Polycrystalline Ceramic Seals vs Glass Seals for Hermetic DC Connectors

By Contributed Article | March 13, 2018

Five Reasons to Consider Polycrystalline Ceramic Seals Instead of Glass Seals for Hermetic DC Connectors

Rick Kalkowski, Marketing Administrator, PA&E, a Division of Hermetic Solutions Group

Advancements in materials give designers new options. Even classic designs such as the venerable DC connector can be updated and improved by considering all available materials. Switching from glass to polycrystalline ceramic brings advantages to hermetic seals and influences other elements of the design, ultimately leading to better performance. 

1.  More Pin Material Options

The glass sealing process for hermetic DC connectors requires the use of extremely heat-resistant pin materials that can survive the melting temperature of glass fully intact, and Kovar, a ferrous nickel/cobalt alloy, is one of the most popular. Polycrystalline ceramics seal at a lower temperature than glass, which accommodates the use of copper alloy pins, such as beryllium copper and chrome copper, as well as platinum, platinum clad, or Inconel pins.

Yellow ceramic-to-metal seals surround high-reliability BeCu pins in a hermetic Micro-D connector.

Figure 1: Yellow ceramic-to-metal seals surround high-reliability BeCu pins in a hermetic Micro-D connector.

2. Higher Electrical Conductivity

Beryllium copper (BeCu) is a popular pin material in connectors that use ceramic-to-metal seals. In a Micro-D format, for example, a Kovar pin with a 0.018” diameter will carry 0.45A while a BeCu pin of the same pin size will carry 2.58A. BeCu pins also exhibit high contact force, high reliability, and long-lifetime performance, which makes them well suited for use in harsh environments.

3. Crack-Proof Seals

If you’ve ever had a stone strike and crack the top your car windshield, you know that it’s only a matter of time before that crack makes its way across the glass. The same thing happens with glass-sealed connector pins. If a fissure starts at the top of a glass seal, it is very likely to follow a path to the bottom and result in a hermetic failure. Ceramics like PA&E’s patented Kryoflex® polycrystalline ceramic, which is a multiple-phase derivative of ceramic oxide crystalline silicates, have a randomized crystalline structure that allows the material to effectively relieve stresses within itself to prevent such cracks from propagating and eliminate the hermetic failures associated with cracked glass seals.

4. Weight Savings

Copper alloy pins can be integrated into aluminum connector bodies with a ceramic-to-metal seal to achieve lightweight and highly conductive connector solutions. Direct-seal aluminum connectors are a great option for applications in which light weight, reliable electrical performance, and robust environmental sealing are top priorities, but are best suited for applications without rigorous thermal cycling requirements, as the hermetic performance of this material combination degrades from 1×10-9 He at 1atm to 1×10-5 after repeated thermal cycles.

5. Robust Insulator

Ceramic functions as an excellent electrical insulator at very high temperatures, unlike glass, which conducts more electricity at elevated temperatures. Kryoflex, for example, has a resistivity of 1×1018Ω/cm with easily maintained resistance values in the 50,000MΩ range – even in microminiature designs, and its standoff voltage capability is such that breakdown will always occur in the air media surrounding the terminal, never through the dielectric.

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