Attaining Low Moisture Levels in Hermetic Packages

Because of occasional high moisture contents (>5000 ppmv) in hermetically sealed, side brazed multilayer ceramic packages, an investigation was carried out to elucidate this effect. The furnaces used for solder sealing of the metal lid on the package were found to have less than 20 ppmv moisture in the nitrogen used as the ambient, so the initial atmosphere in the packages was very dry. Using carefully calibrated moisture sensor chips in the packages it was found that empty packages (no chips) contained from 1000 to 5000 ppmv of water vapor after furnace sealing, regardless of any prebaking of the package or any attempt to prevent readsorption of moisture before sealing. When a chip was die bonded into the package with a gold/silicon eutetic, the moisture level dropped to 100-1000 ppmv after sealing. When a gold/tin eutectic was used for the die bonding, the moisture level dropped even further to less than 20 ppmv after sealing. These low levels of moisture were achieved with no prebaking of parts and no attempt to avoid moisture adsorption before going into the sealing furnace. The proposed mechanism to explain the above observations is the reaction of moisture, always present when a package is heated during the sealing cycle, with silicon that dissolves from the chip in the gold/silicon or gold/tin eutectic used for die bonding: Si + 2H2O ¿ SiO2 + 2H2 With the gold/silicon eutectic, the only silicon available for this reaction is that on the surface of the eutectic material.