Metrological Drift in Precision Bulk Metal Foil Resistors
Bulk metal foil resistor drift stems from thermal stress relaxation and epoxy moisture swelling, manageable through hermetic packaging and thermal burn-in.
Vacancy Migration
Lattice Movement
The movement of empty atomic sites through a crystalline lattice under the influence of thermal or electrical energy alters the structural integrity of thin-film conductors. The phenomenon of vacancy migration leads to localized resistance changes.
Acceleration Factor
High current densities and elevated temperatures accelerate the rate at which these lattice defects move through the material. During operation, vacancy migration is driven by the electron wind in a process known as electromigration, where momentum transfer from electrons pushes metal atoms along the conductor. This action accumulates vacancies at the grain boundaries, which increases the resistance of the path.
Metrological Influence
For high-precision sensor elements, any alteration of the conductor geometry causes a shift in the calibrated output of the transducer. If vacancy migration is allowed to occur unchecked, the sensor will exhibit a slow drift that cannot be corrected by software compensation. Maintaining the current density below the threshold limits established by the material specification is essential to prevent this failure.
Traceable Testing
Verification of resistance drift under accelerated conditions is documented in the reliability qualification report. Test structures are monitored continuously under high current and temperature loads to determine the mean time to failure. This data determines the maximum operating limits of the instrumentation in the field.