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

Abstract 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 PdSn 4 and Ni 3 Sn 4 , small grains of Ni 3 Sn 2 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 Ni 3 Sn 4 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 Ni 3 Sn 4 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 PdSn 4 and Ni 3 Sn 4 , and also the dynamical phenomena, such as faceting, competitive growth and coalescence of Ni 3 Sn 4 scallops during interfacial reaction. Non-parabolic growth kinetics is discussed in terms of the existing theories and characteristics of the evolving microstructure.