The reliability of microelectromechanical systems (MEMS) in shock environments

As a first step toward formulating guidelines for the design of dynamically reliable MEMS, we analyze the mechanical response of, and formulate failure criteria for, a large class of shock loaded microsystems. MEMS are modeled as microstructures attached to elastic substrates, and the shocks are modeled as pulses of acceleration applied to the substrate over a finite time duration. The relevant time scales in the analysis are the acoustic transit time, the time period of vibrations, and the duration of the applied shock load. For many MEMS structures and shock loads (with durations in the range 50-5000 /spl mu/s), the substrates respond as rigid bodies and are expected to be immune to stress-wave-induced damage. Time-domain criteria, obtained to distinguish between the impulse, resonant, and quasistatic responses of the microstructures, correlate well with the experimentally observed responses of different MEMS devices. The formulation of displacement-based and stress-based failure criteria is discussed, along with their sensitivity to the applied strain rate. A case study, in which these results are applied to evaluate the reliability of a packaged surface-micromachined device, is presented.