Kovar (FeNiCo)

Basic Properties

Kovar (Fe-Ni-Co alloy, Chinese grade 4J29, UNS K94610, ASTM F15) is a controlled-expansion alloy designed for hermetic glass-to-metal and ceramic-to-metal sealing.

• Matched thermal expansion: the coefficient of expansion follows borosilicate glass and alumina ceramics from room temperature to about 450 °C, so seals stay vacuum-tight through thermal cycling.
• Hermetic sealing: a thin, tenacious oxide film forms during pre-oxidation in wet hydrogen, allowing glass to wet and bond to the metal.
• Curie temperature around 435 °C: magnetic below it, non-magnetic above it.
• Good machinability and weldability: turned, milled, drilled, stamped and deep drawn, then brazed or resistance welded.
• Moderate corrosion resistance: needs nickel or gold plating for humid, marine or long-life electronic service.
• Drawbacks: nickel and cobalt content makes it costly; excessive cold work or wrong annealing cycles spoil the expansion match.

Common Surface Finishes for Kovar

1. Nickel plating: solderable, corrosion-resistant base layer, standard before glass sealing or brazing.
2. Gold over nickel: low contact resistance and stable RF performance for microwave packages.
3. Electroless nickel: uniform coverage on deep housings, connectors and feedthrough bodies.
4. Bright hydrogen anneal: oxide-free, stress-relieved surface for critical sealing faces.
5. Passivation and pickling: removes machining contamination before plating.

Main Applications

• Hermetic electronic packages: TO headers, IC lids, microwave and RF housings.
• Vacuum feedthroughs, connectors and glass-to-metal seals for sensors and detectors.
• Laser diodes, photodiodes, MEMS and crystal oscillator packages.
• Aerospace and cryogenic electronics, X-ray tube and high-vacuum instrument parts.
• Precision instruments where dimensional stability across temperature matters.

Material parameters