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Thermoelectric Device Packaging for High Reliability Aerospace Applications
Keywords: thermoelectric coolers, aerospace, reliability
Modern military aircraft increasingly use optical components and systems to process information. Thermoelectric coolers improve performance in terrestrial applications by providing active temperature control of the optical components. Traditionally, thermoelectric coolers are not used in aerospace applications because of concerns over reliability. While, thermoelectric coolers have a long history of reliable use in land based optical communication systems, aircraft environmental requirements are much more severe. These concerns highlight the need for a better understanding for the role of packaging and joining materials in thermoelectric/optical component reliability and the ability to model and predict internal stress. Test specimens were prepared using different size thermoelectric devices, plate materials and solder types and thicknesses. Thermoelectric devices with sizes up to 15 mm were tested with plate CTE values from 6 to 10 ppm/C. Solder joints included Tin/Lead and Indium/Silver based solders with different thicknesses. The specimens were thermal cycled with different temperature ranges to determine the point of failure. The most severe range being +125 to -55C, a typical qualification requirement for aerospace components. Each specimen was modeled using multiphysics FEA software. Since many of the material properties are poorly defined, the goal was to demonstrate that a single set of parameters could predict failure over the range of tests performed. The results revealed thermoelectric coolers can easily survive over 1000 thermal cycles for aerospace qualification. Indium mounting solders performed the best and allowed the greatest devices size and difference in thermoelectric to package CTE. FEA modeling identified a critical stress level that was a consistent indicator of survival in the tests performed. Finally, the FEA methodology was extended to a unique planar-multistage thermoelectric device to explore the range of its application.
Mike Gilley, Sr. Member Technical Staff
Marlow Industries, Inc.
Dallas, TX

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