What Are Hermetic Seals?

Hermetic seals are gas-tight and moisture-impermeable enclosures that protect sensitive components from environmental contamination while allowing electrical, optical, or mechanical signals to pass through the sealed boundary. A device is considered hermetically sealed when its packaging prevents both the ingress of water vapor and other reactive gases and the egress of internal gases or fluids over the intended service life. Hermeticity is expressed quantitatively as a leak rate, typically in units of atmospheric cubic centimeters per second (atm cc/s), with acceptance thresholds defined by application standards. The term derives from the Greek god Hermes Trismegistus, whose alchemical tradition associated sealed vessels with transformation and preservation.

Hermetic sealing is distinguished from non-hermetic approaches such as conformal coatings or potting compounds by its reliance exclusively on inorganic materials at the sealed interfaces. Glass, ceramic, and metal bonds do not absorb or transmit moisture over time in the way that polymer encapsulants do, making them suitable for applications requiring decades of reliable service under temperature cycling, radiation exposure, or continuous immersion.

Sealing Methods and Materials

The primary hermetic sealing techniques are glass-to-metal sealing, ceramic-to-metal sealing, and fusion bonding between glass or ceramic components. In glass-to-metal sealing, borosilicate or aluminosilicate glass is fused to a metal pin or feedthrough in a furnace process, creating a bond whose coefficient of thermal expansion is matched between the materials to prevent stress fracture during temperature cycling. Two types of glass-to-metal seals exist: compression seals, where the metal casing is harder than the glass and compresses it on cooling, and matched seals, where the expansion coefficients are closely matched. Ceramic packages, used widely in integrated circuit packaging, are brazed to metal lids or bases and offer higher thermal conductivity and better dimensional stability than glass. The SCHOTT hermetic packaging technical overview describes the manufacturing processes for glass-to-metal and ceramic-to-metal seals and identifies which methods suit specific application environments.

Testing Standards and Hermeticity Thresholds

Hermeticity testing is governed primarily by MIL-STD-883, Test Method 1014, originally developed for military and aerospace electronic packages and subsequently adopted for medical implants and space applications. Two test regimes are used: fine leak testing, using helium or a helium-krypton mixture as a tracer gas in conjunction with a mass spectrometer leak detector for small cavities, and gross leak testing using bubble detection in fluorocarbon fluids for larger packages. Internal moisture content, measured in parts per million (ppm), is the critical reliability parameter: packages with internal moisture above 5,000 ppm risk condensation on internal conductors under temperature cycling, accelerating corrosion and electromigration. The Hermetic Cover Seal Process Technology document from MicroCircuit Laboratories describes the sealing processes and test procedures used to meet MIL-STD-883 TM 1014 requirements for military microcircuit packages.

Applications in Electronic and Medical Packaging

Hermetic sealing is essential wherever electronics must operate reliably in harsh or biologically sensitive environments over long service intervals. Military electronics in radar systems, guided weapons, and communications equipment require hermeticity to withstand humidity, salt fog, and the temperature swings of outdoor deployment. Implantable medical devices, including cardiac pacemakers, cochlear implants, and neurostimulators, use hermetically sealed titanium canisters with glass-to-metal feedthroughs to isolate the electronics from body fluids indefinitely. Space hardware uses hermetic packages to protect against the vacuum and radiation environment of orbit. The Glenair hermetic connector product documentation illustrates the range of connector and feedthrough configurations used to maintain hermeticity at multi-pin electrical interfaces in aerospace and defense systems.

Applications

Hermetic seals have applications in a wide range of engineering domains, including:

• Military and defense electronics exposed to field environmental conditions
• Implantable medical devices requiring long-term biocompatibility and moisture exclusion
• Space and satellite electronics for vacuum and radiation environments
• Automotive sensors and control electronics in engine compartments
• Optoelectronic components including laser diodes and photodetectors
• Industrial sensors and pressure transducers in process control