PCB Nickel Plating Process

1. Functions and Characteristics

PCB (it is the abbreviation of Printed Circuie Board) uses nickel plating as the substrate plating for precious metals and base metals. And for some single-sided printed boards, it is also often used as a surface layer. For some surfaces that are worn under heavy load, such as switch contacts, contacts or plug gold, using nickel as the gold substrate plating can greatly improve the wear resistance. When used as a barrier layer, nickel can effectively prevent the diffusion between copper and other metals. Dumb nickel/gold composite coating is often used as an anti-etching metal coating, and can meet the requirements of hot-press welding and brazing. Only nickel can be used as an anti-corrosive coating of ammonia-containing etchant, without hot-press welding PCBs that require bright plating. Light nickel/gold plating is usually used. The thickness of the nickel plating layer is generally not less than 2.5 microns, usually 4-5 microns.

The deposited layer of PCB low-stress nickel is usually plated with a modified Watt nickel plating solution and some sulfamate nickel plating solutions with stress-reducing additives.

PCB nickel plating has bright nickel and dumb nickel (also called low-stress nickel or semi-bright nickel), which usually require uniform and fine plating, low porosity, low stress, and good ductility.

2. Nickel Sulfamate (Nickel Ammonia)

Nickel sulfamate is widely used as the substrate plating on metallized hole plating and printed plug contacts. The deposited layer has low internal stress, high hardness, and extremely excellent ductility. Add a stress relief agent to the plating solution, and the resulting plating layer will be slightly stressed. There are many different formulations of sulfamate baths. The typical formulations of nickel sulfamate baths are as follows. Due to the low stress of the coating, it has been widely used, but the stability of nickel sulfamate is poor and its cost is relatively high.

3. Modified Watt Nickel (Sulfur Nickel)

Modified Watt Nickel formula, using nickel sulfate, together with the addition of nickel bromide or nickel chloride. Due to internal stress, nickel bromide is mostly used. It can produce a semi-bright, slightly internal stress, good ductility coating. This coating is easily activated for subsequent electroplating, and the cost is relatively low.

4. The Role of Each Component of The Plating Solution:

• Main Salt:

Nickel sulfamate and nickel sulfate are the main salts in the nickel liquid. The nickel salt mainly provides the nickel metal ions required for nickel plating and also acts as a conductive salt. The concentration of nickel plating solution varies slightly with different suppliers, and the allowable content of nickel salt varies greatly. The content of nickel salt is high. Higher cathode current density can be used, with fast deposition speed, 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. The nickel salt content is low and the deposition rate is low, but the dispersing ability is very good, and it can obtain a fine and bright crystalline coating.

• Buffer:

Boric acid is used as a buffer to keep the PH 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. And when PH value is too high, due to the continuous precipitation of H2, PH value of the liquid layer close to the cathode surface rises rapidly, resulting in Ni The formation of (OH)2 colloid, and the inclusion of Ni(OH)2 in the coating increases the brittleness of the coating. At the same time, the adsorption of Ni(OH)2 colloid on the electrode surface will also cause the retention of hydrogen bubbles on the electrode surface. The porosity of the coating increases. Boric acid not only has a PH buffering effect, but also can increase the cathode polarization, thereby improving the performance of the plating solution and reducing the “scorching” phenomenon under high current density. The presence of boric acid also helps 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 all use soluble anodes. The nickel anode is very easy to passivate 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 experimentation, it is found that CI-chloride ion is the best nickel anode activator. In the nickel plating bath containing nickel chloride, nickel chloride not only serves as the main salt and conductive salt, but also acts as an anode activator. In the electroplating nickel solution that does not contain nickel chloride or its low content, a certain amount of sodium chloride needs to be added according to the actual conditions. Nickel bromide or nickel chloride is also commonly used as a stress relief agent to maintain the internal stress of the coating and give the coating a semi-bright appearance.

• Additives:

The main component of the additive is the stress reliever. The addition of the stress reliever improves the cathodic polarization of the plating solution and reduces the internal stress of the coating. As the concentration of the stress reliever changes, the internal stress of the coating can be changed from tension. The stress changes to compressive stress. Commonly used additives are: naphthalenesulfonic acid, p-toluenesulfonamide, saccharin and so on. Compared with the nickel coating without stress relief agent, adding stress relief agent to the plating solution will obtain a uniform, fine and semi-bright coating. Usually the stress relief agent is added at an ampere hour (the current general-purpose combination special additives include anti-pinhole agents, etc.).

• Wetting Agent:

During the electroplating process, the precipitation of hydrogen on the cathode is inevitable. The precipitation of hydrogen not only reduces the current efficiency of the cathode, 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, such as sodium lauryl sulfate, sodium diethylhexyl sulfate, sodium n-octyl sulfate, etc., should be added to the plating solution. 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 the hydrogen bubbles on the electrode is reduced, thereby making the bubbles easily leaving the surface of the electrode prevents or reduces the generation of pinholes in the coating.