Aging Behaviour and Environmental Impact of Under Bump Metallurgies for Wafer Level Balling

Under bump metallurgies (UBMs) have been assessed in terms of robustness and environmental impact, respectively through aging tests after wafer level balling with SAC305 (Sn 96.5%, Ag 3 %, Cu 0.5 % (wt.%)) and life cycle assessment (LCA). Three UBMs are considered: Ti 200 nm / Ni 700 nm / Au 100 nm (UBM A) deposited by sputtering, Cu 2 mu m / Ni 2 mu m / Au 200 nm (UBM B) deposited by electrodeposition (ECD), and Cu 10 mu m (UBM C) deposited by ECD. Configurations A and B have higher shear strength than configuration C after multiple reflows and 1000 h at 150 degrees C high temperature storage (HTS) thanks to Ni barrier avoiding Cu/ Sn interaction that leads to Cu3Sn intermetallic compound (IMC) growth along with voids. After HTS, configuration A is free of defects and has strong Ti/solder or Ti/IMC interfaces. On the other hand, LCA results using standard databases reveal a greater environmental impact for UBM A than UBM B (single score three times larger), and UBM C (single score more than four times larger). This is mainly related to Au resource depletion. UBM B has less impact than UBM A due to a selective deposition process (ECD), enabling to use less Au than in UBM A configuration where patterning is secured by Au etching. Based on these results, UBM B configuration appears to be the best trade-off between robustness and environmental impact. Nevertheless, Au impact is discussed as a function of its sourcing, with highlights on using high-value Au scraps or electronic scraps.