Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyD. ResultsFEM analyses were performed for different solderthickness in the range of 20 µm to 130 µm to determine theinfluence of the die attach thickness on the thermal shutdown time. The solder void configurations were used as anadditional parameter. The current limit was considered to be5.5 A, which corresponds to a power dissipation of 77 Wgenerated in the switch. The ambient temperature was 27 °C.In Fig. 5, the simulated influence of solder thickness andvoid configurations on the thermal shut down time isdepicted. The results have shown that:• This method is reasonably sensitive for a die attachthickness of less than 80 µm, if a group of devices withthe same or similar irregularities are tested.• However, in the same range it is difficult to distinguishbetween a thinner die attach having voids and a thickerdie attach containing no voids at all.• On the contrary this method has good sensitivity to bothsmall and larger voids in the die attach thickness rangefrom 100 µm to 130 µm.tsd [ms]1716No irrregularitesIrregularity 0415Irregularity 0514Irregularity 0613Irregularity 0712Irregularity 01111098765420 30 40 50 60 70 80 90 100 110 120 130Die attach thickness [µm]Fig. 5 Die attach thickness vs. shut down time. The die attach irregularitiesare used as a parameter. The black solid curve means no irregularities in thedie attach. Other curves correspond to void shapes according to Fig. 4.The investigation has been performed for a broad range ofdie attach thicknesses. In production, this range is usuallycentered around 50 µm. If a die attaching process with wellcontrolled die attach thickness is available, this method maybe applied to sort out imperfect devices with good accuracy.However, in the range of 20 µm to 90 µm, this would requirean accuracy of the die attach process of about 5 µm or less.For a die attach thickness larger than 90 µm, a sufficientprocess accuracy might be about 10 µm, but a bettershutdown time measurement resolution will be required.In order to show the applicability of this method to the realSPS devices, measurements of shut down time on real SPSdevices were performed under similar conditions asconsidered in the simulations except for ambienttemperature. The channel 2 (Ch2) of the samples wasswitched on in short circuit at an ambient temperature of−40 °C. The block diagram of the measurement setup isshown in Fig. 6.Voltage supplyPulse generatorV+I DIn2CurrentprobeDUTOut1C1SynchronizationV DSC2Detection ofshut down timeOscilloscopet sdGNDFig. 6 Block diagram of the measurement setup to measure shut down time."DUT" indicates the SPS device under test, V+ is the positive voltage andV DS is the voltage across the power switch. In2 is a signal to activate theDUT channel 2 and is also used to trigger the current measurement recordedby the oscilloscope channel C2.SAM pictureCh1Ch2#1#2#3#4#50 20 40 60t sd[ms]Fig. 7 Measurement of shut down time of the channel 2 for 5 samples whereeach sample has a different content of voids in the die attach. The channel 2is heated up in short circuit at an ambient temperature of −40 °C. Theirregular white areas in the SAM pictures indicate voids in the die attach.The SAM picture of the sample 1 shows a die attach without voids. In thesame picture the approximate position of the die and DMOS transistors isdepicted.©EDA Publishing/THERMINIC 2008 206ISBN: 978-2-35500-008-9