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High-power packages show a characteristic three-dimensional heat flow resulting in large lateral changes in chip and case surface temperature. This paper proposes an unambiguous definition for the RthJC junction-to-case thermal resistance as a key parameter of such packages based on a transient measurement technique ensuring high repeatability even at very low Rth values. The technique is illustrated on thermal transient measurements of high-power MOSFET devices. It is also presented how the same measurement results can be used for die attach quality analysis. Finally, a comparative method is shown for measuring the differences of Rth values among samples with many times higher resolution compared with a direct RthJC measurement.