Maintenance of Plating Solution in PCB Nickel Plating Process

The maintenance of the plating solution needs to consider the following aspects:

1. Temperature

Different nickel processes have different plating bath temperatures. The effect of temperature changes on the nickel plating process is more complicated. In higher temperature nickel plating solution, the nickel plating layer obtained has low internal stress, and good ductility. The internal stress of the plating layer 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 nickel salt solution will be hydrolyzed, and the generated nickel hydroxide colloid will trap the colloidal hydrogen bubbles, causing pinholes in the coating and reducing the cathode polarization at the same time. Therefore, operating temperature is very strict and should be controlled within the specified range. In actual work, room temperature controller is used to maintain the stability of its operating temperature according to the optimal temperature control value provided by the supplier.

2. PH Value

Pactical results show that PH value of the nickel plating electrolyte has a great influence on the performance of the coating and that of the electrolyte. In the strong acid electroplating bath with PH≤2, there is no deposition of metallic nickel, but light gas is precipitated. Generally, PH value of PCB nickel plating electrolyte is maintained between 3-4. The nickel plating solution with higher PH value has higher dispersion power and higher cathode current efficiency. However, when PH is too high, the cathode will continuously release light gas during the electroplating process, which will increase the PH value of the coating near the cathode surface. When it is greater than 6, there will be light nickel oxide colloids that will cause hydrogen bubbles to stay and cause pinholes in the coating. The inclusion of nickel hydroxide in the coating will also increase the brittleness of the coating. The nickel plating solution with lower pH has better anode dissolution, which can increase the content of nickel salt in the electrolyte, allowing the use of higher current density, thereby enhancing production. However, if the PH is too low, temperature range for obtaining bright coatings will be narrowed. Add nickel carbonate or basic nickel carbonate, PH value will increase; add sulfamic acid or sulfuric acid, PH value will decrease. Check and adjust PH value every four hours during the working process.

3. Anode

Conventional nickel plating on PCBs that can be seen at present all use soluble anodes, and it is quite common to use titanium baskets as anodes to install 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 put into the anode bag woven of polypropylene material to prevent the anode mud from falling into the plating solution. And we should regularly clean and check whether the holes are unblocked. The new anode bag should be soaked in boiling water before use.

4. Purification

When there is organic contamination in the plating solution, it should be treated with activated carbon. However, this method usually removes a part of the stress relief agent (additive), which must be supplemented. The treatment process is as follows:

(1) Take out the anode, removing 5ml/l impurity water, and heat (60-80°C) to inflate (stir with air) for 2 hours.

(2) When there are many organic impurities, first add 3-5ml/lr of 30% hydrogen peroxide, and stir for 3 hours.

(3) Add 3-5g/l powdered activity under constant stirring, continue air stirring for 2 hours, turn off the stirring and let it stand for 4 hours, add filter powder and use the spare tank to filter and clean the tank.

(4) Clean and maintain the anode and hang it back. Use a nickel-plated corrugated iron plate as the cathode, and drag the cylinder for 8-12 hours at a current density of 0.5-0.1A/dm2 (It is also often used when there is inorganic pollution in the plating solution that affects the quality.)

(5) Replace the filter element (usually use a set of cotton cores and a set of carbon cores to filter continuously in series, changing periodically can effectively delay the processing time and improve the stability of the plating solution), analyze and adjust various parameters, and add additives for wetting agent for plating.

5. Analysis

The plating solution should use the key points of the process regulations stipulated in the process control, periodically analyze the composition of the plating solution and the Hull cell test, and guide the production department to adjust the parameters of the plating solution based on the obtained parameters.

6. Stirring

Nickel plating process is the same as other electroplating processes. The purpose of stirring is to accelerate the mass transfer process to reduce concentration changes and increase the allowable current density upper limit. Stirring the plating solution also has a very important effect, which is to reduce or prevent pinholes in the nickel plating layer. Because during the electroplating process, the plating ions near the cathode surface are poor, and a large amount of hydrogen is precipitated, which causes PH value to rise to produce nickel hydroxide colloid, which causes the retention of hydrogen bubbles and produces pinholes. The above phenomenon can be eliminated by strengthening the stirring of the remaining plating solution. Commonly used compressed air, cathode movement and forced circulation (combined carbon core and cotton core filtration) stirring.

7. Cathode Current Density

Cathode current density has an impact on cathode current efficiency, deposition rate and coating quality. The test results show that when nickel is plated with an electrolyte with a lower PH, in the low current density area, the cathode current efficiency increases with the increase of the current density. In the high current density area, the cathode current efficiency has nothing to do with the current density. When used high PH electroplating bath nickel, the cathode current efficiency has little relationship with the current density.

Like other plating species, the cathode current density range selected for nickel plating should also depend on the composition, temperature and stirring conditions of the electroplating solution. Due to the large PCB area, the current density of the high-current area and the low-current area are very different, and 2A/dm2 is generally appropriate.