Interfacial reactions between Sn-20 wt.%In-2.8 wt.%Ag (Sn-20In-2.8Ag) Pb-free solder and Cu substrate at 250, 150, and 100 C were investigated. A scallop-type eta-Cu6Sn5 phase layer and a planar epsilon-Cu3Sn phase layer formed at the interface at 250 degrees C. The indium content in the molten solder near the interface was increased with the formation of the eta-Cu6Sn5 phase; and the eta-Cu6Sn5, Ag2In, Cu2In3Sn, and gamma-InSn4 phases formed from the solidification of the remaining solder. At 100 and 150 degrees C, only the eta-Cu6Sn5 phase was found at the interface. However, unusual liquid/solid reaction-like interfacial morphologies, such as irregular elongated intermetallic layers and isolated intermetallic grains, were observed in the solid-state reactions. These eta phase layers had less Sn content than the Sn-20In-2.8Ag alloy, resulting in an excess Sn-rich gamma-InSn4 phase accumulating at the interface and forming porous eta layers on top of the initially formed dense eta layers at 150 degrees C. At 100 degrees C, large elongated eta grains were formed, whereas the interfacial layers remained almost unchanged after prolonged reaction. Based on the experimental evidence, the growth of the eta phase was proposed to follow a diffusion-controlled mechanism at 250, 150 and 100 degrees C, while that of the epsilon phase was probably controlled by the reaction. (C) 2013 Elsevier B.V. All rights reserved.