Au-Sn fluxless SLID bonding: Effect of bonding temperature for stability at high temperature, above 400 °C

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300-350°C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350-400°C, with 95% of the tested samples surviving 400°C without bond delamination. This is more than 100°C higher than the melting temperature of the commonly used eutectic Au-Sn bond (80 wt% Au, melting at 278°C). The Au-Sn system is particularly interesting since it is oxidation resistant, allowing fluxless bonding. With the SLID process, the metal system is applicable for true high-temperature applications. The bonded samples had electroplated Au-Sn layers (Process A), or were made by sandwiching a Au-Sn eutectic preform between Au layers (Process B). In both cases, the overall composition was 7 wt% Sn (11 at% Sn), thus being a surplus of Au relative to the Au₅Sn phase. This phase has a melting point up to 519°C, and is the desired bonding phase for a Au-Sn SLID process to tolerate high temperatures after bonding. Microscopy and EDS analysis of cross-sections identified indeed Au₅Sn as the only bonding phase (in addition to Au). It is remarkable that also samples bonded with as low temperature as 300°C (below the melting point of Sn-rich intermetallics) show this structure and the corresponding high-temperature stability.