Crack Growth Prediction in High-Power LEDs from TTA, SAM and Simulated Data

Crack growth in solder joints is one of the major issues for reliability of automotive lighting applications. Most modern high-power LED SMT packages are based on ceramic submounts. While this thermo-mechanical setup reduces mechanical stresses in the die it causes mechanical stress in the solder joint when the package is soldered onto printed circuit boards (PCB), due to the mismatch of the coefficients of thermal expansion (CTE) between the ceramic submount and PCB. Therefore, temperature changes during operation cause cracks in the solder joint, increasing the thermal resistance of the assembly. This in turn leads to increased operating temperatures of the LED causing a reduction of component lifetime. Cracks can be identified by acoustic microscopy (SAM) and Transient Thermal Analysis (TTA). However, while SAM allows to access the location of the cracks, TTA measures solely an averaged impact on the thermal resistance due to the crack. However, SAM is often not applicable due to package and board design. This paper correlates simulated and measured transient thermal data to SAM images to analyze the impact of the crack position on the TTA data. Digital twins (DT) of high-power LEDs, i.e., transient thermal FE models, are calibrated to TTA data by using an optimizer. Different crack patterns and sizes are introduced to the DT to analyze the impact of the crack location to the TTA data. It is found that the increase of the thermal resistance and the increase of the thermal capacitance depends on the distance of the crack to the heat generating die. Using this effect, it is possible to classify experimental TTA data towards standardized crack patterns, i.e., whether the crack is located in the thermal or in the electrical pads. A large data set of TTA and SAM data of LEDs which were aged by temperature shock testing was used to validate the method. In a first approach, an overall prediction accuracy of 70% was achieved to determine whether the crack is function-critical or mainly located in the electrical pads.