Metallurgy and stability of the Sn/Cu interface for lead-free flip chip application

Abstract

Eutectic tin copper and pure tin are two possible candidates to replace tin lead alloy as the solder bump materials for the flip-chip process. Without lead, the stability of the Sn/Cu interface during reflow suffers from the rapid dissolution of copper into molten tin. The requirements of multiple reflow during the flip-chip manufacturing steps further complicate the issue. This study aims to better understand the metallurgy and stability of the bump/copper substrate interface and compares the results to a Sn/Ni interface. A tin-copper bump with 0.7-1 wt% of copper and pure tin bumps were electroplated using a fountain plating machine on silicon wafers with copper and nickel as the under bump metal. The samples were pre-annealed at various temperatures and multiple reflows were performed using a 5-zones reflow-oven. The cross-sections of the interfaces were studied by scanning electron microscopy and scanning Auger microscopy. It was found that copper dissolution into the eutectic tin copper and pure tin solders during the reflow process resulted in the formation of Cu/sub 6/Sn/sub 5/ intermetallic at the interface. The ball shear test result suggests that the presence of the intermetallic at the interface did not adversely affect the bonding strength of the bump/copper interface. In this study, we developed a pre-annealing process to control the intermetallic region that can significantly slow down the copper dissolution during reflow.