Abstract: Objective The aim is to investigate on atomic diffusion behaviour between Sn-0.7Cu solder and Cu substrate and growth mechanism of intermetallic compound (IMC). Methods Sn-0.7Cu/Cu solder joints were prepared by reflow soldering and subjected to high-temperature aging for 0 h, 50 h, 150 h, 250 h, 350 h and 500 h. Morphology of solder joints was observed by metallurgical microscope and scanning electron microscope (SEM). Diffusion behaviour of IMC layer between Cu atoms and Sn atoms was also simulated by LAMMPS software. Results The results show that interfacial IMC layer gradually transformed from scallop-shaped Cu₆Sn₅ into planar Cu₃Sn with the increase of aging time. Cu substrate to IMC layer shows steady-state diffusion, while the solder to IMC layer shows unsteady diffusion. Sn-0.7Cu/Cu interface diffusion coefficient is 4.82×10⁻⁹ cm²/s, being reduced in comparison to 5.05×10⁻⁹ cm²/s of Sn/Cu interface. Conclusion It is confirmed that the addition of Cu element can effectively inhibit the diffusion of Sn atoms. This study provides an important theoretical basis for the design of high-reliability solder.