1 – Uses for hermetic connectors
Why do you need a sealed hermetic connector to begin with?
The main reason to select a sealed connector over a non-sealed variant is to protect from environmental impact:
Hermetically sealed connectors are often associated with military and aerospace applications. In aerospace, the connectors need to protect electronics against multiple influences: High altitudes, changing atmospheric pressures, severe weather conditions and rapidly changing temperature gradients.
The U.S. Department of Defense has standardized connectors for military uses: “MIL-STD” (military standard) or “MS / MIL-SPEC” (military specifications). Connectors according to MIL-DTL-38999 and MIL-DTL-26482 among other are available as hermetic variants. Meanwhile MIL-SPEC connectors are also used for many industrial and commercial applications due to their robust design.
The protection against moisture is very important for many sensor applications. Ingress of moisture can degrade sensor amplifier electronics or cause measurement errors/drift. In the worst case going unnoticed and leading to false sensor readings! For Sensors / Industrial Automation, metric M8 and M12 connectors according to IEC 61076-2 have established themselves as the industry standard – with hermetic variants available on the market.
Another important hermetic connector use are vacuum electrical feedthroughs. Completely air-tight connectors are needed for a vacuum chamber to reach high-vacuum / ultra-high-vacuum. Mil-spec hermetic connectors according to MIL-DTL-26482 / MIL-C-26482 are often used as a standard in vacuum chamber applications.
The hermeticity of these connectors is usually determined with a helium leak tester. A pressure differential of 1 bar is applied by a vacuum pump. Then helium is sprayed onto the connector and the leakage through the seal is measured. For a connector to be considered “hermetic”, the helium leak rate needs to be below 1e-7 to 1e-8 mbar*l/s. As the helium molecule is really small, connectors which fulfill this criterium are also completely airtight and watertight.
Now, let’s get into the different sealing methods in more detail!
2 – Epoxy or Silicone based polymer sealed connectors
The first technique we are going to consider is potting. Typically, the potting compounds which are used are epoxy resins or silicone-based compounds. Often a regular non-hermetic connector design with plastic insulators is used as a basis. The potting compound is then backfilled into the rear cavity of the connector. An appropriate curing process is used at the end to remove the binder from the potting compound and to form the hermetic seal.
Advantages: The main advantage is that the potting sealing process is relatively simple. It also does not require extensive specialized equipment or tooling. Hence it can be easily integrated into most production environments. The relatively low-temperature sealing process allows a great flexibility with materials. Mismatches due to thermal expansion are comparatively small as long as temperatures stay low.
Drawbacks: While the seal can be good enough for vacuum applications, hermiticity is generally a few orders of magnitudes below of what can be achieved with glass or ceramic sealing methods. The reason for this is the higher diffusivity of gases and moisture through the polymer seal. Potting compounds can also absorb moisture and will outgas to some extent. As an additional hurdle it must be mentioned that the environmental operating conditions (temperatures, pressure, media compatibility) are also limited.
3 – Glass to Metal sealed connectors
These connectors use glass for the purpose of electrical insulation and hermetic sealing. The manufacturing process is performed in specialized furnace equipment at around 1000°C. The molten glass fills the cavities at the desired areas (fixturing) and forms the stable glass seal during cooling. A low dew-point reducing or inert atmosphere protects the metal parts from oxidation / discoloration during the sealing process. Glass-to-Metal seals are designed to be hermetic either by compression or matched(chemical) sealing.
Advantages: Glass-sealing is by many considered as the “gold standard” for hermetic connectors. The main reason lies in the high reliability of the glass seal. For most applications there is virtually no degradation and the high hermeticity will be maintained over years or decades. Glass-to-metal sealed connectors can sustain high pressure differentials, high temperatures and are also physically very robust.
Drawbacks: Connectors using a glass sealing must follow general design rules: For a reliable seal the inner design needs to be circular and materials need to be selected according to the thermal expansion requirements. Some popular materials (e.g. Aluminum) can’t be sealed with standard glasses due to the low melting point.
4 – Ceramic to Metal sealed connectors
With this technology ceramics (usually alumina) is used instead of glass as the insulator material. The Sealing between the ceramic insulation and the metal shell and pins is however not performed directly, but with an intermediate metallic brazed layer. To enable wetting of the brazing filler-metal, the ceramic is typically first metallized. Classically, a molybdenum-manganese layer is brush painted onto the ceramic body and then sintered in a wet hydrogen atmosphere. To enhance the wetting behavior, this surface is then nickel plated. Subsequently the connector assembly can be brazed in a dry hydrogen atmosphere or under high-vacuum conditions.
Advantages: Ceramic to Metal sealing is considered the high-end technology and can have superior performance compared to glass in some applications: By using elastic filler-metals, the resistance to thermal and mechanical shocks is higher. Also, depending on filler-material, higher application temperatures can be sustained.
Drawbacks: Costly manufacturing process.
5 – Conclusion & our opinion
So, which one should you select?
It of course depends on actual requirements of your application and needs to be considered carefully. Very generally speaking these are our thoughts:
Potting is probably the most accessible way to seal a connector. If the design needs to be airtight or watertight in a more basic way (e.g. ingress protection according to IP-classes), then a potted connector can be a good option. We generally would not recommend to use this technique however if a reliable hermetic connection is needed. Many don’t consider epoxy sealed connectors to be true hermetic connectors. From the failure modes and reliability issues we have seen at customer sites we tend to agree.
Between ceramic-metal and glass-metal sealed connectors is comes down to this: Do you need the added performance of ceramic-to-metal sealing (High mechanical / thermal shocks, very high operating temperatures or very high voltage ratings)? If the answer is yes then there is not really an alternative.
However, most applications we have seen do not. Then, glass-to-metal sealing is a much more economical technology which is just as reliable.
Do you have a project which might benefit from a hermetically sealed connector?
Please get in touch with us now so we can discuss!
The Dietze Group helps its customers with custom interconnection solutions from concept to series production. Our products are designed in close collaboration with our customers and engineered to meet the application’s specific needs.