Mechanical Force
Mechanical force exerted on internal components by the protective soft polymer fill during thermal cycling or curing. Measuring silicone gel encapsulation stress is necessary for ensuring the long-term reliability of sensitive mems structures. The gel protects the die from moisture and contaminants but its coefficient of thermal expansion differs from the substrate.
This difference induces a parasitic pressure on the sensor diaphragm.
Temperature Effect
At low temperatures, the gel becomes firmer and exerts more force. This silicone gel encapsulation stress appears as a zero-point shift in the output signal.
Material Selection
Softer gels with lower Shore hardness values are typically selected to minimize the impact. The chemistry of the gel must also remain stable to prevent the silicone gel encapsulation stress from changing as the material ages. Tests are conducted to verify that the gel does not delaminate from the package walls.
Compensation Strategy
Calibration over temperature is the primary method for removing the predictable portion of this effect. Because the silicone gel encapsulation stress is repeatable, software algorithms can subtract the thermal offset from the measured pressure. Hysteresis introduces errors.
These residual shifts cannot be easily corrected if the polymer behaviour becomes non-linear at extreme temperatures.
