Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyAssessment of Die Attach Quality by Analysisof Circuit Thermal Response SpectrumM. Janicki † *, B. Vermeersch # , J. Banaszczyk # , M. Kaminski *, G. De Mey # , A. Napieralski *†Radon Institute for Computational and Applied Mathematics, Austrian Academy of Sciences,Altenbergerstrasse 69, 4040 Linz, Austria* Department of Microelectronics and Computer Science, Technical University of Lodz,Al. Politechniki 11, 93-590 Lodz, Poland#Department of Electronics and Information Systems, University of Ghent,Sint Pietersnieuwstraat 41, 9000 Ghent, BelgiumAbstract-This paper investigates the possibility of assessingthe die attach quality by the spectral analysis of the recordeddevice time response. The conducted analyses of the measuredand the simulated time constant spectra of the thermal responseallowed the explanation of the anomalous electrical oscillationsobserved during the operation of a power converter.I. INTRODUCTIONThe advanced dynamic thermal analysis tools, developedbased on the Network Identification by Deconvolution (NID)method [1], became standard instruments in thermal designand management of electronic systems. Recent publicationsdemonstrate also that some of these tools, in particular thestructure functions, can be applied to solve problems notnecessarily strictly related to the thermal analysis such as thefault detection, identification of material physical propertiesor structure geometry [2]-[4].Here, the time constant spectra of the dynamic thermalresponses will be used to obtain information on the internalstructure of the Schottky diodes used in a power converter.This information allowed the explanation of the anomalousbehaviour of the converter. The following section defines theparticular problem encountered during the operation of theconverter. Then, the possible cause for the observed effectis explained based on the results of thermal measurementsand analyses.II. DC-DC CONVERTER OPERATIONPower converters are important electronic componentscommonly encountered in virtually any electrical appliance.The most important parameter for the assessment of theiroperation is the energy efficiency, which is directly relatedto the losses generated in the circuit. Typically, these lossesconsist of the conduction and the switching losses. Becauseof the current tendency to increase the switching frequencyof the converters, the latter ones are dominant in state-of-theartapplications. For these reasons, the dynamic behaviourof active devices, such as diodes and transistors, is of greatimportance for the converter performance [5].The converter considered in this paper was operated in theContinuous Current Mode and equipped with a Power FactorCorrection circuit (CCM-PFC). From energy performancepoint of view, the crucial role in such a converter is playedby the diode because its turn-off current flows by the shunttransistor, thus increasing its drain current. Hence, the diodereverse recovery charge affects the portion of the energystored in the output capacitor, which is then dissipated on theswitching side, and consequently influences the converterefficiency [6].Thus, the development of commercially available siliconcarbide unipolar Shottky diodes has created new possibilitiesfor power circuit designers. Theoretically, silicon carbidewith its high breakdown voltage and thermal conductivityoffers much better performance [7]. Unfortunately, so far theadvantages from the introduction of silicon carbide devicesare limited by other factors such as the lack of appropriatepackages or the immaturity of the silicon carbide technology.This paper will illustrate this problem based on the exampleof two different silicon carbide diodes used in the converter.A. Circuit DescriptionThe circuit, presented in Fig. 1, was assembled accordingto the application note [8] and contained an integrated dioderectifying bridge, a boost converter and the integrated PFCcontroller ICE1PCS01. The circuit supplied with 190-260 VAC voltage was capable of yielding 550 W power at 400 Voutput voltage. Such a circuit, equipped additionally withan EMI input filter and a forward converter, could constitutea typical example of a state-of-the-art Switched Mode PowerSupply (SMPS) circuit.The shunt transistor T1 was a commercial 21 A, 500 V,power MOS transistor. As the boost diode D1, two typesof silicon carbide Schottky diodes provided by differentmanufacturers were used. Both diodes were placed in theTO-220 packages and had the same forward current andvoltage blocking ratings of 4 A and 600 V. During all themeasurements the diode forward current was set to 2A andthe converter switching frequency was 130 kHz [9].©EDA Publishing/THERMINIC 2008 43ISBN: 978-2-35500-008-9