Choosing Hermetic Seals for High-Current and Extreme-Environment Applications

Applications in aerospace and defense, energy and subsea exploration place extraordinary demands on electrical interconnects. These systems operate under extreme thermal fluctuations, high pressure or harsh chemical exposure, and they depend on connectors and feedthroughs that maintain both structural and electrical integrity throughout their service lives.

Many hermetic-sealing technologies perform well within their intended operating range. However, challenges emerge when applications require higher current densities, wider temperature ranges, greater chemical resistance or materials that are incompatible with conventional glass-to-metal sealing approaches. In these situations, the sealing technology itself is a design constraint.

Engineers should understand where those limitations arise so they can determine whether a traditional glass-to-metal seal is sufficient or they should consider an alternative approach.

When Requirements Exceed Conventional Hermetic Seals’ Limits

As electrical systems become more compact, engineers need to deliver more power through smaller connectors and feedthroughs.

However, material selection can limit how much current a hermetic seal can support. Most hermetic glass seals only allow pins made from lower-conductivity nickel-iron-based alloys. While these materials work well in many applications, they can be a limiting factor when designs require higher current capacity in a compact footprint.

Using highly conductive materials can remove a significant design constraint for high-current hermetic connectors and electrical feedthroughs.

CERAMAX^®, a devitrifying high-expansion-rate crystallized glass (ceramic), is one alternative. It can be directly fused or compression-sealed to a wide variety of metals, including:

• Nickel-plated beryllium copper
• Copper alloys
• Stainless steel
• Titanium
• Inconel

This flexibility gives engineers more options when they select conductor materials for pin designs. For example, sealing directly to copper-alloy pins can dramatically increase current ratings. A standard 0.018-inch-diameter copper-alloy contact sealed with CERAMAX^® can carry up to 10 amps.

When Thermal Extremes Threaten Seal Integrity

Temperature can be just as challenging to manage as electrical load. Extreme temperatures or repeated thermal cycling threatens applications that include thermal batteries, downhole energy systems, cryogenic imaging equipment and aerospace electronics.

After fusing, traditional hermetic glass seals remain in a vitreous, or amorphous, state. CERAMAX^® is different. It is a specially formulated silicate glass that devitrifies, or crystallizes, when heated to its fusing temperature. Once it is devitrified, the material becomes a ceramic. That means CERAMAX^® can be reheated to very high temperatures without thermal flow or loss of seal integrity.

These temperatures can be significantly higher than the original fusing temperature. Once fused to a hermetic metal, CERAMAX^® creates a ceramic-to-metal hermetic seal through a chemical bond with the metal’s natural oxide layer. The resulting seal can operate from cryogenic temperatures as low as -269° C to temperatures as high as 450° C. That operating range suits applications that experience severe thermal cycling or extreme operating conditions.

When Electrical Isolation Cannot Be Compromised

Many high-reliability systems must withstand environmental stress while maintaining electrical isolation over long operating periods.

CERAMAX^® meets and exceeds MIL-STD-883 requirements, maintaining insulation resistance greater than 5,000 megohms at 500 VDC. The material also demonstrates dielectric-withstanding performance of 100 VAC/mil with no dielectric breakdown occurring within the CERAMAX^® body itself. Any eventual breakdown would occur across the external air path rather than through the sealing material.

These characteristics are particularly important in power feedthroughs and other applications where electrical leakage or dielectric failure could compromise system performance.

When Environmental Conditions Create Multiple Failure Risks

Some demanding applications combine several obstacles. High pressure, aggressive chemicals, vibration, shock and thermal cycling all stress hermetic interfaces.

CERAMAX^® is immune to the harsh chemicals commonly used in electronic integration and hermetic-packaging applications. Its temperature tolerance and mechanical stability complement that chemical resistance, helping it perform in environments with multiple stress factors.

Examples include:

• Multipin, high-density aerospace and defense connectors exposed to vibration and temperature cycling.
• Hermetic bulkhead feedthroughs in high-pressure oil and energy exploration environments.
• Thermal battery headers that demand high-current power transmission.
• Cryogenic imaging and targeting systems that operate at liquid-helium temperatures.
• Hermetic feedthroughs for precision instrumentation such as accelerometers, thermocouples and pressure sensors.

In each case, the goal is not simply maintaining a hermetic seal. It is maintaining that seal while also meeting electrical, thermal and mechanical requirements that push conventional technologies to their limits.

When Engineers Should Consider Alternatives to Glass-to-Metal Sealing

When conventional glass-to-metal seals meet the application’s electrical, thermal and environmental requirements, engineers may have little reason to consider an alternative. The calculation changes when current capacity, operating temperature, material selection or environmental exposure begin constraining the design. In those situations, CERAMAX^® offers another option, particularly for applications that require high current, wide temperature tolerance, chemical resistance and long-term hermetic reliability.