The functionality of glass-to-metal seals is to provide a way to hermetically isolate an electrical conductor from one environment to another. In addition to providing the seal, the glass acts as an insulator between one pin to another, and from the housing.

In today's industry, there are basically two types of glass-to-metal seals from
which to choose:
Matched Seals
During the engineering process of a matched seal, housing, pins, and glass components are chosen to have similar Coefficients of Thermal Expansion (COTE) for a range of temperature between ambient and the glass transformation point. A chemical bond between the glass and an oxide formed on the metal components contributes to the strength of a matched seal. Kovar housings and pins, along with borosilicate glass are typically the components used in a matched seal.
  1) Kovar COTE 49 x 10-7 / °C

2) Fusite K glass COTE 45 x 10-7 / °C

3) Kovar COTE 49 x 10-7 / °C
Compression Seals
When the housing material's thermal expansion rate tends to be higher than that of the glass, a compression seal is created. During the manufacturing process, the housing will contract around the glass upon solidification. This action generates a favorable homogenous radial compression stress on the glass at the glass to metal interface. This engineered expansion mismatch produces a robust seal, which can withstand thermal and mechanical stresses. Compression seals are generally made using steel or stainless steel housings, nickel iron pins, and barium alkali glasses.

Various applications benefiting from such glass-to-metal seals are as follows:

• Compressor, Vacuum components
• Sensor or transducer feed-thrus for harsh environments
• High temperature/High pressure military or automotive applications
• Motor protector components
• Battery Feed-thrus
  1) 446 Stainless Steel COTE 108 x 10-7 / °C

2) Fusite T glass COTE 92 x 10-7 / °C

3) Cold Rolled Steel COTE 135 x 10-7 / °C
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