Abstract: In-situ particle-reinforced composite solder was considered as an effective method to enhance the properties of lead-free solders.In-situ Cu₆Sn₅ particles were added into the eutectic Sn-3.5Ag solder to form in-situ metal particles enhanced SnAg-based composite solder.The microstructure and mechanical properties of the composite solder joint during reflow and isothermal aging were investigated,and the fracture mode of composite solder joint and the role of in-situ Cu₆Sn₅ particles were analyzed.Experimental results show that the microstructure and dimensions of in-situ Cu₆Sn₅ particles in composite solder matrix have changed during reflow and isothermal aging.So the shear strengths of composite solder joints have influenced by the sizes of in-situ Cu₆Sn₅ particles.Besides,the deformation of composite solder joint is dominated by shear bonding mode,and the in-situ Cu₆Sn₅ reinforced particles can retard the fracture path of shear bonds.