Cobalt-Graphene NanoSheets enhanced Sn–0.3Ag–0.7Cu composite solder: Study on microstructure, crystal orientation relations and mechanical properties

Abstract

Microstructural modification and mechanical strength enhancement of lead-free Sn–Ag–Cu solder alloys using reinforcing particles have been a focus of the researcher's attention. In this research, Cobalt-decorated Graphene NanoSheets (Co-GNSs) as core/shell dopant particles with concentrations ranging from 0 to 0.5 wt% were added to the low silver Sn–0.3Ag–0.7Cu (SAC0307) solder and the influence of the composite formulation on the microstructure and mechanical properties of the solder bulk was comprehensively examined. Additionally, copper specimens were soldered using the composite solders, and the microstructure of the solder/copper substrate, along with its correlation to the tensile-shear strength of the soldered joint, was investigated. The presence of Co-GNSs within the solder bulk altered the morphology of the primary Cu6Sn5 intermetallic phase and decreased its volume percentage and average size. The Co-GNSs also impacted the crystallographic orientation relationships by promoting the growth of {301} cyclic twinning and increasing the sub-grains along twin boundaries in the solder matrix. Results of the nanoindentation test revealed that addition of up to 0.1 wt% Co-GNSs enhanced the hardness and elastic modulus of the composite solder by ∼17 % and ∼42 %, respectively, compared to the non-composite SAC0307 solder. The strengthening mechanisms were discussed, considering grain and sub-grain features, as well as the presence of second phases and particles within the solder bulk. In the soldered joints, the highest shear strength was achieved in the soldered sample containing 0.1 wt% Co-GNSs, which was associated with the formation of a thin layer of (Cu,Co)6Sn5 and Cu3Sn intermetallics at the solder/copper substrate interface coupled with the refinement of Ag3Sn, and Cu6Sn5 bulk intermetallics. However, the presence of a high amount of Co-GNSs reduced the influence of particles on the microstructure and mechanical properties due to the agglomeration phenomenon. Since adding Co-GNSs did not have a significantly negative impact on the melting point and electrical resistance of the solder, reinforcing lead-free tin-based solders with Co-GNSs can lead to the production of solders with high potential for electronic packaging.