Interfacial reactions of fine-pitch Cu/Sn-3.5Ag pillar joints on Cu/Zn and Cu/Ni under bump metallurgies

The growth behaviors of various interfacial intermetallic compounds formed at fine-pitch Cu/Sn-Ag pillar bumps fabricated on Zn- and Ni-sputtered Cu under bump metallurgies (UBMs) were investigated in this study. After flip-chip bonding, a scallop-type layer of the intermetallic compound Cu6Sn5 was formed at the pillar joint on the Cu and Cu/Zn UBMs, while the joint on the Cu/Ni UBM exhibited a layer of the intermetallic compound (Cu, Ni)6Sn5. During thermal aging, Cu6Sn5 transformed into Cu3Sn, and Kirkendall voids were formed at the Cu/Cu3Sn interface. The thickness of the Cu3Sn layer in the Cu/Ni UBM was lower than those in the Cu and Cu/Zn samples. In addition, the Cu/Ni UBM had a lower void area ratio than did the Cu and Cu/Zn samples, owing to the low growth rate of Cu3Sn. The die shear strength of the Cu/Ni UBM was higher than those of Cu and Cu/Zn UBMs. Finally, with an increase in the aging time, the fracture site moved from the Cu6Sn5 region to the Cu/Cu3Sn interface for the Cu and Cu/Zn UBMs. However, in the case of the Cu/Ni UBM, fracturing occurred in the Cu6Sn5 or Cu3Sn region.