One of the main functions of Glass-to-metal sealing is to hermetically pass a conductor from one environment to another. That conductor may conduct electricity, light or gases. The glass used in glass to metal sealing provides the hermetic seal and as glass is an excellent insulator is often also used to provide electrical isolation.
Glass-to-metal hermetic seals available in industry today are typically one of two types:
Matched Seals
Strength of a Matched Seal
In a matched hermetic seal the housing and glass materials have closely matched coefficients of thermal expansion (CTE). The strength of a matched seal comes from a chemical bond between the glass and an oxide on the metal parts.
Kovar and Glass
Matched seals can be made with various materials but are typically made with Kovar and borosilicate glass. The resultant seal is effectively stress/strain free over a large range of operating temperatures. With little inherent stress metal members can be smaller or thinner than possible with compression seals and in some cases can be eliminated completely. These seals are most often used where the good CTE match with electronic materials (ceramic substrates and silicon) is beneficial or excellent temperature cycling performance is required
Compression Seals
Compression hermetic seals are made by graduating expansion coefficients from the housing material through the glass and the pin. Each section applies a compression stress creating a strong reliable part which can be tailored for high pressure, UHV and highly aggressive environments.
