Engineer’s Nightmare: The helium leak detector is screaming, but the weld looks fine. The culprit? An invisible micro-crack in the glass-to-metal seal. Here is why it happened—and how to stop it.
Glass-to-metal seals (GTMS) are the unsung heroes of vacuum systems. But when they fail, they fail catastrophically. In our failure analysis lab at Vinstronics, we see the same three root causes responsible for 90% of hermetic failures in the field.
Whether you are troubleshooting a Hermetic RF Feedthrough or a discrete power terminal, identifying the type of crack is the first step to fixing the problem.
1. Radial Cracks (The “Thermal Shock” Killer)
The Symptom: A straight crack radiating from the center pin to the outer metal shell, like a spoke on a wheel.
The Cause: This is a classic sign of CTE (Coefficient of Thermal Expansion) Mismatch. If the metal pin expands significantly faster than the glass during a temperature spike (e.g., soldering or baking), the glass is pulled apart by tensile stress.
This is why material selection is non-negotiable. As we explained in our previous deep-dive on The Science of Matched Sealing, using Kovar (4J29) alloy ensures the metal and glass expand in unison, preventing these radial stress cracks entirely.
The Fix: Switch from Compression Seals (Stainless Steel) to Matched Seals (Kovar) for any application undergoing thermal cycling.
2. Meniscus Climb (The Manufacturing Defect)
The Symptom: Chipping or cracking at the very top edge where the glass meets the pin (the “fillet” or meniscus).
The Cause: This is often a manufacturing process error. If the glass “wicks” or climbs too high up the pin during the furnace firing, the glass at the edge becomes razor-thin. This thin glass is extremely fragile and will shatter under the slightest vibration or mechanical shock.
- Ideal Geometry: A controlled, low-profile meniscus.
- Defect: High-climbing glass creating a weak point.
At Vinstronics, we strictly control the oxidation profile and furnace atmosphere to ensure the glass meniscus remains robust and uniform, meeting MIL-STD-883 visual inspection criteria.
3. Installation Stress (The “Heavy Hand”)
The Symptom: Cracks that appear immediately after soldering a wire to the pin or tightening the flange.
The Cause: Even the robust Kovar-Glass bond has limits.
- Overheating: Soldering for too long (>5 seconds) allows heat to travel down the pin into the glass seal, causing localized thermal shock.
- Bending Pins: Applying side-load force to a pin (e.g., bending it to fit a cable) crushes the glass insulator.
Best Practice: Use heat sinks when soldering and ensure proper cable strain relief. For critical applications, refer to the NASA Workmanship Standards for interconnect cabling.
Summary: How to Prevent Leaks
| Defect Type | Root Cause | Solution |
| Radial Crack | Thermal Mismatch | Use Matched Kovar Seals |
| Meniscus Chip | Poor Process Control | Qualified Supplier (Vinstronics) |
| Pin Damage | Improper Soldering | Heat sinks & Strain relief |
Conclusion: Reliability is a Choice
Most hermetic failures are not accidents; they are predictable consequences of physics. By understanding the difference between a Radial Crack (Design issue) and a Meniscus Chip (Process issue), you can pinpoint the solution faster.
Don’t let a $50 connector ruin a $50,000 vacuum system.
Need a Failure Analysis?
Our engineers can help identify the root cause of your current leak.
