<p>In this paper, Cu/In-Sn-Zn-Bi-xAg/Cu (x = 0, 20, 40, 55, 70 wt.%) 3D packaging solder joints with varying contents of nano-Ag particles were prepared by low-temperature transient liquid phase (TLP) bonding. The influence of Ag content on the microstructure and shear properties of In-Sn-Zn-Bi solder joints was investigated. The results indicated that the introduction of Ag particles significantly regulated the microstructure of the solder joints. Without Ag addition, the interfacial intermetallic compounds (IMCs) consisted of a double layer of Cu₆(Sn,In)₅ and Cu₅Zn₈, and the in-situ reaction zone was composed of an InBi phase, an Sn-rich phase, and a Zn-rich phase. After Ag incorporation, the interfacial IMC transformed into a single layer of Cu₃(Sn,In) phase, while the in-situ reaction zone comprised an Ag₃(Sn,In) phase, a Zn-rich phase, a Bi-rich phase, and a small amount of unreacted Ag particles. With the increase in Ag particle content, both the thickness of the interfacial IMC and the shear strength of the solder joints first increased and then decreased, whereas the porosity first decreased and then increased. The properties were achieved at an Ag content of 55 wt.% (IMC thickness of 2.3&#xa0;μm, porosity of 0.94%, shear strength of 23.27&#xa0;MPa), which endowed the solder joints with the best comprehensive performance. Notably, such superior performance is achieved under mild bonding conditions—enabling low-temperature joining with high-temperature service capability—demonstrating the high bonding efficiency of the TLP process. These findings provide valuable guidance for the design of reliable low-temperature interconnects in advanced electronic packaging. Moreover, as the Ag content increased, the fracture mechanism of the solder joints transformed from brittle fracture to ductile fracture, and then reverted to brittle fracture at 70 wt.%.</p>