Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyBoth advantages and disadvantages of this method arepresented. The method is evaluated on a FEM model of areal two channel Smart Power Switch using transient nonlinearthermal FEM simulations and by measurements.II.EVALUATION OF THE MEASUREMENT METHODA. Principle of the methodTo apply this method to an SPS, the SPS has to contain anover-current protection with current limitation and a thermalprotection. The thermal protection has to involve a thermalsensor directly integrated on the chip and a well-definedover-temperature shut down level T sdth . The die attachquality test is based on the heat removal from the device.The device is switched on in a load short circuitconfiguration. Under this condition the over-currentprotection activates the current limitation. Inside the device alarge amount of heat is generated and the temperatureincreases rapidly dependent on the irregularities in the dieattach. At a certain time the thermal protection detects thatthe over-temperature shut down level is exceeded at thetemperature sensor and turns off the switch, as shown inFig. 2. Because the temperature at the sensor depends on theirregularities, the shut down time t sd will be affected as well.Voltage 14VINT sdthTimeproperties are taken from manufacturers' datasheets for everymodeled part respectively. Thermal material properties ofthe silicon chip are modeled as temperature dependentaccording to the material models presented in [4], which wehave validated for simulations of Smart Power Switchesin [5].Fig. 3 The 3D FEM model of the SPS device from two viewing angles.C. Modeling of die attach imperfectionsIn the presented simulations one particular kind ofimperfection is investigated, namely solder voids in the dieattach. Different void configurations are illustrated in Fig. 4.To reduce modeling time and complexity, the die attachvoids are modeled as cuboids and share the same positionand shape in the horizontal plane as the power switch activeregions.INSPS deviceT sensorQuick selfheating upTimeI switchT AmbientStartt sdThermal shutdown(a)Fig. 2 Schematic principle of the measurement method, (a) electricalscheme (IN is an input signal to activate the SPS device), (b) temperatureevolution during short circuit event at the integrated temperature sensor andconsequent thermal shut down as a response of the thermal protection.B. SPS device and its FEM modelFor the investigation, a two channel Smart Power Switchin a P-DSO-12 power package was chosen. The die attach inthis package is created by a reflow soldering process. Thisdevice is based on a contemporary smart power technology[3] and incorporates all features needed for applying themeasurement method. The thermal shut down level of thedevice is approximately 185 °C and the current limit is in therange of 5.5 A to 6.7 A. The nominal supply voltage is 14 V.The device was modeled in a FEM simulator (FlexPDEversion 4) using the extrusion technique to generate 3Dstructures out of 2D regions (see Fig. 3). The geometricalmodel involves a couple of simplifications. No bond wiresand pins are modeled. From all the integrated circuits in thedevice only active regions of the power transistors aremodeled. This can be justified because in this die attachrelatedanalysis the heat is generated only for severalmilliseconds and therefore remains concentrated in thepackage for the regarded time span. Thermal material(b)Geometrical model Irregularity 01Irregularity 04 Irregularity 05Irregularity 06 Irregularity 07Position oftemperaturesensorFig. 4 The black thin layer in the figure denoted as “Geometrical model” isthe die attach. Other figures show investigated die attaches containingsimplified solder voids (white areas indicate voids in the die attach). Thepictures show a horizontal cross-section of the die attach in the FEM model.©EDA Publishing/THERMINIC 2008 205ISBN: 978-2-35500-008-9