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16 SMT Magazine • March 2015 Electrical conductivity of a metallic TIM may, however, be a significant advantage for certain types of applications where having full electrical contact is valuable for device perfor- mance. Primary examples are flange-mount RF power transistors, where the die is mounted on a CTE-matched flange or baseplate directly in the electrical path. Signal performance of a GaN RF device, for example, may be improved with a metallic Heat-Spring TIM, when compared to the electrical performance in an identical appli- cation with a thermal grease. applications requiring Strike angle and repeated contact Certain types of products have been devel- oped to address very specific requirements in the semiconductor test and burn-in industry, where a gimbaled test head with a TIM in place on the head surface must make repeated contact with hundreds or thousands of devices under test, such as bare-die microprocessors for test and binning prior to package assembly. In these designs, the gimbaled test head requires that a TIM survive these many repeated contacts with the head surface positioned at different angles, depending on the format and size of the devic- es under test. This is the "strike angle" problem that has proven so difficult for so many types of TIM materials. The relative robustness of metallic TIM ma- terials has proven to be very valuable in these applications. handling and Placement The use of a metallic TIM as a preform or shim requires a simple placement procedure, which can be either applied manually or adapt- ed for vacuum nozzle placement. Die-cut metal- lic TIM preforms can be packaged in tape and reel or other easily handled formats. alloys, Formats, and Shapes The Heat-Spring product family includes TIMs manufactured from indium and indium alloys; indium with a clad metal on one sur- face for specialized application requirements, such as semiconductor test and burn-in; and from other metals, such as tin. Use of a tin alloy can achieve a very cost-effective TIM preform. Such materials have been tested and character- ized using the same thermal performance and life and reliability standards as the indium met- al Heat-Spring products. A tin alloy, having a higher flow stress than indium metal, will resist flattening of the patterning under pressure. At higher clamping pressures, the load is spread more evenly and this reduces potential for sub- strate bowing. Metallic TIMs, whether flat indium foils or the patterned TIM, can be die-cut to an unlim- ited number of footprint outlines for different package types and are made available in a vari- ety of thicknesses. Environmental Impact When an indium metallic TIM is selected, the material can also be reclaimed in a rework program, if processors or semiconductor mod- ules are upgraded at a later date. Summary Metallic thermal interface materials have been used in electronics assemblies for decades, but typically have not been mentioned in in- dustry discussions of thermal interfaces. Recent developments of patterned metallic interface materials, especially with indium and metals such as tin, have made additional tools avail- able to the systems OEM thermal engineer for solving demanding thermal interface problems. These metallic TIMs have been applied in a range of applications where high performance is required, not simply as measured by thermal resistance, but also for system life and reliability in challenging operating conditions. SmT DEvELOPmEnTS WITh mETaLLIc ThErmaL InTErFacE maTErIaLS continues Feature Tim Jensen is senior product man- ager, engineered solders, Indium Corporation. Dave Saums is an indium Corporation advisor and founder/principal of DS&A llC.

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