A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn4 intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn4 intermetallic was observed after reaction only at 200°C. The PdSn4 grains were always dispersed in the bulk solder within about 10 µm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni3Sn4 intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni3Sn4 intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni3Sn4 had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni3Sn4 intermetallic were studied. In the thickness regime of 0.14 µm to 1.2 µm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni3Sn4 intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.