Glass-to-metal seals.
5 ppm/°C CTE.
Hermetic packaging.

Borosilicate glass has CTE of 5 ppm/°C. Kovar has matched 5 ppm/°C CTE. When heated together to ~900 °C (glass softens), they bond chemically. On cooling, both contract at the same rate — no residual stress at the interface. Result: hermetic seal that doesn't crack with thermal cycling. If mismatched CTE were used, thermal contraction differential would create stress and crack the glass.

Kovar is necessary specifically for glass-to-metal seals. For non-sealed applications (general electronic enclosures, structural parts), stainless steel is cheaper and better. Don't specify Kovar unless you're making a glass-metal seal, ceramic-metal seal, or CTE-matched electronic package. Kovar is a specialty alloy for specialty applications.

Kovar work-hardens (like stainless), low thermal conductivity (heat concentrates at tool), magnetic (can cause chip adherence issues). Solutions: sharp carbide tooling, slow cutting speeds (50-80 m/min), flood coolant, demagnetize workpiece if issues arise. Tolerance ±0.025 mm achievable. Our semiconductor packaging customers routinely order precision Kovar parts with success.

Kovar raw material: $30-60/kg. About 5-10× mild steel but similar to nickel alloys. Available in standard bar, plate, and sheet sizes. 3-4 week lead time typical for non-stocked sizes. For semiconductor and hermetic packaging orders, Kovar is standard and readily available.

Kovar has moderate corrosion resistance — better than mild steel, worse than stainless. For electronics applications (typically enclosed, controlled environments), corrosion isn't usually critical. For parts exposed to humidity or chemistry, specify nickel or gold plating. Never use bare Kovar in corrosive service — it will rust.

Kovar for borosilicate glass. Alloy 52 for soft glass. Alloy 46 for hard glass. Alloy 39 for alumina ceramic. Platinum and palladium for specialty precision seals (expensive). For polymer-to-metal seals, standard metals work — CTE matching only matters for glass and ceramic sealing. For semiconductor chip attach, matched-CTE alloys ensure long-term reliability over thermal cycling.