Kovar Seal Rings for Medical Device Hermetic Packages
Machining a Kovar seal ring for a medical electronics package is, at the lathe or the mill, no different from machining any other Kovar part. What makes it a different job is everything that happens between the last cut and the moment the ring is sealed or brazed: cleaning, deburring and controlling the surface so the joint forms evenly. This guide covers that machining and surface finish practice. For the underlying reason Kovar is chosen for a hermetic seal in the first place, matched expansion to borosilicate glass and alumina, see Kovar glass to metal seal parts, which is not repeated here. This site is a matching service between buyers and independent machine shops. It does not manufacture parts, and neither it nor a matched shop claims FDA registration, ISO 13485 certification or any other regulatory or quality credential.
Typical seal ring and housing geometries
Most Kovar seal parts in medical electronics packaging fall into a small set of shapes:
- Seal rings and eyelets. Short turned rings, often under 20 mm diameter, that a glass preform fuses into to carry a pin or a window.
- Housing shells. Turned or milled cans for sensors, connector modules and small implantable or wearable electronics, with a step or a flange that a lid seam welds or braze to.
- Lids and covers. Flat or slightly domed plates with a matching flange, machined to a flatness callout so the sealing surface contacts evenly around the perimeter, often joined by brazing or seam welding.
- Feedthrough carriers. Bodies that hold a separate feedthrough assembly and then seal or weld into the larger housing, distinct from the feedthrough itself, which is covered in Kovar hermetic feedthrough machining for medical devices.
The shared feature across all of them is a sealing interface, a ring face, a bore or a flange edge, that carries the tightest tolerance and the surface finish requirement on the drawing, while the rest of the part is ordinary turning or milling.
Size is usually the other distinguishing feature. A ring or eyelet for a small sensor housing can be under 10 mm in diameter and a few tenths of a millimeter thick at the wall, which changes the fixturing problem more than the cutting parameters. Small parts flex under clamping force the way larger ones do not, so a shop turning a batch of seal rings often runs lighter cuts and a softer collet or a wax mount rather than a hard three-jaw chuck, to avoid distorting a thin section that then springs back out of round once it is released.
Machining practice, in brief
Kovar, defined by ASTM F15, cuts like Invar 36 and Alloy 42: soft in the annealed state, gummy, prone to work hardening if a tool rubs, and it produces long chips that need breaking. Shops use sharp carbide or HSS with positive rake, keep the feed up so the edge always takes a real chip and run flood coolant for heat and chip control. The full cutting recipe is in Invar 36 machining and applies to Kovar without change. What a shop adds for a seal part is attention to what the surface looks like when the tool leaves it, not just where the dimension lands.
Cleanliness before brazing or sealing
A seal ring that is dimensionally perfect but contaminated will still leak. Three things matter between the mill and the furnace:
| Contamination source | Why it matters | Typical control |
|---|---|---|
| Sulfur or chlorine extreme pressure additives in coolant | Reacts with the metal or the furnace atmosphere during anneal or braze | Choose a coolant free of these additives for sealing parts |
| Cutting fluid residue on the surface | Carbonizes in the furnace, leaves a film the glass or braze alloy will not wet | Ultrasonic degrease before anneal or braze |
| Burrs at the seal edge or bore | Leaves a void or an uneven gap under the glass or braze alloy | Deburr and inspect the seal edge before cleaning |
None of this is exotic, but it has to be planned before the part is cut, not discovered after a batch fails to seal. State on the drawing or the RFQ that the part is for brazing or glass sealing, and let the shop choose a coolant and a cleaning process that fits.
Dimensional control on the seal interface
The dimension that decides whether a joint holds is rarely the one a first-time buyer thinks to tighten. The table lists what typically matters most, and typical values a stable Kovar part achieves; the shop’s own process and your design requirement govern the final callout.
| Feature | What it controls | Typical value |
|---|---|---|
| Concentricity of bore to OD | Even glass or braze gap around the ring | 0.01 to 0.03 mm typical, tighter with extra passes |
| Flatness of a sealing flange | Even contact for seam weld or braze | 0.02 to 0.05 mm typical over the flange diameter |
| Surface finish on the seal face or bore | How evenly glass or braze alloy wets | Ra 0.8 µm or better typical |
| Edge break at the seal interface | Lets the glass meniscus or braze fillet form without a stress riser | Small consistent chamfer or radius, stated on the drawing |
A ring held to a tight bore tolerance but with an uneven flatness on the mating flange will still leak unevenly around its circumference. General tolerance background for the alloy family is in Invar machining tolerances, and Kovar’s corrosion behavior after machining, relevant if the ring sits unplated for any length of time, is covered in Invar passivation.
Worked example: a tolerance stack on a seal ring assembly
Take a hypothetical seal ring, 10 mm bore, that seals a glass preform carrying a 2 mm pin, with a target radial glass gap of 0.20 mm around the pin. If the ring bore holds ±0.02 mm and the pin position within its own carrier holds ±0.03 mm, and the two errors are treated as worst case rather than statistical, the radial gap could vary by up to ±0.05 mm around its 0.20 mm target, a spread of 25 percent.
| Contributor | Tolerance, mm | Effect on radial gap |
|---|---|---|
| Ring bore diameter | ±0.02 | ±0.01 radial |
| Pin position in carrier | ±0.03 | ±0.03 radial |
| Ring concentricity (bore to OD, if OD locates in a fixture) | ±0.02 | ±0.02 radial |
| Worst case radial gap variation | ±0.05 (about 25% of 0.20 mm target) |
This is a hypothetical illustration of how tolerances stack, not a specification for any real part. A design that depends on a consistent glass gap benefits from tightening whichever contributor is cheapest to control, often the ring’s own bore and concentricity, rather than trying to hold every dimension to the same tight number. The device manufacturer’s own qualification testing, not this calculation, determines whether a given gap and its variation are acceptable.
Where the buyer’s responsibility starts
A machined, clean, well toleranced Kovar seal ring is a component. Whether that component is suitable for use in a specific regulated medical device depends on the device manufacturer’s own material qualification, process validation, sterilization compatibility testing and quality system, none of which a machine shop or a matching service can establish on the buyer’s behalf. Sterilization method, any plating applied after sealing and any overmold or coating added downstream can all affect how the finished assembly performs, and each of those is evaluated by the device manufacturer as part of the whole device, not by the raw machined ring in isolation. If your program requires supplier documentation, traceability or a specific quality certification, request it directly from the shop and verify it before committing production quantities. The reference page of Invar and Kovar specifications lists the material specs a mill certificate should cite. A fuller checklist of what to verify in a supplier is in Supplier quality requirements for Invar and Kovar medical parts, and other considerations across the vertical are on the medical device manufacturing overview.
What to put on the RFQ
A Kovar seal ring quote is accurate only when the shop knows the part is going to be sealed. Include:
- Alloy and spec. Kovar to ASTM F15, UNS K94610, with supply form.
- Seal features and their tolerances. Bore, flange or face, with concentricity, flatness, surface finish and edge break called out.
- Delivery condition. Machined and cleaned only, or annealed and oxidized to a referenced procedure.
- Coolant restriction. State that the part is for brazing or glass sealing.
- Quantity and expected repeat volume.
When you send an RFQ with the drawing, we match it with up to three shops that machine Kovar seal parts and understand what a sealing operation needs from the machined surface. Each shop quotes you directly, and you carry out your own supplier qualification before releasing production parts.
Frequently asked questions
What is a Kovar seal ring used for in a medical device?
A short turned or milled ring that a glass preform fuses into, or that a lid brazes or seam welds to, forming a hermetic joint on a sensor housing, a connector shell or a hermetic package for electronics. The physics of why Kovar is chosen for the joint, matched expansion to glass and alumina, is covered in Kovar glass to metal seal parts, and applies here without repeating it.
Is machining a Kovar seal ring different from machining any other Kovar part?
The cutting is the same: sharp positive rake tools, steady feed, flood coolant, because Kovar is gummy and work hardens. What is different is everything downstream of the mill. A seal ring has to arrive at the sealing or brazing operation clean, deburred and free of a heavy cold worked layer, because that surface becomes the joint.
What coolant should a shop use on a part destined for brazing or glass sealing?
One that cleans off completely and leaves no sulfur or chlorine residue, since both can contaminate a braze joint or a glass seal furnace. The specific coolant is the shop's choice; the requirement to state on the RFQ is that the part is for sealing so the shop selects and cleans accordingly.
Does using a hermetically qualified Kovar ring make the assembly medical grade?
No. A well machined, clean Kovar ring is a component that can hold a seal. Whether the resulting assembly is suitable for a given medical device depends on the device manufacturer's own design validation, biocompatibility testing and quality system, not on any property of the machined ring by itself.
Does this site or a matched shop certify parts as FDA cleared or ISO 13485 compliant?
No. This is a matching service between buyers and independent machine shops. Neither the site nor any matched shop is represented as FDA registered, FDA cleared or ISO 13485 certified. If a certification or a documented quality system is a requirement for your program, request the documentation from the shop directly and verify it before you rely on it.
Reviewed against
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.
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