Introduction of Nickel Electroplating Process for Printed Circuit Board (PCB) (3)

5. Maintenance of Plating Solution

a) 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 optimal temperature control value provided by the supplier.

b) PH value – The 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 PCB nickel plating electrolyte is maintained between 3-4. 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.

c) Anode – The conventional nickel plating of PCBs that can be seen at present uses soluble anodes, and it is quite common to use titanium baskets as anodes with nickel corners inside. 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.

d) Purification – When there is organic contamination in the bath, 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 (60-80°C) and aerate (gas-stirring) for 2 hours.

(2) When there are 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 to stir for 2 hours. Turn off the stirring and let it 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 has the influence of inorganic pollution) quality.

(5) Change 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 additives to wet agent to try plating.

e) Analysis – The plating solution should use the key points of the process regulations specified in the process control, regularly analyze the components of the plating solution and the Hull cell test, and guide the production department to adjust the parameters of the plating solution according to the obtained parameters.

f) 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.

g) 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. 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 electroplating solution. Due to the large area of ​​the PCB board, the current density in the high current area and the low current area is reduced. The difference is very large, generally 2A/dm2 is appropriate.