Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between Sn–Pb solder and Cu/Ni/Pd metallization Original Research Article

The interfacial microstructure and the kinetics of interfacial reaction between eutectic Sn–Pb solder and electroplated Ni/Pd on a Cu substrate have been studied by scanning, transmission and analytical electron microscopies. Besides PdSn4 and Ni3Sn4, small grains of Ni3Sn2 with a hexagonal structure are also observed after long-time aging of the diffusion couples at 125°C. The presence of intermetallic phases is correlated with the diffusion paths in the calculated Pd–Pb–Sn and Ni–Pb–Sn isothermal sections. The growth kinetics of the Ni3Sn4 scallops in the submicrometer length scale was analyzed with an Arrhenius type of equation. The thickening kinetics yields a time exponent n=3.1 and an apparent activation energy (Qh) of 25,750 J/mol, while the radial growth kinetics data yield a time exponent m≈6.6 and an apparent activation energy (Qd) of 15,300 J/mol. The radial size distributions (RSDs) of Ni3Sn4 scallops were also quantified. The parameters describing RSDs are consistent with the theories of coarsening in two-phase systems containing a very high volume fraction of the second phase. Selective etching of solder revealed the three-dimensional morphology of PdSn4 and Ni3Sn4, and also the dynamical phenomena, such as faceting, competitive growth and coalescence of Ni3Sn4 scallops during interfacial reaction. Non-parabolic growth kinetics is discussed in terms of the existing theories and characteristics of the evolving microstructure.