Stabilizing bulk and interfacial intermetallic compounds in Cobalt-decorated graphene nanosheet reinforced Sn–Ag–Cu composite solder: thermal aging kinetics and mechanical reliability

Cobalt decorated graphene nanosheet (CoGNS) reinforced Sn-0.3Ag-0.7Cu composite solder was developed to address reliability limitations caused by intermetallic compound (IMC) coarsening and interfacial layer growth during thermal aging. During isothermal aging at 110 °C for 50–1000 h, CoGNS incorporation promoted formation of a Co substituted (Cu,Co) 6 Sn 5 phase that chemically stabilized IMCs, while graphene nanosheets physically obstructed diffusion pathways. This dual mechanism reduced bulk IMC coarsening by 51% and decreased interfacial (Cu,Co) 6 Sn 5 and Cu 3 Sn growth rates by approximately 33% and 60%, respectively. Kinetic modeling confirmed that CoGNS reinforcement lowered effective interdiffusion coefficients for these layers by approximately 60% and 78%, demonstrating strong suppression of atomic transport across the solder/Cu interface. EBSD analysis showed CoGNS constrained Sn grain boundary mobility, promoting a more stable substructured grain state after prolonged aging. Mechanically, the composite maintained 13–25% higher hardness and up to 73% greater creep strength after prolonged aging. The tensile shear strength was higher in the as reflowed and early aging conditions, while the difference became small after 320 h. This work presents a combined chemical and physical reinforcement strategy in which Co substitution stabilizes the (Cu,Co) 6 Sn 5 IMC through chemical interactions, while graphene nanosheets can physically obstruct atomic diffusion pathways, collectively improving the microstructural stability and mechanical reliability of low Ag solder joints during thermal aging.