Multi-scale analysis of polysilicon MEMS sensors subject to accidental drops: Effect of packaging

The effect of packaging on the impact-carrying capacity of micro electro-mechanical systems (MEMS) is investigated, with specific reference to a translational accelerometer. By exploiting the small ratio between the masses of MEMS and package/die (typically 10 − 3 or less) a decoupled two-scale, finite element approach is adopted: at the package/die length-scale the dynamics of whole device after the impact against a flat target surface is studied; at the sensor length-scale the response of the MEMS to the drop-induced loading is investigated, and MEMS details where the stress state can exceed the tensile strength of polysilicon are identified. Two drop orientations are considered, here termed bottom and top; in the first case, package and die strike the target with their bottom surfaces; in the second case, they fall upside-down, and strike the target with their top surfaces. By comparing the simulation outcomes in terms of maximum attained tensile stress, it results that package does not always lead to benefits in term of capability of the studied sensor to sustain drops. In the bottom drop configuration, e.g. MEMS failure may be triggered by the package.