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

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 (Q_h) of 25,750 J/mol, while the radial growth kinetics data yield a time exponent m≈6.6 and an apparent activation energy (Q_d) 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.