Why Kovar, and why still — a century after it was invented
Kovar was developed by Westinghouse in 1936 to solve a specific problem: making a leak-tight, thermally stable joint between a metal conductor and a borosilicate glass envelope for vacuum tubes. The alloy matches the thermal expansion coefficient of hard glass across the sealing temperature range, which means cooled seals don't crack. Nearly 90 years later, with most of the original vacuum-tube market gone, Kovar is still in active production because the same property matters for modern hermetic electronic packages, microwave feedthroughs, and high-reliability space-flight instrumentation.
Our typical Kovar customer is building hermetic packages for high-reliability electronics (space, medical implantable, defense), microwave or RF feedthroughs, or specialized vacuum-tube components for accelerator and radar applications. Orders range from 10-piece prototypes to 10,000-piece production runs of Swiss-turned pins.
Hydrogen-fired stock — the non-obvious requirement for glass-seal work
Raw Kovar stock as milled contains free carbon and surface oxides that cause two problems in sealing applications. During glass sealing at ~1000 °C, free carbon outgasses CO into the softened glass and produces bubbles in the seal. During plating, surface oxides prevent uniform deposition. Both problems disappear when the stock is hydrogen-fired before machining: a 2-hour anneal at 950 °C in a dry H₂ atmosphere decarburizes the surface layer and reduces the oxide to clean metal.
We keep hydrogen-fired Kovar in stock in common bar sizes (3mm to 32mm round) and plate thicknesses (1.5mm, 2.0mm, 3.0mm, 6.0mm). For larger sizes or unusual forms, mill lead time is 3–5 weeks. For mechanical Kovar parts that don't require sealing or plating — pressure-sensor body shells with bolted joints, for example — mill-finish bar stock is fine and cheaper.
Machining parameters
Kovar machines similarly to 410 stainless at moderate hardness (~28 HRC annealed). We run it at ~45 m/min surface speed roughing and 60 m/min finishing, feed per tooth 0.08–0.12 mm, flood coolant at 20 bar. TiAlN-coated carbide tooling is our default. The alloy work-hardens moderately — climb-cut exclusively, don't dwell, and don't let coolant drop out. Chip evacuation matters on pin features; we use peck drilling at 1–1.5 diameter peck depth to keep the bore clean.
For small-diameter Swiss-turn work (pin features 3–8 mm diameter), we run high surface speeds (80–100 m/min finish) with sharp-edge carbide and light depth of cut. These parts typically hold ±0.013 mm diameter tolerance in production, which is the tight side of what glass-sealing houses usually request.
Plating and post-machining finishes
Kovar is rarely shipped bare for electronic applications. Standard finishes we provide through plating partners:
- Gold over nickel — 50 µin Au over 50 µin Ni min, per MIL-G-45204 and MIL-C-26074. Default for hermetic electronic packages and high-reliability RF.
- Electrolytic nickel — 100–300 µin matte per MIL-QQ-N-290. For solder-preparation on lower-cost packages and for corrosion protection on mechanical parts.
- Silver (Ag) for braze prep — 100 µin for customers doing downstream silver brazing of Kovar to ceramic or copper.
- Electroless nickel — uniform plating for complex geometry where electrolytic plating thickness varies too much.
Plating adds 5–7 days to lead time. For high-reliability applications we document plating thickness, coverage, and adhesion per MIL-STD-105. Tell us the plating spec up front; retrofitting after machining is possible but doubles cost.
Quality documentation
Every Kovar lot ships with: ASTM F15 mill certificates linking heat number to your PO, chemistry report (Ni, Co, Mn, Fe, C, Si — trace elements matter for CTE consistency), mechanical property report, CTE certification for the specific heat if required, hydrogen-firing certification if applicable, plating thickness and adhesion reports if applicable, first-article inspection, and Certificate of Conformance. For space-flight and defense we add process traveler documentation and supplier chain of custody back to the original Carpenter Technology or Metal Products Group mill.
What to send for a Kovar quote
Required: STEP file, PDF drawing with tolerances, material callout (ASTM F15 is the standard), whether hydrogen-fired or mill-finish stock, plating specification if applicable, target quantity, target delivery. Helpful: application context (glass-to-metal seal, hermetic package, microwave feedthrough, vacuum tube), specific glass being sealed to if relevant, and any MIL-spec documentation requirements. For the high-reliability customers we typically serve, we can match the full certification and testing scope typical of space and defense programs.