Common Failure Modes in Glass to Metal Seals

1. Cracking Due to Thermal Expansion Mismatch

What happens:
When the thermal expansion coefficient (CTE) of the glass and metal don’t match properly, heating and cooling cycles induce mechanical stress — leading to micro-cracks or catastrophic glass fracture.

Root Cause:

• Incompatible CTE between sealing glass and metal alloy

• Improper sealing geometry or joint design

• Rapid thermal cycling

Prevention Tips:

• Use matched-seal systems (e.g., glass with CTE ~5.5 × 10⁻⁶/K for Kovar)

• Employ compression seal designs where the metal compresses the glass

• Simulate CTE behavior using FEA tools before prototyping

2. Air or Moisture Leakage (Loss of Hermeticity)

What happens:
Tiny pathways or porosities form at the interface or through glass voids, resulting in slow gas ingress or moisture absorption, which degrades electronics or battery performance.

Root Cause:

• Improper glass melting or wetting during seal processing

• Surface contamination

• Inadequate seal geometry

Prevention Tips:

• Maintain clean sealing surfaces and high-purity processing environments

• Conduct helium leak testing to meet MIL-STD-883 or IEC 60068 standards

• Optimize seal profile to avoid sharp edges or thin-wall regions

3. Devitrification of the Glass Phase

What happens:
If glass is held at sealing temperature for too long or exposed to improper cooling, it can partially crystallize — losing transparency, becoming brittle, and introducing structural defects.

Root Cause:

• Excessive sealing time or temperature

• Use of glass not optimized for thermal profile

• Contaminants acting as nucleation sites

Prevention Tips:

• Use low-devitrification sealing glasses (e.g., SiO₂-B₂O₃ or Bi₂O₃-ZnO systems)

• Follow recommended heating and cooling curves

• Avoid prolonged soak times above 700–800°C

4. Corrosion at Metal-Glass Interface

What happens:
Environmental exposure (e.g., salt spray, chemicals) may cause the metal near the seal line to oxidize, weakening the bond and eventually resulting in seal failure.

Root Cause:

• Inadequate surface preparation

• Poor environmental sealing

• Electrochemical reactions between dissimilar metals

Prevention Tips:

• Apply protective coatings (e.g., Ni plating, passivation) near the seal zone

• Use corrosion-resistant metals such as Kovar, Inconel, or titanium

• Store and transport components in dry, clean environments

5. Mechanical Fatigue and Vibration Failure

What happens:
Repeated vibration, pressure changes, or bending forces can cause cracking of the glass or loosening of the seal — especially in large-diameter or unsupported feedthroughs.

Root Cause:

• Inadequate mechanical support

• Improper mounting or thermal insulation

• Low-impact or high-G loading during service

Prevention Tips:

• Reinforce seals with ceramic sleeves or metal flanges

• Use vibration-damping mount designs

• Validate performance with accelerated fatigue testing

How to Evaluate Failure Risk: Best Practices

Engineering Tips for Long-Lasting Seals

• Choose sealing glass with CTE compatibility to your metal (e.g., 5–6 × 10⁻⁶/K for Kovar)

• Use compression seal geometry when metal strength allows

• Maintain cleanroom-level conditions during sealing to avoid voids or inclusions

• Implement quality control testing (leak, thermal, vibration) during prototyping and production

• Store and transport seals in controlled humidity packaging

Conclusion

Glass-to-metal seals are among the most reliable and effective sealing technologies used in high-performance systems — but only when properly engineered. By understanding common failure modes such as CTE mismatch, cracking, devitrification, corrosion, and fatigue, you can drastically reduce risk and extend the lifespan of your sealed components.

Whether you're designing for aerospace systems, battery modules, or medical sensors, following best practices in GTMS design and testing ensures your seals remain hermetic, durable, and failure-resistant — even in the most demanding environments.