The density of packaging in electronic enclosures is increasing rapidly and the real estate available for packaging high power ASICs and processors is decreasing very rapidly as well. This work presents a study where packages are mounted below the board and the ASICs are cooled using the sheet metal tray as the heat spreader. The heat is transferred from the processor or the chip to the sheet metal tray through metallic blocks or heat spreaders by conduction and then the sheet metal tray is cooled convectively by airflow between two successive boards. There are various issues to be encountered when we do such a design for reverse mount packages like the maximum stress exerted on the ASIC, the kind of interface material that should be used to minimize the stress induced on the ASIC and also keeping in mind the thermal impedance of the interface material. The study presents simulation and experimental results that illustrate the concept of using the sheet metal tray as a heat spreader and also discuss potential applications of this concept. Considerations and precautions to be taken while executing this concept are also addressed.

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