Study On The Microstructure, Properties And Thermomigration Behavior Of ZnO Reinforced Sn3.0Ag0.5Cu Lead-free Solder

With the electronic products constantly moving towards miniaturization and high performance, the traditional lead-free Sn Ag Cu(SAC) solders were challenged by the reliability issues such as electromigration and thermomigration failure, which are becoming increasingly common in solder joints under harsh service condition. In such case, novel solder alloys with better performance and higher reliability are strongly demanded for electronic interconnection.In this study, Zn O nanoparticles doped SAC composite solders were successfully fabricated through powder metallurgy method. After the retained ratios of Zn O reinforcements were investigated, the solderability, microstructural evolution, and mechanical properties of composite solders were systematically studied. A thermomigration(TM) device with pure temperature field were designed and implemented, and then the thermomigration behavior of SAC and SAC-1Zn O solder were contrastively studied.The element detection results revealed that only 12% of the initially added Zn O nanoparticles were retained in the final composite solder joints, which may relates to their falling off from solder powders during ball milling and being excluded from solder matrix during reflowing. With an increase in the amount of reinforcements, the melting temperature of composite solders was negligibly altered while the undercooling was considerably decreased. The wettability of composite solders was first improved and then deteriorated with the addition of Zn O nanoparticles; the optimal value was achieved when the doping ratio reached 0.5 wt. %. After Zn O addition, refined microstructure were obtained in the composite solder matrix and impeded interfacial IMC growth was achieved in the composite solder joints during isothermal aging. According to the mechanical results, an addition of 1 wt. % Zn O nanoparticles would give rise to a 17.9% improvement in microhardness and a 10.1% enhancement in shear strength, whereas the fracture mode of solder joints was little affected.The lab-made thermomigration device induced a temperature gradient of 1020 °C/cm in samples, which could effectively trigger thermomigration in solders. The observation of interfacial microstructure at both ends of solder billets proved that the thermomigration behavior in composite solder was retarded by the presence of Zn O nanoparticles. The explanation is that the added Zn O reinforcements could serve as a diffusion barrier to Cu element, thus lowering the diffusion flux of Cu atoms in the solder.