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 (Cu,Ni)6Sn5, while the decrease in intermetallic growth rate at t>1000 h can be attributed to diffusion processes leading to (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 formation and growth. Ni diffuses toward the solder bulk and saturates at 175–200°C, while Cu diffuses from the under bump metallization (UBM) toward the solder bump at 125–150°C. 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 125–150°C. The activation energy for total intermetallic growth was calculated at 0.2 eV.
In-situ characterization of solidification and microstructural evolution during interrupted thermal fatigue in SAC305 and SAC105 solder joints using high energy X-ray diffraction and post-mortem EBSD analysis