A hermetic package is a sealed cavity that keeps the outside atmosphere away from the die inside it, to a leak rate you can qualify and put on a drawing. The word sounds binary — sealed or not — but it is a rate, not a state. A package is hermetic to a specified leak rate or it is not hermetic at all, and the difference between "the lid is on" and "the cavity is qualified" is the whole subject.
Hermetic means a number, not a lid
Every seal leaks a little. The question a hermetic specification answers is how slowly — slowly enough that the cavity atmosphere the part was sealed with is still the atmosphere it has at the end of its service life. That rate is measured, not assumed, and the standard method for measuring it is MIL-STD-883 Method 1014, which runs a fine-leak test for slow paths and a separate gross-leak test for large breaches. We cover how that testing works, and why one test cannot catch both kinds of leak, in what Method 1014 actually proves.
The consequence for a buyer: a leak rate quoted without the internal cavity volume beside it is half a specification. A small cavity tolerates less absolute leakage than a large one for the same service life, because the same flow changes a small internal atmosphere faster.
What actually makes the seal
A hermetic package is a materials problem before it is a process problem, because the seal has to survive every temperature change in service without the joint working itself open. The parts that do that are chosen for how they expand, not for how strong they are.
- Kovar — a nickel-cobalt-iron alloy whose thermal expansion is close to that of certain hard glasses and ceramics. It is the default lid and body material precisely because it moves at nearly the same rate as what it is sealed to.
- Ceramic bodies — alumina and aluminum nitride packages give an inherently inorganic cavity, with nothing inside to outgas.
- Glass-to-metal seals — how an electrical connection crosses a sealed wall without opening a leak path. A conductor passes through a glass insulator sealed into a metal body. That component, and why the feedthrough count is a mechanical constraint, is covered in glass-to-metal seals and Kovar lids.
- The closure itself — solder and epoxy seals, eutectic seam sealing in Au-Si, Au-In and Sn-Ag systems, seam welding, and controlled inert-atmosphere backfill of the cavity before it closes.
One rule runs through all of it: a hermetic build wants nothing organic sealed inside the cavity. An epoxy seal or an organic die attach can outgas for the life of the part, so the seal ends up holding the wrong atmosphere in perfectly. That is why eutectic and metal-system attach show up so often in hermetic work.
When you need one — and when you do not
Hermeticity buys protection from moisture and reactive gas. So you need it when the die cannot tolerate either over its service life:
- Moisture-sensitive or corrosion-prone die and metallization, especially over long field life.
- MEMS and sensors with a moving element or a reference cavity that a changing internal atmosphere would detune.
- Optoelectronics and other devices where surface contamination shifts performance.
- Harsh-environment and long-life hardware where a plastic overmold's slow moisture ingress is not acceptable.
And you do not need it when a well-chosen plastic or open construction already meets the mission. Hermetic sealing adds cost, cavity design, and a qualification step; specifying it for a part that never sees moisture stress buys process complexity and no reliability. The honest question is what the die cannot survive, not whether hermetic sounds safer.
Where to start
Three facts scope a hermetic build:
- What the die cannot tolerate — moisture, oxygen, or a shifting cavity atmosphere — which is what decides whether you need a hermetic cavity at all.
- What is inside the cavity, because anything organic changes the attach and seal strategy regardless of the leak numbers.
- The internal cavity volume and the environmental profile, since the acceptable leak rate depends on both.
We run Kovar lids, glass-to-metal seals, seam welding, eutectic seam sealing and solder or epoxy seals with inert-atmosphere backfill in Halethorpe, Maryland, on US soil, with hermetic testing available on request. More on our hermetic packaging services page.
Answered.
What does hermetic mean for a semiconductor package?
It means the sealed cavity holds its internal atmosphere to a specified leak rate for the part's service life. Hermeticity is a qualified rate, not a yes/no state — a package is hermetic to a leak rate or it is not hermetic. The rate is measured, most commonly by MIL-STD-883 Method 1014 fine and gross leak testing.
What materials are hermetic packages made from?
Materials chosen for matched thermal expansion so the seal survives temperature cycling: Kovar lids and bodies, alumina and aluminum nitride ceramics, and glass-to-metal feedthroughs to carry connections through the wall. The closure is a solder, epoxy, seam-weld or eutectic seam seal in Au-Si, Au-In or Sn-Ag, usually over an inert-atmosphere backfill.
Why does a hermetic cavity avoid organic materials inside?
Because an organic material sealed inside the cavity — an epoxy seal or an organic die attach — can outgas for the life of the part. The seal then works perfectly while holding the wrong atmosphere in. Eutectic and metal-system attach are common in hermetic builds for exactly this reason.
When do I actually need a hermetic package?
When the die cannot tolerate moisture or reactive gas over its service life: moisture-sensitive or corrosion-prone die, MEMS and sensors with a reference cavity, optoelectronics, and harsh-environment long-life hardware. If a well-chosen plastic or open construction already meets the mission, hermetic sealing adds cost and a qualification step for no reliability gain.
Do you offer hermetic packaging and sealing in the US?
Yes — Kovar lids, glass-to-metal seals, seam welding, eutectic seam sealing and solder or epoxy seals with inert-atmosphere cavity backfill, in Halethorpe, Maryland, on US soil, with hermetic leak testing available on request.