The electromigration (EM) performance of Pb-free solder joints is investigated in terms of solder composition (Sn-0.5 Cu vs. Sn-1.8Ag) and joining path to Cu vs. Ni(P) under bump metallization (UBM). In the double-sided joints of Ni(P)/solder/Cu, the microstructure of solder joints and the interfacial intermetallic compound (IMC) layers are significantly affected by solder composition and joining path. The as-reflowed microstructure of Sn-0.5 Cu joints consists of small columnar grains with thin IMC layers at both UBM interfaces, independent of the joining path, while Sn-1.8Ag joints have a few large grains of low-angle grain boundaries with thick IMC layers when joined first to Ni(P) UBM, but a few 60° twins with thin IMC layers when joined first to Cu UBM. The EM stressing under high current densities at I50°C reveals that Sn-1.8Ag joints have a superior lifetime compared to Sn-0.5 Cu joints. In addition, the EM lifetime of Sn-1.8 Ag joints reflowed first on Ni(P) UBM is the longest among four groups of the solder joints examined.