Thermomigration-driven Cu Diffusion and asymmetrical growth of intermetallics in Cu-In system for reliable 3D interconnects

The development of three-dimensional integrated circuit (3D IC) packaging demands high-reliability interconnects capable of withstanding temperature-induced diffusion processes. In this study, the effect of a temperature gradient on intermetallic compound (IMC) growth and thermomigration behavior in Cu/In/Cu joints was systematically investigated during solid–liquid interdiffusion (SLID) bonding. Reflow was conducted at 220 °C under a temperature gradient of 86 °C cm -1 to analyze the interfacial reactions between Cu and In and the evolution of IMCs over different reflow durations. Compared with isothermal bonding, the temperature gradient led to asymmetrical growth of the Cu 11 In 9 and CuIn IMCs, characterized by a thinner layer at the hot end and a thicker layer at the cold end. This asymmetry resulted from thermomigration, where Cu atoms diffused preferentially toward the cold side. The heat of transport ( ) for Cu in molten In was calculated to be 1.48±0.44 kJ mol -1 at 220 °C, indicating a lower driving force for thermomigration compared with conventional lead-free solders. These findings clarify the role of temperature gradients in governing atomic transport and IMC evolution in Cu-In systems and provide quantitative insight into optimizing Cu/In-based SLID bonding for reliable 3D interconnect applications.