In this work, contact angle, spreading area, and isothermal aging tests were conducted to study the difference between Sn-3Ag-0.5Cu lead-free solder and its composite solder at different multiwalled carbon nanotube reinforcement volume fractions. The material interaction between the solder and the substrate at different aging temperatures and times was investigated using scanning electron microscopy elemental analysis. The experimental results indicated that the composite solder had a lower contact angle as well as good spreading area. An intermetallic compound layer was found between the solder and the copper substrate, and the thickness of this reaction layer increased with increasing aging temperature and time. Meanwhile, the intermetallic compound layer of multiwalled carbon nanotube reinforcement composite solder was thicker than that of the Sn-3Ag-0.5Cu lead-free solder. The composite solder with 0.1 vol% multiwalled carbon nanotube reinforcement addition exhibited better comprehensive properties than composite solders with other reinforcement volume fractions.