feed-through

A feed-through is a conductor or bundle of conductors that passes through a hermetically sealed barrier, typically a wall, bulkhead, or enclosure lid, while maintaining the integrity of the seal. The electrical signal or power must cross from one volume to another (often from atmospheric pressure to vacuum, or between pressurized chambers), but the barrier itself cannot be breached. The feed-through accomplishes this by embedding the conductor within an insulating material that fuses to the barrier material, usually ceramic or glass, creating an airtight joint at the boundary.

The most common design uses a ceramic body, typically alumina (Al2O3) or steatite, with embedded conductors. A kovar or molybdenum pin passes through the ceramic, and the ceramic is brazed or sintered to a metal ring that seals against the bulkhead. Alternatively, glass-to-metal seals are used, where borosilicate glass is fused around a metal pin; this approach appears frequently in scientific instruments and vacuum systems. Single-pin feed-throughs handle one signal or ground path. Multi-pin versions carry multiple conductors through a single sealed connection point, reducing the number of holes in the bulkhead.

Feed-throughs must be rated for the pressure differential they will experience. Vacuum systems typically require leakage rates below 10^-9 torr-liters per second; high-voltage applications demand dielectric strength of several kilovolts per millimeter across the insulator. Temperature cycling is a common failure mode, since the conductor, insulator, and housing expand at different rates. This stress concentrates at the ceramic-to-metal boundary, where micro-cracks can initiate and propagate, eventually leading to leakage or electrical failure. Material selection reflects this: kovar (29% nickel, 17% cobalt iron alloy) is chosen specifically because its thermal expansion coefficient closely matches ceramic, reducing strain.

Where feed-throughs appear

The term appears most often in vacuum technology, high-voltage equipment, and aerospace systems. Mass spectrometers, electron microscopes, and particle accelerators rely on them. Power supplies that must feed current into sealed chambers use feed-throughs. High-pressure reactors and autoclaves employ them to bring instrumentation leads and power into the vessel. Military and aerospace applications demand extreme reliability and often specify military specifications such as MIL-DTL-27039 or MIL-DTL-38999 for connectors that incorporate feed-through geometry.

The name reflects the fundamental function: the electrical path feeds through the barrier without violating it. Related terms include bushings (which are less stringent) and hermetic connectors (which emphasize the sealing function). A feed-through differs from a simple terminal or grommet in that it must guarantee an absolute seal, not merely insulate or strain-relieve. In practice, the choice between brazed ceramic, glass-to-metal, or other technologies depends on the operating voltage, temperature range, pressure differential, and frequency of thermal cycling the application will endure.