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A method of decreasing the ductile–brittle transition temperature (DBTT) of Sn-3.5Ag solder by aging treatment has been found to allow the solder to serve at lower temperatures. Charpy impact tests under various temperatures (from −150°C to 20°C) were performed on Sn-3.5Ag solder after 150°C aging treatment, and the morphology of second-phase particles was also acquired, compared with the in situ aging test to analyze the influence of second-phase particles. The results show that with the increase of aging time, the DBTT of the impact curve first decreases and then increases, and the DBTT of impact curve is the lowest at 48 h. Combined with the in situ aging morphology of second-phase particles, precipitation, fusion, and coarsening of Ag3Sn particles are the reason for the change in DBTT. In summary, aging treatment can decrease the DBTT value of Sn-3.5Ag solder, so that the solder can have a greater toughness area. That is, the solder after aging treatment will be able to achieve service at lower temperatures. [ABSTRACT FROM AUTHOR]