Updated June 29, 2008

Click here to go to our page on temperature coefficient of expansion

Low expansion alloys are often used in microwave packaging, particularly when hermeticity is required. For over 100 years engineers have been playing around with alloys to control the temperature expansion coefficient (TCE).

Here's another site with a ton of TCE data

Invar

Invar is the original low-expansion iron alloy. The word Invar was derived from "invariant" by its inventor, Swiss Charles Édouard Guillaume. For his work he received the Nobel Prize in Physics in 1920. The applications of low expansion alloys are many, particularly when you consider that glasses are naturally low TCE. Observatory telescope mirrors are mounted on Invar supports so that they don't distort with the ambient temperature.

Invar's composition is denoted Fe36Ni (36% nickel, balance is iron).

Kovar™

Kovar is the most popular iron alloy used in packaging. Kovar is very similar to Invar, but with a touch of cobalt: Fe29Ni17Co, with a trace of some other elements thrown in. Kovar is a trademark of Carpenter Technology Corporation, but the trademark has been diluted over the years so that you almost never see the little ™ symbol next to it. Shame on us all for our lack of respect!