Thermomigration-induced asymmetric intermetallic growth in Ag/In joints under temperature gradients for low-temperature packaging

Low-temperature solders, such as indium (In), are becoming increasingly vital for advanced electronic packaging, enabling energy efficient manufacturing and reliable fine-pitch interconnects. This study investigates the microstructural evolution and intermetallic compound (IMC) growth mechanisms at the interface between molten In solder and Ag under-bump metallization (UBM) during near-isothermal and temperature gradient reflow processes at 180 °C. Under near-isothermal conditions, symmetric IMC growth is observed. In contrast, reflow under a temperature gradient of 40 °C/cm induces asymmetric IMC formation, with accelerated growth at the cold end. Two IMC phases—AgIn 2 and Ag 9 In 4 —are identified, with AgIn 2 being the dominant phase under both conditions. The observed asymmetry is driven by thermomigration of Ag atoms from the hot end to the cold end. The molar heat of transport ( Q *) for Ag in molten In is calculated as 22.98 kJ/mol, confirming strong diffusion under temperature gradients. This work provides a quantitative understanding of IMC evolution under temperature gradients and informs the design of low-temperature bonding strategies tailored for energy-efficient, reliable, and scalable advanced packaging manufacturing.