A promising lead-free material for flip-chip bumps: Sn-Cu-RE

In advanced electronic packaging, flip-chip (FC) bumps are required to be reliable and inexpensive. The Sn-0.7%Cu alloy has been considered as a lead-free material for FC bumps. Various small amounts of rare earth (RE) elements, which are mainly Ce and La, have been added to the Sn-0.7%Cu alloy to form new alloys. It was found that the new alloys exhibit mechanical properties superior to that of the Sn-0.7%Cu alloy. In particular, the addition of up to 0.5% of RE elements is found to refine the effective grain size and provide a fine and uniform distribution of Cu₆Sn₅ in the solidified microstructure. After aging at high temperature, the microstructure of Sn0.7%Cu-0.5%RE alloy is more stable than that of the Sn-0.7%Cu alloy. Tensile, creep and microhardness tests were, conducted on the solder alloys. It was found that significant improvements of the tensile strength and creep resistance were obtained with RE elements addition. These results have made the Sn-Cu-RE alloy to be very attractive as a suitable material for FC bumps.