the quality of the PCB board; in Comparative Example 20, single-stage spraying was carried out without ceramic membrane filtration, the rinsing efficiency was low, the particulate matter and ions were not intercepted, resulting in obvious color difference. However, the effects of the final products of the PCB boards obtained according to the above cleaning process are all better than those of the blank control group.

[0060] Experimental Example 3 Quality inspections were carried out on the PCB boards of the examples and Comparative Examples 5, 9, 13, 15, and 21. The test results are as follows: The surface states of the PCB boards of Examples 1-5 are all good; Pink circles are generated on the surfaces of the PCB boards obtained according to the cleaning processes of Comparative Examples 5, 9, 13, 15, and 21.

[0061] Among them, the pH value of Comparative Example 5 is too low, the alkaline degreasing is insufficient, and the residues of grease and organic matter are not completely removed. The residues promote the oxidation of the copper surface in the subsequent process, forming cuprous oxide; the content of sulfuric acid in Comparative Example 9 is low, the acid activation is insufficient, and the oxide layer of the coating is not fully dissolved. The residual oxides continue to oxidize in the subsequent process, generating oxides with the characteristics of pink circles; in Comparative Example 13, the acidity is too strong to corrode the coating surface, releasing metal ions and accelerating the oxidation reaction. At the same time, the protection of the solder mask layer is damaged, resulting in local oxidation to form pink circles; in Comparative Example 15, dilute sulfuric acid is not used for treatment, and the oxide layer of the coating is not effectively removed by dilute sulfuric acid. The residual metal oxides react unevenly with the organic film during the OSP treatment, and oxidation occurs in the exposed areas; in Comparative Example 21, the oxygen concentration is too high during the purging process. The high-oxygen environment promotes the oxidation of the metal during vacuum drying, forming a metal oxide layer, presenting a pink color. The above results show that the PCB boards obtained by the cleaning process according to the present invention have good cleaning effects under the conditions of the examples.

[0062] Experimental Example 4 The PCB boards obtained by the cleaning processes of all the examples and comparative examples were subjected to surface cleanliness detection, and the final detection results are shown in Table 4.

[0063] Table 4 Cleanliness test results The PCB board obtained by the cleaning process according to the present invention has a surface cleanliness of level 10 under the conditions of the examples, and the cleaning effect of the PCB board corresponding to the comparative cleaning process has decreased. Among them, in Comparative Example 1, the conductivity was not properly controlled, resulting in a high ion residue; in Comparative Example 2, deionized water was used for rinsing throughout the process, and the total inorganic ion residue reached 7.5 μg / cm², and the surface dirt amount was ≥ 5 μg / cm²; in Comparative Examples 3-5, during the processes of alkaline degreasing, acid etching, and chelation, by adjusting the acid etching time, system pH, and chelation conditions in the treatment process, it was found that the surface cleanliness of the PCB board was finally reduced. Among them, too long acid etching time led to surface roughness, and the dirt amount was 0.75 μg / cm², but due to the mixing of metal ions into the coating, it was actually brittle and discolored; in Comparative Examples 6-10, the parameters of the S12 to S14 processes were adjusted, and the results showed that swelling treatment must be carried out in S12, and then epoxy neutralization and rinsing processes must be passed through. Otherwise, the cumulative effect of the remaining pollutants later will significantly reduce the surface cleanliness; after the acid activation of S13, rinsing treatment should be carried out, and the pH of the solution should be maintained in a strongly acidic environment; at the same time, mixed plasma is used for auxiliary cleaning to make the surface cleanliness reach the optimum; in Comparative Example 11, hydrochloric acid introduced Cl⁻, corroded the copper surface, increased the total inorganic ion content, and corroded the components of the PCB board, and dirt accumulated on its surface; the results of Comparative Example 12 showed that the effect of using citric acid was better than that of oxalic acid because the corrosion effect brought by oxalic acid was long-term, reducing the surface cleanliness of the PCB board; the results of Comparative Example 13 showed that the pH before solder mask cleaning should be maintained within a reasonable range; the results of Comparative Examples 14 and 15 showed that it was necessary to add a surfactant and perform acid treatment during the cleaning before surface treatment (organic solderability protection OSP); in Comparative Example 16, alkaline degreasing and acid activation were not carried out, and the residual grease and oxides led to an increase in the dirt amount; the results of Comparative Examples 17-19 showed that during the main cleaning stage of the assembled finished board, by using supercritical carbon dioxide and ethanol entrainer, and adjusting the time of pressure reduction, the residual dirt amount could be significantly reduced, and the use of supercritical carbon dioxide and entrainer could minimize the dirt amount; in Comparative Example 20, a single-stage spray was used instead of a multi-stage method, and at the same time, it was not filtered, resulting in a reduction in surface cleanliness; in Comparative Example 21, too high oxygen treatment concentration would also increase the dirt amount.

[0064] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A cleaning process for a PCB board, characterized in that: The cleaning process is as follows: Before assembly, during the inner layer manufacturing stage, it is cleaned by immersing it in an aqueous solution containing sodium hydroxide and non-ionic surfactant. After impregnation, acid etching, chelation, and rinsing are carried out; before hole metallization, swelling, reduction neutralization, and the said rinsing are carried out for cleaning; before electroplating, acid activation treatment and the said rinsing are carried out for activation cleaning; before solder mask, it is soaked with a water-based cleaning agent, the said rinsing, and auxiliary cleaning are carried out; before surface treatment, it is cleaned by using a solution containing dilute sulfuric acid and fluorocarbon surfactant, and then the said rinsing is carried out; After assembly, alkaline degreasing, micro-etching, acid activation, and rinsing pre-treatment are carried out; in the main cleaning stage, supercritical carbon dioxide containing ethanol is added, and after dynamic circulation in the reaction kettle, the pressure is released and the temperature is reduced; in the post-treatment stage, three-stage countercurrent rinsing, filtration, and drying are carried out to obtain the said PCB board.

2. The cleaning process of a PCB board according to claim 1, characterized in that: The conductivity of the water after the said rinsing during the inner layer manufacturing stage is 150 - 500 μS / cm; the conductivity of the water after the said rinsing in the cleaning before hole metallization is 60 - 140 μS / cm; the conductivity of the water after the said rinsing in the activation cleaning before electroplating is 20 - 30 μS / cm; the conductivity of the water after the said rinsing in the cleaning before surface treatment is 10 - 30 μS / cm; the conductivity of the water after the said rinsing in the pre-treatment is 5 - 10 μS / cm; the conductivity of the water before the said filtration in the post-treatment stage is 0.5 - 1.5 μS / cm.

3. The cleaning process of a PCB board according to claim 1, characterized in that: The temperature of alkaline degreasing during the cleaning in the inner layer manufacturing stage is 40 - 55 °C; the acid etching and the chelation are carried out simultaneously; the treatment temperature of the acid etching and the chelation is 25 - 35 °C.

4. The cleaning process of a PCB board according to claim 1, characterized in that: In the cleaning before hole metallization, dimethylformamide ethanol solution is used for the said swelling; the temperature of the swelling is 50 - 60 °C, and the time is 5 - 10 min; the reduction neutralization is to reduce with a basic solution of potassium permanganate and sodium hydroxide at 60 - 70 °C for 4 - 8 min, and then soak and neutralize in a solution containing sulfuric acid and hydrogen peroxide.

5. The cleaning process of a PCB board according to claim 1, characterized in that: The raw materials for preparing the said water-based cleaning agent in the cleaning before solder mask include ionized water, treatment acid, and sodium polyacrylate; the treatment acid is one of citric acid, hydrochloric acid, and oxalic acid.

6. The cleaning process of a PCB board according to claim 1, characterized in that: In the cleaning before solder mask, the auxiliary cleaning is to clean with a mixed plasma of argon and oxygen at a total gas flow rate of 200 - 500 sccm.

7. The cleaning process of a PCB board according to claim 1, characterized in that: In the main cleaning stage, the addition amount of the said ethanol is 3 - 5 wt% of the supercritical carbon dioxide; the pressure control of the dynamic circulation in the reaction kettle is 12 - 18 MPa; the flow rate is 8 - 12 L / min; the time for pressure release is 0.8 - 1.5 h.

8. A cleaning process for a PCB board according to claim 1, characterized in that: The spraying pressure of the three-stage countercurrent rinsing in the post-treatment stage is 0.1 MPa - 0.3 MPa; the temperature of the drying is 60 - 80 °C.

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