A hermetic package seals a die inside an airtight cavity made of metal, ceramic, or glass, blocking the moisture and contaminants that kill high-reliability electronics. Plastic packages can’t do this — organic compounds are permeable to water vapor. This guide covers the two families, ceramic and metal-can, plus how each is sealed, how hermeticity is proven under MIL-STD-883, and when the cost is worth paying.
[IMAGE 1: cutaway of a TO metal-can and a CERDIP ceramic package side by side, showing die cavity, lid, seal ring, and leads | alt: “Hermetic packages compared: metal-can TO package and CERDIP ceramic package cutaways showing die cavity and seal”]
Key takeaways
- A hermetic seal is not zero-leak. It’s a leak rate low enough that the internal atmosphere stays dry for the design life — verified, not assumed.
- The two families are metal-can (TO) and ceramic (CERDIP, side-braze, LCC, ceramic QFP/PGA). Both use inorganic, moisture-impermeable materials.
- Kovar (54% Fe, 29% Ni, 17% Co) dominates because its CTE (~5.0–5.5 ppm/°C) matches sealing glass and alumina, so the seal survives thermal cycling.
- Seal methods trade off temperature and tightness: weld sealing gives the lowest leak rates; 80Au20Sn solder seals at 280 °C but typically leaks about a decade more; CERDIP seal glass fuses near 430–470 °C.
- Hermeticity is graded by helium fine-leak and fluorocarbon gross-leak testing per MIL-STD-883 Method 1014, with internal moisture capped at 5,000 ppm (Method 1018).
What makes a package hermetic
A hermetic package is an enclosure sealed with metal, ceramic, or glass so that gases and liquids cannot reach the die cavity. Because the materials are inorganic, it blocks moisture far better than plastic and tolerates higher temperatures. Hermeticity is specified as a maximum leak rate, since every real seal leaks at some rate — the goal is to keep it negligible over the product’s life.
That last point trips up newcomers. “Hermetic” does not mean perfectly sealed. It means the leak rate is slow enough that the cavity stays dry longer than the device needs to operate, which is why hermeticity is a measured number, not a yes/no property.
Hermetic vs plastic: why the cost exists
Plastic (non-hermetic) packages are cheaper, lighter, and fine for most consumer and industrial parts. Their weakness is moisture. Epoxy mold compound absorbs water vapor, and over years that moisture drives galvanic corrosion of bond wires and metallization, and enables ionic contamination.
Hermetic packages are specified when failure is expensive or unreachable: aerospace and defense, space (vacuum, radiation, no repair), medical implants, downhole and high-temperature electronics, MEMS, and optoelectronics where a clear or vacuum cavity is required. A key reliability metric is internal moisture: MIL-STD-883 Method 1018 caps internal water vapor at 5,000 ppm (0.5%), the level below which surface corrosion is suppressed for the device lifetime. Plastic can’t hold that; a sealed ceramic or metal cavity can.
Metal-can (TO) packages
The transistor-outline (TO) metal can is the oldest hermetic format: a metal header carrying glass-sealed lead feedthroughs, with a metal cap welded over the die. Common bodies include the small cylindrical TO-18, TO-46, TO-5, and TO-39, and the large power TO-3 with a bolt-down flange.
The header is usually Kovar, chosen so its thermal expansion matches the glass feedthroughs. Sealing is done by one-shot resistance (projection) welding — a short, high-energy pulse melts the cap-to-header joint locally without heating the die, which lets the assembler control the internal atmosphere and backfill dry nitrogen. TO cans still house discrete transistors, precision references, laser diodes, photodiodes, crystal oscillators, and MEMS sensors, where a small hermetic cavity beats any plastic option.
Ceramic hermetic packages
Ceramic packages use an alumina (Al₂O₃, typically 90–96%) body with metallized traces and a sealed lid. They scale to far higher pin counts than metal cans and are the backbone of hermetic ICs.
| Package | Construction | Seal method | I/O range | Notes |
| CERDIP | Alumina base + cap around a leadframe | Seal glass (glass frit) | ~8–48 | Lowest-cost hermetic DIP; glass is the weak point |
| Side-braze ceramic DIP | Multilayer alumina, pins brazed to sides | Solder/weld metal lid | ~8–64 | Better electricals; row spacing 0.300–0.900 in |
| Ceramic flatpack (Cerpac) | Alumina body, unformed leads two sides | Seal glass or solder | ~10–224 | Lightweight; lead spacing 0.015–0.050 in |
| CERQUAD / CQFP | 92% alumina, leads four sides | Seal glass or solder | 24–256+ | Flat, J-bend, or gull-wing leads |
| LCC / CLCC | Leadless, castellated pads 4 sides + bottom | 80Au20Sn combo lid | 8–100 | Pad pitch 50 mil or 40 mil |
| Ceramic PGA | Multilayer alumina, pin array underneath | Solder/weld metal lid | up to ~559 | High I/O, strong thermal path |
CERDIP is the classic hermetic dual in-line package: a leadframe sandwiched between a preglazed ceramic base and cap, fused with seal glass. It is inexpensive but the glass seal is mechanically the weakest element. The side-brazed package instead brazes metal pins to a multilayer ceramic body and seals a metal lid over a metallization ring, giving cleaner high-frequency performance at higher cost. The leadless ceramic chip carrier (LCC) replaces leads with metallized castellations and is common in surface-mount military assemblies, often sealed with a gold-tin combo lid.
[IMAGE 2: exploded view of a side-brazed ceramic package showing ceramic base, metallization seal ring, solder preform, and Kovar lid | alt: “Side-brazed ceramic hermetic package exploded view: ceramic base, seal ring, solder preform, Kovar lid”]
The materials that make the seal work
Hermetic sealing is a CTE-matching problem. Join two materials whose expansion coefficients differ too much, and thermal cycling cracks the seal.
Kovar is the workhorse: an Fe-Ni-Co alloy, roughly 54% iron, 29% nickel, 17% cobalt, with a CTE near 5.0–5.5 ppm/°C. That matches borosilicate sealing glass and sits close to alumina at about 7 ppm/°C (Kovar contracts about 6.4 ppm/°C versus alumina’s 7.1 ppm/°C over the seal range). Kovar also welds cleanly and is nickel-plated for corrosion resistance.
Glass-to-metal seals come in two forms. A matched seal pairs materials of similar CTE — Kovar body with a matched glass such as Corning 7056 alkali borosilicate — leaving low residual stress and suiting rectangular shells. A compression seal deliberately uses a high-CTE body (often stainless steel) that shrinks onto the glass on cooling, squeezing it into a mechanically stronger, higher-pressure joint. Matched seals are easier to make in complex shapes; compression seals win on ruggedness.
How hermetic packages are sealed
The seal method sets the achievable leak rate, the process temperature the die must survive, and the cost.
| Method | Materials | Process temp | Typical leak result | Best for |
| Resistance seam / one-shot weld | Kovar lid + seal ring | Localized; die stays cool | Lowest leak rates | Metal cans, ceramic w/ Kovar frame |
| Gold-tin (80Au20Sn) solder seal | AuSn preform, Au/Ni-plated lid | 280 °C (eutectic) | ~1 decade above weld sealing | LCC, ceramic combo lids |
| Seal-glass (frit) sealing | Solder glass, preglazed ceramic | ~430–470 °C, ~10–20 min | Good, glass-limited | CERDIP, low-cost ceramic |
| Brazing | Kovar ring to alumina | High (braze alloys) | Excellent | Seal rings, feedthroughs |
| Laser / e-beam weld | Kovar, refractory metals | Localized | Excellent | Irregular shapes, refractory metals |
Seam welding rolls two electrodes around the lid perimeter, fusing a Kovar cap to the seal ring with minimal heat to the die. 80Au20Sn is a hard, high-strength eutectic that melts at 280 °C and can seal under a reducing atmosphere without flux; it is roughly 10–20× stronger than tin-lead solders, per Indium Corporation. One trade-off worth knowing: seam AuSn solder sealing typically yields fine-leak rates about a decade higher than pure weld sealing, per MicroCircuit Laboratories. CERDIP seal glass fuses near 470 °C, which is too hot for some sensitive dies and is why the glass, not the ceramic, usually limits CERDIP reliability.
How hermeticity is verified
You cannot ship a hermetic part on faith. MIL-STD-883 Method 1014 (Seal) defines the accept/reject procedure, and it runs in two stages.
Fine leak detects slow leaks using helium as a tracer. Parts are pressurized (“bombed”) in helium, then moved to a mass-spectrometer chamber under vacuum; escaping helium is measured against a limit computed from the package’s internal free volume. Helium is used because its small atom passes fine defects, it’s inert, and it’s rare in air so it’s easy to detect. Mass-spec systems read down to about 5 × 10⁻⁹ atm·cc/s and can resolve leaks near 10⁻¹² atm·cc/s.
Gross leak catches catastrophic leaks the fine test misses, typically by fluorocarbon immersion — heating the part in a liquid and watching for a bubble stream. The standard leak rate is defined for dry air at 25 °C with 1 atm on the high side. Reject limits scale with internal free volume: small cavities are held near 5 × 10⁻⁸ atm·cc/s helium, while typical aerospace practice calls for 1 × 10⁻⁷ atm·cc/s or better, per Glenair’s hermeticity guidance. Fine leak is always run before gross leak.
Two more controls matter. Method 1018 limits internal water vapor to 5,000 ppm, verified by cavity gas analysis. And for procurement, hermetic microcircuits are typically bought to QML standards — MIL-PRF-38535 for monolithic ICs and MIL-PRF-38534 for hybrids — with discretes under MIL-STD-750 / MIL-PRF-19500.
Master comparison table
| Family | Seal method | Mount | I/O range | Rel. cost | Typical use |
| Metal-can (TO) | One-shot weld | THT / SMT | 2–~12 | Low–med | Transistors, sensors, laser/photodiodes, oscillators |
| CERDIP | Seal glass | THT | 8–48 | Low | Legacy mil ICs, EPROM with quartz window |
| Side-braze ceramic DIP | Solder/weld lid | THT | 8–64 | Medium | Precision analog, RF, mil-grade logic |
| Ceramic flatpack (Cerpac) | Seal glass / solder | SMT | 10–224 | Medium | Lightweight avionics, space |
| CERQUAD / CQFP | Solder / seal glass | SMT | 24–256+ | Med–high | High-pin mil/aero ICs |
| LCC / CLCC | AuSn combo lid | SMT | 8–100 | Medium | Surface-mount military assemblies |
| Ceramic PGA/BGA | Solder/weld lid | THT / SMT | up to ~559 | High | High-I/O processors, FPGAs, ASICs |
How to choose a hermetic package
Run the decision in this order.
- Confirm you actually need hermeticity. If a plastic part meets the moisture, temperature, and reliability spec, use it. Reserve hermetic packages for harsh, high-reliability, or unreachable applications.
- Match I/O to the family. A few pins fit a metal can. Tens to hundreds of pins push you to ceramic DIP, flatpack, CQFP, or ceramic PGA/BGA.
- Set the assembly method. Through-hole leans CERDIP, side-braze, or PGA; surface mount leans LCC, Cerpac, or CQFP.
- Check the die’s temperature limit against the seal. A die that cannot survive ~470 °C seal glass points to weld or 280 °C AuSn sealing instead.
- Specify verification and sourcing up front. State the Method 1014 leak limit, the Method 1018 moisture cap, and the QML class (38535 or 38534) in the requirement, not after tape-out.
[IMAGE 3: decision-path flowchart from “need hermeticity?” through I/O, mount, die temperature, to a package family and seal method | alt: “Hermetic package selection flowchart: need, I/O count, mounting, die temperature, seal method”]
Cost, lifecycle, and second-source reality
Hermetic parts cost several times their plastic equivalents because of ceramic tooling, precious-metal plating, controlled-atmosphere sealing, and per-unit leak screening. Budget for that early.
Sourcing is narrower. Hermetic and QML parts come from a smaller supplier base, lead times run long, and obsolescence is common as commercial volume moved to plastic decades ago. Two lifecycle traps: pure-tin plating on leads can grow tin whiskers, so mil parts specify tin-lead or gold finishes; and gold-aluminum intermetallics (“purple plague”) can embrittle bonds if the thermal budget is abused. Where a hermetic second source is thin, qualify the package and seal method — not just the die — so an alternate assembler is viable.
Design mistakes that cause hermeticity failures
- CTE mismatch at the seal. Pairing a high-expansion lid or body with the wrong glass cracks the seal under thermal cycling. Match to Kovar-compatible materials.
- Porous or thin seal glass. CERDIP seals fail through voids and poor leadframe-to-glass adhesion. Control the glass process and seal geometry.
- Loose particles inside the cavity. Conductive debris causes intermittent shorts; PIND (particle impact noise detection) testing per MIL-STD-883 screens for it.
- Trapped moisture at seal. Sealing in humid ambient exceeds the 5,000 ppm internal limit. Seal under dry nitrogen and verify with cavity gas analysis.
- Gold-plating embrittlement. Excess gold in a solder joint or abused thermal budget forms brittle intermetallics. Control plating thickness and reflow profile.
- Over-torquing a TO-3 or stud package. Crushing the flange distorts the header and can breach the glass feedthroughs.
[IMAGE 4: micro-cross-section of a glass-to-metal feedthrough showing crack initiation at a CTE-mismatched interface | alt: “Glass-to-metal seal cross-section showing crack from CTE mismatch in a hermetic package feedthrough”]
FAQ
What is a hermetic package?
A hermetic package is a sealed enclosure — metal, ceramic, or glass — that keeps gases and liquids out of the die cavity. Because the materials are inorganic and moisture-impermeable, it protects electronics far better than plastic and tolerates higher temperatures. Hermeticity is specified as a maximum leak rate, since every real seal leaks slightly.
What is the difference between hermetic and non-hermetic packaging?
Non-hermetic (plastic) packages use epoxy mold compound that slowly absorbs moisture, which over years corrodes bond wires and metallization. Hermetic packages seal the die in an airtight metal or ceramic cavity that blocks moisture and withstands higher temperatures. Plastic is cheaper and lighter; hermetic is for harsh, high-reliability, or long-life applications.
What materials are used for hermetic sealing?
The common body is Kovar, an iron-nickel-cobalt alloy whose thermal expansion matches sealing glass and alumina. Ceramic packages use alumina bodies. Seals use borosilicate glass (such as Corning 7056), solder glass for CERDIP, and 80Au20Sn gold-tin solder for lids. Kovar’s CTE-matching is what keeps seals intact through thermal cycling.
How is hermeticity tested?
Under MIL-STD-883 Method 1014, in two stages. A fine-leak test pressurizes the part in helium and measures escaping gas with a mass spectrometer, resolving leaks below 1 × 10⁻⁷ atm·cc/s. A gross-leak test immerses the part in hot fluorocarbon and watches for bubbles. Reject limits depend on the package’s internal cavity volume.
What is a CERDIP package?
A CERDIP (ceramic dual in-line package) is a hermetic DIP made from an alumina base and cap fused around a metal leadframe with seal glass. It is the lowest-cost hermetic DIP and was widely used for military-grade logic and windowed EPROMs. Its seal glass, which fuses near 470 °C, is usually the weakest reliability element.
Why are hermetic packages used in aerospace and military electronics?
These applications face wide temperature swings, vibration, vacuum, radiation, and no option for repair. Moisture ingress into a plastic part would cause corrosion and failure over the long service life. A hermetic ceramic or metal package keeps the cavity dry and stable, and the parts are qualified to MIL-STD-883 and QML standards that guarantee screened reliability.
The bottom line
Reach for a hermetic package only when the environment or reliability requirement earns the cost — then specify it completely. For a handful of pins in a harsh or sealed-for-life application, a welded metal can with dry-nitrogen backfill is the simplest choice. For higher pin counts, pick the ceramic family that matches your mounting and I/O, and let the die’s temperature limit choose the seal: weld or 280 °C AuSn for sensitive parts, seal glass where cost rules. Whatever you choose, write the Method 1014 leak limit, the 5,000 ppm moisture cap, and the QML class into the requirement so hermeticity is verified on every unit, not hoped for.
Internal links: electronic component packages (types overview) · moisture sensitivity level (MSL) · MIL-spec / QML component sourcing · MEMS and sensor packaging · crystal and oscillator packages.
External primary sources: TI/National app note SNOA280 “Hermetic Packages” (ti.com); SCHOTT hermetic packaging guide (schott.com); MIL-STD-883 Method 1014 Seal (q-tech.com PDF); Indium Corporation 80Au20Sn properties (indium.com); Inseto gold-tin soldering in electronic packaging (inseto.co.uk).
