Sn/In-Sn bonding interfaces obtained by the transient liquid phase (TLP) reaction were investigated for applications in low-temperature assembly. Bonding was achieved at temperatures below 150 °C with a low bonding pressure. Although a few voids were formed at the interfaces after bonding, the In-Sn metallization enabled low-temperature bonding due to the low eutectic temperature. A bonding mechanism to achieve this novel In-Sn low-temperature assembly was also proposed.