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

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.