Plating Q&A: When to Add Replenishers

Q. Why is our sodium hypophosphite increasing, and should we skip an addition of a replenisher?

A. With any electroless nickel system based on sodium hypophosphite chemistry, if the sodium hypophosphite is higher than the target, the applicator should never skip the addition of a replenishment component to force the hypophosphite back into range. Replenishments are blended with many critical chemistry components, often in the parts per million ranges.

Typically, replenishment components contain higher concentrations of stabilizers or stabilizing chemistries, important for deposit and solution performance. Skipping replenishments completely will put the EN chemistry in jeopardy of slower deposition rates, solution instability, decomposition or greater tank plate out potential. From the resulting deposit, the risk would be higher porosity, reduction in corrosion protection including neutral salt spray failures or poor deposit smoothness, brightness or uniformity.

In most cases, there's less harmful impact to making a partial addition of hypophosphite replenishment chemistry—but never skip additions. Higher concentrations of sodium hypophosphite in EN systems have less detrimental impact than that of lower stabilizers and rate accelerators.

Understanding how consumption/efficiency of hypophosphite is impacted by operations is also important in hypophosphite analysis results. We know the hypophosphite is consumed during the plating process, but it is often forgotten that it can be non-productively consumed if not monitored properly. There are many poor operating practices that will increase consumption.

First, the hypophosphite-reducing agent concentration influences the overall system plating efficiency and is consumed in a given ratio to the nickel metal during plating. The hypophosphite-to-metal ratio is one of the most important aspects. More importantly, the hypophosphite component is balanced with stabilizers and other chemistry constituents that impact hypophosphite efficiency in the deposition reactions. 

Without going into detail for discussion of the EN chemistry reactions, here are some basic guidelines. A medium phosphorus EN system requires that 5 g of sodium hypophosphite will be consumed for every 1 g of nickel deposited. As the phosphorus contents of systems related to their design vary, so do the resulting sodium hypophosphite efficiencies. High phosphorus (>10 percent) EN systems typically range from 5 to 6 g of hypophosphite required for each 1 g of nickel deposited. This is why high phosphorus systems have higher ranges of hypophosphite, typically 36–40 g/L while the low phosphorus EN systems (<4 percent) require 4–5 g of hypophosphite for each 1 g of nickel deposited and traditionally have operating ranges from 24–30 g/L.

Air agitation introduces oxygen into the bath increasing the potential for increased decomposition to orthophosphite. This is especially true in cases of long bath idle periods at operating temperature.

Low solution loading, surface area to tank volume, less than 0.1 ft²/gal increases the relative concentration of stabilizers per workload area. This result decreases the efficiency of the hypophosphite, causing more consumption to plate a given amount of nickel. Conversely, higher loading makes the system more efficient, so one might see hypophosphite concentrations increase.

High stabilizer concentration ranges in the bath will lower the efficiency of the deposition reaction, causing more hypophosphite to be consumed plating a given amount of nickel. Low levels of certain sulfur stabilizers in mid-phos EN systems will lower the efficiency of the reaction, causing increased hypophosphite usage. Both nitrate and copper contamination will consume sulfur stabilizers.

Brad Durkin is the director of international product management for Coventya. He can be reached at b.durkin@coventya.com

Originally published in the December 2015 issue.