(1) Effect of plating bath chemical composition
The influence of nickel salts, reducing agents, complexing agents, buffers and stabilizers, etc., in the electroless nickel plating solution on the electroless plating process and the properties of the coating has been discussed in 6.1.2. In short, the influence of the main components in the electroless nickel plating solution is very important and complex. In the actual operation of electroless nickel plating, it is necessary not only to maintain a certain chemical component within the optimum range, but also to keep other various related chemical components and process parameters within the corresponding optimal range.
(2) Effect of temperature
The bath temperature has a significant influence on the deposition rate of the plating, the stability of the bath, and the quality of the plating.
The catalytic reaction of electroless nickel plating can generally only be achieved under heating conditions. Many individual electroless nickel plating reaction steps have obvious reaction speeds only above 50° C., especially acidic hypophosphite solutions, and the operating temperatures are generally 85～ Between 95 °C. The plating rate increases as the temperature increases, and the deposition rate doubles as the general temperature increases by 10°C. However, it should be pointed out that the high bath temperature will make the bath unstable and self-decomposition. Therefore, the proper temperature should be selected according to the actual situation, and try to maintain this temperature. The general alkaline bath temperature is lower, and its deposition rate at a lower temperature is faster than that of an acidic bath, but the temperature increases, the plating speed is not as fast as the acidic bath.
In addition to affecting the plating rate, the temperature also affects the coating quality. As the temperature increases and the plating speed is fast, the phosphorus content in the coating decreases, the stress and porosity of the coating increase, and the corrosion resistance decreases. Therefore, it is very important that the temperature control in the electroless nickel plating is uniform. It is advisable to maintain the working temperature of the solution within ±2°C. If the temperature fluctuates too much during the plating process, a plated coating will occur. The quality of the coating is not good and affects the adhesion of the coating. .
(3) Effect of pH
The pH value has a great influence on the plating solution, the process and the coating. It is an important factor that must be strictly controlled in the process parameters.
In the acidic electroless nickel plating process, the pH value has a significant influence on the deposition rate and the phosphorus content of the coating. As the pH rises, the deposition rate of nickel increases and the phosphorus content of the coating decreases. The change of pH value will also affect the stress distribution in the coating. The plating solution obtained with a high pH value has a low phosphorus content and exhibits a tensile stress. On the contrary, the plating solution obtained with a low pH has a high phosphorus content and exhibits compressive stress.
There is an optimal pH range for each specific electroless nickel solution. In the electroless nickel plating process, with the deposition of nickel-phosphorus, H+ is continuously generated and the pH value of the plating solution continuously decreases. Therefore, the pH value of the plating solution must be adjusted in time to maintain the pH value of the plating solution so that the fluctuation range can be controlled. Within the 0.2 range of soil. Adjust the pH of the plating solution. Use diluted ammonia or sodium hydroxide. Use caution when stirring. When using different lye to adjust the pH of the bath, the influence on the bath is also different. When adjusting the pH of the plating solution with NaOH, only the acid-base neutralization reaction occurs, and the H+ generated during the reaction is neutralized. When the pH of the plating solution is adjusted with ammonia, except for the neutralization plating solution H+, the plating solution contains The ammonia molecule and the Ni2+ in the plating solution and the complexing agent also generate a complex complex, which reduces the concentration of free Ni2+ in the bath, effectively inhibits the precipitation of nickel phosphite, and improves the stability of the bath.
(4) Effect of stirring
Proper agitation of the bath will increase bath stability and plating quality. The first stirring can prevent the local overheating of the plating solution, prevent the concentration of the local components from being excessively high when the plating solution is added, and the local pH value changes drastically, which is favorable for improving the stability of the plating solution. In addition, the stirring speeds up the speed at which the reaction product leaves the surface of the workpiece, which is advantageous for increasing the deposition rate, ensuring the quality of the plating layer, and the defects such as pores are not likely to occur on the surface of the plating layer. However, excessive agitation is also undesirable, because excessive agitation can easily result in localized partial plating of the workpiece and nickel deposits on the walls and bottom of the container, even causing the bath to decompose in severe cases. In addition, the stirring method and strength also affect the phosphorus content of the coating.
(5) Effect of loading
The loading of the bath refers to the ratio of the plating area of ​​the workpiece to the volume of the bath used. Electroless nickel plating. The loading has a great influence on the stability of the bath. The allowable loading is related to the plating conditions and the composition of the bath. Each plating solution is required to have the best loading during the development process. During the plating, the workpiece should be placed according to the regulations and the concentrate should be added in time so that the best plating effect can be obtained. The loading of the general bath is 0.5 to 1.5 dm2/L. If the loading is too large, that is, if the catalytic surface is too large, the deposition reaction is violent, and the precipitation of nickel phosphite is easy to affect the stability and plating performance of the plating solution. When the loading is too small, tiny impurity particles in the plating solution become catalytic activity. The center initiates the deposition, which easily leads to the decomposition of the bath. Therefore, in order to ensure the best effect of plating, the load should be controlled within the optimal range.
(6) Effect of chemical bath aging
Electroless nickel plating solution has a certain service life. The life of the bath is expressed as the cycle time of the bath, that is, the exhaustion of all Ni2+ in the bath and the addition of Ni2+ to the original concentration for one cycle. As the plating progresses, the reducing agent is continuously added, and the concentration of HPO32-increases. After a certain amount exceeds the solubility of NiHP03, NiHPO3 precipitate will be formed and the plating solution will appear cloudy. Although the addition of a complexing agent can suppress the precipitation of NiHPO3, with the prolongation of the cycle, even if there is a large amount of complexing agent, precipitation cannot be suppressed, the deposition rate of nickel decreases sharply, and the plating performance deteriorates. This indicates that the bath has reached Life expectancy should be abandoned.
(7) Plating on different base materials
Electroless nickel plating can be directly deposited on catalytically active metallic materials (such as nickel, cobalt, palladium, rhodium) and metallic materials (such as iron, aluminum, magnesium, tantalum, and titanium) having a lower potential than nickel. The latter type of metal is the first to rely on the chemical substitution in the solution to make contact with nickel on its surface, since nickel itself is a catalyst, so that the deposition process can continue.
For metal materials (such as copper, brass, silver, etc.) that have no catalytic effect and the potential is higher than that of nickel, the initiation plating method can be used. That is, the surface of the plating part is contacted with a clean wire or aluminum wire to make it a short-circuit battery. At this time, the plated member is used as a cathode, and a nickel layer is first deposited on the surface to allow the electroless nickel plating reaction to proceed; the direct current can also be instantaneously applied as the plating. Another method is to soak the parts to be first soaked in a dilute solution of acidic palladium chloride (for example, immersed for 20s in a solution of 0.1g/LPdCl2 and 0.2g/L HCl), and after thorough rinsing, electroless nickel plating .
Electroless nickel plating on non-metallic materials must be subjected to a special pre-treatment to remove oil stains and mold release agents, followed by chemical sensitization and activation (usually using the nucleation of palladium) to make it catalytically active. In order to be immersed in electroless nickel plating nickel plating.
The influence of nickel salts, reducing agents, complexing agents, buffers and stabilizers, etc., in the electroless nickel plating solution on the electroless plating process and the properties of the coating has been discussed in 6.1.2. In short, the influence of the main components in the electroless nickel plating solution is very important and complex. In the actual operation of electroless nickel plating, it is necessary not only to maintain a certain chemical component within the optimum range, but also to keep other various related chemical components and process parameters within the corresponding optimal range.
(2) Effect of temperature
The bath temperature has a significant influence on the deposition rate of the plating, the stability of the bath, and the quality of the plating.
The catalytic reaction of electroless nickel plating can generally only be achieved under heating conditions. Many individual electroless nickel plating reaction steps have obvious reaction speeds only above 50° C., especially acidic hypophosphite solutions, and the operating temperatures are generally 85～ Between 95 °C. The plating rate increases as the temperature increases, and the deposition rate doubles as the general temperature increases by 10°C. However, it should be pointed out that the high bath temperature will make the bath unstable and self-decomposition. Therefore, the proper temperature should be selected according to the actual situation, and try to maintain this temperature. The general alkaline bath temperature is lower, and its deposition rate at a lower temperature is faster than that of an acidic bath, but the temperature increases, the plating speed is not as fast as the acidic bath.
In addition to affecting the plating rate, the temperature also affects the coating quality. As the temperature increases and the plating speed is fast, the phosphorus content in the coating decreases, the stress and porosity of the coating increase, and the corrosion resistance decreases. Therefore, it is very important that the temperature control in the electroless nickel plating is uniform. It is advisable to maintain the working temperature of the solution within ±2°C. If the temperature fluctuates too much during the plating process, a plated coating will occur. The quality of the coating is not good and affects the adhesion of the coating. .
(3) Effect of pH
The pH value has a great influence on the plating solution, the process and the coating. It is an important factor that must be strictly controlled in the process parameters.
In the acidic electroless nickel plating process, the pH value has a significant influence on the deposition rate and the phosphorus content of the coating. As the pH rises, the deposition rate of nickel increases and the phosphorus content of the coating decreases. The change of pH value will also affect the stress distribution in the coating. The plating solution obtained with a high pH value has a low phosphorus content and exhibits a tensile stress. On the contrary, the plating solution obtained with a low pH has a high phosphorus content and exhibits compressive stress.
There is an optimal pH range for each specific electroless nickel solution. In the electroless nickel plating process, with the deposition of nickel-phosphorus, H+ is continuously generated and the pH value of the plating solution continuously decreases. Therefore, the pH value of the plating solution must be adjusted in time to maintain the pH value of the plating solution so that the fluctuation range can be controlled. Within the 0.2 range of soil. Adjust the pH of the plating solution. Use diluted ammonia or sodium hydroxide. Use caution when stirring. When using different lye to adjust the pH of the bath, the influence on the bath is also different. When adjusting the pH of the plating solution with NaOH, only the acid-base neutralization reaction occurs, and the H+ generated during the reaction is neutralized. When the pH of the plating solution is adjusted with ammonia, except for the neutralization plating solution H+, the plating solution contains The ammonia molecule and the Ni2+ in the plating solution and the complexing agent also generate a complex complex, which reduces the concentration of free Ni2+ in the bath, effectively inhibits the precipitation of nickel phosphite, and improves the stability of the bath.
(4) Effect of stirring
Proper agitation of the bath will increase bath stability and plating quality. The first stirring can prevent the local overheating of the plating solution, prevent the concentration of the local components from being excessively high when the plating solution is added, and the local pH value changes drastically, which is favorable for improving the stability of the plating solution. In addition, the stirring speeds up the speed at which the reaction product leaves the surface of the workpiece, which is advantageous for increasing the deposition rate, ensuring the quality of the plating layer, and the defects such as pores are not likely to occur on the surface of the plating layer. However, excessive agitation is also undesirable, because excessive agitation can easily result in localized partial plating of the workpiece and nickel deposits on the walls and bottom of the container, even causing the bath to decompose in severe cases. In addition, the stirring method and strength also affect the phosphorus content of the coating.
(5) Effect of loading
The loading of the bath refers to the ratio of the plating area of ​​the workpiece to the volume of the bath used. Electroless nickel plating. The loading has a great influence on the stability of the bath. The allowable loading is related to the plating conditions and the composition of the bath. Each plating solution is required to have the best loading during the development process. During the plating, the workpiece should be placed according to the regulations and the concentrate should be added in time so that the best plating effect can be obtained. The loading of the general bath is 0.5 to 1.5 dm2/L. If the loading is too large, that is, if the catalytic surface is too large, the deposition reaction is violent, and the precipitation of nickel phosphite is easy to affect the stability and plating performance of the plating solution. When the loading is too small, tiny impurity particles in the plating solution become catalytic activity. The center initiates the deposition, which easily leads to the decomposition of the bath. Therefore, in order to ensure the best effect of plating, the load should be controlled within the optimal range.
(6) Effect of chemical bath aging
Electroless nickel plating solution has a certain service life. The life of the bath is expressed as the cycle time of the bath, that is, the exhaustion of all Ni2+ in the bath and the addition of Ni2+ to the original concentration for one cycle. As the plating progresses, the reducing agent is continuously added, and the concentration of HPO32-increases. After a certain amount exceeds the solubility of NiHP03, NiHPO3 precipitate will be formed and the plating solution will appear cloudy. Although the addition of a complexing agent can suppress the precipitation of NiHPO3, with the prolongation of the cycle, even if there is a large amount of complexing agent, precipitation cannot be suppressed, the deposition rate of nickel decreases sharply, and the plating performance deteriorates. This indicates that the bath has reached Life expectancy should be abandoned.
(7) Plating on different base materials
Electroless nickel plating can be directly deposited on catalytically active metallic materials (such as nickel, cobalt, palladium, rhodium) and metallic materials (such as iron, aluminum, magnesium, tantalum, and titanium) having a lower potential than nickel. The latter type of metal is the first to rely on the chemical substitution in the solution to make contact with nickel on its surface, since nickel itself is a catalyst, so that the deposition process can continue.
For metal materials (such as copper, brass, silver, etc.) that have no catalytic effect and the potential is higher than that of nickel, the initiation plating method can be used. That is, the surface of the plating part is contacted with a clean wire or aluminum wire to make it a short-circuit battery. At this time, the plated member is used as a cathode, and a nickel layer is first deposited on the surface to allow the electroless nickel plating reaction to proceed; the direct current can also be instantaneously applied as the plating. Another method is to soak the parts to be first soaked in a dilute solution of acidic palladium chloride (for example, immersed for 20s in a solution of 0.1g/LPdCl2 and 0.2g/L HCl), and after thorough rinsing, electroless nickel plating .
Electroless nickel plating on non-metallic materials must be subjected to a special pre-treatment to remove oil stains and mold release agents, followed by chemical sensitization and activation (usually using the nucleation of palladium) to make it catalytically active. In order to be immersed in electroless nickel plating nickel plating.