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.
Hygroscopic Swell Strain
Material Strain
Dimensional expansion occurring in polymer matrices and fiber-reinforced composites when they absorb atmospheric moisture is a well-documented mechanical behavior. This hygroscopic swell strain alters the geometry of the component as a function of the relative humidity of the surrounding air. The resulting deformation can compromise the alignment of precision optical and mechanical assemblies.
Expansion Coefficient
Measurement of this strain relies on defining a coefficient that relates the change in linear dimensions to the change in moisture concentration. This coefficient is critical when modeling hygroscopic swell strain in electronic packages where multiple materials are bonded together. The strain is evaluated by tracking sample mass and dimensions across a range of humidity levels.
Stress Generation
Restraining a material that is attempting to expand due to moisture absorption creates high internal mechanical forces. In micro-sensor designs, hygroscopic swell strain can lead to delamination of protective coatings or shifts in sensor calibration offsets. These forces are especially problematic in devices exposed to fluctuating field environments where moisture levels vary rapidly.
Metrological Tracking
High-accuracy displacement sensors like capacitive dilatometers or optical encoders measure these sub-micron shifts under controlled temperature and humidity conditions. Engineers calculate the hygroscopic swell strain by isolating the moisture-induced changes from any thermal expansion effects that occur simultaneously in the test chamber. This decoupling is achieved by performing the measurements under isothermal conditions where only the humidity is varied.
Such precise tracking ensures that material selection for sensitive sensor housings prevents performance degradation in humid climates.