摘要
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—AgIn2 and Ag9In4—are identified, with AgIn2 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.
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