Hermetic Applications |
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H ermetic connectors are designed for use in harsh application environments. The BAE Systems cover photo of the SKYEYE Unmanned Aerial Vehicle is just such an application. Sensitive photographic, sensor and surveillance equipment mounted in the SKYEYE must be able to operate in severe weather conditions, at high altitudes, under extremes of atmospheric pressure and in rapidly changing temperature gradients. Hermetic connector devices interconnecting the SKYEYE's black box equipment are selected for their ability to protect the controlled equipment environment by maintaining an air-tight seal between severe flight conditions and the UAVs sensitive payload.
Hermeticity is generally defined as the state or condition of being air or gas tight. In interconnect applications, hermetic refers to packaging technology designed to prevent gasses from passing through pressure barriers via the connector. The reason this is important is to prevent any moisture in the leaked gas from condensing inside the pressurized enclosure. The point at which moisture will condense is called the "dew point"—or the precise moment when humidity, pressure, and temperature allows condensation to form. When an electric current must pass through a high-pressure differential barrier, the potential exists for gases, moisture, and in some rare cases particulate matter, to also penetrate the barrier and, as described above, to form condensation in the equipment enclosure. In the receptacle cabling on the pressurized side of the barrier this may result in dielectric breakdown, corrosion, and loss of insulation resistance between conductors (a properly built plug assembly on the non-vacuum side is adequately sealed with conventional environmental protections and so is impervious to moisture ingress). The classic hermetic application is a receptacle feed-through penetrating a pressurized bulkhead, or a pressurized equipment housing— such as is found in inertial navigation units in aircraft. The introduction of moisture-laden air into such an enclosure may be enough to produce false readings and other malfunctions in the device. The ultimate purpose of hermetic sealing then is not merely to "avert the ingress of air or gas into pressurized environments to prevent corrosion resulting from dew point condensation", but more precisely to insure malfunctions do not occur in sensitive electronic systems due to said ingress. Hermetic connectors must perform their magic at extremely high pressure differentials, often as high as 20,000 psi, in order to prevent fluids and high pressure in one area from impacting normal environmental conditions and pressures in another. As with other connector families, hermetic customers may specify the connector coupling style (threaded, bayonet, etc.) pin or socket gender and layout, contact termination type (solder cup, flat eyelet or PCB termination), conductive or non-conductive finish, polarization and so on. Glenair customers may also choose from a broad range of contact densities and package sizes, including standard-density MIL-DTL-38999 Series I, II, III and IV, our .76 in. contact spacing Series 80 "Mighty Mouse" Connector, and both Micro-D and D-Subminiature rectangulars. Glenair's complete in-house hermetic capability also affords us the ability to produce a wide-range of special purpose hermetic connectors designed to meet individual and unique customer specifications.
Connector Hermeticity Although moderately effective sealing may be produced with simple techniques such as epoxy potting, fused glass-to-metal seals are usually specifi ed in high-pressure applications. Glass is an excellent insulator, bonds well to metallic surfaces and is extremely corrosion resistant. And because of its robust mechanical strength and resistance to radical changes in temperature and pressure, glass seals are extremely resistant to any cracking which may introduce leaks into the hermetic package. Fused glass seals may be produced from various recipes of ground, non-crystalline solids such as silicates, borates and phosphates. When heated to high temperature and then cooled, these materials fuse into an amorphous solid called glass. In hermetic connector manufacturing, the glass material is typically introduced as a pre-formed glass seal insulator tooled to precise dimensions. The glass must be exactingly selected for each application according to its ability to form a strong bond with the chosen metal materials.
Electrical properties, such as dielectric withstanding voltage and strength are also considered as is thermal and shock stability. Depending on the style of connector being produced (rectangular versus circular, for example) two distinct categories of glass-seal hermetics may be specified. These are known as Matched and Mismatched (or Compression) Seals. In Matched Seal hermetics, the thermal expansion of the glass and metal materials are relatively close, usually within 10% of each other. This results in a product in which the stress in the glass is relatively small, since the expansion and contraction of both materials during manufacture is closely matched. 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. At ambient temperatures, the glass is chemically wetted (bonded) to the metal shell and contacts, but under little or no pressure or stress. Matched Seals can withstand high thermal and mechanical shocks, and are generally easier to manufacture than Mismatched (Compression) Hermetic Seals. Kovar®, a combination of iron, nickel and cobalt, is the material of choice for Matched Seal hermetic receptacles—both shells and contacts. Kovar® is a low-expansion metal with a coeffi cient of expansion rating matched to the glass material that forms the hermetic seal. In Mismatched (Compression) Seals, the thermal expansion/contraction of the metal exceeds that of the glass. During the firing process, the metal materials, usually stainless steel, expand at a greater rate than the glass. During cooling, the metals contract back into the already solidifying glass to form an extremely robust compression bond. This type of seal is consequently the most frequently specifi ed for extreme, high-pressure applications since the seal produced is reliable to pressures as high as 14,000 psi (1000 bars). The total potential for leakage in a hermetic connector is the sum of any permeation which may occur via the metal materials themselves (through cracks or open pores), and any leakage that may occur via the seal. An additional source of leakage—uncontrolled from the connector manufacturerˇ¦s perspective—results from sub-standard mounting of the hermetic package on the bulkhead or enclosure. Depending on the surface material of the bulkhead, hermetic receptacles may be welded or soldered in place. Low temperature brazing is also possible in certain applications as is the use of adhesive sealants. Finally, mechanical mounting seals such as O-rings equipped jam-nut mounts or drilled mounting flanges are used in applications where the cost or diffi culty of welding or soldering is impractical. Regardless of the choice of mounting technology, care must be given to ensure inadvertent leakage paths are not introduced to the system. Vapor condensation in pressurized enclosures may also be affected by the material makeup of devices located inside the enclosure. Materials such as silicones, adhesives, lubricants and Teflon insulation can all outgas water vapor, and so contribute to the total vapor pressure inside the housing. As discussed above, this rise in vapor pressure will directly impact the condensation dew point of the protected environment.
Hermetic seals are qualified via various methodologies including helium testing and dye penetrant. The purpose of both types of tests is to detect and measure leakage under pressure. 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. In helium testing, a pressure differential between the internal volume of the package and the external environment is created. The resultant pressure gradient causes the helium to diffuse through the connector shell, contacts and/or glass seals. Quantitative and qualitative measurements are then taken using appropriate sensing instruments. Manufacturing CapabilityHermetic connectors are constructed from a core component-set that includes the connector shell, the vitreous glass insert and the selected contacts. Matched hermetic shells may be machined from Kovar®, an iron-nickel-cobalt alloy with a co-efficient of expansion closely balanced to the glass inserts. Titanium shells and Kovar® contacts are also appropriate for matched-seal hermetics. Stainless and cold-rolled steels with 52 nickel-alloy contacts are suitable for compression-seal hermetics. Contacts used in hermetic connectors must be fabricated from Kovar® or from other high-grade materials that can withstand high-heat and bond effectively to the vitreous glass seal. The individual parts are mounted into special fixtures which hold the parts in exact alignment during an exothermic atmosphere firing process. A conveyor belt transports the work through the furnace chamber, where a reducing atmospehere prevents oxidation of the metal components. As discussed above, a gas-tight hermetic seal is formed around all contacts, and between the glass seal and the connector shell, when the vitreous glass is melted in the furnace and then cooled under controlled conditions. After firing, helium testing and fi nish plating (passivation in the case of stainless steel) are completed and the remaining connector components such interfacial seals, O-rings, jam-nuts and so on are assembled to the connector body. Quality control is a critical component of hermetic connector manufacturing. Connectors are not only subjected to a rigorous leak test, but are also visually inspected to insure all components are seated in their correct positions and no surface imperfections or micro-cracking is in evidence. The connectors are also subjected to electrical testing as required by military and industry standards and by customer requirements. Dating back to our initial hermetic order, Glenair has been in the hermetic connector business for close to 30 years. Today, our capabilities are an arm and a leg greater than they once were, but our commitment to high-quality and outstanding availability has remained constant. As you can see from the wide range of commercial and military standard hermetic products we now produce, Glenair is positioned to service an incredibly broad range of both commercial and military standard hermetic packaging requirements.
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