Polyimide Stress Relaxation Spectrum Shift Factors under Automotive Cycling Limits
Polyimide stress relaxation under automotive thermal cycling follows non-linear shift factors, driving zero-offset calibration drift in precision sensors.
MEMS Zero Offset Drift
Static Error
Temporal changes in the output signal of a micro electromechanical system at zero input define this stability metric. Factors contributing to mems zero offset drift include internal stress relaxation in the silicon structure and changes in the ambient temperature. This phenomenon results in a gradual departure from the factory calibration.
Thermal Influence
Expansion differences between the sensor die and the package material introduce parasitic forces onto the sensing element. Because mems zero offset drift is often repeatable with temperature cycles, compensation algorithms can mitigate much of the error. Uncompensated sensors show measurable bias when moving from a cold start to steady state operation.
Long-term Stability
Aging of the mechanical components and the electronic conditioning circuitry leads to a stochastic variation in the baseline signal over months or years. A qualification engineer measures mems zero offset drift by monitoring the sensor output in a vibration isolated and thermally controlled environment for several thousand hours. This testing identifies the Allan variance which characterizes the noise processes.
Moisture ingress into the package can also alter the capacitive or piezoresistive properties of the bridge.
System Compensation
Software filters use historical data to nullify the effect of the drift in real time. The mems zero offset drift determines the frequency at which a system must be re zeroed.