Sn/Cu solder joints serve as electrical pathways and mechanical supports in microelectronic packaging. This study demonstrates that the Cu plating formula has a strong effect on the incorporation of impurities in the Cu deposit and, consequently, affects the microstructural evolution of the Sn/Cu interface. An additive formula with polyethylene glycol (PEG) and chloride ions (Cl-) results in a high-level incorporation of impurities (carbon, oxygen, sulfur, and chlorine) in the Cu deposit. Segregation of impurities to the Sn/Cu interface occurs during thermal aging, which induces the formation of voids and Cu-impurity compounds (CuO, Cu2O, and CuS2) embedded in the Cu6Sn5 and Cu3Sn phase layers, forming an unusual alternating layer structure that is structurally loose and unstable. The addition of bis(3-sulfopropyl) disulfide (SPS) along with PEG and Cl- helps restore the microstructural integrity of the Sn/Cu interface by suppressing the incorporation of impurities. This suppression strongly depends upon the SPS concentration and the PEG molecular weight. The aging temperature also has a significant effect on the microstructural evolution of the Sn/Cu interface, which is determined by the change of the dominant diffusion species. (C) 2016 The Electrochemical Society. All rights reserved.