The role of Bi³⁺‐complex ion as the stabilizer in electroless nickel plating process

Abstract

Bi 3þ -complex ion is presented here as a less toxic stabilizer for use in electroless nickel plating (ENP) to replace the existing Pb 2þ ion stabilizer. The asymmetric derivatives of EDTA are identified to be a type of coordination ligands that can combine with Bi 3þ ions to form soluble complexes in the acidic ENP solution. In the ENP system studied the Bi 3þ -complex ion displays a critical stabilizer concentration of about 10 À5 mol/L, that is, the percolation concentration over which the ENP rate drops sharply. Besides the experimental measurement, deposition rates of both Ni and P are also simulated by using a kinetic model that has been derived from the double electric layer theory. The Bi 3þ -complex ion, behaving like conventional Pb 2þ ion, stabilizes ENP bath through the chemical replacement reaction at the surface of Ni deposition layer and results in a passive plating surface. This investigation also verifies the properties of the EN deposit, which are insignificantly affected by the length of service time of the plating solution by employing Bi 3þ -complex ion stabilizer.