The most critical factor to achieve a consistent nickel deposit is to control the nickel bath within a narrow process window. All variables have to be within a predetermined range and must not deviate too much, as the coating quality will deteriorate rapidly if they do.
Operating Temperature Window
For the vast majority of Electroless Nickel baths an ideal operating temperature is between 85°C and 95°C. By running at too low a temperature the deposition rate will decrease, resulting in an unsatisfactory, non-uniform deposit. Operating at too high a temperature will result in the bath undergoing spontaneous decomposition – an irreversible process that can cause massive damage and is best avoided. It is normal for a reputable plating house to monitor bath temperature on an ongoing basis, not just at the start of a run.
pH Monitoring and Adjustment
The pH of the bath is critical and typically must be maintained between 4.4 and 5.0 for a mid-phosphorus deposit. The pH will naturally decrease as plating continues, and must be periodically adjusted with a controlled addition of ammonia or sodium hydroxide. Typically, the pH is checked every 30 to 60 minutes and then adjusted as required with small additions of the solution required to bring the pH back up to the desired level.
Replenishment Rate
The major components of the electroless nickel bath, nickel and reducing agent, are consumed as metal is deposited onto parts. The rate at which replenishment solution is added to the bath is therefore calculated on the basis of the amount of metal that has been plated. This is typically expressed as metal turnovers.
Bath Turnover Ratio
A bath’s turnover is calculated by determining the equivalent volume of original bath solution that has been added to the solution as it has been used to plate metal. Typically a bath is retired after 5 to 8 turnovers as the by-products of the solution begin to negatively affect the plated metal’s properties.
Signs of Bath Breakdown
Cloudy bath conditions and an excessive amount of sediment within the tank solution can indicate that the process is nearing the end of life. In addition to this, plating is sometimes found to be forming on the walls of the plating tank as opposed to on the work pieces. This can occur very quickly with some solutions, thus the need to continuously monitor the process for any signs of deterioration in addition to periodically checking the key bath chemistry variables. It is also worth noting that handling the bath chemistry itself is controlled by workplace exposure limits for nickel compounds in the form of fumes. A useful reference for Electroless Nickel Plating is https://www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating/.
Good bath control is key to achieving a consistent, durable electroless nickel deposit that will pass inspection.
