Influence of Co addition on microstructure evolution and mechanical strength of solder joints bonded with solid–liquid electromigration

The influence of solid–liquid electromigration on Cu-xCo/Sn-3.0Ag-0.5Cu/Cu-xCo (x = 0, 30 and 50 wt.%) joints bonded at 260 °C with electric density of 2.89 × 102 A/cm2 for 10 h was investigated. The experiment revealed that the (Cu,Co)6Sn5 intermetallic compound (IMC) was dominant phase in bonding joints with the addition of Co. Besides, the lamellate (Co,Cu)Sn2 grains nucleated around the Co-rich phases. Additionally, the current direction was at an angle to the orientation of prismatic (Cu,Co)6Sn5 grain [100] direction compared to that of the Cu6Sn5 [100] direction aligned with the electric current direction. The analysis results of nanoindentation show that the hardness and the Young’s modulus increased with the Co content in the substrates. Moreover, the additional Co caused the refinement of IMCs grain and the improvement of tensile strength of solder joints.