In this study, varying amounts of Ni-CNTs were incorporated into the Sn-Ag-Cu matrix to form the composite solders. The interfacial intermetallic thickness formed on the Ni/Au metallized Cu substrate was determined under the as-soldered and isothermally aged (at 150°C for up to 42 days) conditions. The results of the interfacial intermetallic thickness showed that for the case of the unreinforced solder joint, the intermetallics grew more significantly than that of the composite solder joints, under both as-soldered and isothermally aged conditions. Moreover, the shear test results revealed that the as-soldered and aged composite solder joints exhibited improved ultimate shear strength as compared to that of their unreinforced counterparts. The improvement was observed to be most significant with the addition of 0.05 wt.% of Ni-CNTs, whereby the ultimate shear strength increased by 29% under the as-soldered condition and by 28% after isothermally aged for 42 days. It was concluded that the presence of Ni-CNTs as reinforcements in the Sn-Ag-Cu solder served to inhibit the growth of interfacial intermetallics and also to enhance its mechanical performance.