Glasses have been used for many years as a sealing medium in the electronics industry, particularly where a hermetic and electrically insulating seal is required. Good seals require a chemical bond between the glass and metal, or the glass to be placed in compression - preferably both.
For glass to metal seals, selection of materials is usually based on the need to achieve a particular coefficient of thermal expansion for the given fabrication route and anticipated temperature ranges.
Seals are generally described in terms of: the geometry of the metal parts, type of metal or alloy, type of glass, type of joining method, the fabrication technique, or relative thermal expansion of the glass and metal. The most frequently used classification is based on the last of these.
Using a classification system based on relative thermal expansion, systems can be described as either matched (i.e. the coefficients of thermal expansion (CTE) of the glass and metal are similar), or unmatched (where the CTEs are different). This primary classification is then sub-divided based on type of glass, type of metal, etc. For matched seals, sealing is a result of good chemical bonding at the glass/metal interface. Matched seals can be made to metals, ceramics and other glasses.
Matched seals are sub-divided according to the expansion of the glass (and by definition metal). Glasses with expansions lower than 6x10-6/°C are designated hard glasses, boro-silicates and vitreous silica for example. Glasses with expansions higher than 6x10-6/°C are designated soft glasses, soda glasses for example.
For unmatched seals, two main groups exist: ductile and compression.
Ductile seals are most commonly used to join glass to copper, also known as the Housekeeper technique. Despite its high CTE, copper is effective because of its ductility. If thinned enough, the copper is able to comply with the glass to produce a seal, shown schematically in the Fig. The technique has also been adapted for Kovar and stainless steels, but in these cases chemical bonding is also required.
Compression seals rely on forming a mechanical bond by establishing a compressive hoop force from the metal on to the glass.
- good joint strengths
- good temperature capability to 1000°C
- capable of producing many joints simultaneously
- inexpensive constituents
- simple, flexible process.
- unable to cope with large CTE mismatches
- precise control of glass composition requires
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