Polymer Die Attach Selection Criteria for Low Drift Inertial Measurement Units
Polymer die attach selection governs MEMS IMU bias drift by balancing storage modulus, glass transition temperature, and long-term viscoelastic stress relaxation.
Proof Mass Anchor Distortion
Sensor Error
Micro-scale mechanical deformation occurring at the structural anchor points of a suspended inertial sensor causes an unintended shift in sensor alignment. In silicon accelerometers and gyroscopes, proof mass anchor distortion changes the boundary conditions of the mechanical suspension. This variation alters the nominal tension in the spring system and degrades the sensor accuracy.
Stress Transmission
Packaging materials and solder bonds have different coefficients of thermal expansion than the silicon sensor die. These differences generate mechanical stress that propagates through the substrate, causing proof mass anchor distortion during temperature cycles. This stress changes the spring constant and leads to a shift in the sensor’s zero-g bias and sensitivity.
Temperature Effect
Thermal gradients across the silicon sensor die cause localized expansion that affects the symmetry of the suspension. This asymmetry results in proof mass anchor distortion, which is the primary driver of temperature-induced bias drift in MEMS devices. This drift is especially difficult to calibrate because it varies depending on the rate of temperature change.
Mitigation Strategy
Advanced packaging designs decouple the sensor die from the outer housing using compliant silicone adhesives or glass-frit sandwich structures. These compliant layers absorb the mounting and thermal stresses before they can cause proof mass anchor distortion in the silicon sensor. Mechanical engineers verify the effectiveness of these isolated mounts by subjecting the assembled sensor package to rapid thermal cycles and measuring the shift in bias.
This verification ensures that the sensor meets its bias stability specifications in the field, even when installed on engines or in automotive engine bays.