Controlling Phosphorus Content in Electroless Nickel Coatings

Q: We have a mid-phosphorus electroless nickel (MPEN) bath on our line, and the Technical Data Sheet shows a range of 6-9 percent weight of phosphorus (w/w P) in the EN film. We have a job that specifies a narrower range of 7-8 percent w/w P. We generally use our MPEN bath for eight metal turnovers. How can we ensure that the coating will contain the necessary amount of phosphorus to meet the specification over the entire life of the bath?

A: Electroless nickel deposition is an electrochemical process in which a species referred to as a reducing agent is oxidized at a catalyst, in this case the nickel layer itself. This oxidation yields electrons that are then employed to reduce the nickel ions to form the bulk coating. The nature of the process and the type of reducing agent employed yields alloys rather than depositing a pure nickel layer.

Electroless nickel formulations are designed to yield phosphorus contents in the alloy in general ranges. These usually include the four main types of EN coatings: high-phosphorus (less than 10.5 percent w/w), mid-phosphorus (6-9 percent w/w), low-mid-phosphorus (3- 6 percent w/w) and low-phosphorus (less than 3 percent w/w). Each of these coatings offer different functional properties, such as high corrosion resistance, high wear resistance, solderability and hardness. It is important to understand what factors influence the amount of phosphorus that is co-deposited in the film. Some of the major contributing factors can, and must, be controlled by the applicator to ensure that the desired alloy composition is achieved and thus the intended properties of the coating are exhibited. These major factors for consideration include, but are not limited to:

• Formulation of EN chemistry.
• Concentrations of critical constituents (nickel and sodium hypophosphite).
• Age of solution (metal turnover rate, MTO)
• pH and temperature of operation.

Formulation. The composition of an eIectroless nickel formulation can be the single largest contributor to the determination of the alloy of the film deposited. Suppliers specially formulate EN chemistries to generate deposit layers that achieve the desired alloy composition. It is critically important for both the applicator and the supplier to understand the specific requirements so the right formulation can be selected. Here, we focus our attention on the subsequent factors that the applicator can directly influence.

Concentrations of critical constituents. The two main reactants in the overall reaction that provide the deposited film are the nickel ion and the reducing agent. In the majority of commercial electroless nickel processes available today, these ions are provided by nickel sulfate and sodium hypophosphite. The standard concentrations in most commercially available formulations are 6.0 g/L nickel sulfate and 30.0 g/L sodium hypophosphite. There are formulations that operate outside of these standards, but it is generally true that if formulations are maintained at +/-10 percent of these optimum concentrations, the appropriate amount of phosphorus should be contained within the layer. Operation at lower than optimum concentrations can slow the rate of reaction (plating rate) and lead to higher than desired phosphorus co-deposition. Conversely, higher concentrations can lead to accelerated plating rates, which could suppress phosphorus co-deposition, but more importantly may lead to instability of the chemistry. 

Age of solution. Another important oxidation byproduct of the hypophosphite ion is referred to as the orthophosphite ion. As a bath is used, this builds in solution at a rate of about 24-30 g/L per MTO. Therefore, at advanced bath ages, the concentration of orthophosphite can be significant (for example, at 8 MTO, the orthophosphite concentration will be about 240 g/L). Orthophosphite ions can be a source of elemental phosphorus for eventual reduction and incorporation into the coating; thus, as the orthophosphite builds in solution, the phosphorus content in the alloy will increase. The buildup of other byproducts and introduced species in the EN solution (such as sodium ions, sulfate ions and complexors to maintain solubility of nickel) which impede the diffusion of nickel ions and hypophosphite ions to the catalytic surface may decrease the localized concentration of both of these critical reactants. As these localized concentrations decrease, the reaction rate (or rate of deposition) will decrease, and thus the phosphorus in the deposit will increase.

This is a condition of all hypophosphite-reduced EN formulations, so this effect must be counteracted by another factor to avoid excursions of phosphorus co-deposition. Another method to counteract this effect is steady-state operation via so-called “bleed and feed” or with ion specific membranes to remove these byproducts.

pH and temperature. The pH of operation can have a significant impact on both the rate of deposition and phosphorus content. A byproduct of the oxidation reaction referred to above is H+ hydrogen ions. As H+ increases, the pH increases, which accelerates the reaction rate and decreases the phosphorus content. Generally, buffers are formulated into the chemistry to moderate these pH fluctuations, but alkali (such as ammonium hydroxide, potassium carbonate or sodium hydroxide) must also be added to neutralize the H+ and mitigate the above effect. As an example, an increase of only 0.2 pH units can suppress phosphorus content by as much as 1.0 percent w/w in some chemistries.

Temperature is a main driving force behind the rate of reaction, and the plating rate increases exponentially with increasing temperature. Generally, electroless nickel plating baths are maintained in the range of 85-90^oC (185-194^oF). Within this recommended range, there is very little effect on phosphorus co-deposition. Operating outside this range is not recommended, as other operational and functional components of the EN chemistry can be significantly affected. 

Due to the requirement of the electroless nickel formulation to yield a phosphorus content in the range of 7-8 percent w/w P for the life of the solution (8 MTOs), and considering the above information, the most effective way to control phosporus co-deposition is to incrementally increase the pH of operation. This will accelerate the plating rate and counteract the natural rate reduction that occurs as a result of the aging of the EN solution.

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Ambrose Schaffer, CEF, is research and development manager for functional coatings at Coventya. Visit coventya.com.