SummaryThis manuscript introduces a 3D Finite Element Model (FEM) to predict aging in packaged MEMS resonators which encounter stress relaxation as the primary source of frequency drift. Currently, the 10-year instability is estimated by fitting a logarithmic function to a shorter range of measured data (usually 1,000 hours). However, this standard approach is not based on a physical phenomenon and cannot be used to predict aging due to stress relaxation. The developed model considers the encapsulation and mount epoxies as linear viscoelastic materials and relates the resultant transient deformation to the MEMS frequency shift. Under these conditions, the 3D FEM can describe any complex package structure, as we verify experimentally for a piezoelectric MEMS resonator encapsulated by two types of Land Grid Array (LGA) package: full-glob and cap-glob.