Defense and military electronics rely on ceramic-to-metal hermetic seals to protect sensitive electronic components from moisture, pressure, vibration, and thermal extremes over service lives that can exceed 20 years. Ceramic-to-metal brazed feedthroughs and hermetically sealed packages are used in radar systems, missile guidance electronics, undersea sonar equipment, aircraft avionics, and communications hardware. These seals are qualified to MIL-STD-883, MIL-STD-810, and related standards governing hermeticity testing, thermal cycling, shock, and vibration.
Why Hermetic Sealing Matters in Defense Applications
Military electronics operate in environments that would quickly degrade consumer or commercial-grade electronic assemblies. A naval sonar transducer operates at the bottom of the ocean under hundreds of atmospheres of hydrostatic pressure. A missile guidance unit experiences several hundred g of shock during launch. An aircraft avionics box cycles from -55°C on the ground in Arctic conditions to +125°C in the engine bay. An undersea repeater must function for 25 years without any maintenance.
In all of these cases, a failed hermetic seal allows moisture ingress. Moisture at the surface of an unprotected circuit board causes corrosion, ionic contamination, and eventually electrical shorts or opens. Even a few percent increase in relative humidity inside the package over years of service can degrade analog circuits, reduce insulation resistance, and cause time-dependent failures that are difficult to diagnose. A properly designed ceramic-to-metal hermetic seal prevents this by providing a leak rate so low that meaningful moisture ingress does not occur over the life of the equipment.
Standards That Govern Military Hermetic Seals
MIL-STD-883
MIL-STD-883, Test Methods and Procedures for Microelectronics, includes Method 1014 for hermetic seal testing. Method 1014 specifies both fine leak testing (helium mass spectrometer) and gross leak testing (bubble or fluorocarbon tracer) for hermetically sealed microelectronic packages. Acceptance criteria are specified based on the package’s internal volume, with tighter leak-rate limits required for larger packages.
MIL-STD-810
MIL-STD-810, Environmental Engineering Considerations and Laboratory Tests, covers the full range of environmental tests, including temperature, humidity, altitude, shock, vibration, rain, salt fog, and immersion. Hermetically sealed assemblies are typically tested to demonstrate that the seal survives the relevant environmental exposure and that the leak rate remains within specification after testing.
MIL-PRF-28861 and Related Connector Standards
Military connector standards specify hermeticity requirements for hermetically sealed connectors and feedthroughs used in military systems. These standards define allowable leak rates, test methods, qualification testing sequences, and quality conformance inspection requirements.
Ceramic-to-Metal Seal Design for Military Applications
Material Selection
High-purity alumina (96–99.5% Al₂O₃) is the most common ceramic for military hermetic feedthroughs and packages because of its combination of high dielectric strength, low dielectric loss, chemical inertness, and well-established metallization and brazing processes. Beryllia (BeO) is used in some high-power RF applications where thermal conductivity is critical, but strict handling requirements due to powder toxicity have led many programs to move away from it in newer designs. Aluminum nitride (AlN) is increasingly the preferred choice where higher thermal conductivity than alumina is required, offering a safer handling profile with comparable process compatibility.
Kovar and Low-Expansion Alloys
Kovar (UNS K94610), a nickel-iron-cobalt alloy, is the standard metal partner for alumina ceramic seals in military electronics because its thermal expansion coefficient closely matches that of alumina over the -55°C to +125°C military temperature range. This match minimizes differential thermal stress at the braze joint during thermal cycling. For applications with wider temperature ranges or specific corrosion requirements, other controlled-expansion alloys are used.
Braze Alloy Selection
Silver-copper eutectic (72% Ag / 28% Cu, melting point 779°C) is the most common braze alloy for ceramic-to-metal military hermetic assemblies. It provides reliable flow, good wetting of metallized ceramic, and a finished joint with good strength and hermeticity. Gold-based brazes are used in applications requiring higher corrosion resistance or compatibility with specific plating processes. Active metal brazes using titanium or zirconium allow direct bonding to unmetallized ceramic in some specialized designs.
Application Examples
- Radar transmit/receive modules: high-voltage and RF feedthroughs in hermetically sealed T/R module packages maintain hermeticity through millions of operating hours.
- Missile guidance IMUs: hermetically sealed accelerometer and gyroscope packages prevent moisture ingress that would shift bias and scale factor.
- Undersea sonar transducers: ceramic-to-metal seals at the transducer housing wall provide electrical connections that withstand hydrostatic pressure at depth.
- Submarine electronics: hermetically sealed chassis and connector penetrations protect electronics in high-humidity, salt-atmosphere shipboard environments.
- Airborne EW systems: hermetic packages protect sensitive receiver electronics from the wide temperature and pressure swings of high-altitude flight.
Testing and Qualification
Military hermetic assemblies are subjected to fine leak testing per MIL-STD-883 Method 1014 using helium tracer gas and a mass spectrometer detector, with acceptance limits typically ranging from 5×10-8 to 5×10-9 atm·cc/sec, depending on package volume. Gross leak testing uses fluorocarbon immersion or bubble detection to identify large leaks. Thermal shock, thermal cycling, shock, and vibration testing verify that the brazed joint withstands the application’s mechanical and thermal environment.
MPF Products for Defense Applications
MPF Products manufactures ceramic-to-metal hermetic feedthroughs and assemblies for defense and military electronic applications, with documentation packages including helium leak test records, material certifications, and dimensional inspection data. Custom configurations are available for specialized connector interfaces, unusual pin counts, or specific material requirements driven by military qualification programs.