Analysis of microstructures and fractures in Ag-In transient liquid phase bonded joints

Silver (Ag) and indium (In) are increasingly investigated as transient liquid phase (TLP) bonding materials for die attachment in next generation power modules. In this study, we systemically investigated the relationship between the microstructural evolution, shear strength, and fracture behavior of Ag-In TLP bonded joints. During bonding, Ag9In4 was the primary phase formed between the solid Ag and liquid In. The Ag9In4 joints exhibited brittle fractures with an intergranular fracture mechanism. The shear strength increased as bonding time and temperature increased (from 26.9 MPa to 53.0 MPa), attributed to the improved cohesiveness of grain boundaries. As the bonding process progressed, zeta (Ag3In) formed and developed between Ag and Ag9In4 owing to the solid-solid interdiffusion. The zeta (Ag3In) joints, obtained from the conditions of 250 degrees C and 40 min, exhibited the highest shear strength of 82.87 MPa. Here, the ductile fracture was characterized with an intergranular fracture mechanism. This ductility was partially attributed to the relatively low hardness of zeta (Ag3In) (2.1 GPa). The high strength and ductility of zeta (Ag3In) found in this study present differ from the conventional understanding of the inherently brittle intermetallic compounds. Therefore, the study findings form the basis for enhancing the reliability of power modules.