Thermal stability evaluation of die attach for high brightness LEDs

The long term thermal stability of the die attach is a crucial issue for high brightness light emitting diodes (HB LEDs), which affects the junction-to-case thermal resistance, luminous flux and life time seriously. In this paper, an improved power and temperature cycling method is proposed to evaluate the thermal stability of the die attach materials for HB LEDs. The structure function method is adopted to monitor the degradation of the die attach level thermal resistance during the cycling process instead of the traditional junction-to-case thermal resistance measurement, which provides more accurate, quick and intuitive results. The experimental results indicate that the forming of solder voids is the main degradation mechanism of the die attach for HB LEDs, which is also supported by the scan acoustic microscope (C-SAM) measurement. Comparing thermal stability of different die attach materials, Au/Sn eutectic soldered LED samples present better performance than Ag paste soldered samples in this experiment.