Mechanical Force
Interfacial mechanical load acting parallel to the surface of a sensor chip results from the differential thermal expansion of the silicon die and the plastic package. In MEMS pressure sensors, packaging shear stress introduces unexpected deformations that alter the piezo-resistive or capacitive transduction structures. This stress arises during the cooling phase of the epoxy molding process.
The effect becomes negligible when compliant gel-filled or ceramic housings are substituted for rigid plastic encapsulation.
Sensor Distortion
Silicon and packaging polymers expand and contract at different rates when exposed to temperature changes. This thermal mismatch generates packaging shear stress that is transmitted to the active sensing area. The resulting deformation changes the mechanical tension of the diaphragms or cantilevers.
This alteration shifts the sensor’s null offset, which manifests as temperature-dependent output drift.
Mitigation Design
Designing the mounting structure to isolate the sensor die is the primary method to control this drift. Engineers often use soft silicone adhesives or die-attach materials with low elastic modulus to absorb the thermal expansion mismatch. This layout prevents the packaging shear stress from reaching the active silicon die.
It allows the sensor to maintain its calibrated accuracy over a wide temperature range.
Verification Testing
Environmental chambers are used during testing to verify the effectiveness of the mechanical isolation. The sensors are cycled across their entire operating temperature range to measure offset drift. Sensors with low drift are accepted for shipment.
