<p>This paper explores how aging time and Bi addition impact the growth kinetics of intermetallic compounds (IMCs) at the interface of Sn-0.7Cu-0.8Zn ternary alloy solder joints, utilizing scanning electron microscopy and pull-shear tests to promote theoretical research and development of lead-free solder. For the solder joints of Sn-0.7Cu-0.8Zn and Sn-0.7Cu-0.8Zn-5Bi, the main component of the IMC formed at their interfaces is Cu₆Sn₅. The additional Bi is present as both a solid solution and Bi-rich phase within the solder matrix. When the aging time is increased, the IMC at the solder joint interfaces experiences growth in thickness and grain size, with the growth rates differing to certain extents. Specifically, the addition of 5 wt.% Bi enhances the IMC growth coefficient of the Sn-0.7Cu-0.8Zn/Cu solder joint interface from 0.15&#xa0;μm/h^1/2 to 0.21&#xa0;μm/h^1/2, and its grain size is also larger than that of the IMC at the interface of the Sn-0.7Cu-0.8Zn solder joints. Mechanical property testing reveals that with the progression of aging time, the tensile-shear strength of the solder joints decreases. However, the incorporation of Bi results in a notable enhancement of shear strength within a specific aging period. Nonetheless, the addition of Bi transforms the fracture behavior of solder joints. Instead of experiencing ductile failure within the solder matrix, joints start to break in a brittle manner at the IMC interface. As aging time progresses, these interfaces become increasingly prone to cracking, causing a sharp decline in the shear strength of the joints.</p> Graphical abstract