Kovar Machining for RF and Microwave Hermetic Connectors

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

A catalog RF connector, an SMA, N-type or 3.5 mm interface bought from a distributor, covers the great majority of RF and microwave designs and should be the first choice whenever it fits. Custom Kovar machining enters the picture when it does not: a bulkhead feedthrough with a flange pattern that matches your housing instead of a standard panel cutout, a multi-pin hermetic connector with a pin count or spacing no catalog part offers, or a shell that has to carry a thread, a step or a seal diameter specific to your assembly. This guide covers when a custom Kovar connector body is the right call, how it is machined, and what a manufacturer needs on the RFQ to quote it correctly.

Why a custom body instead of a catalog connector

Standard RF connectors exist because most interfaces are standardized: the mating geometry, the impedance profile and the mounting pattern are fixed by the interface spec. A custom hermetic connector is usually driven by something outside that standard, most often one of these:

• A non-standard mounting flange or bulkhead pattern that has to match an existing housing, chassis cutout or waveguide flange rather than a round panel-mount hole.
• A pin count, pin spacing or pin diameter that a stocked multi-pin hermetic connector does not offer, common on custom sensor or power feedthrough assemblies built alongside an RF path.
• An unusual overall envelope, where space inside the housing limits the connector’s outside diameter or length below what a catalog part allows.
• A combined mechanical and hermetic function, where the same part is a structural bulkhead, a pressure or vacuum seal and an RF or DC feedthrough at once.

None of these change the electrical interface itself, which is usually still a standard coaxial or waveguide mating geometry. What changes is everything around it, and that is a machining job, not a catalog lookup.

Where Kovar fits in the connector

Kovar, the iron-nickel-cobalt alloy covered by ASTM F15 (UNS K94610) and listed alongside Invar 36 and Alloy 42 on the Invar specs reference, is used for the section of a hermetic connector that seals to glass: a feedthrough insert, a pin header or, on smaller connectors, the whole shell. Its typical mean CTE of 5.0 to 5.5 ppm/°C from 30 to 200 °C tracks borosilicate sealing glass and alumina ceramic closely enough that the seal survives repeated thermal cycling without cracking, a property covered in detail in Kovar glass to metal seal machining. Two common constructions show up in RF hardware:

1. Full Kovar shell. The connector body, from the mounting flange to the seal bore, is machined as one Kovar part. Simpler assembly, one CTE to track, but a heavier and more expensive part if most of the shell does not need to be Kovar.
2. Kovar insert in a stainless or brass shell. A separate Kovar feedthrough or window is machined, sealed with its glass or ceramic, then pressed, brazed or welded into an outer shell made from a cheaper, easier-to-machine material. This keeps Kovar to the section that actually needs it.

Either construction is normal. The drawing and the RFQ should state which one is intended so the quote reflects the right material split.

Typical connector features and tolerances

Most custom RF connector bodies combine ordinary turned and milled features with one or two seal-critical dimensions. The table lists typical values seen on hermetic RF connector drawings; the governing drawing and the shop’s quote decide the real numbers for a given part.

Feature	Typical tolerance	Typical surface finish	Why it matters
Outside diameter, mounting shoulder	±0.025 to ±0.05 mm	As machined	Fits the panel or housing bore
Seal bore for glass bead or preform	±0.01 to ±0.025 mm	Ra 0.8 µm or better	Sets the glass gap, covered in Kovar seal parts
Pin diameter through the seal	±0.005 to ±0.01 mm	Ra 0.4 to 0.8 µm	Concentricity to the bore controls glass thickness around the pin
Mounting flange thread or bolt pattern	Per ASME Y14.5 position callout	As machined	Matches the housing’s existing pattern
Overall length	±0.05 to ±0.1 mm	As machined	Usually non-critical unless it sets a cavity dimension

Only the seal bore and pin features usually justify tight tolerance callouts. Everything else on the shell can carry ordinary shop tolerances, and calling out unnecessary precision on the whole part inflates the quote without improving the connector.

Catalog connector vs custom Kovar body

Factor	Catalog connector	Custom Kovar body
Lead time	Days to a few weeks from stock	Weeks, prototype then production
Tooling and NRE	None	Programming, fixturing, sometimes a seal fixture
Fit to existing housing	Only if the housing was designed around it	Matches your flange, pin count and envelope
Unit cost at low volume	Low	Higher, driven by machining and sealing steps
Unit cost at production volume	Stays low	Narrows the gap as fixturing amortizes
Design control	Limited to catalog options	Full control of geometry and pin layout

The decision is rarely close once the mechanical fit is wrong. A catalog connector that does not fit the housing is not a cheaper option, it is a redesign of the housing instead of the connector.

Machining and sealing sequence

A hermetic connector body goes through roughly the same sequence as other Kovar seal parts, described in Kovar glass to metal seal machining, with the connector-specific steps called out:

1. Turning the shell or insert, working from bar stock, holding the mounting features and leaving stock on the seal bore for finishing.
2. Drilling and reaming pin holes, or turning individual pins if the connector uses machined rather than wire pins, with attention to burrs at the seal interface as covered in Invar drilling and tapping, which applies to Kovar as well.
3. Cleaning and decarburizing anneal, removing cutting fluid residue and surface carbon before the part goes anywhere near glass.
4. Glass sealing, usually at a dedicated sealing house, fusing the preform to the pin and bore.
5. Final machining or trimming, if the mounting flange or outer diameter is finished after sealing to protect the seal during the hot process.
6. Plating, typically nickel or gold over the exposed Kovar surfaces, after sealing.

Steps 4 and 6 are frequently subcontracted by the machine shop to a sealing and plating partner rather than run in house. Ask directly who performs each step and whether the shop manages that chain or expects you to.

Protecting the design while sourcing prototypes

A custom connector drawing usually carries information a buyer would rather not spread across an unlimited number of shops: the housing interface it mates to, the pin layout and sometimes the module it belongs to. Before sending a drawing out for quotes, confirm what the shop or matching service does with it, covered in Invar and Kovar NDA and IP protection, and keep the distribution to the number of shops you actually need quotes from rather than a broad blast to every shop in a directory.

Once a shop has proven it can hold the seal quality and the flange tolerance on this connector, most programs still want a qualified second source before committing to single-supplier production, particularly on a telecom or RF program where a supply interruption stops a whole module line. The same first article and sample logic used to qualify the primary shop applies to a second source, described in second source qualification for Invar and Kovar parts: a small sample order measured against the same drawing and, ideally, against a part from the first shop, rather than trusting a second shop’s own claim that it can match the process.

Small-run economics: what drives the price

A custom Kovar connector’s unit price at prototype quantities is driven less by the raw material, which is a small fraction of the cost on a small part, and more by three fixed costs that do not scale down with quantity: programming the turning and drilling operations, building any dedicated fixture for the seal bore or pin holes, and qualifying the sealing house’s process on a new geometry. A 5-piece order and a 25-piece order of the same design often carry a similar per-piece machining cost, because the fixed costs are spread over too few parts either way to see much difference. The gap narrows meaningfully once an order reaches the low hundreds, where the same fixture and the same qualified seal process amortize over enough parts to matter. Buyers planning a production transition should ask the shop to quote a representative production quantity alongside the prototype quantity, even before the design is frozen, so the expected cost curve is visible early rather than discovered after several small orders.

Worked example: bulkhead connector prototype run

Take a hypothetical 8-pin hermetic bulkhead connector for a telecom RF module, with a custom hex flange that matches an existing housing cutout no catalog connector was designed around. The buyer needs 15 prototype units for qualification testing, with a production run of several hundred expected six months later if qualification passes.

Stage	Typical quantity	What is being validated
First article, 2 to 3 pieces	2 to 3	Fit to the housing, pin layout, seal integrity
Prototype lot	12 to 15	RF performance and environmental qualification
Production lot	Hundreds	Cost and lead time at volume

This staged approach is illustrative, not a quote, and a real program’s stage sizes depend on its own qualification plan; see prototype vs production Invar and Kovar machining for how the fixturing and heat treatment built for a prototype run typically carries forward into production without starting over.

Back to the RF and telecom hardware overview

This article is one piece of a broader pattern in RF, microwave and telecom hardware sourcing, covered on the RF, microwave and telecom hardware overview, where custom hermetic parts, waveguide sections and alloy selection all come up on the same programs.

What to put on the RFQ

• Construction type. Full Kovar shell or a Kovar insert in another shell material.
• Mating interface and mounting pattern. Reference the standard interface if one applies, plus the custom flange or bolt pattern drawing.
• Pin count, spacing and diameter, with which features are seal-critical.
• Delivery condition. Machined only, sealed, or sealed and plated, and who performs each step.
• Quantity now and expected production volume, so the shop can plan fixturing that carries forward.
• Environmental and qualification requirements, such as pressure, vacuum or thermal cycling the connector must survive.

Send your drawing and pin layout through the RFQ form, and we match it with up to three shops that machine Kovar hermetic connector bodies and can carry the part from prototype through a production quote.

Frequently asked questions

Reviewed against

• ASTM F15, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy
• Wikipedia, Kovar

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

• Kovar Hermetic Packages for Telecom Electronics Manufacturers · Materials
• Machining Kovar for Glass to Metal Seals: What the Seal Needs · Materials
• Invar for Antenna and RF Waveguide Structures · Materials
• What to Include on an Invar RFQ: A Drawing Checklist · Choosing a Supplier

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