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

Abstract

The bumps 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.