Cold bump pull tests performed on wafer level chip scale packages using SAC105 solder bumps show an increase in the occurrence of brittle failure modes with aging temperature and time. Fast intermetallic growth at 0–1000 h can be attributed to , while the decrease in intermetallic growth rate at can be attributed to diffusion processes leading to and formation and growth. Ni diffuses toward the solder bulk and saturates at , while Cu diffuses from the under bump metallization (UBM) toward the solder bump at . Interactions between Cu and Ni atoms lead to saturation of their atomic % gradients due to intermetallic formation. Sn diffusion from the solder toward the UBM occurs at . The activation energy for total intermetallic growth was calculated at 0.2 eV.