Interfacial microstructure and shear strength of Sn-Ag-Cu based composite solders on Cu and Au/Ni metallized Cu substrates

Abstract

Nano-sized, non-reacting, non-coarsening CeO2 particles with a density close to that of solder alloy were incorporated into Sn-3.0wt%Ag-0.5wt%Cu solder paste. The interfacial microstructure and shear strength of Au/Ni metallized Cu substrates were investigated, as a function of aging time, at various temperatures. After solid state aging at low temperature, an island-shaped Cu6Sn5 intermetallic compound (IMC) layer was found to be adhered at the interfaces of the Cu/Sn-Ag-Cu solder systems. However, after a prolonged aging, a very thin, firmly adhering Cu3Sn IMC layer was observed between the Cu6Sn5 IMC layer and the Cu substrate. On the other hand, a scallop-shaped (Cu, Ni)-Sn IMC layer was found at the interfaces of the Sn-Ag-Cu based solder-Au/Ni metallized Cu substrates. As the solid-state aging time and temperature increase, the thicknesses of the IMC layers also remarkably increased. In the solder ball region of both systems, a fine microstructure of Ag3Sn and Cu6Sn5 IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solders doped with CeO2 nanoparticles was inhibited, due to an accumulation of surface-active CeO2 nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joints doped with CeO2 nanoparticles had higher shear strengths than that of the plain Sn-Ag-Cu solder joints, due to a well-controlled fine IMC particles and uniformly distributed CeO2 nanoparticles.