| Traditional SnPb solder was the priority of electronic packaging materials for its better wetability and lower melting temperature. Because lead is a toxic element, especially the WEEE and RoHs was announced in Euerope Union and other countries and area, lead-free trend becomes more and more obvious. Besides, solder joints become smaller as the miniaturization of electronics. As the medium of support, electrify and cooling, solder joints endured much more mechanical, electrical and thermal load. Problems such as creep, thermal fatigue and electromigration were occurred, which required higher service reliability for lead-free solder.Two means of impreovment were used for lead-free solder at present. One was alloying, the other was compositing. In this research, composite solders were fabricated by mechanically mixing the Sn-3.5Ag solder paste with micron scale of 10 vol% Cu and Ni particles. Microstructure, mechanical and processing properties were studied. Thermomemchanical fatigue and residual shear strength were emphasized.The results show that:The morphology and dimension of intermetallic compound (IMC) at the Cu substrate-solder interface and around the reinforcement particles varied with different Cu and Ni additions. The reasons for the veriation of IMC morphology were the difference of diffusion coefficients and activation energies between Cu/Sn and Ni/Sn. Shear strengths of Cu particle reinforced composite solder joints increased approximately 33%, while the Ni particle reinforced composite solder joints increased about 20%. Voids, which were caused by the bad fluidity of melting solder, were the reason for the decrease of shear strength of solder joints. Spreading area of composite solders decreased about 15%. The wetting angle of Cu particle reinforced composite solder increased from 11°to 18°.Thermal damage was observed at the surface of solder joints in both in Sn-3.5Ag and Cu reinforced composite solder joints. Thermomemchanical fatigue cracks in Sn-3.5Ag solder joints initiated at the interface between solder and Cu substrate and extended in solder matrix with the extension angles of about 45°. Cracks in Cu reinforced composite solder joints initiated at the substrate/solder interface, was not extended in the matrix, instead, near the Cu substrate/solder interface. The cracks between solder matrix and the interfacial IMC were getting more significant as the thermal cycles were increased. Some cracks were observed at the particle/solder interface in the composite solder joints. It is possible that the reinforcement particles and its surrounding IMC block the cracks from propagation through the reinforcements. No thermal cracks were observed in Ni paticles reinforced composite solder joints, which were shown that the Ni reinforced composite solder joints have better thermomechanical property.The effects of different ramp rates on microstructure of Cu reinforced composite solder were investigated in this research. The results show that thermal damage at the IMC/solder was more serious at the faster ramp rate, while damage at Cu particle and its surrounding IMC was more serious at the lower ramp rate. The reason for this phenomenon was probably that contribution to damage at low temperature is more significant than high temperature in TMF cycles.The simple shear strength of the composite solder joints decreased with the increase of the TMF cycles. The decreasing rate was sharply reduced after 100 cycles and dropped when the cycling increased. Faster ramp rate has brought the solder joints with less time for crack healing. Therefore, less residual strength was resulted at a faster ramp rate. The decreasing trend was similar in both ramp rates.
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