The impact of reflow soldering induced dopant redistribution on the mechanical properties of CNTs doped Sn58Bi solder joints

This research was intended to establish a scientific relationship between the changes of the weight percent of CNTs in the solder blocks and the changes of mechanical properties of the doped solder joints. The impact of reflow soldering induced dopant redistribution has been detailedly discussed. It was proved to be a principle reason that led to the changes of the weight percent of CNTs in the doped solder joints, and finally influenced the mechanical properties. The drag force caused by the outward flow of the solder flux and the buoyancy force caused by the density gap were the main inducing factors for the redistribution. By experimental methods, CNTs doped Sn58Bi solder pastes with the doping amount of 0.025, 0.05, 0.100, 0.150 and 0.200 wt% were studied to analyze this phenomenon. Our findings showed that with doping amount of 0.100 wt%, the doped solder block seemed saturated. After doping more CNTs, the final weight percent of CNTs was hardly increased. Also, the excess CNTs preferred to aggregate at the near surface region of the solder block, making the surface became darker and coarser. To analyze the real conditions, CNTs doped Sn58Bi solder joints with different original doping amount were studied to detect the changes of mechanical properties. According to the results, the highest shear stress was found to be 77.3 MPa with the doping amount of 0.050 wt%. After the doping amount overpassed 0.100 wt%, the shear stress decreased sharply. Also, via morphology observation, the doped CNTs were found aggregated at the interface of the solder joints seriously with the doping amount of 0.200 wt%.