The interfacial reaction between pure tin and substrate with the composition of Cu-4.3 at% Ti (C1990HP) was investigated using the reaction couple technique from 240 °C until 270 °C in the range 0.5~4.0h. The SEM images show the Cu6Sn5 and small precipitated Ti2Sn3 phase formed at the Sn/C1990HP interface. In addition of Ti substantially increased the amount of intermetallic compound (IMC) at the interface which separated parts of Cu6Sn5 compounds with the inner region containing more Ti than the outer. The existence of Sn/C1990HP on the liquid/solid state reaction indicates that spalling occurred with changes in reaction time and temperature. With increased reaction temperature and time, the grain produced an abnormal condition resulting in Cu6Sn5 not accumulating at the interface and spalling into the solder in addition to grain ripening and an increase in total layer thickness. The hexagonal prism-shaped Cu6Sn5 phase is found on the top of the C1990HP substrate when the Cu6Sn5 layer detaches. The reaction phase formation, detachment, and split mechanisms are proposed in this study.

The dissolution behavior of the Ni substrate and Ni3Sn4 phase was studied in the following molten lead-free solders: Sn, (wt.%) Sn–3.0Ag–0.5Cu (SAC), Sn–0.7Cu (SC), Sn–58Bi (SB) and Sn–9Zn (SZ) at 240[Formula: see text]C, 270[Formula: see text]C and 300[Formula: see text]C. The dissolution rate of the Ni substrate in solder decreased from Sn, SAC, SC, SB, to SZ. The thick Ni5(Zn, Sn)[Formula: see text] phase formed at the SZ/Ni interface hindering the Ni dissolution. Ni3Sn4 phase dissolution rate in molten solder decreased from Sn, SZ, SAC, SC, to SB at 240[Formula: see text]C and SC, Sn, SAC, SZ, to SB at 300[Formula: see text]C. The (Cu, Ni)6Sn5 phase spilling was observed at the SAC/Ni3Sn4 and SC/Ni3Sn4 interfaces. Zn in the SZ solder reacted with the Ni3Sn4 phase to form the (Ni, Sn)5Zn[Formula: see text] phase at the interface.

To improve solder-joint reliability, the Sn–3.0Ag–0.5Cu (SAC) solder had minor Ge and Pd additions to form composite solders. The results indicated that the Cu6Sn5 and Cu3Sn phases formed at the SAC–[Formula: see text]Ge/Cu interface ([Formula: see text], 0.5, 1.0, 2.0 wt.%) and the Ag3Sn phase precipitated in the solder. Cu6Sn5 and Cu3Sn phases formed at the interface and PdSn4 phase in the solder when the SAC–1.0Pd and SAC–2.0Pd reacted with the Cu. The SAC–1.0 Pd solder had the lowest liquidus temperature, and Ge addition improved the wettability.