Fluxless bondings of silicon to alumina substrate using electroplated eutectic Au-Sn solder

Large 6 mm times 9 mm silicon dice have been successfully bonded on alumina substrate with electroplated Au80Sn20 eutectic alloy. Eutectic AuSn is one of the best known hard solders having excellent fatigue-resistance and mechanical properties. A fluxless bonding process in 50 militorrs of vacuum environment is presented. Vacuum environment is employed to prevent tin oxidation during the process. The oxygen content is expected to be reduced by a factor of 15,200, comparing to bonding in air. One of the challenges in silicon-to-alumina bonding is the large mismatch in thermal expansion between silicon of 2.7 times 10-6 ppm/degC and alumina of 7 times 10 -6 ppm/degC. Electroplating method is used to build multi-layer solder. It is an economical alternative to vacuum deposition method and can produce thick solders. Joints fabricated are examined using scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX). It is found that proper bonding condition is needed to turn the stacked layers into a uniform AuSn eutectic alloy. Nearly void-free joints are achieved and confirmed by a scanning acoustic microscope (SAM). To evaluate the reliability of the solder joint and the bonded structure, samples go through thermal cycling test to determine failure modes. Microstructure changes of the solder joints during thermal cycling test are also investigated