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•Matched Seals
To create a matched hermetic seal, housing and glass materials are selected to have similar coefficient of thermal expansion. The strength of a matched seal comes primarily from a chemical bond between the glass and an oxide formed on the metal parts. Matched seals are most often made with Kovar housings and pins, and borosilicate glass.
•Compression Seals
A compression hermetic seal is made with the housing material thermal expansion rate much higher than that of the glass. Upon solidification of the seal during the manufacturing process, the housing will contract around the glass, applying a desirable compression stress on the glass bead. The strength of the glass-to-metal seal is reinforced mechanically as well as chemically, creating a stronger, more reliable part. Compression seals are often made using steel or stainless steel housings, nickel iron pins, and barium alkali glass.