1. **The Role and Characteristics of Nickel Electroplating Process on PCB Boards**
Nickel plating is employed on PCB boards as a substrate coating for both precious and base metals, and it is also commonly used as a surface layer for some single-sided printed boards. For surfaces that experience significant wear, such as switch contacts, connector contacts, or plug gold, nickel serves as an effective backing layer under gold to substantially enhance wear resistance. When used as a barrier, nickel efficiently prevents diffusion between copper and other metals. Nickel/gold combination coatings are frequently utilized as etching-resistant metal coatings, capable of meeting the requirements for hot-press welding and brazing. Nickel alone can also act as a corrosion-resistant coating for ammonia-based etchants without requiring hot-press welding. PCB boards with bright plating are typically required to have bright nickel/gold plating. The thickness of nickel plating is generally not less than 2.5 microns, with typical values ranging from 4 to 5 microns. The deposited layer of low-stress nickel on PCBs is usually plated using a modified Watts nickel bath or certain sulfamate nickel baths with stress-reducing additives. Nickel plating on PCB boards includes both bright nickel and matte nickel (also known as low-stress nickel or semi-bright nickel), which are expected to have uniform and meticulous plating, low porosity, low stress, and good ductility.
2. **Nickel Sulfamate (Ammonia Nickel)** Nickel sulfamate is widely used as a substrate coating for metallized hole plating and printed plug contacts. The deposited layer obtained has low internal stress, high hardness, and excellent ductility. Adding a stress reliever to the bath will result in a slightly stressed coating. There are various sulfamate baths with different formulations, with a typical nickel sulfamate bath formula shown in the table below. Although the low stress of the coating makes it widely used, the stability of nickel sulfamate is poor, and its cost is relatively high.
3. **Modified Watt Nickel (Sulfur Nickel)** The formulation of Modified Watt Nickel uses nickel sulfate, along with the addition of nickel bromide or nickel chloride. Nickel bromide is typically used due to its lower internal stress. This produces a semi-bright coating with slightly internal stress and good ductility. Additionally, this coating is easily activated for subsequent electroplating, and its cost is relatively low.
4. **Role of Each Component of the Plating Solution:**
1) **Main Salts**—Nickel sulfamate and nickel sulfate are the main salts in the nickel solution. Nickel salts primarily provide the nickel metal ions required for plating and also serve as conductive salts. The concentration of nickel salts varies slightly with different suppliers, and the allowable content of nickel salts can differ significantly. Higher concentrations of nickel salt allow for a higher cathode current density and a faster deposition rate, often used for high-speed, thick nickel plating. However, if the concentration is too high, cathodic polarization decreases, dispersion ability worsens, and carry-out loss of the plating solution increases. Lower concentrations result in a slower deposition rate but better dispersing ability, leading to a finer crystal and brighter coating.
2) **Buffer**—Boric acid is used as a buffer to maintain the pH value of the nickel plating solution within a specific range. Practice has shown that if the pH value is too low, cathode current efficiency decreases. Conversely, if the pH is too high, the pH value of the liquid layer near the cathode surface rises rapidly due to continuous H2 precipitation, leading to the formation of Ni(OH)2 colloid, which increases the brittleness of the coating. Additionally, the adsorption of Ni(OH)2 colloid on the electrode surface can trap hydrogen bubbles, increasing coating porosity. Boric acid not only buffers pH but also enhances cathodic polarization, improving bath performance and reducing “burning” at high current densities. It also helps improve the mechanical properties of the coating.
3) **Anode Activator**—Except for sulfate-type nickel plating solutions that use insoluble anodes, other nickel plating processes use soluble anodes. Nickel anodes are prone to passivation during electrification. To ensure proper anode dissolution, an anode activator is added to the plating solution. Chloride ions (Cl-) are known to be effective activators for nickel anodes. In solutions containing nickel chloride, nickel chloride serves both as the main salt and an anode activator. For solutions low in nickel chloride or lacking it, sodium chloride should be added as needed. Nickel bromide or nickel chloride is also commonly used as a stress reliever to maintain the internal stress of the coating and provide a semi-bright appearance.
4) **Additives**—The primary component of additives is stress relievers. Adding stress relievers improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. Adjusting the concentration of stress relievers can shift the internal stress of the coating from tensile to compressive. Common additives include naphthalene sulfonic acid, p-toluenesulfonamide, and saccharin. Compared to nickel coatings without stress relievers, the addition of these additives results in a uniform, fine, and semi-bright coating. Typically, stress relievers are added based on ampere-hours (A-hours), with specialized additives including anti-pinhole agents also being used.
5) **Wetting Agent**—Hydrogen evolution on the cathode is inevitable during electroplating. This hydrogen evolution not only reduces cathode current efficiency but can also cause pinholes in the coating due to trapped hydrogen bubbles. To reduce or prevent pinholes, a small amount of wetting agent should be added to the plating solution. Examples include sodium lauryl sulfate, sodium diethylhexyl sulfate, and n-octane. These anionic surfactants adsorb onto the cathode surface, reducing the interfacial tension between the electrode and the solution. This decreases the wetting contact angle of hydrogen bubbles on the electrode, facilitating their release from the surface and preventing or mitigating pinhole formation.
5. **Maintenance of Plating Solution**
5.1 **Temperature**—Different nickel processes require different bath temperatures. The effect of temperature changes on the nickel plating process is complex. Higher temperatures generally result in a nickel coating with lower internal stress and better ductility, with the internal stress stabilizing at around 50°C. The typical operating temperature is maintained between 55–60°C. Excessive temperatures can lead to hydrolysis of nickel salt, causing nickel hydroxide colloid to retain colloidal hydrogen bubbles, resulting in pinholes and reduced cathodic polarization. Therefore, the working temperature must be strictly controlled within the specified range, using a temperature controller to maintain stability according to supplier recommendations.
5.2 **pH Value**—The pH value of the nickel plating electrolyte significantly affects coating performance and electrolyte stability. In strongly acidic electroplating solutions with pH ≤ 2, no metallic nickel is deposited, and only light gas is precipitated. Generally, the pH value for nickel plating solutions for PCB boards is maintained between 3 and 4. Nickel baths with higher pH values offer better dispersion power and higher cathode current efficiency. However, excessively high pH values can lead to rapid increases in pH near the cathode surface due to light gas precipitation, resulting in pinholes and increased brittleness from nickel hydroxide inclusion. Conversely, lower pH values enhance anode dissolution, allowing higher nickel salt content and higher current densities, which improves production rates. However, very low pH values narrow the temperature range for obtaining bright coatings. To adjust pH, add nickel carbonate or basic nickel carbonate to increase pH, or sulfamic acid or sulfuric acid to decrease pH. Check and adjust the pH value every four hours during operation.
5.3 **Anode**—Current PCB nickel plating processes use soluble anodes, often titanium baskets with built-in nickel corners. This setup allows for a large and stable anode area and simplifies maintenance. The titanium basket should be placed in a polypropylene anode bag to prevent anode sludge from contaminating the plating solution and should be regularly cleaned and inspected for blockages. New anode bags should be soaked in boiling water before use.
5.4 **Purification**—For organic contamination in the bath, activated carbon treatment is required. However, this method may also remove some stress relievers (additives), which need replenishment. The treatment process involves:
1) Removing the anode, adding 5 ml/l of impurity-removing water, heating to 60-80°C, and aerating (gas-stirring) for 2 hours.
2) For substantial organic impurities, first add 3-5 ml/l of 30% hydrogen peroxide and stir for 3 hours.
3) Add 3-5 g/l of powdered activated carbon while stirring continuously, then let it stand for 4 hours. Filter using filter powder and a spare tank.
4) Clean and maintain the anode hanger, using a corrugated iron plate as the cathode. Operate at a current density of 0.5-1.0 A/dm² for 8-12 hours.
5) Replace the filter element (a series of cotton and carbon cores for continuous filtration), analyze and adjust parameters, and add wetting agents before resuming plating.
6. **Analysis**—The plating solution should follow process regulations and be regularly analyzed. Conduct Hull cell tests and adjust parameters based on the results to guide production adjustments.
7. **Stirring**—Like other electroplating processes, stirring is crucial to accelerate mass transfer, reduce concentration variations, and increase the allowable current density. Proper stirring also helps reduce or prevent pinholes in the nickel plating layer by addressing hydrogen bubble retention and pH changes. Common stirring methods include compressed air, cathode movement, and forced circulation combined with filtration.
8. **Cathode Current Density**—The cathode current density affects current efficiency, deposition rate, and coating quality. Lower pH electrolytes show increased efficiency with higher current densities in low-density regions, while in high-density regions, efficiency is less affected by current density. Optimal cathode current density varies based on solution composition, temperature, and stirring conditions, generally ranging around 2 A/dm².
6. **Troubleshooting and Solutions**
1) **Pitting**—Pitting often results from organic contamination, with large pits indicating oil contamination. Poor agitation can trap air bubbles, causing pits. Use wetting agents to mitigate this issue. Small pits are commonly referred to as pinholes. Issues like poor pretreatment, inadequate metal quality, insufficient boric acid, and low bath temperature can cause pinholes. Process control and anti-pinhole agents are critical for stabilization.
2) **Roughness and Burrs**—Roughness usually indicates dirty solution, which can be corrected with thorough filtration. High current density can cause anode slime and impurities to introduce roughness and burrs.
3) **Low Bonding Force**—Incomplete deoxidation of the copper coating can lead to poor adhesion between copper and nickel, causing peeling. Interruptions in current or low temperatures can also cause peeling.
4) **Brittleness and Poor Weldability**—B
Nickel plating is employed on PCB boards as a substrate coating for both precious and base metals, and it is also commonly used as a surface layer for some single-sided printed boards. For surfaces that experience significant wear, such as switch contacts, connector contacts, or plug gold, nickel serves as an effective backing layer under gold to substantially enhance wear resistance. When used as a barrier, nickel efficiently prevents diffusion between copper and other metals. Nickel/gold combination coatings are frequently utilized as etching-resistant metal coatings, capable of meeting the requirements for hot-press welding and brazing. Nickel alone can also act as a corrosion-resistant coating for ammonia-based etchants without requiring hot-press welding. PCB boards with bright plating are typically required to have bright nickel/gold plating. The thickness of nickel plating is generally not less than 2.5 microns, with typical values ranging from 4 to 5 microns. The deposited layer of low-stress nickel on PCBs is usually plated using a modified Watts nickel bath or certain sulfamate nickel baths with stress-reducing additives. Nickel plating on PCB boards includes both bright nickel and matte nickel (also known as low-stress nickel or semi-bright nickel), which are expected to have uniform and meticulous plating, low porosity, low stress, and good ductility.
2. **Nickel Sulfamate (Ammonia Nickel)** Nickel sulfamate is widely used as a substrate coating for metallized hole plating and printed plug contacts. The deposited layer obtained has low internal stress, high hardness, and excellent ductility. Adding a stress reliever to the bath will result in a slightly stressed coating. There are various sulfamate baths with different formulations, with a typical nickel sulfamate bath formula shown in the table below. Although the low stress of the coating makes it widely used, the stability of nickel sulfamate is poor, and its cost is relatively high.
3. **Modified Watt Nickel (Sulfur Nickel)** The formulation of Modified Watt Nickel uses nickel sulfate, along with the addition of nickel bromide or nickel chloride. Nickel bromide is typically used due to its lower internal stress. This produces a semi-bright coating with slightly internal stress and good ductility. Additionally, this coating is easily activated for subsequent electroplating, and its cost is relatively low.
4. **Role of Each Component of the Plating Solution:**
1) **Main Salts**—Nickel sulfamate and nickel sulfate are the main salts in the nickel solution. Nickel salts primarily provide the nickel metal ions required for plating and also serve as conductive salts. The concentration of nickel salts varies slightly with different suppliers, and the allowable content of nickel salts can differ significantly. Higher concentrations of nickel salt allow for a higher cathode current density and a faster deposition rate, often used for high-speed, thick nickel plating. However, if the concentration is too high, cathodic polarization decreases, dispersion ability worsens, and carry-out loss of the plating solution increases. Lower concentrations result in a slower deposition rate but better dispersing ability, leading to a finer crystal and brighter coating.
2) **Buffer**—Boric acid is used as a buffer to maintain the pH value of the nickel plating solution within a specific range. Practice has shown that if the pH value is too low, cathode current efficiency decreases. Conversely, if the pH is too high, the pH value of the liquid layer near the cathode surface rises rapidly due to continuous H2 precipitation, leading to the formation of Ni(OH)2 colloid, which increases the brittleness of the coating. Additionally, the adsorption of Ni(OH)2 colloid on the electrode surface can trap hydrogen bubbles, increasing coating porosity. Boric acid not only buffers pH but also enhances cathodic polarization, improving bath performance and reducing “burning” at high current densities. It also helps improve the mechanical properties of the coating.
3) **Anode Activator**—Except for sulfate-type nickel plating solutions that use insoluble anodes, other nickel plating processes use soluble anodes. Nickel anodes are prone to passivation during electrification. To ensure proper anode dissolution, an anode activator is added to the plating solution. Chloride ions (Cl-) are known to be effective activators for nickel anodes. In solutions containing nickel chloride, nickel chloride serves both as the main salt and an anode activator. For solutions low in nickel chloride or lacking it, sodium chloride should be added as needed. Nickel bromide or nickel chloride is also commonly used as a stress reliever to maintain the internal stress of the coating and provide a semi-bright appearance.
4) **Additives**—The primary component of additives is stress relievers. Adding stress relievers improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. Adjusting the concentration of stress relievers can shift the internal stress of the coating from tensile to compressive. Common additives include naphthalene sulfonic acid, p-toluenesulfonamide, and saccharin. Compared to nickel coatings without stress relievers, the addition of these additives results in a uniform, fine, and semi-bright coating. Typically, stress relievers are added based on ampere-hours (A-hours), with specialized additives including anti-pinhole agents also being used.
5) **Wetting Agent**—Hydrogen evolution on the cathode is inevitable during electroplating. This hydrogen evolution not only reduces cathode current efficiency but can also cause pinholes in the coating due to trapped hydrogen bubbles. To reduce or prevent pinholes, a small amount of wetting agent should be added to the plating solution. Examples include sodium lauryl sulfate, sodium diethylhexyl sulfate, and n-octane. These anionic surfactants adsorb onto the cathode surface, reducing the interfacial tension between the electrode and the solution. This decreases the wetting contact angle of hydrogen bubbles on the electrode, facilitating their release from the surface and preventing or mitigating pinhole formation.
5. **Maintenance of Plating Solution**
5.1 **Temperature**—Different nickel processes require different bath temperatures. The effect of temperature changes on the nickel plating process is complex. Higher temperatures generally result in a nickel coating with lower internal stress and better ductility, with the internal stress stabilizing at around 50°C. The typical operating temperature is maintained between 55–60°C. Excessive temperatures can lead to hydrolysis of nickel salt, causing nickel hydroxide colloid to retain colloidal hydrogen bubbles, resulting in pinholes and reduced cathodic polarization. Therefore, the working temperature must be strictly controlled within the specified range, using a temperature controller to maintain stability according to supplier recommendations.
5.2 **pH Value**—The pH value of the nickel plating electrolyte significantly affects coating performance and electrolyte stability. In strongly acidic electroplating solutions with pH ≤ 2, no metallic nickel is deposited, and only light gas is precipitated. Generally, the pH value for nickel plating solutions for PCB boards is maintained between 3 and 4. Nickel baths with higher pH values offer better dispersion power and higher cathode current efficiency. However, excessively high pH values can lead to rapid increases in pH near the cathode surface due to light gas precipitation, resulting in pinholes and increased brittleness from nickel hydroxide inclusion. Conversely, lower pH values enhance anode dissolution, allowing higher nickel salt content and higher current densities, which improves production rates. However, very low pH values narrow the temperature range for obtaining bright coatings. To adjust pH, add nickel carbonate or basic nickel carbonate to increase pH, or sulfamic acid or sulfuric acid to decrease pH. Check and adjust the pH value every four hours during operation.
5.3 **Anode**—Current PCB nickel plating processes use soluble anodes, often titanium baskets with built-in nickel corners. This setup allows for a large and stable anode area and simplifies maintenance. The titanium basket should be placed in a polypropylene anode bag to prevent anode sludge from contaminating the plating solution and should be regularly cleaned and inspected for blockages. New anode bags should be soaked in boiling water before use.
5.4 **Purification**—For organic contamination in the bath, activated carbon treatment is required. However, this method may also remove some stress relievers (additives), which need replenishment. The treatment process involves:
1) Removing the anode, adding 5 ml/l of impurity-removing water, heating to 60-80°C, and aerating (gas-stirring) for 2 hours.
2) For substantial organic impurities, first add 3-5 ml/l of 30% hydrogen peroxide and stir for 3 hours.
3) Add 3-5 g/l of powdered activated carbon while stirring continuously, then let it stand for 4 hours. Filter using filter powder and a spare tank.
4) Clean and maintain the anode hanger, using a corrugated iron plate as the cathode. Operate at a current density of 0.5-1.0 A/dm² for 8-12 hours.
5) Replace the filter element (a series of cotton and carbon cores for continuous filtration), analyze and adjust parameters, and add wetting agents before resuming plating.
6. **Analysis**—The plating solution should follow process regulations and be regularly analyzed. Conduct Hull cell tests and adjust parameters based on the results to guide production adjustments.
7. **Stirring**—Like other electroplating processes, stirring is crucial to accelerate mass transfer, reduce concentration variations, and increase the allowable current density. Proper stirring also helps reduce or prevent pinholes in the nickel plating layer by addressing hydrogen bubble retention and pH changes. Common stirring methods include compressed air, cathode movement, and forced circulation combined with filtration.
8. **Cathode Current Density**—The cathode current density affects current efficiency, deposition rate, and coating quality. Lower pH electrolytes show increased efficiency with higher current densities in low-density regions, while in high-density regions, efficiency is less affected by current density. Optimal cathode current density varies based on solution composition, temperature, and stirring conditions, generally ranging around 2 A/dm².
6. **Troubleshooting and Solutions**
1) **Pitting**—Pitting often results from organic contamination, with large pits indicating oil contamination. Poor agitation can trap air bubbles, causing pits. Use wetting agents to mitigate this issue. Small pits are commonly referred to as pinholes. Issues like poor pretreatment, inadequate metal quality, insufficient boric acid, and low bath temperature can cause pinholes. Process control and anti-pinhole agents are critical for stabilization.
2) **Roughness and Burrs**—Roughness usually indicates dirty solution, which can be corrected with thorough filtration. High current density can cause anode slime and impurities to introduce roughness and burrs.
3) **Low Bonding Force**—Incomplete deoxidation of the copper coating can lead to poor adhesion between copper and nickel, causing peeling. Interruptions in current or low temperatures can also cause peeling.
4) **Brittleness and Poor Weldability**—B
