Thermomechanical stress in GaN-LEDs soldered onto Cu substrates studied using Finite Element Method and Raman spectroscopy

Local thermomechanical stress can cause failures in semiconductor packages during long time operation under harsh environmental conditions. This study helps to explain the packaging induced stress in blue GaN-LEDs soldered onto copper substrates using AuSn alloy as lead-free interconnect material. Based on the finite element method, a virtual prototype is developed to simulate the thermomechanical behavior and stress in the LED and in the complete LED/AuSn/Cu assembly considering plastic and visco-plastic strain. The investigations are in the temperature range between 180 and -50 °C. To validate the model, the simulation results are compared to experimental data collected with Raman spectroscopy. Studies of the E_2^H phonon mode of GaN semiconductor are elaborated to understand the induced thermomechanical stress. The model enables evaluation of the stress in the interfaces of the assembly, which otherwise cannot be accessed by measurements. It serves to predict how assemblies would perform, before committing resources to build a physical prototype.