Mechanical Distortion
A physical phenomenon occurs when two bonded materials with different thermal expansion coefficients expand at different rates during temperature changes. In precision instrumentation, structural thermal expansion mismatch generates localized mechanical stresses that can deform the sensor substrate. This deformation introduces unwanted measurement errors that are separate from the intrinsic sensitivity of the transducer.
The magnitude of this distortion depends on the temperature range and the geometry of the bonded components.
Sensor Performance
High-accuracy sensors are vulnerable to strain-induced biases when mounted to a housing made of a dissimilar material. If a silicon sensor is directly attached to an aluminum chassis, the structural thermal expansion mismatch will cause the sensor package to bend as temperature fluctuates. This bending is detected by the sensing elements as a physical load, which leads to significant output drift.
To prevent this, isolating mechanical linkages or intermediate mounting plates must be integrated into the housing design.
Material Optimization
Engineers address this issue by selecting materials with closely matched coefficients of thermal expansion for the sensor housing and substrates. For example, using specialized alloys like Invar or Kovar alongside ceramic substrates reduces the structural thermal expansion mismatch. This selection minimizes the thermal strain transmitted to the active sensing element, which preserves the baseline calibration of the instrument.
The choice of bonding adhesives or solders also helps, as compliant materials can absorb some of the differential expansion and prevent micro-cracks from forming in the joint over time.
Assembly Qualification
Environmental stress screening is used to verify the integrity of the bonded assembly under cyclic thermal conditions. These tests expose the sensor assembly to repeated temperature swings to check for mechanical separation or calibration drift. Any assembly showing significant output changes during the thermal test is rejected.
This screening ensures that only structurally stable units are deployed in the field.
