Cross-interaction of under-bump metallurgy and surface finish in flip-chip solder joints

Abstract

The cross-interaction of the under-bump metallurgy (UBM)/solder interface and the solder/surface-finish interface in flip-chip solder joints was investigated. In this study, the UBM on the chip side was a single layer of Cu (8.5 µm), and the surface finish on the substrate side was a 0.2-µm Au layer over 5-µm Ni. It was shown that, after two reflows, the Ni layer of the surface finish had been covered with (Cu1−xNix)6Sn5. This shows that the effect of cross-interaction of the two interfaces is important even during the reflow stage. During subsequent solid-state aging at 115°C, 135°C, and 155°C, the formation of (Cu1−xNix)6Sn5 over the Ni layer was found to have the effect of reducing the Ni consumption rate. At the same time, the Cu consumption rate of the UBM was accelerated. The results of this study show that the selection of the UBM and the surface finish has to be considered together because the cross-interaction of the two interfaces plays an important role.