Abstract
Microstructure and shear strength of Sn-Zn lead-free solders and Au/Ni/Cu UBM joint under thermal aging conditions were investigated. The samples were aged isothermally at 373 K and 423 K for 300, 600, and 900 hours. The IMCs at the interface between solder and UBM were examined by FESEM and TEM. The results showed the shear strength was decreased with aging time and temperature. The solder ball with highly activated flux had about 8.2% increased shear strength than that of BGA/CSP flux. Poor wetting and many voids were observed in the fractured solder joint with of the latter flux. The decreased shear strength was caused by IMC growth and Zn grain coarsening. In the solder layer, Zn reacted with Au and then was transformed to the β-AuZn compound. At the joint interface, although AuZn grew first, γ -Ni5Zn21 compounds were formed with aging time.
