Drop simulation and stress analysis of MEMS devices

Drop testing of micro-machined accelerometers from the height of a table top to a solid surface shows that a moderate impact can result in severe damage of transducer elements. The relative high stiffness of the accelerometer device in combination with a high contact stiffness of the solid surface cause extremely high acceleration pulses at the impact. This paper presents a detailed analysis of the consequences of dropping a micro-machined transducer structure to a solid surface. The analysis is composed of experimental testing and numerical simulation. Impact forces are measured for bare sensor chips and molded sensor devices by means of an instrumented drop test. Structural simulation models are generated for micro-machined transducers. These models consider the dynamics of the deformation behavior of moveable elements including a travel stop and associated possible impact inside the sensor element. Maximum stresses are calculated in critical regions of the transducer. Weibull theory and statistical distributions of material strength are considered in order to predict the probability for crack initiation due to stress concentrations.