Characterization of lead-free solder and sintered nano-silver die-attach layers using 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 paper, a measurement system for thermal impedance is developed to evaluate three die-attach materials. Thanks to its high temperature sensitivity (10 mV/°C), 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. To check the degradation of the die-attachment, three samples using three die-attach materials were thermally cycled from -40°C to 125°C. 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%).