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 eut ectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on C u substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructur e was characterized by imaging and energy dispersive x-ray analysis in scan ning 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 d ependent on the reaction temperature. In the case of Sn-3.5Ag solder, we di d not observe any Pd-bearing intermetallic after reaction even at 230 degre es C. In the case of Sn-57Bi solder the PdSn4 intermetallic was observed af ter reaction at 150 degrees C and 180 degrees C, while in the case of Sn-38 Pb solder the PdSn4 intermetallic was observed after reaction only at 200 d egrees C. The PdSn4 grains were always dispersed in the bulk solder within about 10 mu m from the solder/substrate interface. At higher reaction tempe ratures, there was no Pd-bearing intermetallic due to increased solubility in the Liquid solder. The presence or absence of Pd-bearing intermetallic w as 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 in termetallic was observed at the solder-substrate interface by SEM. The pres ence of Ni3Sn4 intermetallic was consistent with the expected diffusion pat h based on the calculated Ni-Sn-X (X = Ag, Bi, Pb) isothermal sections. Sel ective etching of solders revealed that Ni3Sn4 had a faceted scallop morpho logy. Both the radial growth and the thickening kinetics of Ni3Sn4 intermet allic were studied. In the thickness regime of 0.14 mu m to 1.2 mu m, the g rowth kinetics always yielded a time exponent n > 3 for liquid-state reacti on. The temporal law for coarsening also yielded time exponent m > 3. The a pparent activation energies for thickening were: 16936J/mol for the Sn-3.5A g solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38 Pb solder during Liquid-state reaction. The corresponding activation energi es for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni3Sn4 intermetallic layer d uring solid-state aging of the Sn-57Bi/substrate diffusion couples. The kin etic parameters associated with thickening and radial growth were discussed in terms of current theories.