The effects of Zn additions to Sn-0.7Cu and Sn-3.8Ag-0.7Cu (all in wt.% unless specified otherwise) Pb-free solders on the interfacial reactions with Cu substrates were investigated. The study was focused on the intermetallic compound (IMC) growth, Cu consumption and void formation as a function of thermal aging and solder composition. Four different kinds of Cu substrates (high-purity Cu, oxygen-free Cu, vacuum-sputtered Cu, and electroplated Cu) were prepared to compare their interfacial reaction behaviors with Zn-added solders. Thermal aging was performed at 150 degrees C for up to 1000 h to accelerate the interfacial reactions between solders and Cu substrates. Growth of IMCs (CU6Sn5 and CU3Sn) in Zn-added solders was slower than those without Zn additions. The growth of the CU3Sn phase, in particular, was drastically reduced in the Zn-added solders on all four Cu substrates. On an electroplated Cu substrate, numerous voids were observed in the CU3Sn phase for Sn-Cu and Sn-Ag-Cu solders aged at 150 degrees C for 1000 h. However, these voids were largely eliminated in the Zn-added solders. On the other three Cu substrates, the conditions which produce a high density of voids were not found after aging both solders with and without Zn. The Cu consumption with Zn-added solders was also significantly lower. The beneficial effects of Zn additions on interfacial reaction behaviors are reported, and the corresponding mechanisms in suppressing void formation and Cu consumption due to Zn additions will be discussed.