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Abstruct -Thermal-mechanical stresses occur in plastic IC packages. These stresses affect Hall plate magnetic sensitivity via the piezo-Hall effect. In this paper the short and long-term stability problems of Hall plates encapsulated in SOP and TSSOP packages are considered. A sensitivity shift is observed when reflow soldering, temperature cycling, or humidity testing are performed. Moreover, this shift is not stable in time and a slow relaxation is observed. This parameter shift is seen as a serious reliability failure of the Hall plates. Sensitivity drift is related to packaging stress drift due to the viscoelastic flow of the molding compound. The normal in-plane stress drift is calculated from the high-accuracy magnetic measurements.