Microstructure and mechanical properties of as-reflowed Sn58Bi composite solder pastes

The differential scanning calorimetric curve of the composite SnBi solder paste with SAC-BN particles presents two endothermal areas representing the eutectic SnBi and SAC-BN alloys, respectively. The differential scanning calorimetric curve of the reflowed solder shows only one endothermal area at the liquidus temperature of 170 °C. With the increasing amount of SAC-BN particles in the composite solder pastes, the concentration of Sn-rich phases increase significantly. The grain size of β-Sn matrix grows due to adding SAC-BN particles into the composite solder pastes in the range of 0 to 8 wt%. Further addition of the doped particles leads to grain refinement in the solder bulks. Based on the microstructure transformation, the hardness and the elastic modulus of the solder bulks decrease as the percentage of doped particles varies from 0 to 8 wt%, but increase when the doped percentage is higher than 8 wt%. The solder bulk with 8 wt% SAC-BN particles addition shows the minimum hardness and elastic modulus, which are 164.1 MPa and 16.8 GPa, respectively.