Bi Behaviour in Au–(In–Bi) SLID Bonding

Au–(In–Bi) solid-liquid interdiffusion bonding (SLID) has shown potential for assembly of temperature-sensitive materials, such as poled piezoelectric and composite materials. Successful bonds can be made using Au–(In–Bi) SLID bonding at a low temperature of 120 °C. This work focuses on studying the behaviour of Bi in the Au–(In–Bi) SLID system. Thin-film (1 μm) eutectic In–Bi (78.5 at% In) is deposited as the intermediate layer for bonding by electron-beam evaporation. Bonding at 120 °C in atmosphere produced bonds with Bi inclusions. The Bi inclusions were characterized at various bonding times using cross-sectional measurements. The inclusions indicated poor bonding to Au, meaning asymmetric bonds where Bi inclusions are in direct contact with Au are mechanically weaker compared to a symmetric bond where there is no contact between the Au and Bi inclusions. The average size of the Bi inclusions in the bondline increased with bonding time. However, the large variance between the observed number of inclusions means more analysis is needed to confirm that this is mainly a result of coalescence as opposed to further segregation of the Bi from the liquid In–Bi phase as bonding time is increased.