Interfacial Reactions Between Electrodeposited Sn-Cu, Sn-Ag-Cu Solders and Cu, Ni Substrates

Abstract

Interfacial reactions between two near eutectic Pb-free solders, Sn-1.1Cu and Sn-3.6Ag-1.8Cu, and two common base materials, Cu and Ni, were studied by characterizing the formation and growth of intermetallic compounds (IMCs) during reflowing and aging using SEM, XRD, and TEM. A continuous bilayer of thin, uniform Cu3Sn and thick, nonuniform Cu6Sn5 was formed at the interfaces of both Sn-1.1Cu/Cu and Sn-3.6Ag-1.8Cu/Cu, with the Cu3Sn layer adjacent to the Cu substrate. This is attributed to the diffusion of Cu from the Cu substrate to the solder to first form Cu6Sn5, then Cu3Sn. The inclusion of Ag in the Sn-3.6Ag-1.8Cu solder film inhibited the diffusion of Cu and, therefore, the growth of Cu3Sn. For both Sn-1.1Cu/Ni and Sn-3.6Ag-1.8Cu/Ni, an intermetallic film of (Ni,Cu)3Sn4 was formed at the interface, and the film had three distinct morphologies: a continuous planar layer at the Ni interface, followed by long, thin needles and large, polygonal crystals. The layers and the crystals were thinner at the Sn-3.6Ag-1.8Cu/Ni interface, indicating that the addition of Ag slowed down the growth of the (Ni, Cu)3Sn4 films. At the Sn-3.6Ag-1.8Cu/Ni interface, Ag3Sn particles were also observed and they coarsened with aging time. No separate Ag particles were observed.