Accumulative roll bonding as a route to cobalt-reinforced Sn–Ag–Cu composite solder: Microstructure, interfacial reactions, and mechanical reliability

A key challenge in composite solders is achieving uniform particle distribution to prevent agglomeration and ensure consistent reinforcement. The accumulative roll bonding (ARB) method offers a solution but may influence solder properties. In this study, Sn-0.3Ag-0.7Cu (SAC0307) foils reinforced with 0-1 wt% Co particles were fabricated by ARB and characterized before and after soldering. Differential scanning calorimetry and resistivity tests showed minor effects: a 2 °C melting reduction and 5% resistivity increase, indicating comparable physical properties to SAC0307. Large cast β-Sn grains (123 ± 25 μm) were refined nearly tenfold (13 ± 8 μm) and elongated into ribbons through ARB, primarily via recrystallization stimulated by cobalt particles. The highest tensile strength and hardness occurred at 1 wt% Co, reflecting improved reinforcement distribution. After soldering, moderate Co content (0.2-0.4 wt%) produced the most uniform dispersion, with Co acting as nucleation sites to refine β-Sn and promote Cu 6 Sn 5 solidification. At the solder/Cu interface, Co partially substituted Cu in Cu 6 Sn 5 , forming (Cu,Co) 6 Sn 5 that enhanced fracture toughness. Maximum shear strength (∼19.3 MPa, 38% improvement) was achieved at 0.4 wt% Co, attributed to refined β-Sn and toughened intermetallics. Overall, ARB-fabricated Co reinforced SAC solders showed uniform reinforcement, controlled microstructure, and improved mechanical reliability.