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24-26 September 2008, Rome, Italy0.40.35PrototypeDefect0.30.25C0.20.150.10.0500 2 4 6 8 10 12 14 16 18RFig. 9.Structure functions for the fourth heat source[7] M. E. Van Valkenburg, Modern Network Synthesis, Wiley, New York,1960.[8] M. Kline, I. W. Kay, Electromagnetic Theory and Geometrical Optics,John Wiley & Sons, 1965.[9] L. Codecasa, D. D’Amore, P. Maffezzoni, “Physical Interpretation andNumerical Approximation of Structure Functions of Components andPackages,” Proc. SEMI-THERM XXI, Vol. 1, pp. 146-153, 2005.©EDA Publishing/THERMINIC 2008 13ISBN: 978-2-35500-008-9
24-26 September 2008, Rome, ItalyTransient Dual Interface Measurement of the Rth-JCof Power PackagesDirk SchweitzerInfineon Technologies AGAm Campeon 1-1285579 Neubiberg, GermanyAbstract - The accurate and reproducible measurement of thejunction-to-case thermal resistance Rth-JC of powersemiconductor devices is far from trivial. In the recent timeseveral new approaches to measure the Rth-JC have beensuggested, among them transient measurements with twodifferent interface layers between the package and a heat-sink.The Rth-JC can be identified either in the structure functionsor at the point of separation of the two Zth-curves or theirderivatives. Further investigations revealed however that thelatter approach is restricted to power packages with solder dieattach and cannot be applied to devices with thermally lowconductive glue die attach since an internal heat flow barrierfalsifies the measurement result. After recapitulating thetransient dual interface measurement and its evaluation usingthe derivatives of the Zth curves, a detailed investigation of thismethod by means of Finite Element simulations is presentedherein.I. INTRODUCTIONDue to the ever increasing power dissipation levels ofpower semiconductors a low junction-to-case thermalresistance Rth-JC is regarded as more and more important.For devices operating near the thermal limit it is no longersufficient to state just an upper limit for the Rth-JC value anda lower Rth-JC is also a competitive advantage for thesemiconductor manufacturer. On the other hand it must beensured that the data-sheet Rth-JC doesn’t underestimate thereal value. Thus accurate and reproducible methods tomeasure the Rth-JC are required. Unfortunately theserequirements are not easy to meet, which is reflected also bythe fact that to date no JEDEC standard for the measurementof Rth-JC has been defined. The traditional measurementmethod follows the definition of Rth-JC in JESD51-1 [1]:Thermal resistance, junction-to-case: The thermalresistance from the operating portion of a semiconductordevice to outside surface of the package (case) closest to thechip mounting area when that same surface is properly heatsunk so as to minimize temperature variation across thatsurface.The traditional method thus requires the measurement ofthe junction temperature T J , the case temperature T C , and thepower dissipation P. The junction-to-case thermal resistanceis calculated using:TJ− TCRthJC=(1)−PThough widely accepted it should be noted that, strictlyspeaking, eq. (1) is not even well-defined. Neither junctionnor case temperatures are truly uniform in the chip and casearea respectively. It is not exactly specified whether T J andT C refer to the maximum or to some average values of thetemperature distributions on chip and package surface.Besides the ambiguities of this definition, it involves thedifficulty to accurately measure the package casetemperature while it is in close contact with a heat sink. Ahole or groove in the heat sink must be provided for thethermocouple wires; this however will distort thetemperature field and therefore have an impact on theresults. Furthermore it is difficult to ensure that thethermocouple actually measures the case temperature andnot the temperature of the heat-sink or some average value inbetween. Although the reproducibility of results obtainedwith the same measurement set-up is often quite good –meticulous mounting of thermocouple and device provided –different cold-plate set-ups are likely to produce deviant Rth-JC values.To overcome these problems, transient measurementmethods have been proposed, e.g. by Siegal [2] whosuggested to evaluate the heating curve. While in [2] theproblem of determining the thermal interface resistancebetween package and heat sink remains unsolved, Szabo etal. presented a method to identify the Rth-JC comparing thestructure functions obtained from two transientmeasurements with different interface layers betweenpackage and heat sink [3] [4], referred to herein as transientdual interface measurement (TDIM). The latter approach inprinciple allows distinguishing between the Rth-JC of thepackage and the thermal interface resistance. However, itsaccuracy is limited due to numerical problems arising duringthe generation of the structure functions, and the evaluationof the structure functions can be quite difficult. A thoroughdiscussion of the chances and limits of the structure functionmethod can be found in [5]. Also in [5] we presented analternative evaluation of the TDIM measurement, based onthe comparison of the derivatives of the Zth curves. Incontrast to the structure function method the Rth-JC can bedetermined automatically by a computer program. While our©EDA Publishing/THERMINIC 2008 14ISBN: 978-2-35500-008-9
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