This study investigated the effects of solid-state aging on Sn–3Ag–0.5Cu (SAC305) and Sn–9Zn (SZ) solder joints with two Cu electroplated layers: PC-Cu and PCS-Cu. Distinct surface features and impurity levels were evident in PC-Cu and PCS-Cu owing to the use of different additives in the electroplating solutions. The SAC305 solder joints exhibited notable voids and cracks at the joint interface during aging, which were attributed to the high impurity content of PC-Cu. In contrast, low-impurity PCS-Cu did not exhibit void formation. Regardless of whether PC-Cu or PCS-Cu was used, no interfacial voids were observed in the SZ solder joints, implying that the SZ solder effectively suppressed voids. Mechanisms related to void suppression were also proposed. The presence of Zn in the solder changed the interfacial reactions, forming a Cu 5 Zn 8 intermetallic compound (IMC) in addition to the Cu–Sn IMCs in the SAC305 solder joints. Inhibited grain-boundary diffusion in high-impurity scenarios hinders voids owing to the effective substitution of Cu vacancies by Zn. With Zn consumption near the soldering interface, the Cu 5 Zn 8 IMC detached and a Cu 6 Sn 5 phase was formed between the Cu–Zn IMC and Cu. Meanwhile, PCS-Cu exhibited localized Cu 6 Sn 5 growth. In addition, SZ showed less influence of Cu impurities on the interfacial IMC thickness compared to SAC/PC-Cu with rapid IMC growth. The findings revealed the void formation mechanisms and highlighted the void-suppressing features of SZ in solder joints.