Microstructural observation of Bi67In reacting with Cu for microelectronic interconnects

•A combination of Bi and In solder significantly reduces the bonding temperature and carbon emissions.•In segregation between scallop and layer Cu11In9 induced CuIn2 formation in the middle of Cu11In9 after aging.•CuIn2 formation originated from the In segregation because the peritectic reaction from Cu11In9+In to CuIn2 after aging.•Scallop Cu11In9, layer CuIn2, and wavy Cu11In9 formation from solder to Cu side after aging. Low-temperature solders are considered eco-friendly because of lowered wasted power, warpage, electromigration. In addition, chips are becoming increasingly integrated so as to achieve superior performance. Semiconductor packaging has moved from two-dimensional to three-dimensional integrated-circuits with vertical stacking for increased functionality. Thermal budget impacts electronic device manufacture, specifically in heterogeneous integration. Therefore, reduction of process temperatures is necessary. Bi and In were selected in this study because they are commercial solders with low-melting points. Further, the microstructures and interfacial reactions of Bi–In solders have not been extensively studied. This study entailed a systematic examination of the interfacial reactions occurring between Bi67In and Cu metallization upon reflow and subsequent aging was performed. It was observed that In segregates in the middle region of Cu11In9 after reflow. The phase transformations from Cu11In9 to CuIn2 start in the In segregation region. This remarkable phenomenon warrants in-depth research. [Display omitted]