Abstract: To develop high-performance low-temperature lead-free solders for advanced electronic packaging, a series of medium-entropy SnBiZnIn-based solders were fabricated via a melting method. This study investigates the effects of varying Zn contents (31.66%, 15.83%, and 7.91% in mole fraction) on the microstructure, thermal properties, wettability, and interfacial reactions with Cu substrates of SnBiZn5In solders. Isothermal aging experiments were conducted to evaluate the growth kinetics of the interfacial intermetallic compound (IMC) layers. The results show that reducing the Zn content refines the solder microstructure, improves elemental distribution uniformity, enhances wettability, and decreases the thickness of the interfacial IMC layer. At a Zn content of 7.91%, the Sn₅₅Bi₃₂Zn₈In₅ solder exhibits optimal comprehensive performance, with a low melting point of 122.03 °C, the formation of a dense Cu₅Zn₈-type IMC layer approximately 1.93 μm thick at the interface after reflow soldering, and a contact angle of only 28.9°. Aging experiments further reveal that solders with lower Zn content possess higher activation energy for IMC layer growth, indicating stronger resistance to atomic diffusion and improved aging stability. This solder composition offers a promising solution for low-temperature interconnects in 3D integrated circuits (3D ICs) and holds significant potential for engineering applications.