TWI Knowledge Summary

Ceramics joining - glass to metal seals

by Alan Taylor

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 (giving a good mechanical bond) - preferably both.

For glass to metal seals, the materials are usually selected in terms of their required engineering properties and also to provide a particular coefficient of thermal expansion (CTE). 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 the latter of these.

Using the classification system based on relative thermal expansion, systems can be described as either matched (i.e. the CTEs of the glass and metal are similar) or unmatched (where they are different). This primary classification is then sub-divided, based on type of glass, type of metal, etc; this is shown in Figure 1.

Fig.1. Types of glass-metal seal

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. A typical design is shown in Figure 2.

Fig.2. Configuration for a matched glass-metal seal

Matched seals are sub-divided according to the expansion of the glass (and obviously the metal). Glasses with expansions lower than 6 x 10 ^-6 /°C are designated as hard glasses, for example boro-silicates and vitreous silica. Glasses with expansions higher than 6 x 10 ^-6 /°C, such as soda glasses, are designated as soft glasses.

For unmatched seals, two main groups of seals exist: ductile and compression.

Ductile seals (of which the Housekeeper is the best-known example) are most commonly used to join glass and copper. 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; this is shown schematically in Figure 3. The technique has also been adapted for the use of Kovar and stainless steels, but in these cases chemical bonding is also required.

Fig.3. Schematic configuration of a ductile or Housekeeper seal

Compression seals rely on forming a mechanical bond by establishing a compressive hoop force from the metal onto the glass, this is shown schematically in Figure 4.

Fig.4. Schematic diagram showing a compression seal

There are two main sub-divisions: matched and reinforced. Matched compression seals have pins and glass with the same CTE values, the compressive force is generated by the outer case (frame) which has a considerably higher expansion. Matched compression seals can be further divided by specifying the glass type; i.e. hard or soft. Reinforced compression seals used low expansion pins with higher expansion glass and very high expansion outer case.

Advantages of glass to metal seals

• Good joint strengths
• Good temperature capability
• Capable of producing many joints simultaneously
• Inexpensive constituents
• Simple, flexible process.

Disadvantages of glass to metal seals

• Unable to cope with large CTE mismatches
• Precise control of glass composition needed.

Also, you can use the Weldasearch literature database to supplement what you find in JoinIT.

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