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 atmasphere 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: AW (mghch square) = 23 m'] exp (-0AIRT) 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. 1.0 INTRO DUCTION Cerdip EPROMs (Ceramic Dual-in-Lime Package Electrically Programmable Read-only Memory) are hermetically packaged between two monolithic, dry pressed alumina caps joined together by a low temperature melting solder glass. The solder glass is either vitrifying or devitrifying. Vitreous glasses resemble ordinary glass. They are relatively transparent and can be reworked (remelted) repeatedly without degradation in physical properties. They contain no nucleating agents and have low alpha particle