Multi-step transient liquid phase bonding of Ag-In-Sn composite solders joints for low interfacial thermal resistance and enhanced mechanical integrity

Power semiconductor devices are placing increasingly stringent demands on the performance of packaging materials, driving the development of advanced solder technologies. In this study, Ag-In-Sn composite solder was employed using the transient liquid phase (TLP) bonding process, through which the interrelations among interfacial bonding behavior, shear strength, thermal resistance, and microstructural evolution of the bonding joints were characterized. Notably, when the InSn composite powder addition reaches 20 wt%, the joints exhibit a minimum thermal resistance of 0.9 mm²·K/W and a maximum shear strength of 30.63 MPa. The thermal resistance demonstrates a non-monotonic dependence on InSn composite powder content—first increasing, then decreasing, and rising again—governed by the competing influences of enhanced microstructural densification and the increased incorporation of low-thermal-conductivity InSn composite powder phase. Conversely, the shear strength increases initially due to improved interparticle bonding, facilitated by the infiltration of the molten InSn phase and the formation of strengthening intermetallic compounds (IMCs) such as Ag₃In and Ag₃Sn. However, beyond the densification threshold, the excessive formation of a substitutional solid solution, Ag₃(In,Sn), leads to a pronounced reduction in critical shear stress for dislocation slip. This softening effect ultimately lowers the plastic deformation threshold, resulting in a sharp decline in mechanical integrity. These findings provide fundamental insights into the microstructural design and performance optimization of low-temperature Ag-based solders for high-reliability electronic packaging applications.