Unravelling the synergistic enhancement effect of Ag₃Sn phase and graphene nanosheets addition on the thermal cycling reliability of Sn–xAg–0.7Cu solder joint

In the work, thermal cycling reliability of graphene nanosheets-doped Sn–xAg–Cu/Cu solder joint (x = 1, 2, and 3 wt.%) was investigated. With different Ag content, Ag₃Sn IMCs formed in solder matrix displayed different morphologies, number, and sizes. They played a synergistic role in enhancing the joint’s thermal cycling reliability with graphene nanosheets, the mechanism of which was fully clarified in our work. Experimental results showed that the increased Ag content and trace doping of graphene nanosheets into Sn–xAg–Cu/Cu solder joints enhanced their initial shear strength, thus allowing them to maintain a higher strength level during the whole thermal cycling. Electron Backscatter Diffraction results showed that graphene nanosheets-doped solder joints fractured from the solder matrix, and further from the interfacial IMC interlayer with a mixed feature of ductile and brittle. Increasing Ag content decreased the initial thickness of interfacial IMC layer via weakening the atom affinity of Sn and Cu, while forming more Ag₃Sn strengthening phases. Doping graphene nanosheets further inhibited the growth of initial interfacial IMC layer and controlled the morphologies, number, and sizes of Ag₃Sn phase during soldering and thermal cycling. As a result, they synergistically led to the delayed growth of interfacial IMC layer as well as matrix microstructure after thermal cycling, thus contributing to a slower decrease rate in shear strength. Theoretical calculation also showed a synergism contribution of graphene nanosheets and Ag₃Sn phase, with the largest decrease ratio (19.8%) occurred in the average interfacial diffusion coefficient in Sn–1Ag–Cu.