What You Need to Know about Hermetic Glass to Metal Seals

Hermetic glass-to-metal seals (GTMS) are vital components in many industries where airtight, leak-proof, and durable enclosures are required. These seals are used to create a permanent bond between glass and metal, providing a protective barrier against environmental factors such as moisture, dust, gases, and temperature fluctuations. From electronics to medical devices and aerospace technologies, hermetic seals are indispensable for ensuring the long-term functionality and reliability of high-performance products.

What Are Hermetic Glass to Metal Seals?

A hermetic seal is a type of sealing technique that creates a completely airtight barrier between two materials—in this case, glass and metal. Glass-to-metal sealing is accomplished by melting the glass and allowing it to bond with a metal part at high temperatures. The result is a strong, durable, and hermetic bond that ensures the sealed component is resistant to external elements like moisture, pressure, and gases.

Unlike other types of seals, which may allow for some degree of leakage or contamination, hermetic seals provide a 100% barrier, making them ideal for use in environments where even the slightest leakage could lead to failure. These seals are used extensively in industries such as aerospace, electronics, telecommunications, and medical devices.

The Process of Creating Hermetic Glass to Metal Seals

The process of creating a hermetic glass-to-metal seal involves several stages, each requiring precise control of temperature, material properties, and handling techniques. Let's break down the main steps involved in the creation of hermetic GTMS:

1. Material Selection

Choosing the right materials for both the glass and metal components is crucial to the success of the hermetic seal. The materials must have compatible thermal expansion rates to prevent the seal from failing under temperature changes.

Glass: The glass used for GTMS is often a special lead borosilicate or other specific glass types that can withstand high temperatures and form a reliable bond with metals. The glass should have a low coefficient of expansion (CTE) to minimize cracking risks.

Metal: The metal typically used in hermetic sealing is stainless steel, kovar, or other alloys that have a similar CTE to the glass. This ensures that both materials expand and contract at compatible rates during thermal cycling.

2. Surface Preparation

Before sealing, both the glass and metal surfaces must be cleaned and prepared to remove any dirt, oils, or contaminants. Contaminants on the surfaces can prevent the glass and metal from bonding properly, leading to leaks and weak seals.

Glass Surface: Glass is typically cleaned using an ultrasonic cleaner or a chemical bath to ensure it is free of any residues that might interfere with the bonding process.

Metal Surface: Similarly, the metal surface is cleaned using abrasive methods or chemical cleaning to prepare it for bonding. The metal is often treated to enhance the adhesion properties of the glass.

3. Heating and Sealing

Once the materials are prepared, they are heated to a temperature high enough to melt the glass and allow it to flow and bond with the metal. This step is carefully controlled, as the glass must be molten enough to adhere to the metal but not so hot that it deforms or evaporates.

The temperature and pressure applied during the sealing process are critical to the success of the hermetic seal. If the temperature is too low, the glass may not bond properly. If the temperature is too high, the glass may crack or the metal may deform. The seal is formed when the glass cools and solidifies around the metal, creating a tight, airtight bond.

4. Post-Sealing Inspection

After the sealing process, the hermetic seal is carefully inspected to ensure that it meets the required standards. This can involve visual inspections for defects or cracks, as well as more advanced testing methods such as helium leak testing or pressure testing to ensure the seal is airtight.

Applications of Hermetic Glass to Metal Seals

Hermetic glass-to-metal seals are used across a variety of industries where a high level of protection is required for sensitive electronic components, sensors, and systems. Here are some key applications:

1. Aerospace

In the aerospace industry, hermetic seals are used to protect sensitive components, such as avionics, sensors, and pressure vessels. These seals are critical for ensuring that components are protected from the harsh environmental conditions in space or high-altitude flight, where exposure to extreme temperatures, pressure fluctuations, and radiation is common.

2. Medical Devices

Medical devices often require hermetic glass-to-metal seals to protect sensitive electronics or sensors. Devices like pacemakers, hearing aids, and insulin pumps must operate reliably inside the human body, where exposure to moisture and temperature variations is constant. Hermetic seals ensure that these devices remain sealed and sterile, thus maintaining their performance and safety.

3. Electronics and Telecommunications

In electronics, hermetic glass-to-metal seals are used to protect components such as semiconductor packages, LED lights, and sensors from moisture, dust, and other environmental contaminants. In telecommunications, they ensure that components inside communication devices are shielded from the external environment, guaranteeing signal integrity and long-term functionality.

4. Energy

In the energy sector, hermetic seals are used in applications such as solar panels, high-voltage connectors, and electrical transformers. These seals protect delicate components from external elements that could cause degradation or failure. For instance, hermetic seals are used in photovoltaic cells to prevent moisture from entering and damaging sensitive electronics.

5. Automotive

Hermetic seals are also used in automotive applications, particularly in components that need to be resistant to high temperatures, moisture, and chemical exposure. This includes components like sensors, ignition systems, and control units, where the integrity of the seal is essential for long-term reliability.

Challenges in Hermetic Glass to Metal Sealing

While hermetic glass-to-metal seals are highly effective, the process of creating them is not without challenges. Some of the main issues that can arise during the sealing process include:

1. Thermal Expansion Mismatch

As mentioned earlier, thermal expansion mismatch is one of the most common issues in hermetic sealing. Glass and metal expand and contract at different rates when subjected to temperature changes. If these rates are not properly matched, it can lead to stress on the seal and, eventually, failure.

Solution: Carefully select materials with compatible coefficients of thermal expansion (CTE) to ensure that both the glass and metal expand at similar rates.

2. Contamination

Contaminants on the surfaces of the glass or metal can interfere with the bonding process, leading to weak seals and leaks. Even minor dust or oil can have a significant impact on the performance of the hermetic seal.

Solution: Use a cleanroom environment for the sealing process, and ensure that both materials are thoroughly cleaned before sealing.

3. Cracking of the Glass

Due to the high temperatures involved in the sealing process, the glass can sometimes crack or fracture, especially if there are imperfections in the glass or if the cooling process is too rapid.

Solution: Implement a controlled cooling process to allow the glass to cool gradually, reducing the likelihood of thermal shock and cracking.

4. Leakage

Leakage is the primary concern in hermetic sealing, as even the smallest leak can compromise the entire system. This can occur due to insufficient bonding, improper temperature control, or mechanical stress.

Solution: Conduct rigorous leak testing (such as helium leak detection) and ensure proper bonding through precise temperature and pressure control.

Best Practices for Ensuring Successful Hermetic Seals

To ensure that your hermetic glass-to-metal seals are reliable and durable, consider the following best practices:

1.Select the Right Materials: Carefully choose materials that are compatible in terms of thermal expansion, corrosion resistance, and bonding ability.

2.Use Precise Temperature Control: Maintain strict control over the heating and cooling cycles during the sealing process to prevent cracks and ensure strong bonding.

3.Conduct Rigorous Testing: Perform leak testing, visual inspections, and mechanical tests to ensure that the hermetic seal meets all required standards.

4.Handle with Care: Both the glass and metal components should be handled carefully to prevent contamination or mechanical stress that could lead to seal failure.

5.Monitor Long-Term Performance: Assess the performance of hermetic seals over time to identify any potential degradation or weaknesses that could arise due to environmental exposure or wear and tear.