Kovar Feedthroughs for Implantable Medical Electronics
A hermetic feedthrough is one of the few places on an implantable device where a wire has to cross a sealed wall without breaking the seal. Inside a pacemaker case or a neurostimulator header, a Kovar body carries one or more pins through a glass or ceramic insulator that is fused to it, and the whole assembly then joins the rest of the enclosure. This guide covers how that construction works, what an engineer puts on the drawing and how the Kovar body is machined. It is written as general engineering background for someone specifying or sourcing the metal part, not as a claim about the finished device. This site matches buyers with shops that machine Kovar; it does not manufacture implants, and neither it nor a matched shop makes any claim about FDA clearance, ISO 13485 certification or biocompatibility, which are properties of the tested, finished device and its manufacturer’s quality system, not of a raw alloy.
How a Kovar feedthrough is built
A typical feedthrough has four parts working together:
- The pin. A conductor, often a platinum alloy, tantalum or a nickel alloy chosen for conductivity, corrosion resistance and compatibility with the insulator and the intended manufacturing process.
- The insulator. A glass bead or a ceramic disc, chosen so its expansion tracks the Kovar body through the sealing cycle and through the device’s service temperature range.
- The Kovar body. The machined header or can wall that the insulator fuses into and that later welds, brazes or seals to the rest of the enclosure.
- The seal itself. The fused joint between pin, insulator and body, formed at high temperature and then cooled under control so no crack or gap opens as the parts contract at different rates. The general mechanics of this kind of joint are described in the Wikipedia entry on hermetic seals.
The insulator is what actually blocks gas and fluid. The Kovar body’s job is to hold that insulator without stressing it, both during the sealing process and for the life of the device. That is the whole reason the body is Kovar rather than a more common implant alloy such as titanium.
Why the body is Kovar, not titanium or stainless
Titanium is the material of choice for the enclosure of most implantable devices, valued for its strength and long clinical history. It is a poor match for a glass or ceramic seal, because it expands roughly three times faster than the sealing materials. The table lists typical mean CTE values used for planning; the material certificate and the insulator supplier’s data govern any real design.
| Material | Typical mean CTE, ppm/°C | Interval |
|---|---|---|
| Kovar, ASTM F15 | 5.0 to 5.5 | 30 to 200 °C |
| Borosilicate sealing glass | about 4.6 to 5.2 | 0 to 300 °C |
| Alumina ceramic, 94 to 96 % | about 6.5 to 7.5 | 25 to 300 °C |
| Titanium, Grade 1 to 4 | about 8.6 to 9.2 | 20 to 300 °C |
| 316L stainless | about 16 to 17 | 20 to 100 °C |
Kovar sits close enough to both borosilicate glass and alumina that the seal survives cooling from several hundred degrees down to room temperature without cracking. That is the entire reason it appears inside an otherwise titanium device: the header or can is often a titanium to Kovar transition, laser welded after the glass to metal or ceramic to metal seal is made, so the implant body stays titanium where it contacts tissue and the feedthrough stays Kovar where it has to seal to glass or ceramic. The physics of that seal, the anneal and oxidation steps the Kovar needs before sealing, are covered in Kovar glass to metal seal parts and are not repeated here.
What to specify on the drawing
A feedthrough drawing that only calls out the machined dimensions of the Kovar body is missing the properties that actually decide whether the part works. The table lists what an engineer typically adds, with the purpose of each callout.
| Specification | What it controls | Typical way it is stated |
|---|---|---|
| Pin count and position | Electrical routing, insulator layout | Drawing view with true position tolerance per pin |
| Pin material | Conductivity, corrosion, weld or braze compatibility | Named alloy, referenced to its own material spec |
| Insulator material and seal type | Which glass or ceramic, glass to metal or ceramic to metal | Named material, referenced supplier data sheet |
| Helium leak rate | Hermeticity of the finished seal | Maximum rate in atm·cm3/s, per the device’s own test method |
| Insulation resistance | Electrical isolation between pins and body | Minimum resistance at a stated test voltage |
| Dielectric withstand voltage | Insulation margin under overvoltage | Minimum voltage, stated duration, no breakdown |
| Delivery condition of the Kovar body | Who anneals, cleans and oxidizes before sealing | Machined only, or machined and process ready |
None of these numbers come from the raw Kovar. They come from the finished, sealed and tested assembly, which is why the device manufacturer’s own test program, not the machinist’s mill certificate, is what ultimately proves the part meets them.
Machining the Kovar body
The header or can wall that holds the feedthrough is machined the same way any Kovar part is: sharp positive rake tooling, steady feed so the edge always takes a real chip and flood coolant, because Kovar is soft, gummy and work hardens if a tool rubs instead of cutting. The general recipe is covered in Invar 36 machining and carries over to Kovar unchanged. What differs for a feedthrough body is what happens after the mill: a decarburizing anneal, thorough cleaning to remove coolant residue and a controlled oxidation so the glass or ceramic wets the surface evenly. A body that arrives at the sealing house with burrs, cold work or leftover cutting fluid is the most common cause of a rejected seal, not a defect in the Kovar itself.
Kovar itself is defined by ASTM F15, the iron-nickel-cobalt sealing alloy specification. Bores, counterbores and pin holes that become part of the seal typically hold tolerances of ±0.01 to ±0.025 mm, with a controlled edge break so the glass meniscus forms cleanly. General tolerance guidance for the alloy family is in Invar machining tolerances, and the full alloy comparison is in Invar 36 vs Kovar vs Super Invar.
Worked example: sizing a header bore for a 12-pin device
Take a hypothetical 12-pin feedthrough header, a Kovar disc 14 mm in diameter that seals to a ceramic insulator carrier at roughly 500 °C before cooling to room temperature, a drop of about 480 K. The question is how far the Kovar bore and the ceramic insulator want to shrink relative to each other over that drop, since the mismatch is what stresses the seal.
Shrinkage of a 14 mm diameter over 480 K = 14 mm × CTE × 480.
| Material | Typical CTE, ppm/°C | Bore shrinkage, mm | Mismatch to ceramic at about 7.0, mm |
|---|---|---|---|
| Kovar | 5.3 | 0.036 | 0.011 |
| Alumina ceramic | 7.0 | 0.047 | 0.000 |
| Titanium Grade 2 | 8.9 | 0.060 | 0.013 |
| 316L stainless | 16.5 | 0.111 | 0.064 |
Values are typical and the calculation is illustrative; a real design uses the insulator supplier’s expansion curve, not a single room temperature figure. Kovar and titanium land at a similar mismatch here because alumina’s expansion sits between them, which is why some ceramic to metal seals use titanium bodies directly. Where the seal is glass rather than ceramic, Kovar’s advantage over titanium widens, because borosilicate glass tracks Kovar far more closely than it tracks titanium. This is illustrative sizing only. The insulator supplier and the device manufacturer’s own qualification testing determine the actual geometry and the pass or fail leak rate.
What stays the device manufacturer’s responsibility
The Kovar body is one machined component in an assembly that a device manufacturer designs, seals, tests and submits for regulatory clearance under its own quality system. Machining the body correctly, to the alloy specification and the drawing’s tolerances, does not establish biocompatibility, sterility, electrical performance or regulatory clearance for the finished feedthrough or the device it goes into. Those come from the device manufacturer’s own material qualification, process validation and test data. If your quality system requires a supplier with a specific certification, a documented process history or traceable material lots, ask the shop directly for that documentation and verify it yourself before you release production quantities. See /reference/invar-specs/ for the alloy specifications a mill certificate should reference. For the full detail on what to verify before using a shop for a regulated part, see Supplier quality requirements for Invar and Kovar medical parts. For other Kovar and Invar considerations across medical device manufacturing, see the medical device manufacturing overview.
What to put on the RFQ
A Kovar feedthrough body quote is only accurate when the shop knows what happens to the part after it leaves their door. Include:
- Alloy and spec. Kovar to ASTM F15, UNS K94610, with the supply condition.
- Seal features. Which bores and faces seal to glass or ceramic, with tolerances, edge breaks and position callouts.
- Delivery condition. Machined and cleaned only, or annealed and oxidized to a referenced procedure.
- Pin and insulator interface dimensions, even though those parts are supplied separately, so the shop understands the fit.
- Quantity and expected repeat volume.
When you send an RFQ with the drawing, we match it with up to three shops that machine Kovar for hermetic seal applications. Each shop quotes you directly against your drawing, and you evaluate and qualify that shop’s quality system and documentation yourself before committing production parts to a regulated device.
Frequently asked questions
What does a Kovar hermetic feedthrough do?
It carries one or more electrical conductors through the wall of a sealed enclosure without letting gas or fluid pass. A pin sits inside an insulator, usually glass or ceramic, that is fused into a Kovar body. The Kovar body then welds, brazes or seals into the rest of the enclosure. The result is an electrical path with a hermetic barrier around it.
Why is Kovar used for the feedthrough body instead of titanium or stainless steel?
The body has to expand at close to the same rate as the glass or ceramic insulator through the sealing temperature and, for an implant, through sterilization and storage. Kovar's typical CTE of 5.0 to 5.5 ppm/°C tracks common sealing glasses and alumina ceramic. Titanium and stainless steel expand two to three times faster, which stresses the glass to ceramic joint and raises the risk of a leak path.
Does using Kovar make a feedthrough medical grade or implant grade?
No. Kovar is a controlled expansion alloy defined by its chemistry and thermal properties, not a regulatory classification. There is no material property that makes a raw alloy medical grade, implant grade or biocompatible on its own. Biocompatibility and any regulatory clearance apply to the finished, tested feedthrough assembly and to the device it goes into, established by the device manufacturer's own testing and quality system, not by the alloy a shop machines it from.
Does invarmachining.com or a matched shop certify feedthroughs as FDA cleared or ISO 13485 compliant?
No. This site is a matching service, not a device manufacturer or a certification body, and it does not claim that itself or any matched shop holds FDA registration, FDA clearance, ISO 13485 certification or any other regulatory or quality credential. If your application requires a supplier with a specific quality system or a documented history in regulated manufacturing, you need to request that documentation directly from the shop and verify it before you rely on it.
What should the drawing specify beyond the machined dimensions of the Kovar body?
Pin material, count and position, the insulator material and seal type, the required helium leak rate, insulation resistance and dielectric withstand voltage and the delivery condition of the Kovar body: machined only, or machined and ready for the sealing house's anneal and oxidation steps.
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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