The relation between plating conditions and rate of deposition or composition of plated film was investigated in two kinds of ammoniacal alkaline electroless plating baths (A-T and A-C) for cobalt-nickel alloys of sulfate-hypophosphite system.
The results obtained were as follows.
(1) The preferable conditions for stable baths having relatively high rate of deposition and giving good appearance of plated film were as follows:
Ammoniacal alkaline tartrate bath (A-T) Ammoniacal alkaline citrate bath (A-C)
CoSO₄+NiSO₄ 0.1mol/l 0.1mol/l
NaH₂PO₂ 0.2mol/l 0.2mol/l
Na-citrate - 0.2mol/l
Na-tartrate 0.5mol/l -
(NH₄)₂SO₄ 0.5mol/l 0.5mol/l
pH(NH₄OH) 9.0 9.0
Operating temperature 90°C 90°C
(2) Under the above conditions, a desired film composition having any ratio of nickel to cobalt and containing some amount of phosphorus was obtained at a relatively high rate of deposition by controlling the ratio of concentrations of ions between nickel and cobalt in each bath.
(3) The plating rate in A-T was higher than that in A-C, but the former gave inferior appearance of deposited film to the latter owing to its lower stability. The plating rates in the both baths were higher than those in single baths (for cobalt or nickel plating) under the same conditions.
(4) The utilization efficiencies of hypophosphite in the above baths were usually kept in the range of 20-40% in A-T, and 28-30% in A-C, both of which were higher than that of nickel plating baths and lower than that of cobalt plating baths under the same plating conditions.
(5) The phosphorus content in the plated film almost linearly decreased from 2.4 to 1.9% in A-T or from 5.8 to 1.8% in A-C with the increase of cobalt content in the film.