Mechanical properties and microstructures of Cu/In-48Sn alloy/Cu with low temperature TLP bonding

Silicon carbide semiconductor, which has a wide band gap and high-power conversion efficiency has been widely used. However, since the operating temperature of SiC exceeds 250 °C, SAC 305 with a melting point of 217 °C cannot be used. Therefore, transient liquid phase (TLP) bonding, which is a method to form intermetallic compounds has been attracted for alternative methods. The TLP bonding method based on Sn base materials requires a peak temperature at least 280 °C.In this study, to decrease the bonding temperature, eutectic In-48Sn alloy (Tm=118 °C) was applied. Cu/In-48Sn/Cu was bonded at 170, 200, and 230 °C in air condition with 0.75 MPa. The bonding time were 10, 20, and 30 min. To investigate reliabilities of Cu/In-48Sn alloy/Cu solder joint, High temperature storage test were conducted at 150 °C for 300, 500, and 1000 h. The microstructure of the joints was observed by FE-SEM. Atomic distribution of Cu, Sn and In of joints was analyzed by EPMA mapping analysis.The results show that micro-layered existed in all conditions and due to insufficient bonding time, unmelted In and Sn were observed at 170 °C for 10, and 20 min, which means In and Sn were not able to form high melting point materials. When the time reached 30 min at 170 °C that is a sufficient bonding time to form high melting point materials, there were Cu6(In,Sn)5 small amount of residual Sn, and In remained. As the temperature increases, demanded bonding time to form IMCs decreased. Thus, even at 200 °C for 10 and 20 min, most of Sn and In were formed Cu6(In,Sn)5, and only a small amount of residual Sn and In remained. When the time reached 30 min at 200 °C, Sn, and In formed Cu6(In,Sn)5 entirely without residual Sn and In. However, in case of 230 °C, due to the high temperature, transformation of Cu6(In,Sn)5 to Cu3(In,Sn) gradually over the time. Result of transformation of Cu6(In,Sn)5 to Cu3(In,Sn), micro-layer void and kirkendall voids were observed at 230 °C.Excessive Cu3(In,Sn) leads to excessive kirkendall voids, which were located between Cu and Cu3(In,Sn) layer. These kirkendall voids were created by different diffusion rates in the Cu and Sn. The kirkendall void is reason of degraded the shear strength of the joint.