Abstract: The microscale solder joint in electronic packaging usually serves under the coupled loading of electric current, heat and mechanical stress, which has a distinct inhomogeneous microstructure. Tensile performance of inhomogeneous microscale line-type Cu/Sn-58Bi/Cu solder joints with different current densities and temperatures as well as their size effects are investigated under electro-thermo-mechanical coupled loads. The results show that at low current densities and temperatures, as the height of solder joints decreases, the mechanical constraint of Cu substrate on solder increases, which enhances tensile performance of solder joints; the fracture occurs in solder matrix, showing a ductile fracture mode. These are consistent with the mechanical performance and fracture behaviour of solder joints under tensile load without electric current stressing at room temperature. The local current crowding caused by the non-uniform distribution of current densities in Sn and Bi phases increases the temperature gradient between them. The mismatch strain and stress at the Sn/Bi interface increase due to the difference in coefficient of thermal expansion between Sn and Bi. Therefore, the tensile strength of solder joints under electro-thermo-mechanical coupled loads is lower than that at room temperature without electric current stressing. As the current density and temperature increase, the mismatch strain and stress at the Sn/Bi interface and the solder/IMC layer interface increase, resulting in further decrease in tensile strength of solder joints. Moreover, the fracture location gradually changes from the solder matrix to the solder/IMC layer interface, while the fracture mode shifts from ductile fracture to ductile-brittle mixed fracture.

Key words: inhomogeneous microscale solder joint, electro-thermo-mechanical coupled loads, tensile performance