Role of Cu microstructure during isothermal aging of Cu/Sn/Cu micro solder joints

With the continuous downsizing of solder joints, the influence of the grain characteristics of the under bump metallization (UBMs) on the interfacial reaction is exposed. In this paper, the effect of grain characteristics of Cu UBM on the growth of interfacial intermetallic compounds and the formation of Kirkendall voids in Cu/Sn/Cu micro solder joints during isothermal aging at 150 °C was investigated. Electroplated polycrystalline Cu (EP-Cu), (111) single crystal Cu ((111) Cu), and (111) nano-twinned Cu ((111) nt-Cu) were selected as UBMs. After reflow soldering, the Cu₆Sn₅ grains formed on (111) nt-Cu were near to the orientation \(\text{(2}\stackrel{\mathrm{-}}{1}\stackrel{\mathrm{-}}{1}\text{3)}\) $\text{(2}\stackrel{-}{1}\stackrel{-}{1}\text{3)}$ , while those were randomly formed on both EP-Cu and (111) Cu. During isothermal aging, the Cu₆Sn₅ grains exhibited the highest growth rate on (111) nt-Cu UBM, and the slowest growth rate on (111) Cu UBM, due to the differing number of diffusion routes among the three Cu UBMs with distinct grain characteristics. Furthermore, no Kirkendall voids were detected on the (111) Cu UBM, however, a significant number of Kirkendall voids were identified on the (111) nt-Cu UBM, which can be attributed to the diffusion path and impurities present in the three Cu UBMs. The results will offer theoretical and practical recommendations for the selection of Cu UBMs for micro solder joints in 3D packaging interconnection technology.