Fatigue damage of automotive LEDs: Experimental approach and thermomechanical model

The automotive lighting industry continues to experience significant growth as it embraces new trends focused on higher performance and smarter functionalities. Any new products in automotive must undergo severe accelerated tests in order to ensure its reliability under the various conditions that it may be confronted with during its lifetime. Due to the coefficient of thermal expansion mismatch between the LED package and the PCB substrate, the solder joint fatigue failure was revealed to be one of the major failure modes during accelerated thermal cycling. The present work focuses on analyzing three LEDs (2-pads, 3-pads and 4-pads) configurations subject to thermal cycling test. First, the damage in the solder joints during tests is observed with dye penetration at different stages of cycling. During the tests, monitoring of the variation of ΔV F was made to follow the performance evolution of LED as a function of cycles. The dye penetration observations were analyzed to follow the evolution of the damage. On the 3 configurations of LEDs (2-pads, 3-pads and 4-pads) a model predicting the damage as function of the ΔV F was defined and used to establish a damage law. The second part is focusing on a thermomechanical modeling of the strains on the solder SAC305 joint occurring during thermal cycles for the 3 configurations of LEDs mounted on the type of IMS PCB. Finally, a fatigue analysis is addressed based on the 3 configurations of LEDs on the time to failure in the thermal cycle test. A comparison between three configurations is made and a correlation with test results is performed to evaluate the accuracy of the prediction. • LED modules subject to accelerated reliability testing • Measurable parameter as damage indicator evaluation • Formulation of solder joint damage model • Thermomechanical Model and damage measure based on Plastic Cumulative Strain • Thermal fatigue life predictions and correlation with experimental results