Choosing Invar, Kovar or Alloy 42 for RF and Microwave Housings

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

An RF or microwave housing rarely stands alone. It carries a ceramic substrate, seals a connector or a window, or both, and the alloy that holds the housing together has to expand at a rate compatible with whatever is bonded, sealed or mounted inside it. That is a different question from “which alloy has the lowest expansion,” and it is the question this guide answers: how to choose between Invar 36, Kovar and Alloy 42 specifically for RF and microwave housing designs, based on the substrate and seal requirements rather than the alloy’s expansion in isolation.

The question is match, not minimum

Invar 36’s fame rests on having a very low coefficient of thermal expansion, typically 1.2 to 1.6 ppm/°C near room temperature. That is the right property when a housing’s job is to hold its own shape, for example a precision waveguide reference structure with nothing bonded to it, covered in Invar for antenna and RF waveguide structures. It is often the wrong property when the housing carries a ceramic substrate or seals to glass, because a housing that barely expands while its substrate expands normally puts the bond or the seal under stress every thermal cycle. Kovar and Alloy 42 exist because RF and microwave hardware frequently needs the second behavior: expansion matched to a ceramic or a glass, not minimized.

Typical CTE of common RF and microwave substrates

Substrate CTE varies by supplier, grade and, for some ceramics, orientation, so treat the figures below as typical planning values and confirm against your substrate’s own datasheet before finalizing a housing material.

Substrate	Typical CTE, ppm/°C	Common use
Alumina (94 to 96 %)	6.5 to 7.5	General RF and microwave circuit substrates, hybrid packages
Aluminum nitride (AlN)	4.0 to 5.5	High power RF substrates needing better thermal conductivity than alumina
LTCC (low temperature co-fired ceramic)	3.0 to 7.0, grade dependent	Multilayer RF modules, filters, integrated passives
Borosilicate sealing glass	4.6 to 5.2	Glass-to-metal seals in connectors and packages

The alloy side of this comparison, specification numbers, UNS designations and typical CTE bands, is collected on the Invar specs reference, which is worth keeping open alongside a substrate datasheet while making this decision.

The spread on LTCC is wide because it depends heavily on the specific glass-ceramic formulation, which is one reason a datasheet-specific number matters more for LTCC housings than for alumina or AlN designs.

The three alloys against those substrates

Alloy	Typical CTE, ppm/°C	Fit to alumina (6.5 to 7.5)	Fit to AlN (4.0 to 5.5)	Fit to glass seal (4.6 to 5.2)
Invar 36	1.2 to 1.6	Poor, large mismatch	Poor, large mismatch	Poor, large mismatch
Kovar	5.0 to 5.5	Moderate, within 1.5 to 2.5 ppm/°C	Good, close match	Good, close match
Alloy 42	4.0 to 6.0	Moderate, similar to Kovar	Good, close match	Good to moderate

Invar 36 is included mainly to show why it is usually the wrong choice once a substrate is involved. Kovar and Alloy 42 both land close enough to alumina, AlN and typical sealing glass to be workable housing materials, and the choice between the two usually comes down to secondary factors covered below rather than to CTE alone.

Decision table by housing requirement

Housing requirement	First choice	Why	Watch out for
Precision reference structure, no bonded substrate	Invar 36	Lowest expansion, holds its own geometry	Do not extend this choice to a housing with a substrate bonded inside
Housing with alumina or AlN substrate, no glass seal	Kovar or Alloy 42	Both track the substrate reasonably well	Confirm the substrate’s actual CTE from its datasheet, not a generic figure
Housing with a glass-to-metal seal for a connector or window	Kovar	Established track record sealing to borosilicate glass at sealing temperature	See Kovar glass to metal seal machining for the surface prep the seal needs
High-volume housing with thin stamped sections	Alloy 42	Wider strip availability, generally lower material cost	Machined features on thin Alloy 42 need the fixturing approach in Alloy 42 for lead frames
Housing that must also minimize weight	None of the three is light	All three are denser than aluminum	Weight tradeoff is inherent to the alloy family, not a choice between them

Cost and machinability

Factor	Invar 36	Kovar	Alloy 42
Relative material cost	Baseline	Somewhat higher, cobalt content	Generally lower than Kovar
Stock availability	Widest in bar and plate	Good, common in electronic packaging	Best in strip and sheet, more limited in bar
Machinability	Gummy, work hardening, standard recipe	Same recipe as Invar 36, slightly tougher	Same recipe, usually thinner parts
Typical heat treatment before use	Stress relief, sometimes full stabilization	Decarburizing anneal before sealing	Anneal, often done on the strip before machining

None of these differences should override a genuine CTE mismatch. A cheaper alloy that mismatches the substrate by several ppm/°C is not a cost saving once the bond or seal fails in the field. Cost and machinability are the tiebreakers between Kovar and Alloy 42 once both are acceptable CTE matches, not a reason to substitute Invar 36 where a match is required. The full comparison of all three alloys outside the RF-specific context is in Invar 36 vs Kovar vs Super Invar.

Mixed-material housings

Not every housing has to be one alloy end to end. A common pattern in RF and microwave hardware is a structural frame in one material with a smaller insert or window in another where the substrate or seal actually lives. An Invar 36 reference structure that positions a waveguide feed relative to a reflector, discussed in Invar for antenna and RF waveguide structures, might carry a small Kovar window where a hermetic connector passes through, rather than making the whole structure Kovar to accommodate one feedthrough. The interface between the two materials then needs its own design attention, typically a mechanical joint sized to tolerate the difference in expansion between the two alloys rather than a rigid bond that would crack.

This mixed approach is worth considering whenever only a small fraction of the housing actually touches a substrate or a seal. It avoids paying for Kovar’s cobalt content or Alloy 42’s different stock forms across a whole structure when only one feature needs the matched expansion, at the cost of an extra interface to design and qualify. Whether that tradeoff is worth it depends on the size of the mismatched section relative to the whole housing, and on whether the assembly can tolerate a mechanical joint at all or needs to be one continuous piece for structural or RF shielding reasons.

Confirming the substrate CTE before committing to an alloy

The CTE figures in the tables above are typical handbook or datasheet values, and a substrate supplier’s own datasheet number is itself often a nominal figure across a temperature range rather than a guaranteed value for every lot. On a design where the mismatch budget is tight, it is worth asking the substrate supplier for measured expansion data over your actual operating range, the same way a tight Invar tolerance justifies asking for measured CTE on a heat lot rather than relying on the mill’s chemistry certificate alone, covered in Invar material certification. The cost of that verification is small next to the cost of discovering a substrate lot that runs outside its typical CTE band after housings have already been machined and substrates bonded.

Worked example: housing for an alumina hybrid module

Take a hypothetical RF housing, 60 mm long internally, that carries a bonded alumina hybrid substrate across a −40 to +85 °C operating range, a swing of 125 K. The designer compares Invar 36, Kovar and 6061 aluminum for the housing.

Growth over 60 mm across 125 K = 60 mm × CTE × 125.

Housing material	Typical CTE, ppm/°C	Growth, mm	Mismatch to alumina at about 7.0 ppm/°C, mm
Alumina substrate (reference)	7.0	0.053	0.000
Kovar	5.3	0.040	0.013
Invar 36	1.4	0.011	0.042
6061 aluminum	23.5	0.176	0.123

Values are typical and the calculation is illustrative; a real bond line’s stress depends on the adhesive or braze, not just the raw mismatch. Kovar keeps the housing within about a quarter of the mismatch that Invar 36 would produce, and well inside what aluminum would produce, which is why Kovar is the common choice for a housing bonded directly to an alumina substrate rather than Invar 36’s usual role in structures with no bonded ceramic.

RF conductivity and plating

All three alloys conduct less well than copper or silver, so an RF-active surface, a cavity wall, a launch transition or a connector interface, is typically plated regardless of which of the three alloys is chosen for the housing body. Plating handles the RF loss question; the alloy choice handles the mechanical and thermal compatibility question. Treat them as separate decisions on the drawing rather than assuming the base alloy alone determines RF performance.

Back to the RF and telecom hardware overview

Alloy selection sits alongside custom connectors, hermetic packages and waveguide sourcing as a recurring decision across RF, microwave and telecom hardware programs, covered together on the RF, microwave and telecom hardware overview.

What to put on the RFQ

• Substrate material and its datasheet CTE, not just its name, since grade affects the actual value.
• Whether the housing includes a glass-to-metal seal, and at what temperature it is made.
• Operating temperature range, so the shop and the substrate supplier are working from the same numbers.
• Which surfaces are RF-active and need plating, separate from the base alloy choice.
• Volume expectations, since Alloy 42’s stocked forms may favor it at higher volume where Kovar’s bar stock would not.

Send your housing drawing through the RFQ form, and we match it with up to three shops experienced in machining Invar, Kovar and Alloy 42 for RF and microwave hardware, so the alloy recommendation you get is grounded in parts they have actually cut.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Invar

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Invar Waveguide Component Machining and Supplier Sourcing · Choosing a Supplier
• Invar 36 vs Kovar vs Super Invar: Which Alloy for Your Part · Materials
• Machining Kovar for Glass to Metal Seals: What the Seal Needs · Materials
• Machining Alloy 42 (ASTM F30): Thin Parts, Flatness and Alloy Choice · Materials

All materials guides