Electroless Nickel Plating Bath Control Explained
Operating a plating bath efficiently is more than loading parts and waiting for them to be coated. A number of interdependent parameters need to be constantly monitored in order to achieve the correct properties on a regular basis.
pH Control and Why It Drifts
pH is one of the most important parameters to control when operating a plating bath. Hydrogen ions are released during the reaction, leading to a decrease in pH and a decrease in the coating’s deposition rate. The pH needs to be checked on a regular basis and adjusted with sodium hydroxide solution as required. An acid bath typically needs to be maintained between a pH of 4.5 and 5.0.
Nickel Ion Concentration
The bath will need to be replenished with a Nickel salt concentrate as the Nickel ions are consumed as the coating builds up. Most baths are run in a continuous or semi-continuous manner, with top-ups added on a timed basis or based on analysis of the bath for Nickel content. Failure to maintain an adequate concentration of Nickel in the bath will result in a decrease in the rate of deposition, and potentially the formation of patchy or even powdery deposits.
Phosphorus Content and Deposit Properties
The Phosphorus content of the coating formed is a function of the hypophosphite to Nickel ratio in the plating solution. Typically high phosphorus coatings (above 10%) are corrosion resistant, whilst low phosphorus coatings (below 6%) are very hard and offer good wear resistance. Specifications for Electroless Nickel Plating typically include a range for the Phosphorus content of the coating, and it is essential that the bath chemistry is controlled within strict limits to reproduce the required coating properties.
Temperature Management
Accurate temperature control of the plating bath is crucial. The majority of acid baths are run at temperatures between 85°C and 95°C. It is important to note that a few degrees variation can significantly affect the plating rate. This means that the temperature must be checked using a surface thermometer, coated samples and coating thickness measurements to verify the conditions within the tank.
Stabiliser Levels
Stabilisers – These chemicals stop the bath’s ingredients from decomposing spontaneously, forming a hard layer on the sides and bottom of the tank. Stabilisers are slowly consumed by the chemical reactions occurring in the bath and must be replenished from time to time. The amount needed to stop decomposition must be determined very carefully because too much can have a completely opposite effect: It can stop the plating process completely or cause local pitting.
Load Ratio
Load Ratio: The surface area of all parts in the tank divided by the volume of solution in the tank. This will determine how fast the Nickel and Hypophosphite will be consumed by the solution. To maintain a consistent solution, it is typical to operate within a maximum load ratio per charge to maintain the solution for subsequent cycles.
Control of the plating bath to consistent standards is the key to producing coatings that meet specifications.