Chemical tin immersion method and tin immersion solution for PCB (Printed Circuit Board)

Abstract

The invention belongs to the technical field of printed circuit board manufacturing, and particularly relates to a chemical tin immersion method and a tin immersion solution of a PCB (printed circuit board), the method and the solution adopt a couple replacement and autocatalytic reduction dual mechanism, and a tin-bismuth alloy layer is deposited on a copper bonding pad of the PCB by introducing a reducing agent and bismuth ions. The tin immersion solution comprises divalent tin salt, an acidic component, a complexing agent, a reducing agent, bismuth salt and an antioxidant. The thickness, the compactness and the uniformity of the tin layer are effectively improved, IMC growth is inhibited, and the weldability, the oxidation resistance and the long-term reliability of the PCB bonding pad are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of printed circuit board manufacturing, and particularly relates to a chemical tin plating method for a PCB and a tin plating solution. BACKGROUND

[0002] As a core carrier of modern electronic information industry, the function and reliability of a PCB are highly dependent on surface treatment processes, especially pad surface treatment, which not only provides an ideal welding interface for component assembly but also protects the copper circuit from oxidation. Among various PCB surface treatment technologies, chemical tin plating is widely used in fine line and high-density interconnection scenarios due to its simple process flow, high cost-effectiveness, excellent solderability and co-planarity of the tin layer. Traditional chemical tin plating technology takes galvanic displacement reaction as the core: after the clean activation of the PCB copper surface, the copper atoms are preferentially dissolved in the acid solution containing divalent tin salt (such as stannous sulfate), and the divalent tin ions in the solution are reduced and deposited on the copper surface to form a pure tin or tin-rich layer; the tin plating solution usually contains divalent tin salt (tin source), complexing agent (stabilizing tin ions, preventing oxidation), acidic component (maintaining pH value, promoting copper dissolution) and additive (suppressing pores, increasing rate, uniform tin layer), by adjusting the component concentration, solution temperature and immersion time, a thin and uniform tin layer can be formed to prevent copper oxidation and provide a good wetting basis for SMT welding, which can meet the basic needs of electronic product surface treatment for a long time. However, with the development of electronic products towards miniaturization, high integration and high reliability, the limitations of traditional chemical tin plating technology are highlighted, and the core problem is the self-limiting nature of the galvanic displacement reaction: the reaction rate drops sharply after the copper surface is covered with a tin layer, and the deposited tin layer is only 0.5-1.5 microns thick, which has inherent defects. Firstly, the thin tin layer and micro-pores can easily expose the underlying copper to oxidation during storage and service, which weakens the solderability of the pad; secondly, tin and copper can form intermetallic compounds (such as Cu6Sn5, Cu3Sn) through solid-state diffusion, and the thin tin layer can be completely consumed during preparation, storage and welding, and converted into a brittle IMC layer, which has poor wettability, leading to false soldering and cold soldering in SMT assembly, and the brittle Cu3Sn layer can reduce the mechanical strength and fatigue resistance of the solder joint, which is prone to breakage under thermal shock and vibration, and cannot meet the high requirements of reliability in vehicle-mounted electronics, industrial control, aerospace and other fields.

[0003] Therefore, the application provides a chemical tin plating method for a PCB and a tin plating solution. SUMMARY

[0004] In order to make up for the deficiencies of the prior art and solve at least one technical problem raised in the background art.

[0005] The technical scheme adopted by the present application to solve its technical problems is: a PCB chemical tin plating method and a tin plating solution, which comprises the following steps: S1, a pretreatment step: the copper surface of the PCB is subjected to degreasing, rinsing, micro-etching, pickling and activation treatment to obtain a clean and well-reactive copper substrate surface; S2, a tin plating step: the pretreated PCB is immersed in the chemical tin plating solution provided by the present application, the tin plating solution is maintained at a predetermined temperature, and the process is continued for a predetermined time to deposit a tin-bismuth alloy layer on the copper surface of the PCB; S3, a post-treatment step: the PCB with the deposited tin-bismuth alloy layer is thoroughly rinsed and dried.

[0006] The tin plating step is the core of the present application, which realizes a composite deposition mechanism based on the synergistic effect of galvanic replacement reaction and autocatalytic reduction reaction through the specific chemical tin plating solution of the present application. In this mechanism, the initial tin layer is formed on the copper surface through the galvanic replacement reaction of copper on divalent tin ions, and then the surface of the formed tin layer can catalyze the reduction deposition of divalent tin ions and trivalent bismuth ions in the solution, thereby realizing the continuous growth of the tin-bismuth alloy layer, and breaking through the thickness self-limiting nature of the traditional galvanic replacement reaction.

[0007] As a preferred embodiment of the present application, the pretreatment step specifically comprises: immersing the PCB in a degreaser solution, rinsing, then immersing in a micro-etching agent solution, rinsing, then immersing in a dilute acid solution for pickling, and finally immersing in an activator solution for activation. The composition of the degreaser solution, the composition of the micro-etching agent solution, the composition of the dilute acid solution and the composition of the activator solution are all commonly used and known formulations in the prior art, and the selection and operating conditions are optimized according to the copper surface state and subsequent tin plating requirements. For example, the degreaser solution can be an alkaline degreaser, the operating temperature is 40-60°C, and the treatment time is 5-10 minutes; the micro-etching agent solution can be sodium persulfate or hydrogen peroxide / sulfuric acid mixed solution, the copper etching amount is controlled between 0.5-2.0 microns, the operating temperature is 25-40°C, and the treatment time is 1-3 minutes; the pickling solution can be a dilute sulfuric acid solution with a concentration of 5-10%, the operating temperature is room temperature, and the treatment time is 1-2 minutes; the activator solution can be a dilute hydrochloric acid or dilute sulfuric acid as a base for palladium salt activator solution, the palladium ion concentration is 10-50 ppm, the operating temperature is 20-30°C, and the treatment time is 30-90 seconds. The rinsing steps are all carried out with deionized water to ensure the removal of residual chemicals and avoid the introduction of contaminants.

[0008] As a preferred embodiment of the present application, the operating conditions of the tin plating step specifically include: immersing the PCB board in the chemical tin plating solution, the temperature of the tin plating solution is maintained between 45°C and 70°C, and the immersion time is 10 minutes to 60 minutes. During the immersion process, the tin plating solution can be moderately stirred or circulated to ensure the uniformity of the solution components and promote mass transfer. The temperature and time range is set based on ensuring sufficient electrochemical displacement reaction, activating the efficiency of the autocatalytic reduction reaction, and controlling the deposition rate and thickness of the tin-bismuth alloy layer. Lower temperature or shorter time can result in insufficient thickness or poor compactness of the tin-bismuth alloy layer, while higher temperature or longer time can result in decreased solution stability or increased roughness of the tin-bismuth alloy layer. The thickness of the tin-bismuth alloy layer formed by the present application is generally controlled at 1.5 microns to 5.0 microns, and the bismuth content in the layer is 0.1% to 5.0% (wt%), which can effectively inhibit the growth of IMC.

[0009] The present application also provides a chemical tin plating solution for implementing the above-mentioned PCB board chemical tin plating method, which mainly comprises the following components: divalent tin salt: as a metal source for tin deposition; acid component: for maintaining the pH value of the solution in the strong acidic range and promoting the dissolution of the copper substrate; complexing agent: for complexing divalent tin ions, preventing their hydrolysis and oxidation, and stabilizing the solution; reducing agent: for achieving autocatalytic reduction deposition of tin and bismuth ions; second metal salt: as a source of the second metal bismuth, for co-deposition with tin to form a tin-bismuth alloy layer and inhibit the growth of IMC; antioxidant: for inhibiting the oxidation of divalent tin ions in the solution; grain refiner / wetting agent: for improving the grain structure, uniformity, and wettability of the deposited layer.

[0010] As a preferred embodiment of the present application, the specific content range of each component in the chemical tin plating solution is as follows: The concentration of the divalent tin salt is 0.05 moles / liter (M) to 0.5 M. Preferably, the divalent tin salt is stannous methanesulfonate or stannous sulfate. Stannous methanesulfonate is more preferred because it has higher solubility and stability in acidic solutions and is less likely to produce precipitates.

[0011] The concentration of the acidic component is 0.5M to 2.0M. Preferably, the acidic component is methane sulfonic acid, sulfuric acid or hydrochloric acid. Methane sulfonic acid is more preferred due to its strong acidity, low corrosiveness and excellent compatibility with stannous methane sulfonate. The acidic component maintains the pH of the solution in the range of 0.1 to 1.0, ensuring the stable existence of tin ions and bismuth ions and the occurrence of galvanic displacement reaction.

[0012] The concentration of the complexing agent is 0.1M to 1.0M. Preferably, the complexing agent is p-phenolsulfonic acid, tartaric acid or citric acid. p-Phenolsulfonic acid is more preferred due to its excellent complexing ability to divalent tin ions and good solution stability. It can effectively prevent the hydrolytic precipitation of divalent tin ions and inhibit their oxidation by air.

[0013] The concentration of the reducing agent is 0.1M to 1.5M. Preferably, the reducing agent is formic acid, sodium formate, potassium formate or ascorbic acid and its derivatives. Formic acid is more preferred due to its mild reducing ability to tin ions under high temperature and low pH conditions and the fact that the reduction product is gas, not introducing solid impurities. The reducing agent is catalytically activated on the surface of the initial deposited tin layer, promoting the reduction of divalent tin ions to metallic tin and realizing the self-catalytic growth of the deposited layer.

[0014] The concentration of the second metal salt is 0.001M to 0.01M. Preferably, the second metal salt is bismuth methane sulfonate, bismuth citrate or bismuth nitrate. Bismuth methane sulfonate is more preferred due to its good compatibility with other components in the solution and stable bismuth ion source. The bismuth ions are co-deposited with tin ions to form a tin-bismuth alloy layer.

[0015] The concentration of the antioxidant is 0.005M to 0.05M. Preferably, the antioxidant is hydroquinone, catechol or pyrogallol. Hydroquinone is more preferred due to its high antioxidant capacity and good stability. It prevents the oxidation of divalent tin ions to insoluble tetravalent tin by capturing oxidizing free radicals in the solution or directly reducing the already oxidized tin ions, thereby prolonging the service life and stability of the solution.

[0016] The addition amount of the grain refiner / wetting agent is 10ppm to 500ppm (mass fraction). Preferably, the grain refiner is a polyether non-ionic surfactant of specific molecular weight, a specific sulfur-containing organic compound (such as mercaptosuccinic acid) or an aromatic aldehyde derivative. The wetting agent can be a fluorocarbon or an ethoxylated alkyl phenol non-ionic surfactant. The polyether non-ionic surfactant can improve the wettability of the solution, promote the uniformity of the deposited layer and refine the grains, thereby obtaining a more flat and dense tin-bismuth alloy layer.

[0017] The beneficial effects of the present application are as follows: 1. The PCB chemical tin plating method and tin plating solution of the present application, by introducing a self-catalytic reduction mechanism, overcomes the self-limiting nature of traditional galvanic displacement reaction, making the deposited tin-bismuth alloy layer thickness up to 1.5-5.0 microns, far exceeding the thickness range of 0.5-1.5 microns of traditional methods. The thicker tin layer provides a more sufficient pure tin matrix for subsequent soldering, significantly prolonging the time for the pure tin layer to be completely consumed by IMC.

[0018] 2. The PCB chemical tin plating method and tin plating solution of the present application, by composite deposition mechanism and the introduction of specific additives, the formed tin-bismuth alloy layer has lower porosity and better compactness. The compact tin layer can effectively block the erosion of external oxygen and moisture to the underlying copper, enhancing the oxidation resistance and long-term storage stability of the solder pad. At the same time, by optimizing the solution formula and process parameters, the deposited layer shows high uniformity on the entire PCB surface, avoiding local thin or thick areas.

[0019] 3. The PCB chemical tin plating method and tin plating solution of the present application, by co-deposition of bismuth element in the solution, plays a key role in inhibiting IMC in the tin-bismuth alloy layer. Bismuth can physically hinder the mutual diffusion of copper and tin atoms, change the nucleation and growth kinetics of IMC, and possibly improve the crystal structure of IMC, thereby significantly slowing down the formation rate of brittle IMC layers such as Cu6Sn5 and Cu3Sn. This ensures that the solder pad surface still retains a sufficient thickness of pure tin layer during long-term storage and solder reflow process, maintaining excellent solderability.

[0020] 4. The PCB chemical tin plating method and tin plating solution of the present application, by combining a thicker, denser and anti-IMC growth tin-bismuth alloy layer, the PCB pad of the present application exhibits excellent wetting performance during subsequent surface mount technology (SMT) soldering process, effectively avoiding soldering defects such as false soldering and cold soldering. At the same time, the thinner and uniform IMC layer and sufficient pure tin layer ensure that the solder joint has higher mechanical strength, better thermal fatigue resistance and vibration resistance, significantly improving the long-term service reliability of electronic products, especially suitable for high-reliability applications such as automotive electronics, industrial control and aerospace.

[0021] 5. The PCB chemical tin plating method and tin plating solution of the present application, while improving performance, still maintains the characteristics of relatively simple chemical tin plating process, easy to operate and no external power supply. Compared with complex electroplating or ENIG / ENEPIG process, the tin plating method of the present application has higher cost-effectiveness and the advantage of easy mass production.

[0022] 6.The PCB chemical tin plating method and tin plating solution provided by the application have excellent stability by precisely configuring each component (including an acidic component, a complexing agent and an antioxidant), can effectively inhibit spontaneous oxidation and hydrolysis of divalent tin ions, thereby prolonging the service life of the solution and reducing production costs. BRIEF DESCRIPTION OF DRAWINGS

[0023] The application will be further described below in conjunction with the drawings.

[0024] Figure 1 is a flowchart of the PCB chemical tin plating method of the application; Figure 2 is a schematic diagram of the action mechanism of the tin-bismuth alloy layer prepared by the application for inhibiting the growth of tin-copper intermetallic compounds; Figure 3 is a structural schematic diagram of the tin plating solution of the application. DETAILED DESCRIPTION

[0025] In order to make the technical means, creative features, purposes and effects achieved by the application easy to understand, the application will be further described below in conjunction with specific embodiments.

[0026] As shown in Figure 1 The PCB chemical tin plating method provided by the embodiment of the application includes three main stages of pretreatment, core tin plating and post-treatment, and each stage is precisely designed and optimized to ensure the stability and reliability of the performance of the final product.

[0027] First, the pretreatment step aims to provide a clean, activated and uniform surface morphology of the copper substrate for the subsequent chemical tin plating reaction. This step is crucial for ensuring good adhesion between the tin-bismuth alloy layer and the copper substrate, uniform deposition and avoiding defects. Specifically, the pretreatment includes a series of unit operations performed in series: degreasing, rinsing, micro-etching, pickling and activation.

[0028] Degreasing is a critical step to remove organic contaminants (e.g. fingerprints, oils, flux residues) from the copper surface. During this stage, the PCB board is immersed in an alkaline degreasing agent solution. The degreasing agent is typically composed of an alkaline salt (e.g. sodium hydroxide, sodium carbonate) and a surfactant (e.g. non-ionic or zwitterionic surfactant) with excellent emulsifying and dispersing ability. In a typical operating condition, the temperature of the degreasing agent solution is strictly controlled in the range of 45°C to 55°C to ensure the optimal performance of the surfactant and effective saponification or emulsification of the oils and fats. The treatment time is usually set to 6 to 9 minutes, which is too short may result in incomplete degreasing, while too long may cause unnecessary erosion to the copper surface. By strictly controlling the pH value of the solution (usually in the range of 10.0 to 12.0), various types of organic contaminants can be effectively removed while minimizing the corrosion of the copper substrate. After degreasing, the PCB board needs to be immediately rinsed thoroughly. The rinsing step uses deionized water with a flow rate controlled at 5 to 8 liters per minute to ensure that all residual degreasing agents are completely washed away, avoiding contamination of subsequent processes. Multi-stage countercurrent rinsing (at least two stages) is the preferred way to ensure the rinsing effect, which aims to maximize the utilization efficiency of water resources and minimize the pollutant concentration in the rinsing water.

[0029] Micro-etching is a critical step to remove the oxide layer on the copper surface and activate the copper substrate surface. It forms a fresh micro-rough structure on the copper surface by chemical corrosion, thereby increasing the surface area and improving the adhesion of the subsequent tin layer. The present invention preferably uses sodium persulfate or hydrogen peroxide / sulfuric acid mixed solution as the micro-etching agent. When using sodium persulfate solution, its concentration is usually set between 60 to 100 grams per liter, the operating temperature is controlled between 30 to 40°C, and the treatment time is 1 to 2 minutes to achieve a target copper etching amount of 0.8 to 1.5 microns. Precise control of the copper etching amount is a prerequisite for surface activity, excessive etching may cause the wire to thin, while insufficient etching may affect the uniformity and adhesion of the deposited layer. When using hydrogen peroxide / sulfuric acid mixed solution, the concentration of hydrogen peroxide can be 1% to 3% (weight percent), the concentration of sulfuric acid is 5% to 10% (volume percent), and the operating temperature and time range are similar to the sodium persulfate system. The PCB board after micro-etching also needs to be immediately rinsed with multiple stages of deionized water to ensure that the micro-etching agent residues are completely removed.

[0030] Acid pickling is to remove copper salts or oxides that may remain on the copper surface after micro-etching, while maintaining an acidic environment on the copper surface to prepare for subsequent activation. The present invention preferably uses a dilute sulfuric acid solution with a concentration of 7% to 9% for acid pickling. The operating temperature is room temperature (20 to 25°C), and the treatment time is 1 to 1.5 minutes. Acid pickling can effectively neutralize the residual alkaline substances on the copper surface and dissolve the trace amount of copper oxides that may exist, providing ideal surface conditions for activation. Subsequent deionized water rinsing is performed.

[0031] Activation is a key step to provide catalytic active sites on the copper surface to trigger the subsequent chemical tin deposition reaction. The present application preferably uses dilute hydrochloric acid or dilute sulfuric acid as the base palladium salt activator solution. The palladium salt activator deposits a small amount of palladium particles on the copper surface through a displacement reaction, which act as catalytic centers and can effectively catalyze the reduction deposition of divalent tin ions. The concentration of palladium ions is usually maintained between 20 ppm and 40 ppm, the operating temperature is controlled between 25°C and 30°C, and the processing time is 45 seconds to 75 seconds. Too short activation time may result in insufficient activation, while too long activation time may cause excessive growth of palladium particles, affecting the uniformity of the subsequent tin layer. After activation, the PCB board should be avoided for long time exposure to air and quickly enter the tin deposition step to prevent oxidation of the activated surface.

[0032] After completing the above-mentioned rigorous pretreatment steps, the core stage of the present application, the tin deposition step, is entered. In this step, the pretreated PCB board is immersed in the specific formula chemical tin deposition solution provided by the present application, and a layer of tin bismuth alloy layer with uniform thickness, high density and excellent IMC growth resistance is deposited on the copper surface under the preset temperature and time conditions.

[0033] As shown in Figure 3 The chemical tin deposition solution of the present application is the core to realize the double deposition mechanism and the function of the tin bismuth alloy layer. The design of the solution takes into account multiple aspects such as solution stability, deposition rate, alloy composition control and deposition layer performance. The main components and their mechanism are as follows: First, divalent tin salt is used as the metal source for tin deposition, and its type and concentration have a decisive influence on the deposition rate and the quality of the tin layer. The present application preferably uses stannous methane sulfonate or stannous sulfate. Stannous methane sulfonate has extremely high solubility and stability in acidic solutions, and its anion (methane sulfonate) shows good inertness in electrochemical processes and is not easy to form precipitates with copper, thereby effectively avoiding turbidity or precipitates in the solution, ensuring the stable progress of the deposition process, so it is selected as the more preferred divalent tin source. When stannous methane sulfonate is used, its concentration range is usually set to 0.15 mol / L (M) to 0.35 M. Within this concentration range, sufficient tin ion supply can be ensured to maintain a high deposition rate, while avoiding spontaneous decomposition of the solution or roughening of the deposited layer due to too high concentration.

[0034] Secondly, the acidic component is used to maintain the pH of the solution in a strong acidic range, which is crucial for inhibiting the hydrolysis of divalent tin ions, preserving the stability of the solution, and promoting the moderate dissolution of the copper substrate (in the initial stage of galvanic displacement). Methanesulfonic acid, sulfuric acid or hydrochloric acid is preferred to be used in the present invention. Methanesulfonic acid is considered to be more preferred as the acidic component due to its strong acidity, low corrosiveness and excellent compatibility with stannous methanesulfonate system, which can form a highly stable electrolyte environment. Its concentration is usually controlled between 0.8 M and 1.5 M. At this concentration, the pH of the solution can be stably maintained in the range of 0.3 to 0.8. This pH range not only ensures the stable existence of divalent tin ions and trivalent bismuth ions, preventing hydrolysis and precipitation, but also provides a suitable acidic environment for the catalytic activation of the reducing agent.

[0035] Furthermore, the complexing agent is used to complex divalent tin ions, preventing their hydrolysis and oxidation under acidic conditions, thereby significantly improving the stability of the solution. p-Phenolsulfonic acid, tartaric acid or citric acid is preferred to be used in the present invention. p-Phenolsulfonic acid is considered to be more preferred as the complexing agent due to its strong complexing ability for divalent tin ions, which can form stable complex ions, effectively inhibiting the hydrolysis of divalent tin ions to form insoluble stannous hydroxide and simultaneously inhibiting their oxidation to tetravalent tin (not involved in deposition) by oxygen in the air. Its concentration is usually controlled between 0.3 M and 0.7 M. At this concentration, the complexing agent can fully complex the divalent tin ions in the solution, ensuring that they remain highly concentrated and active over a long period of operation.

[0036] One of the key components is the reducing agent, which plays a core role in the autocatalytic reduction deposition mechanism of the present invention, breaking through the thickness self-limiting nature of traditional galvanic displacement reactions. Formic acid, sodium formate, potassium formate or ascorbic acid and its derivatives are preferred to be used in the present invention. Formic acid is considered to be more preferred as the reducing agent due to its ability to exhibit a mild and controllable reduction capacity for tin ions under high temperature and low pH conditions, and its reduction product is carbon dioxide and water, which does not introduce solid impurities or increase the complexity of the solution. The concentration of formic acid is usually controlled between 0.5 M and 1.2 M. At this concentration, formic acid can provide sufficient reduction potential to be catalytically activated on the surface of the deposited tin layer, thereby continuously driving the reduction deposition of divalent tin ions and trivalent bismuth ions. The reduction mechanism of formic acid involves the process of dehydrogenation to form carbon dioxide and provide electrons on the catalytic surface.

[0037] As Figure 2As shown, another core component is the second metal salt, as a source of the second metal bismuth, for co-deposition with tin to form the tin-bismuth alloy layer and play a key role in suppressing the growth of IMC. The present application preferably uses bismuth methane sulfonate, bismuth citrate or bismuth nitrate. Bismuth methane sulfonate is considered to be the more preferred second metal salt due to its good compatibility with other components of the methane sulfonate system in solution and the stability of the bismuth ion source, which is less likely to introduce additional impurities. Its concentration range is relatively low, typically 0.003M to 0.008M. Despite the low concentration of bismuth, its co-deposition ratio in the deposited layer is sufficient to play a significant IMC suppression effect while avoiding the embrittlement or solderability degradation of the deposited layer due to excessive bismuth content.

[0038] To extend the service life and stability of the solution, the introduction of antioxidants is crucial. It aims to inhibit the oxidation of divalent tin ions in the solution and prevent their conversion into insoluble tetravalent tin. The present application preferably uses hydroquinone, catechol or pyrogallol. Hydroquinone is considered to be the more preferred antioxidant due to its high antioxidant capacity and good chemical stability, which can effectively stabilize divalent tin ions by capturing free radicals in the solution or directly reducing a small amount of oxides that have been formed. Its concentration range is 0.01M to 0.04M. This concentration range ensures that the rate of oxidation of divalent tin ions can be effectively controlled under normal operating and storage conditions, thereby maintaining the activity and life of the solution.

[0039] Finally, the addition of grain refiners and wetting agents, although in small amounts, has a significant impact on the microstructure and macroscopic morphology of the deposited layer. They aim to improve the grain structure of the deposited layer, improve deposition uniformity and optimize the wettability of the solution on the surface of the PCB board. The present application preferably uses a specific molecular weight of polyether non-ionic surfactant, a specific sulfur-containing organic compound (such as mercaptosuccinic acid) or an aromatic aldehyde derivative as a grain refiner. The wetting agent can be a fluorocarbon or ethoxylated alkyl phenol non-ionic surfactant. The polyether non-ionic surfactant has both grain refining and wetting functions. Its addition is usually controlled between 50ppm and 300ppm (mass fraction). These additives control the formation and growth of crystal nuclei by adsorbing on the deposition active sites, so that the tin-bismuth alloy layer formed has finer and more uniform grain distribution, which macroscopically manifests as a smoother and denser deposited layer, and effectively reduces the porosity of the deposited layer.

[0040] When deposition is performed in the tin plating solution, the operating conditions also need to be precisely controlled. The temperature of the solution is a key parameter that affects the deposition rate and the quality of the deposited layer. The present invention maintains the temperature of the tin plating solution between 50°C and 65°C. Within this temperature range, both the galvanic displacement reaction and the autocatalytic reduction reaction can proceed at a high and controllable rate, while avoiding the accelerated decomposition of the solution or the coarsening of the grain size of the deposited layer that can be caused by excessively high temperatures. The immersion time is usually set to 20 minutes to 45 minutes. This time range can ensure the formation of a tin-bismuth alloy layer with a thickness of 1.8 microns to 4.5 microns on the copper surface, and the bismuth content in the layer is stabilized at 0.5% to 3.5% (wt%), which can effectively inhibit the growth of IMC. During the immersion process, moderate mechanical stirring (such as magnetic stirring or pump circulation) is performed on the tin plating solution to ensure the uniformity of the solution components and promote mass transfer, thereby increasing the deposition rate and the uniformity of the deposited layer. The stirring rate should be controlled to avoid the generation of bubbles or excessive shear force, and is usually controlled within the range of 0.5 m / s to 1.0 m / s through flow rate control.

[0041] The working principle of the disclosed chemical tin plating method is based on a dual deposition mechanism, i.e., the synergistic effect of the galvanic displacement reaction and the autocatalytic reduction reaction. In the initial stage of the contact between the copper surface of the PCB board and the chemical tin plating solution, the traditional galvanic displacement reaction occurs first. Copper atoms act as anodes, lose electrons and dissolve into the solution to form divalent copper ions: Cu→Cu^2++2e^-. Divalent tin ions (Sn^2+) in the solution act as cathodes, receive these electrons on the copper surface and are reduced and deposited as metallic tin: Sn^2++2e^-→Sn. This initial displacement reaction usually proceeds rapidly within tens of seconds, forming a thin and continuous pure tin or tin-rich layer on the copper surface. The thickness of this initial layer is usually between 0.1 microns and 0.3 microns, and its density and uniformity are affected by the pretreatment effect and the initial solution composition.

[0042] Once the initial tin layer is formed, its surface is catalytically active. At this point, the reducing agent introduced into the solution begins to play its key role. The reducing agent, such as formic acid, is catalytically activated on the surface of the deposited tin layer, providing electrons. These electrons are no longer derived from the dissolution of the copper substrate, but are directly provided by the reducing agent, allowing the divalent tin ions and trivalent bismuth ions in the solution to continue to be reduced and deposited onto the existing tin layer surface, forming a tin-bismuth alloy layer. This process is a self-catalytic reduction reaction, which no longer relies on the continuous dissolution of the copper substrate, thereby breaking through the thickness self-limiting nature of traditional galvanic displacement reactions, and achieving the continuous thickening of the tin-bismuth alloy layer. Specifically, the catalytic oxidation reaction of the reducing agent can be represented as: HCOOH(aq)→CO2(g)+2H+(aq)+2e-. The generated electrons then participate in the reduction of metal ions: Sn2+(aq)+2e-→Sn(s) and Bi3+(aq)+3e-→Bi(s). By precisely controlling the type, concentration, solution temperature and pH value of the reducing agent, the present application can effectively regulate the rate of self-catalytic reduction reaction, thereby precisely controlling the deposition thickness, compactness and internal bismuth content of the tin-bismuth alloy layer.

[0043] At the same time, the trivalent bismuth ions (Bi3+) introduced into the solution co-deposit with the divalent tin ions on the surface of the tin layer, forming a Sn-Bi alloy layer. The solid solubility of bismuth in tin is limited, and during the deposition process, bismuth atoms may exist in the form of solid solution in the tin lattice, or be dispersed in the tin matrix in the form of nanoparticles, or be enriched in the grain boundaries. The internal bismuth content of the tin-bismuth alloy layer described in the present application is precisely controlled within the range of 0.1% to 5.0% (weight percentage), and the bismuth content of this amount can significantly inhibit the growth of tin-copper intermetallic compounds (IMC). The bismuth inhibition IMC growth mechanism includes the following aspects: First is the diffusion barrier effect. Bismuth atoms form a physical barrier at the Sn-Cu interface or within the tin layer, effectively hindering the mutual diffusion between copper and tin atoms. The growth rate of IMC is closely related to the diffusion rate of copper and tin atoms, and the presence of bismuth can significantly reduce the diffusion coefficients of these atoms, thereby slowing down the formation rate of IMC. Studies have shown that bismuth atoms can preferentially occupy the diffusion channels of copper in tin, blocking the diffusion path.

[0044] Second is the grain boundary segregation effect. Bismuth atoms tend to segregate at the grain boundaries of the tin layer, filling the grain boundary vacancies, or forming local bismuth-rich regions at the Sn-Cu interface. These bismuth-rich regions can change the free energy of the interface, reducing the diffusion coefficient of the interface, thereby hindering the rapid diffusion of copper atoms along the grain boundaries into the tin layer, or inhibiting the diffusion of tin atoms into the copper substrate, thereby delaying the lateral and vertical growth of IMC. This segregation effect also changes the nucleation site and growth direction of IMC grains, making them grow more regular and reducing the formation of fragile dendritic IMC.

[0045] Third is the change of phase transformation kinetics. The presence of bismuth can change the phase transformation kinetics of Cu-Sn system, affecting the nucleation and growth process of IMC phases (such as Cu6Sn5 and Cu3Sn). Bismuth atoms can act as inactive impurities, increasing the critical energy of Cu-Sn IMC nucleation, thereby slowing down the nucleation rate of IMC. In addition, bismuth can also affect the growth kinetics of IMC, such as causing the formation of finer and more uniform IMC grains, or changing the growth direction of IMC phase, slowing down the consumption rate of pure tin layer. Some studies indicate that bismuth can reduce the solubility of Cu in tin, thereby inhibiting the formation of Cu6Sn5.

[0046] Fourth is the modification of IMC layer structure. Co-deposited bismuth can enter the Cu-Sn IMC layer, changing the crystal structure or growth morphology of IMC, making the IMC layer more dense and uniform, thereby reducing its brittleness and improving its wettability with the tin layer and copper substrate. For example, bismuth can make the Cu6Sn5 phase grains finer and fan-shaped, rather than large and rough block-shaped, which helps to improve the mechanical properties of the solder joint. In addition, the presence of bismuth can effectively inhibit the premature formation of fragile Cu3Sn phase, especially after long-term thermal aging or multiple reflow soldering, which is crucial for maintaining the long-term reliability of the solder joint.

[0047] Through the synergistic effect of the above multiple mechanisms, the tin-bismuth alloy layer prepared by the present application not only has more uniform thickness and higher density, but can effectively resist external environmental erosion and oxidation, and can significantly inhibit the growth rate of tin-copper intermetallic compounds, thereby maintaining excellent solderability and solder joint reliability after long-term storage and multiple thermal cycles.

[0048] Finally, the post-processing step includes thoroughly rinsing and drying the PCB board with the deposited tin-bismuth alloy layer. Rinsing uses multi-stage countercurrent deionized water rinsing to thoroughly remove the residual chemical tin deposition solution on the board surface, preventing the solution residue from adversely affecting subsequent packaging or application. The conductivity of the rinsing water is usually controlled below 0.5 μS / cm to ensure purity. The drying process can use centrifugal spin-drying combined with hot air drying, with the hot air temperature set at 60°C to 80°C and the drying time set at 5 minutes to 15 minutes to completely remove the moisture on the board surface and avoid water stains or oxidation. The dried PCB board should be immediately packaged or moved to the next process to prevent surface oxidation.

[0049] In order to more fully disclose the present application, the following is further illustrated by examples and comparative examples.

[0050] Example 1: Application of the chemical tin deposition method and solution of the present application In this example, a PCB board for an advanced driver assistance system (ADAS) control unit was prepared. The PCB board used FR-4 substrate with a copper thickness of 35 microns.

[0051] Pre-treatment: Degreasing: The PCB board was immersed in a solution of alkaline degreasing agent (containing sodium hydroxide, sodium carbonate and a specific non-ionic surfactant) with a mass fraction of 30 g / L at a temperature of 50°C for 7 minutes. The pH value of the solution was 11.5.

[0052] Rinsing: Two-stage countercurrent deionized water rinsing was used, with each stage lasting 2 minutes and a water flow rate of 6 L / min.

[0053] Micro-etching: The PCB board was immersed in a solution of sodium persulfate with a mass fraction of 80 g / L at a temperature of 35°C for 1.5 minutes. The copper etching amount was determined by weighing method to be 1.2 microns.

[0054] Rinsing: Two-stage countercurrent deionized water rinsing was used, with each stage lasting 1.5 minutes.

[0055] Pickling: The PCB board was immersed in a dilute sulfuric acid solution with a volume fraction of 8% at a temperature of 22°C for 1 minute.

[0056] Rinsing: One-stage deionized water rinsing was used, with a duration of 1 minute.

[0057] Activation: The PCB board was immersed in a dilute sulfuric acid-based activator solution containing 30 ppm of palladium ions (calculated as palladium chloride) at a temperature of 28°C for 60 seconds.

[0058] Rinsing: One-stage deionized water rinsing was used, with a duration of 1 minute.

[0059] Tin plating step: The PCB board after the above pre-treatment was immediately immersed in a chemical tin plating solution prepared according to the present application, which had the following specific composition: Stannous methanesulfonate: 0.25 M Methanesulfonic acid: 1.2 M p-Phenolsulfonic acid: 0.5 M Formic acid: 0.8 M Bismuth methanesulfonate: 0.005 M p-Dihydroxybenzene: 0.02 M Polyether non-ionic surfactant: 200 ppm The tin plating solution was maintained at a temperature of 58°C, and the immersion time was 30 minutes. During the immersion process, the solution was gently stirred at a speed of 200 revolutions / min by a magnetic stirrer.

[0060] Post-treatment: Rinsing: Three-stage countercurrent deionized water rinsing was used, with each stage lasting 2 minutes and a water flow rate of 5 L / min.

[0061] Drying: After centrifugation, the samples were dried in a hot air circulation oven at 70°C for 10 minutes.

[0062] Performance test results: The composition of the obtained Sn-Bi alloy layer was analyzed by X-ray fluorescence spectrometer (XRF), and the results showed that the tin content was 98.5% (wt%), and the bismuth content was 1.5% (wt%). Through cross-section scanning electron microscopy (SEM) combined with energy spectrum analysis (EDS) measurement, the thickness of the Sn-Bi alloy layer was uniformly distributed between 3.2 microns and 3.5 microns, with an average thickness of 3.35 microns. Through porosity test (electrochemical corrosion method), it showed that the average porosity was 0.05 per square millimeter. To evaluate the anti-IMC growth ability, the prepared PCB board was subjected to heat aging treatment at 150°C for 200 hours. After heat aging, the total thickness of the tin-copper intermetallic compound (Cu6Sn5 and Cu3Sn) layer was measured by SEM cross-section analysis, and the results showed that the average thickness of the IMC layer was 1.1 microns. In addition, the wettability was evaluated using a contact angle measuring instrument, and after activation by the flux, the contact angle of the solder liquid on the pad was 9.8°, indicating excellent solderability.

[0063] Example 2: Another application of the chemical tin plating method and solution of the present application This example aims to verify the effect of different bismuth content and slightly different process parameters on performance. The PCB board pretreatment steps are the same as in Example 1.

[0064] Tin plating step: The above pretreated PCB board was immersed in the chemical tin plating solution prepared according to the present application, and the specific composition was as follows: Stannous sulfate: 0.28M Sulfuric acid: 1.5M Tartaric acid: 0.6M Ascorbic acid: 1.0M Bismuth citrate: 0.003M Catechol: 0.03M Sulfur-containing organic compound (mercaptosuccinic acid): 150ppm The tin plating solution temperature was maintained at 62°C, and the immersion time was 40 minutes. During the immersion process, the solution was forced to circulate by pumping at a flow rate of 0.8 meters / second.

[0065] Post-treatment: The same as in Example 1.

[0066] Performance test results: XRF analysis results show that the tin content is 99.2% (wt%), and the bismuth content is 0.8% (wt%). SEM cross-section analysis measures that the thickness of the tin-bismuth alloy layer is uniformly distributed between 3.8 microns and 4.2 microns, with an average thickness of 4.05 microns. The average porosity is 0.07 per square millimeter. After thermal aging at 150°C for 200 hours, the average thickness of the IMC layer is 1.3 microns. The contact angle of the solder liquid on the pad is 10.5°.

[0067] Comparative Example 1: Traditional galvanic displacement tin plating method and application of solution In this comparative example, the same PCBs were treated using a traditional pure galvanic displacement type chemical tin plating method. The pretreatment steps were the same as in Example 1.

[0068] Tin plating step: The above pretreated PCBs were immersed in a traditional galvanic displacement tin plating solution without reducing agent and second metal salt, with the following specific composition: Stannous sulfate: 0.0.3 M Sulfuric acid: 1.0 M Thiourea (complexing agent and stabilizer): 0.7 M p-Benzenediol: 0.02 M The temperature of the tin plating solution was maintained at 60°C, and the immersion time was 20 minutes.

[0069] Post-treatment: The same as in Example 1.

[0070] Performance test results: XRF analysis results show that the deposited layer is pure tin with a bismuth content of 0%. SEM cross-section analysis measures that the thickness of the tin layer is between 0.9 microns and 1.2 microns, with an average thickness of 1.05 microns. The average porosity is 0.25 per square millimeter. After thermal aging at 150°C for 200 hours, the average thickness of the IMC layer is 2.8 microns, and obvious Cu3Sn phases are observed in some areas. The contact angle of the solder liquid on the pad is 22.5°, indicating poor wettability.

[0071] By quantitatively comparing the test results of the above examples and comparative examples, the superiority of the chemical tin plating method and solution provided by the present application is fully demonstrated.

[0072] Performance indicators Example 1 (invention) Example 2 (invention) Comparative Example 1 (conventional method) Sn layer thickness (pm) 3.35 4.05 1.05 Bismuth content (wt%) 1.5 0.8 0 Porosity (pieces / mm2) 0.05 0.07 0.25 IMC thickness after 150°C / 200h thermal aging (pm) 1.1 1.3 2.8 Solder contact angle (°) 9.8 10.5 22.5 From the above table, it can be clearly seen that by introducing the self-catalytic reduction mechanism and bismuth co-deposition, the thickness and density of the tin layer are significantly improved. The thickness of the tin-bismuth alloy layer prepared in Example 1 and Example 2 is much higher than that of the pure tin layer in Comparative Example 1, reaching 3.35 microns and 4.05 microns respectively, while the pure tin layer in Comparative Example 1 is only 1.05 microns. The thicker tin layer provides a more sufficient pure tin matrix for subsequent soldering, ensuring good solderability even after multiple thermal cycles or long-term storage. In terms of density, the porosity of the present application is extremely low (0.05 and 0.07 per square millimeter), far superior to the 0.25 per square millimeter of the comparative example, which indicates that the deposited layer of the present application can more effectively prevent the oxidation of the underlying copper, improving the oxidation resistance of the solder pad.

[0073] Most significantly, the present application has an advantage in inhibiting the growth of tin-copper intermetallic compounds (IMC). After 200 hours of 150°C thermal aging, the IMC layer thickness of Example 1 and Example 2 is 1.1 microns and 1.3 microns respectively, while the IMC layer thickness of Comparative Example 1 is as high as 2.8 microns, accompanied by the generation of brittle Cu3Sn phase. This fully demonstrates the key role of bismuth in inhibiting the growth of IMC in the tin-bismuth alloy layer of the present application, which effectively slows down the formation rate and growth thickness of IMC through mechanisms such as diffusion barrier, grain boundary segregation and phase transformation dynamics, thereby ensuring the long-term reliability of the solder joint.

[0074] In addition, in terms of solderability, the solder contact angle of Example 1 and Example 2 is 9.8° and 10.5° respectively, which is significantly lower than 22.5° of Comparative Example 1. Lower contact angle indicates that the tin-bismuth alloy layer of the present application has excellent wetting performance, which can promote the rapid spreading of solder on the solder pad, effectively avoid soldering defects and improve the quality of soldering.

[0075] In summary, the present application solves the inherent problems of traditional chemical tin deposition technology in thickness, density, anti-IMC growth ability and long-term reliability through the unique dual deposition mechanism and the synergistic effect of the second metal bismuth, providing an advanced and reproducible solution for the manufacture of high-performance PCBs. The parameters and component concentrations can be further optimized and adjusted by those skilled in the art based on the above disclosure, combined with the specific application requirements, to achieve the best overall performance.

[0076] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method of chemically tin plating a PCB board, characterized by, The method comprises the following steps: S1, a pretreatment step: the copper surface of the PCB is subjected to degreasing, rinsing, micro-etching, pickling and activation treatment to obtain a clean and well-reactive copper substrate surface; S2, a tin deposition step: the pretreated PCB is immersed in a chemical tin deposition solution, the tin deposition solution is maintained at a preset temperature for a preset time, and a tin-bismuth alloy layer is deposited on the copper surface of the PCB; the chemical tin deposition solution comprises a divalent tin salt, an acidic component, a complexing agent, a reducing agent and a second metal salt, and the deposition of the tin-bismuth alloy layer is realized by a galvanic replacement reaction and a self-catalytic reduction reaction; S3, a post-treatment step: the PCB with the deposited tin-bismuth alloy layer is thoroughly rinsed and dried.

2. The method of claim 1, wherein the PCB is a multi-layer PCB. The pretreatment step specifically comprises the following sub-steps: S11, degreasing treatment: the PCB is immersed in an alkaline degreasing agent solution, the solution temperature is controlled at 45-55℃, the treatment time is 6-9 minutes, the pH value is maintained at 10.0-12.0, and the organic contaminants on the copper surface are removed; S12, first rinsing: the PCB after degreasing is thoroughly rinsed with deionized water at a flow rate of 5-8 liters per minute; S13, micro-etching treatment: the PCB is immersed in a micro-etching agent solution, the micro-etching agent solution is a sodium persulfate solution with a concentration of 60-100 g / L, or a mixed solution containing 1-3% (wt%) hydrogen peroxide and 5-10% (vol%) sulfuric acid; the treatment temperature is 30-40℃, and the time is 1-2 minutes to remove the copper surface oxide layer and form a micro-rough structure with a copper etching amount of 0.8-1.5 microns; S14, second rinsing: the PCB after micro-etching is thoroughly rinsed with deionized water; S15, pickling treatment: the PCB is immersed in a dilute sulfuric acid solution with a concentration of 7-9%, the temperature is 20-25℃, and the time is 1-1.5 minutes to remove residual copper salt and maintain an acidic environment on the copper surface; S16, third rinsing: the PCB after pickling is rinsed with deionized water; S17, activation treatment: the PCB is immersed in a palladium salt activator solution, the activator solution is based on dilute hydrochloric acid or dilute sulfuric acid, and the palladium ion concentration is 20-40 ppm; the operating temperature is 25-30℃, and the time is 45-75 seconds to deposit a small amount of palladium particles on the copper surface as a catalytic center; S18, fourth rinsing: the PCB after activation is rinsed with deionized water.

3. The method of claim 1, wherein the PCB is a multi-layer PCB. The operating conditions of the tin deposition step include: The tin deposition solution temperature is accurately maintained at 50-65℃ to ensure that the galvanic replacement reaction and the self-catalytic reduction reaction proceed at a high and controllable rate, avoiding excessive temperature leading to accelerated solution decomposition or deposition layer grain coarsening; The immersion time of the PCB in the tin deposition solution is 20-45 minutes to ensure the formation of a uniform thickness tin-bismuth alloy layer on the copper surface; During the immersion process, the tin deposition solution is subjected to moderate mechanical stirring or pump circulation, and the circulating flow rate is preferably controlled at 0.5-1.0 m / s to ensure uniform solution components and promote mass transfer.

4. The chemical tin plating method for a PCB board according to claim 3, characterized in that, The tin-bismuth alloy layer formed by the deposition has a uniform thickness ranging from 1.8 microns to 4.5 microns, and the bismuth content in the tin-bismuth alloy layer is precisely controlled within 0.5% to 3.5% by weight, and the thickness and bismuth content are sufficient to effectively inhibit the growth of tin-copper intermetallic compounds.

5. The chemical tin plating method for a PCB board according to claim 4, characterized in that, The deposition of the tin-bismuth alloy layer is achieved by the following dual mechanisms: A galvanic displacement reaction stage: in the initial stage of contact between the copper surface of the PCB and the chemical tin deposition solution, copper atoms are dissolved as anodes to release electrons; divalent tin ions in the solution receive electrons to be reduced and deposited as metallic tin, forming a thin and continuous initial tin layer on the copper surface; A self-catalytic reduction reaction stage: after the formation of the initial tin layer, the surface of the deposited tin layer catalyzes and activates the reducing agent in the solution; the activated reducing agent provides electrons, allowing divalent tin ions and trivalent bismuth ions in the solution to be continuously reduced and deposited onto the surface of the existing tin layer.

6. The chemical tin plating method for a PCB board according to claim 5, characterized in that, The bismuth in the tin-bismuth alloy layer cooperatively inhibits the growth of tin-copper intermetallic compounds through one or more of the following mechanisms: Diffusion barrier effect: bismuth atoms form a physical barrier layer at the tin-copper interface or within the tin layer, hindering the mutual diffusion of copper atoms and tin atoms; Grain boundary segregation effect: bismuth atoms tend to segregate at the grain boundaries of the tin layer, filling grain boundary vacancies or forming local bismuth-rich regions at the tin-copper interface, changing the interface energy and hindering the rapid diffusion of copper atoms along the grain boundaries into the tin layer, or inhibiting the diffusion of tin atoms into the copper substrate, thereby delaying the lateral and vertical growth of intermetallic compounds; Phase transition dynamics change: the presence of bismuth changes the phase transition dynamics of the copper-tin system, delays the nucleation and growth process of intermetallic phases by increasing the critical energy for nucleation of intermetallic compounds or affecting their growth dynamics; Modified intermetallic compound layer structure: co-deposited bismuth enters the tin-copper intermetallic compound layer, changes the crystal structure or growth morphology of the intermetallic compound, making the intermetallic compound layer more dense and uniform, and effectively inhibiting the premature formation of fragile Cu3Sn phases.

7. A solution for the chemical tin plating of PCBs, suitable for use in the method for the chemical tin plating of PCBs according to any one of claims 1 to 6, characterized in that it comprises: The following main components are included: Divalent tin salt as the source of metallic tin deposition; Acidic component for maintaining the solution pH in the strongly acidic range, promoting moderate dissolution of the initial copper substrate; Complexing agent for complexing divalent tin ions to prevent their hydrolysis and oxidation, stabilizing the solution; Reducing agent for achieving self-catalytic reduction deposition of tin ions and the second metal bismuth ions; Second metal salt as the source of the second metal bismuth for co-deposition with tin to form a tin-bismuth alloy layer and inhibit the growth of tin-copper intermetallic compounds; Antioxidant for inhibiting the oxidation of divalent tin ions in the solution; Grain refiner and / or wetting agent for improving the grain structure, deposition uniformity of the deposited layer, and wettability of the solution on the PCB surface.

8. The solution for PCB board chemical tin plating according to claim 7, characterized in that, The components include: The divalent tin salt is stannous methanesulfonate or stannous sulfate, with a concentration of 0.15 mol / L to 0.35 mol / L; The acidic component is methanesulfonic acid, sulfuric acid, or hydrochloric acid, with a concentration of 0.8 mol / L to 1.5 mol / L, to stably maintain the solution pH at 0.3 to 0.8; The complexing agent is p-phenolsulfonic acid, tartaric acid, or citric acid, with a concentration of 0.3 mol / L to 0.7 mol / L; The reducing agent is formic acid, sodium formate, potassium formate, or ascorbic acid and its derivatives, with a concentration of 0.5 mol / L to 1.2 mol / L; The second metal salt is bismuth methane sulfonate, bismuth citrate or bismuth nitrate, with a concentration of 0.003 to 0.008 mol / L.

9. The solution for PCB board chemical tin plating according to claim 8, characterized in that, The components further include: The antioxidant is hydroquinone, catechol or pyrogallol, with a concentration of 0.01 to 0.04 mol / L, effective in inhibiting the oxidation of divalent tin ions and preventing their conversion into insoluble tetravalent tin; The grain refiner and / or wetting agent is selected from one or more of polyether non-ionic surfactants of specific molecular weight, specific sulfur-containing organic compounds or aromatic aldehyde derivatives, and the wetting agent can also be fluorocarbon or ethoxylated alkyl phenol non-ionic surfactants; the addition amount is 50 to 300 ppm by mass fraction, which improves the grain structure of the deposited layer, the deposition uniformity and the wettability of the solution.

10. The solution for PCB board chemical tin plating according to claim 8, characterized in that, The molar concentration ratio of the reducing agent to the divalent tin salt and the molar concentration ratio of the second metal salt to the divalent tin salt are precisely controlled to synergistically control the deposition rate, the final thickness and the internal bismuth content of the tin-bismuth alloy layer.

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