What are Hermetic Connectors?
Hermetic connectors are completely sealed against ingress of air or liquid using a glass, ceramic, or composite sealing device or material. Hermetic sealing offers the highest level of protection available against air, gases, chemicals, moisture, debris, and other contaminants. It is an essential feature for many harsh environment connectors, particularly those used in medical, mil/aero, or space applications.
Connectors that are used in harsh environments where liquids, gases, or other contaminants are present must be completely sealed to protect contacts and materials from exposure to elements that could interfere with the integrity of the connection, resulting in dielectric breakdown or degradation of materials through exposure to elements that result in corrosion or condensation inside of a sealed connection. Hermetically sealed connectors come in all types, including circular, rectangular, RF, fiber optic, and high-power configurations. Hermetic sealing is also used to separate pressure differentials on two sides of a vacuum. In some cases, hermetic sealing can be achieved as a modular or configurable feature through the addition of a modular seal to a non-hermetically sealed connector. There are three levels of hermetic sealing: fine, high, and ultra-high vacuum, according to IEC 60068. Hermetically sealed connectors are tested to withstand pressures of 10,000 psi and higher while maintaining a stable pressure differential both inside and outside the assembly.
The term hermetic sealing derives from the ancient Greek figure Hermes Trismegistus, whose writings on alchemy and material properties included a technical procedure in which a mixture of materials was placed in a glass vessel which was completely sealed by fusing the neck closed.
Common methods for creating a hermetic seal:
Glass-to-Metal Sealing (GTMS): Glass-to-metal seals combine metal and glass to create a long-lasting vacuum-tight seal. When heated, glass begins to liquify; when pressed against metal, it adheres and then hardens, creating an impermeable barrier. Glass is an excellent conductor and allows power and signal to pass through the barrier; this is called a feedthrough. In GTMS, the molten glass adheres to the package and pins and insulates the electrical conductors. This is a highly reliable sealing method. In pressure sealing, a preformed glass seal is positioned against a metal connector housing. When pressure is applied during the production process, the metal expands and forms around the glass to create an extremely strong seal.
Ceramic-to-metal seals are used in components that must sustain high amounts of pressure, or where high temperatures, high voltages, or chemicals may impact a glass seal. High temperature co-fired ceramics (HTCC) are more stable under these conditions than glass. Both glass and ceramic seals have been tested to well over 1,000 psi.
O-rings and polymer/epoxy seals are lightweight, inexpensive ways to enhance a physical barrier. These types of seals are not considered true hermetic seals, as they can handle only limited temperature ranges and their materials remain mobile enough to permit some condensation to form inside cavities.
IP Ratings define various levels of waterproofing for components and are useful for determining a connector’s suitability for exposure to water and other common environmental factors. However, most IP ratings are not sufficient for extreme harsh environment applications, such as space, where a vacuum seal is required, or oil & gas applications, where chemical contaminants are present. IP level 8 indicates hermetic sealing against water in depths of more than one meter.
Shell materials used in hermetically sealed connectors include stainless steel, aluminum alloy, and titanium, especially for medical applications.
Conductors can be made from stranded or solid-plated copper and may be insulated. Hermetic connectors may include brass, copper, or nickel iron pin contacts as well as fiber optic cables.
Devices made from plastic cannot be hermetically sealed, as polymeric materials possess diffusive and absorbent characteristics with a high permeation conductivity over time, and the process through which a hermetic seal is achieved typically involves high heat, which damages or changes polymeric materials.
Temperature ranges for hermetic interconnects are wide — typically -100 °C to 200 °C — to enable them to serve in harsh environments.
Production Hermetically sealed connectors are generally produced in small batches.
Testing MIL-STD-883 Test Method 1014 is used to determine hermeticity. To be considered hermetic, the internal moisture content or condensation inside a connector must not exceed 5,000 parts per million (PPM) over the lifetime of the device. At 5,000 PPM, the dew point is far below the freezing point, rendering any residual moisture into ice crystals that would not cause corrosion.
Specifying Guidance This video series from Martec, a Trexon company, offers guidance on how to specify hermetic connectors.
Standards IEC 60068 defines environmental testing of electronic components to assess their ability to perform under extreme environmental conditions.
Markets, Sectors, and Applications:
Equipment for military, space, marine, medical, cryogenics, oil & gas, Test & Measurement, industrial/OEM markets, and other applications that must perform in extreme harsh environments.