Modeling Package-Induced Effects on Molded Hall Sensors

The consideration of package-induced effects during the design process of microelectromechanical systems (MEMS) sensors is still of major importance in order to estimate its influence on performance and reliability. A method for predicting the influence of the packaging and assembly process on the performance of a Hall sensor is described. The geometry of the device, the behavior of the packaging materials, as well as the packaging process are regarded using this method. At first, the complex material behavior of adhesives and molding compounds was investigated. It is shown, that for modeling the influence of the packaging on the sensor performance correctly, the nonlinear temperature- and time-dependent material behavior has to be taken into account. Differences of the coefficients of thermal expansion of the packaging materials lead to thermo-mechanical stress in the package. Due to the time-dependent material properties, this results in a change of the offset and the sensitivity of the sensor over time and may lead to a sensor working outside its specification. The piezoresistive and piezo-Hall coefficients of semiconductor Hall plates were measured; thereby the stress- and temperature-dependency of the Hall plates were determined. According to the results, package-induced stresses lead to a change of the offset voltage up to 80% of the full-scale while the sensitivity changes by plusmn4 percentage.