Fabrication of Cu6Sn5 single-crystal layer for under-bump metallization in flip-chip packaging

To inhibit the rapid consumption of the copper substrate at the intergranular regions (grain boundaries or solder channels), a Cu6Sn5 single-crystal layer was fabricated via 1 min reflow at 250 °C. The orientation maps showed that the host-controlled growth behavior of the Cu6Sn5 phase existed during this single-crystal-forming procedure. By combining surface morphologies and kinetic analyses, the physical mechanism behind this behavior was identified as grain boundary migration rather than Ostwald ripening. This study provided a strong foundation for the fabrication of Cu6Sn5 under-bump metallization, as well as other similar intermetallic diffusion barriers.