Sealed, Signed, and Delivered
By Jenny Bieksha, Bishop & Associates Inc.

Hermetic packaging of interconnects is commonly utilized to protect the devices from harsh environments, as found in many aerospace and defense applications. Hermetic seals are airtight, preventing the invasion of oxygen, humidity, moisture or other exterior contaminants, to enter the sealed package via the connector. The reason this is important is to prevent any moisture from condensing inside the pressurized enclosure. While the requirement for connector hermeticity was originally driven by military/aerospace electronic applications, there are now a diverse range of markets and applications that require hermetically sealed connectors.

Material Selection
Proper selection of subcomponent materials, material pre-conditioning, and tight process control are required to consistently produce hermetic packages. Connector hermeticity may be negatively affected by the permeability of shell materials and the quality of the sealing technology. Metal materials are chosen due to their relative impermeability to gas, although certain plastics may also be used.

Kovar® (ASTM F-15), stainless steel, titanium, and aluminum are typically chosen for the shell material. All provide an effective barrier against gas ingress and are able to withstand the high heat of the fabrication process. Kovar® is an iron-nickel-cobalt, controlled expansion alloy whose chemical composition is controlled to assure uniform thermal expansion properties. It is the most common material used in “matched” seals.

Contacts used in hermetic connectors must be fabricated from high-grade materials that can withstand high-heat and bond effectively to the vitreous glass seal. Kovar® and Alloy #52 (a 50/50 nickel-iron alloy) are often used as a material for contacts in compression-sealed hermetics.

Types of Hermetic Seals
Glass is an excellent insulator, as it bonds well to metallic surfaces and is extremely corrosion resistant. Glass seals are extremely resistant to any cracking, which may introduce leaks into the hermetic package.

Glass “matched” seals rely on glass-to-metal combinations with similar coefficients of thermal expansion to form an oxide bond that results in a hermetic seal. This design produces a stress-free robust seal well suited to many applications. This is extremely important in glass hermetic connectors such as the Micro-D, since the rectangular shape of the connector shell can exert varying degrees of stress on the glass. Matched seals can withstand high thermal and mechanical shocks, and are generally easier to manufacture.

Glass “mismatched” (compression) seals rely on fused glass-to-metal combinations, where the thermal expansion/contraction of the metal exceeds that of the glass, creating a strong mechanical compression bond that provides the hermetic seal. This design is well suited when the design parameters of the final package fall outside of the matched seal glass and metal combinations. A compression seal is most frequently specified for extreme, high-pressure applications.

Ceramic-to-metal seals are used in applications where units must operate under extreme temperature (hot and cold) and pressure. Unlike glass-to-metal seals that rely on the glass melting to form the seal around the connector’s power and signal pins, ceramic-to-metal seals are laser brazed into the metal.

Leakage
The design and installation of the connectors plays an important role in hermetic sealing. In many situations, leakage can be minimized up front by selecting the proper materials and using better assembly techniques (i.e. welding, brazing) to mount the hermetic package onto the bulkhead or enclosure. Even metal materials are susceptible to gas leakage. Their permeability can be compromised when weld and solder joints are formed between connector shell materials and the base material of the bulkhead.

While various potential failure modes exist, the most commonly observed failure modes are due to trapped water vapor and possible condensation. The best device packaging companies occasionally experience water vapor-induced issues due to material selection, process/supplier changes, and/or equipment degradation.

There are two major mechanisms by which gas can get through a hermetic barrier: leakage and diffusion. Gas leakages are caused by material defects, such as cracks. Diffusion is known to occur generally in most plastics and rubber materials, allowing extremely small quantities of gas to migrate through the hermetic barrier. Other leaks are caused by:

• External leaks result from poor welds or poor usage of seals. These leaks usually are easily found using a helium leak detector.

• Outgassing (also called desorption) is generated from any material that releases gas molecules that were “captured” during the absorption period. This absorption usually takes place when these materials are exposed to atmospheric pressure. The outgassing rate depends upon the surface finish of a material, its chemical composition, and upon the time spent under vacuum. The longer a material is kept under vacuum, the lesser the outgassing. Materials such as silicones, adhesives, lubricants, and Teflon insulation can all outgas water vapor, and contribute to the total vapor pressure inside the housing.

• A virtual leak is a source of gas that is physically trapped within the chamber with only a small, low conductance path from the trapped pocket of gas into the chamber. Gas trapped between assembled components, or contained inside materials, will slowly migrate out of the device into the vacuum area. Virtual leaks are usually the result of a manufacturing defect within the material. There is no way to detect these leaks using a conventional leak detector, as gas applied to the outside will not penetrate into the trapped cavity.

Testing
Hermetic seals are tested via various methodologies, including helium testing and dye penetrant. The purpose of both types of tests is to detect and measure leakage under pressure. In helium testing, a pressure differential between the internal volume of the package and the external environment is created, causing helium to diffuse through the connector shell, contacts, and/or glass seals. The dye penetrant test consists of immersing the sample in a fluorescent dye, pressurizing and examining under an ultraviolet lamp for traces of the dye. The dye penetrant method has the advantage of revealing the exact location of a full-scale leak, while helium testing measures overall leakage of the hermetic device.

Connector Types and Applications
Hermetic connectors consist of COTS and MIL types, including circulars and rectangulars. They range from the size of a match head for a sensor application, to a two-foot diameter bulkhead used on the International Space Station. The classic hermetic application is a receptacle feed-thru penetrating a pressurized bulkhead, or pressurized equipment housing — as found in inertial navigation units in aircraft. High-quality, reliable connections are essential in these applications. The introduction of moisture-laden air into such an enclosure may be enough to produce false readings and other malfunctions in the device.

As many new cost-saving initiatives are being integrated into military and aerospace programs, there will be a growing demand for highly reliable hermetic connectors, both COTS and MIL versions. Equipment used in these applications must operate in severe weather conditions, at high altitudes, under extremes of atmospheric pressure, and in rapidly changing temperature gradients. Well-designed hermetic connectors are a key part of the system needed to protect the controlled equipment environment.

Jenny Bieksha Director, Renewable Energy, Medical, and Military, Bishop & Associates Inc. Jenny Bieksha joined Bishop & Associates in 2008 as its market segment director for the renewable energy, and the test, measurement, and instrumentation markets. She is currently a management consultant specializing in strategic business planning, with an emphasis on the development of program, market, and product plans. Bieksha has more than 20 years of experience in the electronics industry, with a background in market management, business development, channel sales, product management, and operations for ITT Corporation, Delphi Connection Systems, and Hughes Aircraft Company. Bieksha has a bachelor of science degree in marketing from the University of Wyoming, and also holds a certificate as a project management professional. She can be reached at jbieksha@bishopinc.com.