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

Since a die-attach layer has a significant impact on the thermal performance of a power module, its quality can be characterized using thermal performance. In this paper, a measurement system for thermal impedance is developed to evaluate three die-attach materials. Thanks to its high temperature sensitivity (10 mV/degrees C), the gate-emitter voltage of an insulated gate bipolar transistor (IGBT) is used as the temperature-sensitive parameter. The power dissipation in the IGBT remains constant by a feedback loop, regardless of the junction temperature. Experimental results show that the sample using sintered nano-silver for the die-attach has 12.1% lower thermal impedance than the samples using SAC305 and SN100C solders. To check the degradation of the die-attachment, six samples using three die-attach materials were thermally cycled from -40 to 125 degrees C. The experimental results show that, after 500 cycles, the thermal impedance of SAC305 samples and SN100C samples is increased by 12.9% and 13.3%, respectively, which are much higher than that of the sample using the sintered nano-silver for the die-attach (3.1%).