Introduction
Hermetic feedthroughs are essential components that allow electrical or optical signals to pass through sealed barriers without compromising the internal environment. Choosing the right materials is critical because the performance, reliability, and lifespan of a feedthrough depend on the materials used. Factors such as electrical insulation, thermal expansion, mechanical strength, and chemical resistance must all be considered during the design process.
1. Metals for Structural Integrity
Metals are commonly used in hermetic feedthrough for structural components and housing. Stainless steel, Kovar, and other alloys provide mechanical strength, durability, and thermal stability.
- Stainless Steel: Corrosion-resistant and suitable for high-pressure and industrial applications.
- Kovar: Has a thermal expansion coefficient compatible with glass, making it ideal for glass-to-metal seals.
The choice of metal ensures that the feedthrough maintains its structural integrity under stress, temperature changes, and pressure differences.
2. Ceramics for Electrical Insulation
Ceramics are widely used as insulating materials in feedthroughs because of their excellent dielectric properties and thermal stability. They prevent electrical shorts between conductors and the metal housing.
- Alumina (Al₂O₃): High strength, low thermal expansion, and excellent electrical insulation.
- Zirconia (ZrO₂): Offers high fracture toughness and resistance to wear.
Ceramics are essential in high-voltage and high-temperature applications, providing reliable electrical isolation while maintaining a strong seal.
3. Glass for Hermetic Sealing
Glass is commonly used in glass-to-metal feedthroughs because it can form a permanent, airtight bond with metals like Kovar. This combination ensures hermeticity while maintaining electrical insulation.
- Borosilicate Glass: Resistant to thermal shock and chemical attack.
- Fused Silica: High-temperature stability and low thermal expansion.
The selection of glass depends on the operating environment, temperature range, and mechanical requirements.
4. Polymers for Low-Pressure Applications
High-performance polymers are occasionally used for lightweight or low-cost feedthroughs. While they do not offer the same hermeticity as metals, glass, or ceramics, they are suitable for low-pressure and low-voltage applications.
- PTFE (Teflon): Excellent chemical resistance and electrical insulation.
- Polyimide: High thermal stability and flexibility.
Polymer feedthroughs are typically chosen for consumer electronics, moderate industrial systems, and applications where cost or weight is a concern.
5. Factors Influencing Material Selection
Electrical Requirements
Materials must provide sufficient insulation and conductivity for the intended signals or power levels.
Environmental Conditions
Temperature extremes, pressure differences, moisture, and chemical exposure all influence material choice.
Mechanical Stress and Compatibility
Materials must withstand vibration, shock, and thermal cycling. Matching thermal expansion coefficients between metals and insulators (like Kovar and glass) prevents seal failure.
Cost and Manufacturability
While high-performance materials like ceramics and specialized metals offer superior reliability, budget constraints may influence material choice for less critical applications.
Conclusion
Material selection is a critical step in hermetic feedthrough design, as it directly affects electrical performance, sealing integrity, and durability. By carefully choosing metals, ceramics, glass, or polymers based on application requirements, environmental conditions, and mechanical stresses, designers can create reliable feedthroughs that perform consistently in aerospace, medical, industrial, and scientific applications.
