IMC growth of solid state reaction between Ni UBM and Sn-3Ag-0.5Cu and Sn-3.5Ag solder bump using ball place bumping method during aging

The humps for flip chip interconnection are becoming smaller and smaller. Since lead-free solders became popular, Ni-based under bump metallization (UBM) has attracted attention in recent years because of their slower reaction rate than traditional Cu-based UBM. However, there is little data concerning the solid state reaction between small lead-free solder bumps and Ni-based UBM. In this work, Sn-3Ag-0.5Cu and Sn-3.5Ag solder bumps were fabricated with 110-μm-diameter solder balls on electrolytic Ni, and the growth kinetics of intermetallic compound (IMC) layers and the morphology of bumps during long-term aging were investigated. The IMC layer exhibited parabolic growth, and the activation energy values for the Sn-3Ag-0.5Cu or Sn-3.5Ag solder/Ni UBM were obtained. The growth rate accelerated at 463 K or above. (Ni,Cu) ₃Sn₄ or Ni₃Sn₄ IMC was formed mainly at the solder/Ni interface after long-term aging. Large voids were formed at the solder/IMC interface at 463 K or above. The voids are the result of stress by volume expansion due to IMC growth. Coarse Ag₃Sn grains were observed adjacent to the voids and contributed to void initiation.