Controlling intermetallic compound formation reaction between Sn and Ni-P by Zn addition

Effects of Zn addition on the interfacial reaction between Sn and Ni(P) were investigated by systematically varying the Zn concentration in the Sn solder. It was found that the typical Ni-Sn reaction product, Ni₃Sn₄ phase, was changed substantially by adding small amounts of Zn to the Sn. With the Zn addition, the ternary Ni₄(Sn_1-x,Zn_x) phase formed at the interface during reflow and aging according to X-ray diffraction analysis. In the Ni₄(Sn_1-x,Zn_x) phase, the lattice parameters contracted with increasing Zn content, in agreement with the Vegard's law. Since diffusions of the reactive species through the denser ternary intermetallic compound were more unlikely than through the binary Ni₃Sn₄, the Zn-containing solder showed a much slower electroless Ni-P consumption rate than Sn. The decrease in Ni consumption rate increased with the increasing Zn content in Sn. The reason for the decrease was that the growth rate of Ni₄(Sn_1-x,Zn_x) phase was directly determined by substitution of Zn atoms into the Sn sublattice.