KEY TAKEAWAYS

✓
Kovar (ASTM F15) is optimized for glass-to-metal and ceramic-to-metal sealing in hermetic packages; Invar 36 is optimized for dimensional stability over wide temperature ranges.

✓
Kovar’s CTE (~5.1 ppm/°C) matches borosilicate glass and alumina ceramics; Invar 36’s CTE (~1.2 ppm/°C) is one of the lowest of any metallic alloy.

✓
Both alloys work-harden rapidly and require sharp tooling, low cutting speeds, and aggressive coolant strategies — plan for higher machining costs versus standard steels.

✓
Invar 36 is more sensitive to residual stress and requires careful annealing and stress-relief cycles to hold tight tolerances in finished parts.

✓
Specify surface finish and plating requirements upfront — bare Kovar and Invar oxidize and are rarely used uncoated in production electronics assemblies.

Kovar (UNS K94610, ASTM F15) is a nickel-cobalt ferrous alloy developed specifically to match the thermal expansion of borosilicate glass and alumina ceramic. Its nominal composition is approximately 29% nickel, 17% cobalt, and the balance iron. Invar 36 (UNS K93600) is a nickel-iron alloy containing approximately 36% nickel, famous for having the lowest CTE of any commercial metallic alloy near room temperature.
In electronics, CTE mismatch is a primary failure mechanism. Solder joints crack, hermetic seals leak, and optical alignments drift when dissimilar materials expand and contract at different rates across thermal cycles. Both Kovar and Invar exist to solve this problem — but in different ways and for different application contexts. Kovar solves it by matching the expansion of glass and ceramic. Invar solves it by minimizing expansion altogether.
These are specialty engineering alloys, not commodity materials. Lead times, machinability challenges, and cost premiums are all real. Engineers who understand the physics behind each alloy will make better sourcing and design decisions from the start.

The coefficient of thermal expansion (CTE) describes how much a material expands per degree of temperature rise, expressed in parts per million per degree Celsius (ppm/°C). For hermetic electronic packages, the CTE of the metal housing must closely match the glass or ceramic feedthrough it is sealed to — otherwise differential expansion creates stress that fractures the seal over time.
Kovar’s CTE of ~5.1 ppm/°C aligns closely with borosilicate glass (~3.3–5.5 ppm/°C) and alumina ceramic (~6.0–7.0 ppm/°C). This makes it the standard choice for transistor outline (TO) cans, microwave packages, hybrid circuit housings, and fiber optic ferrules where glass-to-metal seals are required.
Invar 36’s CTE of ~1.2 ppm/°C near room temperature is anomalously low due to the Invar effect — a magnetovolume phenomenon where magnetic ordering partially counteracts thermal lattice expansion. This makes Invar the material of choice when you need dimensional stability across temperature ranges, not necessarily a match to glass. Precision optical benches, metrology fixtures, satellite structural components, and shadow masks for display manufacturing are all classic Invar applications.
One critical note: Invar’s low CTE is most pronounced between roughly -100°C and +200°C. Above the Curie temperature (~230°C), the Invar effect diminishes and CTE rises sharply. Always verify your operating temperature range before specifying Invar.

⚠️ WARNING: Invar 36’s ultra-low CTE is temperature-dependent. Above ~230°C (the Curie point), the Invar effect breaks down and CTE increases significantly. Do not assume low expansion at elevated temperatures without verifying your full thermal profile.

Neither Kovar nor Invar behaves like a free-machining steel. Both alloys are gummy, work-hardening, and abrasive on cutting tools — characteristics that demand experienced machinists and dialed-in process parameters. Understanding their mechanical properties upfront helps engineers design for manufacturability and avoid tolerance stack-up surprises.
Kovar has a tensile strength of approximately 517 MPa (75 ksi) in the annealed condition, with yield strength around 275 MPa. It is moderately ductile with elongation around 30%. It machines acceptably with carbide tooling, but its tendency to work-harden means dwell time in the cut must be minimized. Sharp tools and consistent chip load are essential. Kovar can be cold-worked but typically requires annealing to restore properties.
Invar 36 has similar tensile strength (~450–480 MPa annealed) but is notoriously more difficult to machine. It galls, smears, and builds up on tool edges. Cutting speeds must be kept low — typically 30–60% of those used for 304 stainless steel. Flood coolant is mandatory. High-speed steel (HSS) tooling is sometimes preferred for certain operations due to its toughness, though carbide remains common. Chip control is poor; expect long stringy chips.
For both materials, residual stress management is critical. Parts should be rough-machined, stress-relieved, then finish-machined to hold tolerances tighter than ±0.005 inch. Invar is particularly sensitive — skipping stress relief on Invar parts is a reliable way to get spring-back and dimensional drift after final machining.

RULE OF THUMB: For Invar parts requiring tolerances tighter than ±0.005 inch, always include a stress-relief anneal between roughing and finishing operations. Build this into your schedule — it adds time but prevents expensive scrap.

Successful precision machining of Kovar and Invar requires departure from standard steel practices. Shops that treat these alloys like 4140 steel will produce out-of-tolerance parts and burn through tooling budgets. Here are the parameters that matter most.
Cutting speed: Keep surface footage conservative. For Kovar, 60–100 SFM with carbide is a reasonable starting point. For Invar, target 40–80 SFM. Higher speeds accelerate work hardening and built-up edge formation. Feed rate: Use consistent, positive chip loads. A feed that is too light causes rubbing rather than cutting — the fastest path to work hardening and tool failure. Depth of cut: Moderate depths of cut outperform very shallow passes for the same reason. Get under the work-hardened layer from the previous pass.
Tooling geometry: High positive rake angles reduce cutting forces and minimize work hardening. TiAlN-coated carbide end mills and drills perform well on Kovar. For Invar, some machinists prefer uncoated carbide or HSS with bright finish due to Invar’s tendency to adhere to coated surfaces. Coolant: Flood coolant is non-negotiable — both for temperature control and chip evacuation. Mist or dry machining is not appropriate for production runs of these alloys.
Hole making deserves special attention. Drilling Invar produces notorious chip packing. Use peck drilling with frequent retraction, high coolant pressure, and sharp drills. Reaming Invar and Kovar is effective for achieving precise bore diameters. Tapping should use form taps (thread-forming) rather than cut taps where possible, as cut taps clog rapidly in these materials.

Bare Kovar and bare Invar are rarely acceptable in finished electronic assemblies. Both alloys oxidize, Kovar’s oxide layer can interfere with solderability, and neither material has the corrosion resistance required for long-term reliability in most environments. Surface finishing and plating are therefore integral to the design, not an afterthought.
For Kovar, the most common finishing sequence for hermetic packages is electroless nickel plating (typically 3–5 µm) followed by gold plating (typically 0.5–2.5 µm). This sequence provides solderability, bondability for wire bonding, and oxidation resistance. Nickel acts as a diffusion barrier between the iron-based substrate and gold. Some applications use silver plating for RF/microwave applications where conductivity is paramount. Kovar must be thoroughly cleaned and activated before plating — surface oxides formed during machining will cause adhesion failures if not properly removed.
For Invar, finishing options are similar but the application context often differs. Precision structural Invar components (optical benches, metrology hardware) are sometimes left bare or receive a light phosphate conversion coat for corrosion protection without dimensional impact. Invar used in electronics may receive similar nickel-gold plating sequences as Kovar. Hard chrome plating on Invar is uncommon due to hydrogen embrittlement risk.
Joining both alloys requires care. Kovar is designed to be brazed and is routinely joined to ceramics using active metal brazing (with copper-silver-titanium braze alloys) or conventional copper-silver brazes in controlled atmospheres. Invar can be welded (TIG preferred), brazed, or soldered, but welding introduces significant residual stress — post-weld stress relief is strongly recommended. Mechanical fastening of both alloys is straightforward but avoid galvanic couples with dissimilar metals in humid environments.

The decision between Kovar and Invar is usually determined by the primary functional requirement. Use this framework to guide material selection before committing to a design.
Choose Kovar when:

• Your design requires a hermetic glass-to-metal or ceramic-to-metal seal
• You are manufacturing TO-style packages, microwave cavities, or hybrid circuit lids
• You need a CTE match to borosilicate glass (~3.3–5.5 ppm/°C) or alumina (~6–7 ppm/°C)
• The part will be soldered or brazed to ceramic substrates or glass feedthroughs
• Operating temperatures remain below 450°C (above which Kovar’s CTE begins to diverge from glass)

Choose Invar 36 when:

• Dimensional stability across a wide temperature range is the primary requirement
• You are building precision optical systems, laser housings, or interferometric instruments
• The application involves satellite or space structures where thermal cycling must not shift alignment
• You need a tooling material (shadow masks, precision fixtures, metrology standards) that holds geometry across temperature swings
• Operating temperatures remain below ~200°C for reliable low-CTE performance

There are applications where engineers consider both alloys — for example, some microwave package designs use Invar for structural frames and Kovar for sealing rings. In these cases, compatibility of CTE, joining methods, and plating sequences between the two materials must be carefully engineered.

KEY INSIGHT: Kovar and Invar are sometimes used together in the same assembly — Invar for dimensional stability in the structural frame, Kovar for the hermetic sealing interface. Validate CTE compatibility at the joint interface to avoid stress concentration failures.

Kovar and Invar are specialty alloys with meaningful supply chain implications. Neither is stocked in the same breadth as 304 stainless or aluminum 6061. Raw material lead times of 4–8 weeks are not unusual for bar, sheet, or plate, particularly for Invar in larger cross-sections or Kovar in tight-tolerance rod. Engineers who discover this late in program schedules pay premium expedite fees or delay program milestones. Factor material procurement into your schedule early.
From a DFM (design for manufacturability) standpoint, several design choices disproportionately affect cost and yield on these alloys. Avoid thin unsupported walls — both materials are prone to chatter and deflection during machining of thin-wall features. Minimum recommended wall thickness for precision Kovar or Invar parts is typically 0.030 inch for features under 0.5 inch tall. Avoid blind tapped holes in Invar where possible — chip packing and tap breakage rates are high. Through-holes with thread inserts are a more reliable design alternative for high-volume production.
Tolerances tighter than ±0.001 inch on Invar require careful process planning, including multiple stress-relief cycles, temperature-controlled machining environments, and CMM inspection at multiple stages — not just at final inspection. These requirements should be communicated explicitly on the engineering drawing, not assumed.
Nimble’s certified partner network includes shops with demonstrated experience on Kovar and Invar for aerospace and defense electronics applications. AS9100 and ITAR registration means the documentation, traceability, and process controls required for these programs are already in place. A free DFM review before quoting can catch manufacturability issues before they become production problems.

⚠️ PROCUREMENT WARNING: Invar 36 and Kovar raw material lead times frequently run 4–8 weeks. If your program schedule does not account for material procurement, you will miss milestones. Place material orders as soon as design intent is confirmed — do not wait for final drawing release.

Kovar and Invar components appear predominantly in aerospace, defense, and high-reliability electronics programs — environments where inspection rigor and material traceability are contractual requirements, not optional quality practices. Understanding what the inspection and documentation requirements look like helps procurement teams qualify suppliers correctly and avoid costly nonconformance events.
Material certification to ASTM F15 (Kovar) or ASTM F1684 (Invar 36) with chemical composition and mechanical property documentation (mill cert / CMTR) should be a baseline purchase requirement. For ITAR-controlled programs, material sourcing documentation and chain-of-custody records are additional requirements. Confirm that your supplier maintains this documentation and can produce it on demand.
Dimensional inspection of tight-tolerance Kovar and Invar parts should be performed on a CMM (coordinate measuring machine), not with manual gauging alone. CMM inspection provides the measurement uncertainty documentation required for first article inspection (FAI) reports under AS9102 and is essential for establishing whether a process is in control across a production run. For CTE-sensitive assemblies, dimensional inspection should ideally be performed at a controlled temperature (20°C per ISO 1 standard) since the dimensional state of an Invar part at 15°C versus 25°C differs measurably on parts with tolerances in the low-ten-thousandths of an inch range.
Nimble includes CMM inspection with production orders as a standard element of the quality package — not as an upsell. For aerospace and defense procurement teams evaluating suppliers, that baseline expectation matters. Request a quote with a free DFM review to get a full picture of process and quality plan before committing to a supplier.

READY TO SOURCE?

Table of Contents

Related Services

Get a Quote in 24h

Upload your drawing and get a detailed quote with free DFM review.