The effect of thermal cycling on nanoparticle reinforced composite lead-free solder

The effects of thermal cycling on shear strength and fracture mode of the nanosized Sn-3.0Ag-0.5Cu particulates reinforced Sn-58Bi composite solder were investigated in this paper. By using a self developed top-down method named Consumable-electrode Direct Current Arc technique, the Sn-3.0Ag-0.5Cu nanoparticles were successfully manufactured. The primary particle size of Sn-3.0Ag-0.5Cu nanoparticles ranged from 20nm to 80nm. Sn-3.0Ag-0.5Cu nanoparticles with different weight percentages were mixed into commercial Sn-58Bi solder paste in order to develop a composite solder paste which is lead-free and possess high strength and low melting point. Following the conventional surface-mount technology process, the 1206 chip resistor and ENIG/Cu pad were joined by the composite solder. Scanning electron microscope, transmission electron microscope and optical microscope were employed to observe the morphology of nanoparticles, microstructure of solder matrix, fracture mode after shear test and crack after thermal cycling. The experimental results indicated that before thermal cycling all composite solders' shear strength increased greatly compared to Sn-58Bi solder making them comparable to Sn-3.0Ag-0.5Cu solder. The fracture surfaces of all composite solder joints occurred at the interface between the solder matrix and the resistor termination. After thermal cycling, the shear strength of the composite solders was at a constant value. However, when the weight percentages of Sn-3.0Ag-0.5Cu nanoparticles exceeded a certain value, the shear strength of composite solder joints decreased rapidly and the case of solder brittle fracture increased as the nanoparticles content increased.