We have successfully developed a fluxless bonding process to manufacture In-Au microjoint between laser diode and silicon substrate, and studied the interface properties and microstructure of In-Au layer. From XRD, SEM and EDX results indicate that AuIn2 is the main intermetallic phase, which plays an important role on the quality of joints. To study the thermal stability, the flip-chip assemblies were then tested by thermal shock and high temperature storage. From the thermal shock test, we can find that high bonding temperature will result in failure mode, such as the occurrence of cracks at the grain boundary. From thermal aging test, no brittle intermetallic phases and fractures were observed in the solder joints. Therefore, the bonding process has to be optimized because the reliability of solder joint strongly depended on the initial microstructure. The optimal bonding temperature is 200°C after thermal shock test and high temperature storage test.