Growth of Intermetallic Compounds in Cu-(Sn-Bi) SLID Bonds

Abstract Solid–liquid interdiffusion (SLID) is a bonding technique which offers bonds with good mechanical, electrical, and hermetic performance, even for further steps that exceed its process temperature. The technique has been successfully employed in high-temperature applications, MEMS packaging, and 3D integration at wafer level. The recent trends of packaging larger wafer sizes, packages consisting of dissimilar materials, and/or temperature-sensitive materials has led to demands for SLID technologies with bonding temperatures below 200 °C. The Cu-(Sn-Bi) system, using eutectic Sn-Bi, can be bonded at low temperatures and has an expected thermal stability above 600 °C. This work presents the microstructure of Cu-(Sn-Bi) bondlines when bonded at different temperatures and time values, aiming to understand the growth dynamics. A model of the evolution is also proposed based on the phase diagrams of the materials involved in the system. Samples were bonded for 30 min and 2 h, with bonding temperatures ranging from 150 to 300 °C. Cross-sectional micrographs were analyzed and compared to the proposed model. Bonding temperature plays an important role in microstructures in the bondline. Below 186 °C (solid-state phase transition of Cu 6 Sn 5 ), samples presented only Cu 6 Sn 5 ; in contrast, samples bonded above this temperature contained only the Cu 3 Sn phase. Additionally, samples bonded above 271 °C (Bi melting point) showed a different microstructure to the rest.