Effect of graphene doping on microstructural and mechanical properties of Sn-8Zn-3Bi solder joints together with electromigration analysis

In this study, various weight percentages (0, 0.05 and 0.1 wt.%) of graphene nanosheet doped lead-free Sn-8Zn-3Bi solder alloys were investigated in order to analyze the electromigration induced microstructural development and mechanical properties, such as the shear strength and microhardness, as well as the melting characteristics of the novel composite solders. The effect of electromigration on the solder joint was systematically studied by using a newly developed wire-type testing configuration. The samples were stressed under a current density of 5 x 10(3) A/cm(2) at 100 degrees C for different aging periods in order to study the electromigration induced reliability issues. For solders with and without the graphene, gamma-Cu5Zn8 intermetallic compounds (IMCs) were found at the solder and Cu pad interface. The majority of the added graphene nanosheets were proved to be uniformly distributed in the b-Sn matrix. After the graphene addition, needle-like Zn-rich phases with a finer microstructure were discovered in the solder matrix. The growth rate of the IMC layers of the graphene doped solder was slower in comparison to IMC layers in plain solder at the interfaces. With 0.1 wt.% graphene addition, the measured IMC growth rate was decreased from 30.9 x 10(-14) to 24.9 x 10(-14)cm(2)/s. The melting temperature of the doped solder measured by differential scanning calorimeter (DSC) showed little difference from that of the plain solder. The Vickers hardness, up to 29.9 Hv with 0.1 wt.% graphene addition, is 9.1% higher than that of plain solder. The graphene doped solder consistently demonstrated higher ball shear strength as a function of aging time. The ball shear strength value was increased by 10.2 +/- 0.8% than that of plain solders during the whole aging period. The improvement was due to the dispersion-strengthening mechanism, refined microstructure and excellent intrinsic mechanical properties of graphene. Moreover, fracture occurred at the IMC interface of the doped samples showed a ductile fracture pattern with a large distribution of dimples on the rough surface. (C) 2013 Elsevier B.V. All rights reserved.