Role of a Pb2+ stabilizer in the electroless nickel plating system:: A theoretical exploration

The electroless nickel plating process involves multiple chemical reactions that interrelate mutually in intricate ways and take place almost simultaneously. This work attempts to provide a theoretical template for understanding this complicated system from the role of heavy metal stabilizers. Pb2+ ion was selected for the study because it is the most effective species in this category. A theoretical expression of the nickel deposition rate is created by envisioning the Stern-Grahame electrical double layer presumably attached to the plating frontier (the inner Helmholtz plane) as a one-dimensional potential well and the electron transfer from the Fermi level of plating frontier to Ni2+ ions at the outer Helmholtz plane as the quantum tunneling effect. The deposition of P atoms is assumed to go through the oxidative-addition of hypophosphite (H2PO2-) ions at Ni atoms on the plating frontier and on this basis its theoretical deposition rate is developed. Another adsorption state of H2PO2- on the surface Ni atoms is proposed to form by its two oxygen atoms, and the P(I) center supplies electrons to the plating frontier through these two oxygen bridges. Establishing the relationship between Pb2+ ion concentration and the Fermi level of the plating frontier (i.e., inside the solid Ni-P alloy deposition layer on the plating substrate) is an important step for deriving the nickel and element phosphorus deposition rates. The stabilizing effect of the lead ion is attributed to its participation as the neutral atom in the lattice of the Ni-P film at the plating frontier, which results in the ascension of Fermi energy. Thus, the oxidation of hypophosphite at the plating frontier is retarded, which in turn slows down the deposition of both Ni and P that needs electrons. The dependence of the deposition rates of Ni and P on concentrations of Pb2+ ion was experimentally examined. It is found that the theoretical models predict well the experimental trends. The parameters designated in the theoretical model are determined through a self-consistent computation procedure.