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Characterization of Lead-Free Solder and Sintered Nano-Silver Die-Attach Layers Using Thermal Impedance

Fig. 1. Schematic diagram of the measurement system for the thermal impedance.
Since a die-attach layer has significant impact on the thermal performance of a power module, its quality can be characterized using thermal performance. In this study, a measurement system for thermal impedance is developed to evaluate three die-attach materials as shown in Fig. 1(a). In the developed system, thanks to its high temperature sensitivity (10mV/°C as shown in Fig. 1(b)), the gate-emitter voltage of an IGBT is used as the temperature-sensitive parameter. The power dissipation in the IGBT is maintained constant regardless of the junction temperature by a feedback loop. Experimental results show that the sample using sintered nano-silver for die-attach has 12.1% lower thermal impedance than the samples using SAC305 and SN100C solders as shown in Fig.2(a). To check the degradation of the die-attachment, three samples using three die-attach materials were thermally cycled from -40°C to 125°C. As shown in Fig. 2(b), the experimental results show that after 500 cycles, the thermal impedance of SAC305 samples and SN100C samples is increased by 12.8% and 15%, respectively, which is much higher than the sample using nano-silver paste for die-attach (increased by 4.1%).

Fig. 2. K factor of the measurement system.
Fig. 3. Thermal impedance of the samples using three different die-attach materials before thermal cycling.
Fig. 4. the change of the thermal impedance vs number of thermal cycling.
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