hermetic cable assembly | Insights by WEIPU

Hermeticity describes a defined resistance to gas or liquid leakage through a sealed electrical boundary; it is not simply the presence of an O-ring, potting compound, or IP rating. A hermetic cable assembly normally uses a feedthrough or termination in which the conductor insulation, seal, and housing form a controlled barrier. The design must account for leakage through interfaces, material permeability, cracks, voids, and the cable jacket itself. IP ratings under IEC 60529 address protection against ingress under specified test conditions, but an IP67 or IP69K result does not by itself establish vacuum or pressure hermeticity. The buyer should therefore specify an allowable leak rate, pressure differential, test medium, test pressure, and acceptance condition.

Helium is used as a tracer gas because it is chemically inert and detectable at very low concentrations by a mass-spectrometer leak detector. In a vacuum method, the assembly or its sealed volume is connected to the detector and helium is applied to the exterior; a response indicates gas entering through a leak path. A pressure or sniffer method may be appropriate when the part cannot be connected directly to a vacuum system. The test fixture, stabilization time, helium concentration, background level, calibration practice, and search pattern affect the result. A meaningful specification states the leak-rate limit and test conditions, rather than saying only that the part was helium tested. Large cable jackets and porous materials can also create outgassing or permeation effects that require method-specific interpretation.

No single sealing material suits every pressure and temperature profile. Glass-to-metal and ceramic-to-metal feedthroughs can provide rigid, low-permeability barriers, but they require controlled mechanical design because differences in thermal expansion can create stress. Elastomeric seals are useful where repeated mating or movement is required, yet their compression set, gas permeability, chemical compatibility, and low-temperature flexibility must be checked. Epoxy or other potting systems depend heavily on formulation, cure, adhesion, void control, and compatibility with the cable jacket. Pressure cycling can expose microcracks, seal relaxation, and interface movement that a single static test may miss. Specify the temperature ramp, dwell time, pressure range, cycle count, fluids, vibration, and post-test leak criterion before selecting materials.

A sealed feedthrough does not automatically retain the cable’s original voltage or current rating. Current capacity depends on conductor size, contact resistance, terminal geometry, ambient temperature, heat dissipation, bundle density, and the thermal path through the seal and housing. Voltage withstand and insulation resistance can be affected by contamination, voids, humidity, altitude, creepage, clearance, and the dielectric properties of the sealing system. Derating may therefore be required when several circuits operate together or when the assembly is installed in a hot enclosure. The design review should define continuous and peak current, voltage, frequency, conductor count, insulation system, temperature rise limit, dielectric withstand, and insulation-resistance test conditions. Ratings should be supported by the applicable component and assembly test evidence.

Usually, a conventional industrial connector cannot be assumed suitable for hermetic service simply by adding a gasket or sealant. A connector’s mating seal may protect against environmental ingress while still allowing gas leakage through contacts, backshells, cable jackets, threaded joints, or pressure-relief paths. Conversion can also affect contact alignment, creepage, serviceability, torque, thermal behavior, and qualification status. A purpose-designed hermetic feedthrough, sealed bulkhead, or molded cable interface may be more appropriate when a specified leak rate is critical. Standard industrial connectors can remain useful on the atmospheric side of a pressure boundary, provided the complete architecture separates the hermetic barrier from the ordinary mating interface. Any modification should be treated as a new configuration requiring engineering review and validation.

Request a dimensioned interface drawing showing mounting features, sealing surfaces, cable exit geometry, allowable tolerances, conductor identification, mating details, and installation orientation. The technical specification should also identify materials, plating, temperature range, electrical ratings, pressure or vacuum limits, bend constraints, environmental exposures, and the defined leak-rate acceptance limit. Ask for the proposed test procedure covering leak testing, insulation resistance, dielectric withstand, continuity, thermal cycling, pressure cycling, vibration, and any application-specific exposure. Test samples should be traceable to the drawing revision and manufacturing configuration; otherwise, a report may not represent the production assembly. Also confirm sampling level, retest rules, inspection records, MOQ, lead time, and change-control responsibilities before placing a bulk order.