Evaluation of Residual Stress in MEMS Micromirror Die Surface Mounting Process and Shock Destructive Reliability Test

Light detection and ranging (LiDAR) devices based on micro-electromechanical systems (MEMS) micromirrors have been promising sensors for automatic driving due to the advantages of MEMS devices. The packaging plays a vital role in the reliability of MEMS micromirror but introduces the residual stress from the surface mounting process. The shock reliability of micromirrors is influenced by residual stress. In this work, the residual stress from the surface mounting process is investigated through resonant frequency tests. The frequency drift can be 718 and −281 Hz under the residual stress of 107.0 and −25.0 MPa, respectively. As the difference of coefficient of thermal expansion (CTE) rises, the residual stress proportionally increases but decreases with the thickness of the substrate. In addition, a higher curing temperature can introduce a more significant stress level. The dynamic response in a shock environment under different residual stress is analyzed. Drop fall experiments verify the theoretical conclusions that the stiffness hardening effect can improve the shock level tolerance. The tensile stress levels of 41.7 and 107.0 MPa are corresponding to 100 and 200 g, respectively. The influences of residual stress on frequency drift and reliability need to be considered seriously and simultaneously during the packaging design for MEMS micromirrors. This work provides a deep insight into the commercial applications of MEMS-based LiDAR in autonomous driving.