Hermetic Glass-to-Metal Sealing

Reliable, tightly-secured electrical and optical signal transmission: hermetic packaging and sealing has an 80-year history of keeping electrical components and systems connected and protected in major industries worldwide.
What are glass-to-metal seals?

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.
Assembly process of a glass-to-metal seal showing the eyelet, pin and glass joining together to form a hermetic bond

Highly robust electrical insulation

Sealing electrical or optical conductors with glass creates a non-permeable seal. As an inorganic material, glass seals do not suffer from aging and are therefore used when sensitive electronic components need to be protected reliably or long-term.

GTMS Glass-to-Metal Sealed Feedthrough Assembly Graphic

Preventing moisture and gas intrusion

Glass as a sealing material creates a hermetic barrier against moisture intrusion, condensation, and leakage, providing more reliable protection in harsh environments than non-hermetic polymer seals. Glass and metal can be directly sealed together without any additional interface materials, reducing complexity compared to ceramic-to-metal seal alternatives.

Purpose & function

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.

Diagram showing a comparison between matched and compression glass-to-metal sealing types

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

Production process

Glass expertise Glass expertise Glass expertise Glass expertise Glass expertise
  • Glass expertise
  • Glass expertise
  • Glass expertise
  • Glass expertise
  • Glass expertise

Glass expertise

As a glass company at heart, glass expertise and manufacturing capabilities are the ‘secret ingredients’ that set SCHOTT apart as the leader in glass-to-metal sealing. Glass development, melting, milling, and preform manufacturing are all fully managed in-house by many of the world’s top glass formulation and hermetic sealing experts.

Choosing the right sealing glass according to the metal and operating conditions is essential to produce a highly durable glass-to-metal seal. Advanced requirements necessitate the use of special materials. SCHOTT offers the broadest range of standard glasses as well as unique new glass developments. This enables us to meet the exact specification the customer has in mind. The specialty glass melts are further processed into shards, milled into fine glass powder, and finally pressed and sintered to produce glass preforms.

SCHOTT glass preforms have an extremely high dimensional accuracy, mechanical stability, consistent weight, and smooth surfaces – all aspects that play a decisive role in enabling robust and reliably hermetic glass-to-metal seals. Entirely new glass preforms are consistently in development to serve new applications and solve development challenges.
Packaging and feedthrough design/validation Packaging and feedthrough design/validation Packaging and feedthrough design/validation
  • Packaging and feedthrough design/validation
  • Packaging and feedthrough design/validation
  • Packaging and feedthrough design/validation

Packaging and feedthrough design/validation

The development process at SCHOTT is a collaborative approach: we closely work with our customers to enable delivery of the best components possible to suit application needs.

Direct engineering support, complete customization, and rendering of more cost-efficient or less complex designs are all part of the design and development steps for glass-to-metal seals. Often, SCHOTT can actually provide a better product than the original customer concept through the aid of expert input and validation measures.

Validation is a key area for glass-to-metal seals. SCHOTT can perform simulations and tests with varying tools and support systems to verify successful implementation in applications, as well as with customer processes. These include welding, soldering, handling, and various other measures for proof of concept. The bottom line is we take your vision and make it work!
Sealing process

Sealing process

The sealing process transforms the three separate components into a single, vacuum-tight glass-to-metal sealed assembly.

The glass preform, metal eyelet, and metal pin are assembled in a carbon fixture and put through a furnace in a highly controlled and specialized process. The temperature, oven conditions, gas compositions, and heating/cooling behavior of the materials are all essential factors that are expertly optimized according to the assembly requirements.

The end result of a successful sealing process for GTMS is a robust and durable component manufactured according to the exact established design specifications.
Plating Plating
  • Plating
  • Plating


Plating is the final step of the process for the manufacture of glass-to-metal seals.

SCHOTT performs all plating processes in-house – a key differentiating factor in the production process as many GTMS manufacturers outsource this step, leading to longer lead times and potential quality issues.

The internal plating infrastructure at SCHOTT is optimized to enable both prototype quantities as well as high-volume, consistent-quality serial production. We can advise on material types and thickness for the plating process to help optimize efficiency and performance for the final assembly.

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We are certified

Certifications of our GTMS production sites cover ISO 9001, ISO 14001, IATF 16949, EHS, AS9100/EN9100/JISQ9100, ISO 5001, ASME, ATEX and IECEx, CE Plus, KTA 1401, and EffiNet.

Want to talk directly to an expert?

Let’s discuss how glass-to-metal sealing can be a fit for your application.

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