Path Dependent Response
Path-dependent error behavior in sensors occurs when the output value at a specific temperature depends on whether the device was previously hotter or colder. Thermal mechanical hysteresis is a measure of this non-repeatability caused by the build-up of internal stresses between the sensor and its package. As the materials expand and contract at different rates, they can become trapped in a state of tension that does not fully relax.
This results in a shifting baseline for pressure and acceleration measurements.
Internal Stress
Mismatched expansion coefficients lead to a tug of war between the silicon and the ceramic housing. This is the primary driver of thermal mechanical hysteresis in micro-machined devices. The stress is often stored in the die-attach adhesive which acts like a slow-moving spring.
Repeatability Test
Verification of this error requires multiple temperature cycles in a controlled oven. A technician records the sensor output at room temperature after it has been heated and then again after it has been cooled. The difference between these two readings defines the thermal mechanical hysteresis.
Stabilization Time
Errors can sometimes be reduced by allowing the device to soak at a constant temperature for several hours. This gives the internal stresses time to reach a new equilibrium. However, in many field applications, this delay is not practical and the hysteresis must be managed through better mechanical design.
