Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyEvaluation of an Electrical Method for Detection ofDie Attach Imperfections in Smart Power SwitchesUsing Transient Thermal FEM SimulationsV. Košel 1,2 , M. Glavanovics 1 , E. Scheikl 31)KAI - Kompetenzzentrum Automobil- und Industrie- Elektronik GmbH, Villach, Austria2)Slovak University of Technology in Bratislava, Slovakia3)Infineon Technologies, Villach, AustriaAbstract – A method for detection of die attach imperfectionsin Smart Power Switches is evaluated using non-lineartransient thermal FEM simulations and measurements. Thismethod employs smart functions of SPS devices such as overcurrentand over-temperature protection. The protectivethermal shut down function is triggered by the temperature ofthe integrated temperature sensor. The temperature risedepends on power dissipation in the device and on the thermaljunction to ambient impedance which is inter alia related to thedie attach imperfection. The evaluation is performed on a twochannel Smart Power Switch. Results are focused on practicalrelevance and sensitivity of this method to the influence of dieattach thickness and die attach voids.temperature increase and the switch temperature mightlocally reach a critical level. To avoid such a situation in theapplication, SPS devices have to be tested for die attachquality before being delivered to the customer. The possibleirregularities in the die attach are voids, 1D or 2D cracks,cavities at the chip border which are later filled by moldingcompound and also an excessive tilt of the die (see Fig. 1).I. INTRODUCTIONAutomotive industry is well-known for high requirementson electronic devices’ reliability and quality. Whereas forelectronic devices used in domestic appliances the qualityand functionality of the products is usually determinedstatistically on selected samples only, in automotiveelectronics every single device has to be tested for allelectrical parameters that are specified in the datasheet.Additionally, automotive power devices are tested for agroup of physical parameters involving die attach quality.The quality of a die attach is determined by the die attachthickness, by the coverage of the whole backside of the diewith an attaching material (glue or solder) and by the qualityof the interfacial mechanical contact of the die attach to thedie back side and to the leadframe. Even minor localimperfections may be obstacles for the heat to diffuse out ofthe device into the environment.In applications under extreme operating conditions,temperature inside a power switch can exceed 200 °C [1]. Inan Smart Power Switch (SPS), the integrated smart thermalprotection keeps the switch temperature at a level whichguarantees that the switch temperature does not exceed therange of reliable operation. However, if some interface orvolumetric irregularities are introduced in the device’spackage, under certain transient conditions it might happenthat the thermal protection is not able to detect a local rapid(a)(c)Fig. 1 Common imperfections in the die attach, (a) voids, (b) voids andmolded chip border cavities, (c) tilt of the die.In [2], we have presented a method for detection of dieattach irregularities. Both advantages and disadvantages arediscussed. In that method the amount of imperfection wasrelated to a parameter which expressed the efficiency of heatremoval from the device after heating the device up. Thedevice was heated using the intrinsic reverse diode of theDMOS based power switch.In this paper we evaluate another method which can beused to test the die attach quality of Smart Power Switchesdirectly in the production. In contrast to the method from [2],this method uses a decision parameter which is related to theheating phase in the regular operating condition of theintegrated power transistor. The heating phase is terminatedby a shut down triggered by the thermal protection circuit.(b)©EDA Publishing/THERMINIC 2008 204ISBN: 978-2-35500-008-9