Solid state reaction between Sn-rich solders (Sn-3.5Ag and Sn-37Pb) and two types of Ni-based metallization (electroless Ni-P and sputtered Ni) has been studied. The growth rates of the main intermetallic compound (IMC), Ni 3 Sn 4 , at different aging temperatures are obtained and the activation energy calculated. Ni 3 Sn 4 grows faster with Sn-3.5Ag solder than with Sn-37Pb solder under the same aging condition. The activation energy for the IMC growth with Ni-P metallization is higher than that with sputtered Ni metallization. Kirkendall voids are found inside Ni 3 P layer after thermal aging in the solder/Ni-P UBM systems. This is the result of unbalanced element diffusion in solid state reaction between Sn-rich solders and Ni-P metallization. No voids are formed in solder/Ni system.