Depletion and phase transformation of a submicron Ni(P) film in the early stage of soldering reaction between Sn-Ag-Cu and Au/Pd(P)/Ni(P)/Cu

The early stage of soldering reaction between Sn-3Ag-0.5Cu solder and ultrathin-Ni(P)-type Au/Pd(P)/Ni(P)/Cu pad was investigated by field-emission scanning electron microscopy (FE-SEM) in conjunction with field-emission electron probe microanalysis (FEEPMA) and high-resolution transmission electron microscopy (HRTEM). FE-SEM, FE-EPMA, and HRTEM investigations showed that Ni 2 SnP and Ni 3 P were the predominant P-containing intermetallic compounds (IMCs) in the soldering reaction and that their growth behaviors strongly depended on the depletion of Ni(P). The growth of Ni 3 P dominated over that of Ni 2 SnP in the early stage of soldering, whereas the Ni 3 P gradually transformed into Ni 2 SnP after Ni(P) depletion. This Ni(P)-depletion-induced Ni 2 SnP growth behavior is different from the reaction mechanisms reported in the literature. Detailed analyses of the microstructural evolution of the IMC during Ni(P) depletion were conducted, and a two-stage reaction mechanism was proposed to rationalize the unique IMC growth behavior.