What is Kovar, and where is it used?

  • When two connecting materials expand and contract at different rates this can create immense pressure.
  • Kovar is an alloy which expands and contracts at the same rate as borosilicate glass and something you will see in everyday life but never give a second glance.
  • Everyday elements such as x-ray tubes, power tubes, etc. depend upon Kovar for their longevity.
kovar metal alloy
Kovar’s been around for a while! It was originally patented back in 1936

Kovar is an iron nickel cobalt alloy with a coefficient of thermal expansion similar to that of hard glass. This might make sense to those of a more engineering technical mind but in layman’s terms what does this actually mean and what is it used for?

Main benefits of Kovar

The coefficient of thermal expansion is THE most important mechanical  property of Kovar. The alloy is precisely formulated and controlled to produce a coefficient of thermal expansion that is very close to that of hard glass (e.g. borosilicate glass) or ceramic, meaning that it expands and contracts at a similar rate. It is therefore perfect in applications where it is used in conjunction with glass or ceramic materials, especially where a hermetic seal must be maintained across a wide temperature range. There are a vast number of products where metal and glass or ceramic are joined together. You can probably think of many examples of such products in daily life.

Considering that the first US patent application for Kovar was granted back in August 1936 it is perhaps surprising that this unique, and very important alloy is not better-known. The name itself is indeed trademarked by CRS Holdings Inc in Delaware, who are a subsidiary of Carpenter Technology Corp.

Common uses of Kovar

If we take for example a traditional tube light which is commonplace in many workplaces we can then look at how Kovar works in real life situations. We all know from accidentally touching a tube lightbulb that the glass can become extremely hot very quickly. Creating the required lighting using electricity creates immense heat which very quickly transfers to the glass.

Pulsed x-ray tube. The domed section is made from Kovar

Using non-Kovar metals to connect the glass tubing to the metal holders would be extremely dangerous and in real life unworkable. This is because traditional metals have a thermal expansion coefficient very different to that of hard glass and therefore each element, the glass and the metal, would expand and contract at different rates. Inevitably this would lead to increased pressure on the glass tube which would ultimately crack under the added force, or not be able to maintain its hermetic seal.

Kovar is also used in a number of other products such as microwave tubes, transistors, diodes, power tubes, x-ray tubes and indeed any product which requires a glass to metal (or ceramic) seal. It has also been used in integrated circuits, such as the flat pack and dual-in-line package.

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Hermetic package semiconductor assembly examples

Kovar may not be the best known alloy you will ever come across, it is probably something you have never heard of, but when you bear in mind the number of glass to metal seals required in everyday products you will realise just how important it is.



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