Electrodeposition of nickel–phosphorus from a highly conductive citrate bath for wire bonding applications

Ni-P coatings electroplating - A review, Part I: Pure Ni-P alloy

“…Modified Watts electrolyte for Ni-P plating combines the traditional nickel sulfate, nickel chloride and boric acid with a phosphorus oxyacid which is a source of phosphorus. Modified Watts bath containing sodium hypophosphite as a phosphorus source has also been reported in many works [35,36,[42][43][44]. Other than sulfate electrolyte, several other kinds of Ni-P aqueous electroplating baths exist, contingent on the nickel source nature mostly used are sulfamate and sulfonate ones.…”

“…Anode dissolution-If used as a contributing source of Ni in the bath, nickel chloride has two major functions: it appreciably increases solution conductivity thereby reducing voltage requirements and it is important in obtaining satisfactory dissolution of nickel anodes [53,54]. Due to the increase in solution conductivity, plating thickness and cathodic current efficiency are reported to increase with the increase of chloride concentration [44]. In the chloride electrolytes activity of Ni +2 ions is higher than in sulfate electrolyte and metal deposition potential is lower [55,56].…”

“…Electroplating resulted in high cathode current efficiency and hard deposits exhibiting crystal structure of nearly random orientation in case of the bath pH value being between 4 and 6, while when the pH value was 3.5 or less current efficiency was lower and hardness of the obtained deposits decreased. Dadvand and colleagues [44] reported electrodeposition of Ni-P from a citrate bath containing sodium chloride, citric acid, nickel sulfate, ammonia and sodium hypophosphite. They found cathode current efficiency in low current density regions to be twice higher compared to the one achieved when employing modified Watts bath.…”

“…: potassium or sodium carbonate, potassium or sodium hydroxide, ammonia. Dadvand et al [44] advocated the benefits of using ammonia for bath neutralization, as carbonate salts decrease the solution conductivity and alkali metal hydroxides promote the formation of nickel hydroxide complexes.…”

“…In the study by Li et al [76] it was determined that at low bath temperatures high NH 4 Cl content in combination with low NaH 2 PO 2 concentration can contribute to creating thick films (6-8 at.% of phosphorus) with optimum catalytic activity even at high current densities. However, ammonia concentration must be chosen carefully because exceeding the ammonia concentration above the certain limit can negatively affect the cathode current efficiency, hence decrease the plating rate [44]. Bai and Hu [110] observed that for Ni-P electrodeposits with P content from 0 to 28 at.% the ability of catalysing hydrogen evolution decreases with increasing annealing temperature, concluding that annealed deposits are not suitable for HER.…”