PCB Blog - Introduction and Troubleshooting of Nickel Electroplating on PCB board

1. The role and characteristics of nickel electroplating process on PCB board
Nickel plating is used on PCB boards as a substrate coating for precious metals and base metals, and is also commonly used as a surface layer for some single-sided printed boards. For some surfaces that are heavily worn, such as switch contacts, contacts, or plug gold, using nickel as a backing layer of gold can greatly improve wear resistance. When used as a barrier, nickel is effective in preventing diffusion between copper and other metals. Dumb nickel/gold combination coatings are often used as etching-resistant metal coatings, and can meet the requirements of hot-press welding and brazing. Only nickel can be used as a corrosion-resistant coating for ammonia-based etchants without hot-press welding. PCB boards with bright plating are also required, usually with bright nickel/gold plating. Nickel plating thickness is generally not less than 2.5 microns, usually 4-5 microns. The deposited layer of low-stress nickel on the PCB is usually plated with a modified Watts nickel bath and some sulfamate nickel baths with stress-reducing additives. We often say that the nickel plating of PCB boards includes bright nickel and matt nickel (also known as low stress nickel or semi-bright nickel), which usually require 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 on metallized hole plating and printed plug contacts. The obtained deposited layer has low internal stress, high hardness and excellent ductility. Add a stress reliever to the bath and the resulting coating will be slightly stressed. There are a variety of sulfamate baths with different formulations. The typical nickel sulfamate bath formula is shown in the table below. Due to the low stress of the coating, it is widely used, but the stability of nickel sulfamate is poor, and its cost is relatively high.

3. Modified Watt Nickel (Sulfur Nickel)
Modified Watts Nickel formulation using nickel sulfate, along 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; and this coating is easy to activate for subsequent electroplating, and the cost is relatively low.

4. The 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 mainly provide 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 low deposition rate, but has good dispersing ability, and can obtain fine crystal and bright coating.
2) 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. The porosity of the coating increases. Not only does boric acid have a pH buffering effect, but he 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.
3) 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. It is found through experiments that CI-chloride ion is the activator of nickel anode. 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.
4) Additives - the main component of additives 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 reduced. Change from tensile stress to compressive stress. Commonly used additives are: naphthalene sulfonic acid, p-toluenesulfonamide, 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.).
5) Wetting agent - During the electroplating process, 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.

5. Maintenance of plating solution
5.1 Temperature - Different nickel processes use different bath temperatures. The effect of temperature change on the nickel plating process is more complicated. In the nickel plating solution with higher temperature, the obtained nickel coating has low internal stress and good ductility, and the internal stress of the coating becomes stable when the temperature is increased to 50°C. The general operating temperature is maintained at 55--60 degrees C. If the temperature is too high, the hydrolysis of the nickel salt will occur, and the resulting nickel hydroxide colloid will retain the colloidal hydrogen bubbles, resulting in pinholes in the coating, and at the same time reducing the cathodic polarization. Therefore, the working temperature is very strict and should be controlled within the specified range. In actual work, the normal temperature controller is used to maintain the stability of its working temperature according to the temperature control value provided by the supplier.

5.2 PH value - practical results show that the PH value of the nickel plating electrolyte has a great influence on the performance of the coating and the electrolyte. In the strong acid electroplating solution with PH≤2, there is no deposition of metallic nickel, only light gas is precipitated. Generally, the pH value of the nickel-plated electrolyte for PCB board is maintained between 3 and 4. Nickel baths with higher pH have higher dispersion power and higher cathode current efficiency. However, when the pH is too high, due to the continuous precipitation of light gas from the cathode during the electroplating process, the pH value of the coating near the surface of the cathode rises rapidly. Pinholes appear in the coating. The inclusion of nickel hydroxide in the coating will also increase the brittleness of the coating. Nickel plating baths with lower pH have better anode dissolution, which can increase the content of nickel salts in the electrolyte, allowing higher current density to be used, thereby enhancing production. However, if the pH is too low, the temperature range for obtaining bright coatings will be narrowed. Adding nickel carbonate or basic nickel carbonate, the pH value increases; adding sulfamic acid or sulfuric acid, the pH value decreases, check and adjust the pH value every four hours during the working process.

5.3 Anode - The conventional nickel plating of PCB board that can be seen at present uses soluble anodes, and it is quite common to use titanium baskets as anodes with built-in nickel corners. The advantage is that the anode area can be made large enough and does not change, and the anode maintenance is relatively simple. The titanium basket should be placed in an anode bag made of polypropylene material to prevent the anode slime from falling into the plating solution. And should regularly clean and check whether the holes are unobstructed. New anode bags should be soaked in boiling water before use.

5.4 Purification—When the bath has organic contamination, it should be treated with activated carbon. However, this method usually removes a portion of the stress reliever (additive), which must be replenished. Its treatment process is as follows;
1) Take out the anode, add 5ml/l of impurity-removing water, heat it (60-80°C) and aerate (gas-stirring) for 2 hours.
2) When there is a lot of organic impurities, first add 3-5ml/lr of 30% hydrogen peroxide for treatment, and stir for 3 hours.
3) Add 3-5g/l powdered activity under constant stirring, continue gas stirring for 2 hours, turn off stirring and let stand for 4 hours, add filter powder and use a spare tank to filter and clean the tank at the same time.
4) Clean and maintain the anode hanger, use a corrugated iron plate plated with nickel as the cathode, and drag the cylinder for 8-12 hours at a current density of 0.5-0.1 A/square decimeter (when the plating solution is contaminated with inorganic substances, it will affect the quality. is also often used)
5) Replace the filter element (usually a group of cotton cores and a group of carbon cores are used for continuous filtration in series, and periodic replacement can effectively prolong the large processing time and improve the stability of the plating solution), analyze and adjust various parameters, and add additive wetting agent You can try plating.
6) Analysis - The plating solution should use the main points of the process regulations specified in the process control, regularly analyze the plating solution components and the Hull cell test, and guide the production department to adjust the parameters of the plating solution according to the obtained parameters.
7) Stirring - the nickel plating process is the same as other electroplating processes. The purpose of stirring is to accelerate the mass transfer process to reduce the concentration change and increase the upper limit of the allowable current density. Stirring the plating solution also has a very important role in reducing or preventing pinholes in the nickel plating layer. Because, during the electroplating process, the plating ions near the surface of the cathode are depleted, and a large amount of hydrogen is precipitated, which increases the pH value and produces nickel hydroxide colloid, resulting in the retention of hydrogen bubbles and the generation of pinholes. The above phenomenon can be eliminated by strengthening the stirring of the plating solution. Compressed air, cathode movement and forced circulation (combined with carbon core and cotton core filtration) are commonly used for stirring.
8) Cathode current density - Cathode current density has an effect on cathodic current efficiency, deposition rate and coating quality. The test results show that when nickel is plated with an electrolyte with a lower pH, the cathode current efficiency increases with the increase of the current density in the low current density region; in the high current density region, the cathode current efficiency has nothing to do with the current density, and when the The cathode current efficiency has little to do with the current density at higher pH nickel plating solutions. Like other plating species, the range of cathode current density selected for nickel plating should also depend on the composition, temperature and stirring conditions of the plating solution. The density varies greatly, generally 2A/dm2 is appropriate.

6. Troubleshooting and troubleshooting
1) Makeng: Makeng is the result of organic pollution. Large hemp pits usually indicate oil contamination. Poor agitation fails to dislodge the air bubbles, which creates pits. Wetting agent can be used to reduce its influence. We usually call small pits as pinholes. Poor pretreatment, poor metal quality, too little boric acid content, and too low bath temperature will cause pinholes. And process control is the key, anti-pinhole agent should be added as process stabilizer.
2) Roughness and burrs: Roughness means that the solution is dirty, and it can be corrected by full filtration (the PH is too high to form hydroxide precipitation and should be controlled). If the current density is too high, the anode slime and the impure water will bring in impurities, which will cause roughness and burrs in severe cases.
3) Low bonding force: If the copper coating is not fully deoxidized, the coating will peel off, and the adhesion between copper and nickel will be poor. If the current is interrupted, it will cause the nickel coating to peel off at the interrupted place, and it will also peel off when the temperature is too low.
4) The coating is brittle and has poor weldability: when the coating is bent or subjected to a certain degree of wear, the coating is usually brittle. This shows that there is organic or heavy metal pollution. Too many additives, entrained organics and electroplating resists are the main sources of organic pollution. They must be treated with activated carbon. Insufficient addition and high pH will also affect the brittleness of the coating.
5) The coating is dark and the color is uneven: the coating is dark and the color is uneven, which means there is metal pollution. Because copper is usually plated first and then nickel is plated, the copper solution brought in is the main source of pollution. It is important to reduce the copper solution on the hanger to a minimum. In order to remove metal contamination in the tank, especially the copper removal solution, a corrugated steel cathode should be used, at a current density of 2 to 5 amps/square foot, 5 amps per gallon of solution for one hour. Poor pretreatment, poor low coating, too low current density, too low main salt concentration, poor contact of the electroplating power circuit will affect the color of the coating.
6) Coating burn: Possible causes of coating burn: insufficient boric acid, low concentration of metal salt, too low working temperature, too high current density, too high PH or insufficient stirring.
7) Low deposition rate: Low PH value or low current density will cause low deposition rate.
8) Blistering or peeling of the coating layer: poor pre-plating treatment, excessively long interruption time, organic impurity pollution, excessive current density, too low temperature, too high or too low PH, and serious influence of impurities will cause blistering or peeling phenomenon.
9) Anode passivation: The anode activator is insufficient, the anode area is too small, and the current density is too high on PCB board.