Kovar is an Iron-Nickel-Cobalt alloy, best known for having a similar coefficient of thermal expansion (CTE) to Borosilicate glass,quartz glass and other optical materials that allows for glass-to-metal bonds.

Kovar is a specialty alloy with CTE characteristics similar to multiple glass and ceramic components, that can be tailored to have specific non-linear thermal expansion curve to match the material it is mating with. This characteristic, along with good mechanical properties, makes it a frequent choice for temperature-sensitive glass and metal applications.

At present, there are two main methods for connecting glass and kovar alloy: flame fusion sealing and anodic diffusion bonding.