LEDs Lifetime Prediction Modeling: Thermomechanical Simulation for SAC305 and SAC105

The challenge of predicting the reliability of solder joints in high-power LED packages by Finite Element Analysis (FEA) is crucial for the automotive industry. Focusing on the thermo-mechanical stresses between LED package and the PCB, i.e. the aluminum insulated metal substrate (Al-IMS), this paper analyzes the crack formation in the solder joints for seven different ceramic-based LED packages. The model is validated by data from a large experimental study, covering typical two pad and three pad footprint design for LED package and five solder materials. The data set contains experimental data of nearly 1,800 LED packages, including over 40,000 Transient Thermal analyses (TTA) and 9,000 Scanning Acoustic Microscopy (SAM) measurements. The paper investigates different post-processing approaches related to FEA simulations for surface-mounted devices (SMD) LED assemblies, recognizing the limitations of approaches used for Ball Grid Array (BGA) packages due to the large difference in geometry and pad layout. The primary goal of the FEA model is to correctly predict the reliability of different pad designs, i.e. two or three pad footprints for different solders and develop the universal lifetime model. The methodology includes detailed thermomechanical simulations via Ansys. The paper compares the behavior of SAC305 and SAC105. Each LED package undergoes five thermal cycles, ranging from - 40∘C to 125∘C, to assess accumulated creep strain. The Coffin Manson model is utilized for the lifetime prediction of the solder joint. To the best of the author's knowledge, the paper presents for the first time a model which is validated for different pad designs and answers the question: Is a three-pad or a two-pad preferable for solder joint reliability? The results show a good agreement, i.e. R²=96% for critical path averaging, of simulation outcomes for ceramic LED packages with thick film metallization. However, some variability is noted for ceramic LED packages with thin film ceramic substrates, i.e. these packages show a significant amount of error in lifetime predictions. The study enhances understanding of the reliability of different LED package designs and solder materials, aiming to improve prediction accuracy and FEA post-processing techniques to evaluate the SMD packages and develop a lifetime model that can be adapted to different LED designs.