Fluxless silicon-to-alumina bonding using electroplated Au–Sn–Au structure at eutectic composition

Large 6 mm × 9 mm silicon chips have been successfully bonded to alumina substrates with electroplated Au/Sn/Au structure at the eutectic composition. The new bonding process is entirely fluxless, or flux-free. It is performed in vacuum (50 milliTorrs), where the oxygen content is reduced by a factor of 15,200 comparing to air, to inhibit solder oxidation. Eutectic Au80Sn20 alloy is a well established solder that exhibits excellent fatigue-resistance, creep-resistance, and long-term reliability. Despite the popularity of plastic packages and organic substrates, alumina remains an important packaging material for highly reliable products in demanding environment. A major challenge in silicon-to-alumina bonding is the large thermal expansion mismatch between silicon (2.7 ppm/°C) and alumina (7 ppm/°C). The new process developed shows that a large silicon chip can be bonded to an alumina substrate without chip cracking or solder fracture. In this research, we learned the specific bonding procedures necessary to turn the Au/Sn/Au structure into homogeneous eutectic alloy without the chip breaking away from the solder layer. Nearly void-free joints are produced as confirmed by a scanning acoustic microscope (SAM). The joints are studied using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) to evaluate the microstructure and the composition.