Modelling Thermo-Mechanical Stress in GaN-LEDs Soldered on Copper Substrate with Simulations Validated by Raman Experiments

In semiconductor devices manufacturing, various materials with different physico-chemical characteristics are connected and over hundreds of sequenced processing steps are necessary. In this regard, thermomechanical stress due to compressive and tensile strain is a serious aspect inside the device. Residual thermomechanical stress due to large difference in coefficients of thermal expansion between the materials generates reliability problems not only at the bonded interfaces but also for the lifetime of the active regions of the high power semiconductors. In this study, blue LEDs based on gallium nitride (GaN) bonded to a silicon carrier by a gold layer were soldered with eutectic gold-tin (AuSn) on a copper board or an aluminum insulated metal (Al-IMS). For both boards a reflow process in presence of formic acid vapor was used for soldering. The assemblies were studied using Raman spectroscopy. A finite element model was developed to simulate the thermomechanical stress present in the assemblies. Measured and simulated values were compared and evaluated at room temperature, at -50°C and at 180°C. The results suggest guidelines for the optimization of the assembling process of LED-based microelectronic devices.