Crystallization of vitreous seal in hermetic packages

Dull glass in ceramic packages which leads to electrical bridging between leads of tin plated parts has been identified to be due to the formation of metallic lead crystals on the solder glass surface. Controlled sealing atmosphere testing revealed that the presence of small amounts of carbon and hydrogen-containing solvents in the sealing furnace results in the formation of metallic lead crystals. The lead crystals may be dissolved with nitric acid prior to tin plating to eliminate bridging. The glass dissolution in nitric acid is described by the equation: ▵W (mg/inch square)=23[H⁺] exp(-0.4/RT)t. Two other types of crystals, columnar lead oxide and acicular hydrocerrusite are also formed on the solder glass surface but they do not impact the reliability of the package