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The corrosion resistance of solder joints is a critical factor affecting the service life of electronic products during long-term operation. In this study, the corrosion behavior of Sn-0.7Cu-xNi@C (x = 0, 0.04, 0.08, and 0.12 wt.%) composite solders was investigated using a Tafel polarization curve in 3.5 wt.% NaCl solution, and the result demonstrated that it was the Ni@C that enhanced the corrosion resistance of the composite solder. The corrosion rate of the composite solders decreased with increasing Ni@C content and reached the lowest value of 0.205 mm/y when the content of Ni@C reached 0.08 wt.%. Ni@C changed the morphologies of corrosion products Sn3O(OH)2Cl2 from thick flakes to dense fine needles and flakes, which made it more difficult for Cl– to break down corrosion products. Thus, the corrosion resistance of composite solder was improved. The carbon skeleton in Ni@C served as an inert physical barrier to inhibit further corrosion. Furthermore, the potential difference between IMC and β-Sn decreased with the addition of Ni@C, which reduced the corrosion rate of the electric couple and enhanced the corrosion resistance of the composite solder. [ABSTRACT FROM AUTHOR]