How is Hermetic Packaging Achieved? — Glass-to-Metal Sealing (GTMS) Reliable Connection Solution

4. Basic Principle of GTMS: High-Temperature Melting, Cooling to Seal

The principle of glass-to-metal sealing (GTMS) is straightforward: heat the glass to a molten state so it flows and wets the metal surface, forming chemical bonds or micro-mechanical interlocking at the interface, and after cooling, obtain a dense sealing interface.

This technology originated from vacuum tube manufacturing in the early 20th century and was subsequently widely applied in aerospace, military electronics, sensors, and optoelectronic component packaging. After long-term development, it has become one of the most widely applied and technologically mature mainstream solutions in the hermetic packaging field.

But a simple principle does not mean simple execution. What is the most difficult problem in the process?

5. The Greatest Challenge: Coefficient of Thermal Expansion Must Match

Sealing is performed at 900~1100°C. When cooling to room temperature, if the contraction difference between glass and metal is too large, the brittle glass will crack directly.

Through precise dimensional design and thermodynamic matching calculations, two solutions exist:

Matched Seal: Selecting glass and metal pairs with similar coefficients of thermal expansion. The classic combination is Kovar alloy (CTE≈5.3×10⁻⁶/°C) with borosilicate glass, resulting in minimal stress after cooling.

Compression Seal: Selecting metal (such as stainless steel) with a significantly higher expansion coefficient than glass, paired with glass. After cooling, the metal contracts more, applying continuous compressive stress on the glass from the outside — even if micro-cracks exist, they will not propagate to cause leakage.

Matched sealing has low stress; compression sealing has high stress. Each has its applicable scenarios.

With the material problem solved, what does GTMS look like in actual products?

6. Typical Product Form: Feedthrough

The most typical GTMS application is the hermetic feedthrough — a metal pin passes through a pre-drilled hole in the metal housing, with a glass insulator filling the gap between the pin and the housing. After high-temperature melting and cooling, the three components are sealed as one.

This structure simultaneously accomplishes four things:

Sealing — glass fills the gap, molecular-level leak-tight

Insulation — glass resistivity up to 10¹⁴ ©·cm, housing and pins are not electrically connected

Fixation — pins are firmly positioned, resistant to vibration and shock

Transmission — connection between inside and outside of the housing, enabling signal transmission

This is why GTMS feedthroughs have become the standard solution for sensors, connectors, and optical communication modules.

7. Advantages and Application Boundaries of GTMS

GTMS Advantages: mature process, low cost, suitable for mass production, excellent electrical insulation.

GTMS Limitations:

Long-term operating temperature ≤300°C

High glass brittleness, limited shock resistance

Increased dielectric loss at high frequencies (>10GHz)

When application requirements exceed GTMS performance boundaries — such as high-temperature, high-power, millimeter-wave frequency, or high mechanical strength scenarios — Ceramic-to-Metal Sealing (CTMS) solutions with higher temperature resistance, thermal conductivity, and mechanical performance are needed.

[Next Article Preview] In the next article, we will further analyze how Ceramic-to-Metal Sealing (CTMS) meets high-temperature, high-power, and high-frequency application requirements.