Correlation of Scanning Acoustic Microscopy and Transient Thermal Analysis to Identify Crack Growth in Solder Joints

Solder joint reliability is one major issue in the solid-state lighting industry. Most lighting applications use white high-power LEDs with a ceramic submount soldered onto an Al-MCPCB (aluminum metal core PCB). This thermo-mechanical setup relieves the LED die from mechanical stress. However, now the solder joint becomes the bottleneck of the application due to the high CTE (coefficient of thermal expansion) mismatch between aluminum and ceramic. As a result, crack growth is initiated in the solder joint during aging, adverse for the heat dissipation from the LED. This increases the operation temperature of the LED leading to a reduced efficiency, a reduced lifetime, and a change in color. To obtain a better understanding of the thermal impact of solder joint cracks, this paper consolidates transient thermal analysis (TTA) and scanning acoustic microscopy (SAM) with a calibrated transient finite element (FE) model for a high-power LED. TTA data before thermal shock cycling is used to calibrate the FE model by an optimizer tool. Thereby a coefficient of determination of 0.9999 was achieved for the model. On the one hand, this optimized model was used to determine the thermal influence of crack size and location for dedicated sample LEDs after 1000 thermal shock cycles to further validate the accuracy. On the other hand, the optimized model was used to identify the most critical regions for crack growth. Hereby the relevance for heat dissipation of the thermal pad of the LED package was found to be significantly higher than one would expect from the geometrical dimensions.