Experimental Evidence for Formation of Ni-Al Compoundin Flip-Chip Joints Under Current Stressing

The materials interactions on the under bump metallization (UBM) side of flip-chip solder joints during current stressing were studied by using high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM). Flip-chip solder joints with sputtered Al/Ni(V)/Cu UBM were subjected to current stressing at an ambient temperature of 150°C. It was found that a layer of Ni-Al phase, presumably NiAl3 according to energy-dispersive x-ray spectroscopy (EDX) measurements, was observed at the Al/Ni(V) interface. In addition, evidence for the formation of a nonconductive oxide layer at the NiAl3/Ni(V) interface was observed. This nonconductive oxide layer was responsible for diverting electron current away from the porous region.