<p> The interfacial reaction between solder and substrate, particularly the growth mechanism and mechanical response of intermetallic compounds (IMCs), is a key factor in determine the reliability and lifespan of products in electronic packaging and welding. The interfacial reaction characteristics of Cu/Sn-0.3Ag-0.7Cu-<i>x</i>Ni/Cu (<i>x</i> = 0, 0.05, 0.1, 0.15, 0.2, and 0.3 wt%)-soldered joints with different isothermal aging time were studied at 150&#xa0;°C. The microstructure and mechanical properties of the IMC layer at the interface are focused and its mechanism is explored. The results demonstrate that the scallop-like Cu₆Sn₅ phase is preferentially formed during the growing process of the soldering interface without isothermal aging. The fracture of Cu/Sn-0.3Ag-0.7Cu/Cu-soldered joint manifests primarily as a combination of brittle and ductile fractures. After isothermal aging, a lamellar Cu₃Sn phase forms near the Cu matrix, and the fracture mechanism of the IMC layer changes from cleavage to brittle fracture. As the aging time increases, the Cu₃Sn phase thickens, leading to a decrease in tensile strength, which drops by about 44% after 480&#xa0;h. Increasing the Ni content thickens the Cu₆Sn₅ and Cu₃Sn layers, improves the tensile properties of the solder joints, reaching a maximum tensile strength of 59.12&#xa0;MPa at 0.15 wt% Ni, and effectively delays aging failure.</p>