What are glass-to-metal seals (GTMS)?

Hermetic glass-to-metal sealing technology combines metal and glass to create vacuum-tight electrical connectors, packaging, feedthroughs or optical windows/lenses in electronics or electronic systems. Meeting requirements for harsh environments and high performance, hermetic glass-to-metal seals are engineered to deliver uncompromised reliability.

Hermetic Glass-to-Metal Seals – purpose and function

The key function of glass-to-metal seals is to reliably enable pass-through of electrical or optical signals into vacuum-tight housings or enclosures. Simultaneously, the inorganic glass sealing prevents moisture or gas intrusion and leakage that could cause damage or failure of the encapsulated electrical components/systems, semiconductors, electro-chemicals, pyrotechnics, or other substances.

Glass-to-metal seals are engineered to perform reliably in harsh operating conditions, such as extreme temperatures, moisture, pressure or chemicals. They can also fulfill requirements for longevity, performance, and efficiency improvements, as well as innovative designs. At their core, the purpose of glass-to-metal seals is to enable reliable connection and protection for the manufacture of high quality and long-lasting hermetically sealed components or equipment. Versatility is a key characteristic of glass-to-metal sealing technology. This is demonstrated by the multitude of functions, designs, and sizes of GTMS components in a broad range of applications.

Examples can be found in a vast array of products and industries, ranging from high-temperature sensors, oil and gas applications, batteries and capacitors, nuclear reactors, implantable medical devices, optoelectronic packaging, and automotive airbags, among others.

Types of GTMS

One of the key aspects of creating high-quality glass-to-metal seals is the combination of materials with optimal coefficients of thermal expansion (CTE). The expert selection and subsequent processing of metals and suitable sealing glasses is essential for durable gas-tightness of the seal. GTMS products can be segmented into two types based on the glass and metal selection, and the corresponding CTEs of the chosen materials:


1. Matched seals

The CTE of glass and metal in matched seals is balanced. Matched seals are typically used for applications where high-temperature variations are present, or tight pin pitch or dimensional requirements (for example, miniaturized or non-round shapes) are factors. Depending on the design, matched glass-to-metal seals are able to maintain integrity through tens of thousands of thermal cycles and are well suited for semiconductor and optoelectronic assemblies where structural integrity is essential for precise operation of the enclosed electronics.


2. Compression seals

Due to vastly different CTEs of glass and metal in compression seals, the outer metal eyelet shrinks firmly onto the sealing glass during the production process. This compressive force forms a seal with immense bonded physical strength. In fact, the compressive strength of glass is ten to twenty times higher than its tensile strength. This is why compression seals are typically used for designs with requirements for high mechanical robustness. Usage examples can be found in automotive and energy applications, where long-term seal integrity is essential for continued safe operation of electronics in volatile environments.

Knowing what works

As the pioneer in hermetic glass-to-metal sealing, SCHOTT experts can turn to decades of experience, as well as the most versatile portfolio and in-house know-how, to guide customers in exactly what will work and provide solutions to maximize effectiveness in their application. In a situation where we don’t have a solution in the portfolio yet, we can also explore the possibility to develop it for you.

How glass-to-metal seals are produced

Hermetic glass-to-metal seals have an intricate but extremely effective production process with only three involved pieces: the metal eyelet (housing), metal pin (feedthrough), and glass preform (sealing material). To achieve the desired exceptional reliability of the final assembly, knowledge of the fundamental details is essential. Competence in glass types and formulations is a key proficiency in maximizing robustness and reliability, especially when it comes to customized components. It also includes the selection of the right metal and glass combination for the intended end-use and proper execution of the sealing process to maximize integrity of the seal. You benefit from SCHOTT’s in-house development capability to alter or engineer entirely new glass types in order to fulfill unique challenges and enter new applications.

The production process for glass-to-metal seals can be broken down into four main areas:

1. Glass expertise
2. Packaging and feedthrough design/validation
3. Sealing process
4. Plating