New Fluxless Hermetic Sealing Technique Using Electroplated Sn-Rich Soft Solder

New fluxless hermetic sealing technique is reported using electroplated Sn-rich soft solder. Specific type of glass (SCG72) is chosen as a lid material to be sealed onto the alumina ceramic package to deal with thermal expansion mismatch. This glass has nominal coefficient of thermal expansion of 7ppm/degC, which is almost perfectly matching to that of alumina ceramic package. Thick Sn layer is plated over the Cr/Au patterned glass wafer, followed immediately by thin Au layer. This outer Au layer prevents the inner Sn from oxidation. In bonding, the glass lid is placed over the ceramic package having outer layers of Ni/Au. To achieve high quality hermetic sealing joint, a fluxless process is developed in vacuum environment (50 militorrs) to suppress tin oxidation. Comparing to bonding in air, the oxygen content is reduced by a factor of 15,200. Fluxless bonding is desired in many hermetic sealing applications such as MEMS, sensors, photonic devices, and imaging devices. Nearly void-free lid sealed joints are made with Sn-rich composition. Helium leakage tests are performed to evaluate the quality of the hermetic sealing. Scanning Electron Microscope (SEM) with Energy Dispersive X-ray spectroscopy (EDX) analysis is done to evaluate the joint and microstructure. This new sealing process can be applied to nearly all devices that require hermetic sealing.