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With the increase of driving current and power density, the thermal fatigue characteristics of die attach materials have become an important issue impacting the long-term reliability of high-brightness light-emitting diodes (HB LEDs). In this paper, an accelerated power cycling method is used to predict the thermal fatigue life of die attach materials for packaged HB LEDs. The thermal fatigue life of Au80Sn20 eutectic and silver paste is investigated at driving currents of 650, 675, and 700 mA. Changes in the thermal resistance of die attach materials are monitored by the noninvasive structure function method based on the measurement of transient temperature response curves through power cycling experiments. Results of C-mode scanning acoustic microscopy and cross-sectional scanning electron microscopy indicate that delamination is the main failure mechanism of the die attach materials under thermal fatigue stress. Furthermore, the thermal fatigue life of die attach materials can be derived from the relationship between lifetime and temperature difference according to the Coffin-Manson relationship. The results suggest that Au80Sn20 eutectic possesses a longer thermal fatigue life than silver paste at the same temperature difference of thermal fatigue stress.