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The first group contains solder die attach devices in TO stylepackages (4 samples), the other two groups are devices withglue die attach (thermal conductivity of the glue: 1.6 W/mK)in exposed pad packages (6 samples each). The glue dieattach devices in both groups differ only in die attachthickness which was approximately 30μm in the first and50μm in the second group. With each sample we performeda TDIM measurement and determined the Rth-JC using boththe structure function approach and the derivatives of theZth-curves as described above.While both methods yield about the same Rth-JC value forthe solder die attach devices (avg. deviation only -1.4%), theevaluation of the Zth-derivatives results in lower Rth-JCvalues for the devices with thin glue die attach (avg.deviation -5.8%) and even more so for the devices with thickglue die attach (avg. deviation -11.8%). This confirms theresults of the 1D analysis: the higher the thermal resistanceof the internal heat flow barrier, the more the steady-stateRth-JC and the diverging point of the Zth-curves and theirderivatives will differ.VI. CONCLUSIONSFor power devices with thermally high conductive (solder)die attach the point of separation of the Zth-curves allows todetermine the Rth-JC more accurately than the traditionalthermocouple measurement. This has been confirmed bycomparison with Rth-JC values obtained from the structurefunction and by finite element simulations (the latter notbeing presented herein, see [5]). The point of separation isbest defined by the difference |Δ(da/dz)| of the slopes of thetwo Zth-curves, although a theoretically justifiable value forthe limit ε for which the curves are still regarded as equalcould not be defined. This is due to the fact that ε dependson geometry and material parameters of the package as wellas on the thickness and thermal conductivity of the interfacelayers used in the TDIM measurement. The definition of ε asthe maximum amplitude of the “noise” of |Δ(da/dz)| yieldsconsistent and reproducible results but remains neverthelessunsatisfactory since it depends on the measurement equipmentand on the algorithm used to compute the derivative(da/dz). A better definition of ε is subject to further investigation.For devices with an internal heat flow barrier such as lowconductive thermal glue it has been shown that the “transientRth-JC”, defined by the diverging point of the Zth-curves, islower than the steady-state Rth-JC to be measured. Thereforefor these devices we suggest to determine the Rth-JC byevaluation of the structure functions [5].24-26 September 2008, Rome, ItalyREFERENCES[1] Electronic Industries Association, Integrated Circuit ThermalMeasurement Method – Electrical Test Method, EIA / JEDECStandard, JESD 51-1, 1995 [www.jedec.org].[2] B. Siegal, “An alternative approach to junction-to-case thermalresistance measurements”, Electronics Cooling Magazine, vol. 7,No. 2, pp. 52-57, 2001.[3] P. Szabo, O. Steffens, M. Lenz, and G. Farkas, “Transient junctionto-casemeasurement methodology of high accuracy and highreproducibility”, Proc. 10 th THERMINIC, Sofia-Antipolis, pp. 145-150, 2004.[4] O. Steffens, P. Szabo, M. Lenz, and G. Farkas, “Thermal transientcharacterization methodology for single chip and stackedstructures”, Proc. 21 th SEMITHERM, San Jose, pp. 313-321, 2005.[5] D. Schweitzer, H. Pape, and L. Chen, “Transient measurement ofthe junction-to-case thermal resistance using structure functions:chances and limits”, Proc. 24 th SEMITHERM, San Jose, pp.193-199, 2008.©EDA Publishing/THERMINIC 2008 19ISBN: 978-2-35500-008-9