Au-sn SLID bonding: Fluxless bonding with high temperature stability to above 350 °c

A fluxless SLID (solid-liquid inter diffusion) bonding process based on Au and Sn, where the final bond consists of intermetallics with high melting point, is presented. The decomposition temperature of the bond was tested by applying shear force while heating bonded samples. No bond delamination was observed for temperatures up to 350-400degC, which is 100degC higher than the melting temperature of the commonly used eutectic Au-Sn bonds (80 wt% Au, melting at 278degC). The Au-Sn metal system is of great interest since it is oxidation resistant, allowing fluxless bonding. The high temperature stability of the presented process opens the possibility to use Au-Sn bonding for true high-temperature applications. The bonded samples had electroplated Au-Sn layers, with an overall composition of 8 wt% Sn (13 at% Sn), thus being a surplus of Au relative to the eutectic point. The Sn layer was converted to an intermetallic compound prior to bonding. No flux agent or chemical surface treatment was used. SEM/ EDS analysis of cross-sections shows uniform bond lines consisting of a layered structure: Au / Au-Sn-alloy / Au. The bonding alloy, being rich in Au, was identified as the zeta/ zeta' phase (Au 5 Sn). This phase, with a melting point up to 519degC, explains the elevated delamination temperature of the bonded samples. Since the Au-Sn phase diagram does not contain room-temperature phases between the zeta' phase (Au 5 Sn) and the Au phase, the bond is expected to be stable over time.