Investigations on High-Power LEDs and Solder Interconnects in Automotive Application: Part II - Reliability

Thermo-mechanical reliability is one major challenge in solid-state lighting for automotive applications. Mismatches in the coefficients of thermal expansion (CTE) between high-power LED packages and substrates paired with temperature changes induce mechanical stress. This leads to thermal degradation by crack formation in the solder interconnect and/or delamination in the substrate, which in turn increases junction temperature, thus decreasing light output and reducing the lifetime. A reliability study with a total of 1,800 samples - segmented in nine LED types and five solder pastes - is performed to investigate degradation and understand the influence of solder material and LED package design. The results are presented in two papers. Initial characterization of the LEDs was handled in the first paper. This second paper focuses on degradation and lifetime. Overall, more than 40,000 transient thermal analysis (TTA) and 9,000 scanning acoustic microscopy (SAM) measurements were taken to evaluate degradation during accelerated aging of 1,500 thermal shock cycles. Six different failure modes were observed, which were distinguishable by only using TTA data. For the reliability evaluation, crack ratio was determined by SAM images while thermal degradation as well as mean lifetime were determined using TTA data. Multiple observations were made within this study. First: SAM and TTA data correlated very well; Second: Higher silver content and additives in the solder paste reduce crack growth and increases lifetime; Third: Thick film ceramic LEDs reach significant longer lifetimes than thin film ceramic LEDs, and copper lead-frame LEDs reached by far the longest lifetimes; Fourth: A pad design with a greater pad size, smaller gaps and balanced size ratio between electrical and thermal pad is advantageous; Fifth: Voiding (below 10%) has no significant influence on the reliability.