Introduction of PCB Nickel Electroplating Process (2)

4. The Role of Each Component of the Plating Solution

The main salts—nickel sulfamate and nickel sulfate are the main salts in the nickel solution. The nickel salts mainly provide the nickel-metal ions required for nickel plating and also play the role of conductive salts. The concentration of nickel plating solution varies slightly with different suppliers, and the allowable content of nickel salt varies greatly. The high content of nickel salt allows the use of a higher cathode current density and a fast deposition rate and is often used for high-speed thick nickel plating. However, if the concentration is too high, the cathodic polarization will be reduced, the dispersion ability will be poor, and the carry-out loss of the plating solution will be large. Low nickel salt content has a low deposition rate, but has the good dispersing ability, and can obtain fine crystal and bright coating.

Buffer – Boric acid is used as a buffer to maintain the pH value of the nickel plating solution within a certain range. Practice has proved that when the PH value of the nickel plating solution is too low, the cathode current efficiency will decrease; when the PH value is too high, the PH value of the liquid layer close to the cathode surface will increase rapidly due to the continuous precipitation of H2, resulting in Ni The formation of (OH)2 colloid and the inclusion of Ni(OH)2 in the coating increase the brittleness of the coating. At the same time, the adsorption of Ni(OH)2 colloid on the electrode surface will also cause hydrogen bubbles to stay on the electrode surface and the porosity of the coating increases. Not only does boric acid have a pH buffering effect, but it can increase cathodic polarization, thereby improving bath performance and reducing “burning” at high current densities. The presence of boric acid is also beneficial to improve the mechanical properties of the coating.

Anode activator—Except for sulfate-type nickel plating solutions that use insoluble anodes, other types of nickel plating processes use soluble anodes. The nickel anode is easily passivated during the electrification process. In order to ensure the normal dissolution of the anode, a certain amount of anode activator is added to the plating solution. Through experiments, it is found that CI-chloride ion is the best nickel anode activator. In the nickel plating solution containing nickel chloride, in addition to being the main salt and conductive salt, nickel chloride also acts as an anode activator. In the electroplating nickel solution that does not contain nickel chloride or its content is low, a certain amount of sodium chloride should be added according to the actual situation. Nickel bromide or nickel chloride is also commonly used as a stress reliever to maintain the internal stress of the coating and give the coating a semi-bright appearance.

Additive – The main component of the additive is stress reliever. The addition of stress reliever improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. With the change of the concentration of the stress reliever, the internal stress of the coating can be changed from tension compressive stress. Commonly used additives are naphthalene sulfonic acid, p-toluene sulfonamide, saccharin and so on. Compared with the nickel coating without the stress reliever, adding the stress reliever to the bath will result in a uniform, fine and semi-bright coating. Usually, the stress relief agent is added by ampere one hour (currently, the general-purpose combination special additives include anti-pinhole agents, etc.).

Wetting Agent – During the electroplating process of PCB, the evolution of hydrogen on the cathode is inevitable. The evolution of hydrogen not only reduces the cathode current efficiency, but also causes pinholes in the coating due to the retention of hydrogen bubbles on the electrode surface. The porosity of the nickel plating layer is relatively high. In order to reduce or prevent the generation of pinholes, a small amount of wetting agent should be added to the plating solution, such as sodium lauryl sulfate, sodium diethylhexyl sulfate, n-octane. It is an anionic surface active substance, which can be adsorbed on the surface of the cathode, so that the interfacial tension between the electrode and the solution is reduced, and the wetting contact angle of hydrogen bubbles on the electrode is reduced, so that the bubbles are easily away from the electrode surface, preventing or mitigating the generation of plating pinholes.