Evaluating Temperature Coefficient Wander in Thin Film Platinum Resistance Elements
Evaluating temperature coefficient wander in thin film platinum RTDs requires tracking substrate strain, glass passivation diffusion, and multi-point calibration.
Alumina Substrate Mismatch
Thermal Discrepancy
A dimensional difference in thermal expansion between bonded materials creates mechanical strain when the assembly undergoes temperature variations. In electronics packaging, an alumina substrate mismatch occurs when the ceramic substrate expands or contracts at a rate distinct from that of the silicon chip or the board. This difference is typically measured in parts per million per degree Celsius.
Mechanical Strain
Shear forces develop at the bonded boundary as each material deforms according to its specific coefficient. When the alumina substrate mismatch is pronounced, it causes micro-cracking in solder joints or delamination of the active die. Engineers measure these strain values using laser interferometry at specific reference temperatures to map the displacement.
This testing prevents premature failures in thick-film and thin-film hybrid circuits.
Metrological Verification
Measurement of the differential expansion relies on high-resolution dilatometry to compare the ceramic substrate against a certified reference sample of known purity. The reference material, often high-purity platinum or single-crystal silicon, is maintained under strict laboratory conditions to ensure traceability. During the verification process, thermal drift in the displacement sensor can introduce systematic errors that must be mathematically corrected using calibration coefficients.
A tolerance limit is typically set by the packaging designer at the start of the design phase. This limits the maximum allowable deviation to within ten percent of the nominal specification.
Environmental Influence
Operating conditions in the field often diverge from the static environment of the test laboratory, introducing dynamic thermal gradients. These gradients worsen the effect of the alumina substrate mismatch because the temperature across the assembly is non-uniform. Moisture absorption also modifies the dimensions of composite packages, adding further measurement uncertainty.
The strain must therefore be evaluated under both dry and humidified conditions.