Short- and Long-Term Stability of NTC Chip-Scale Thermistors

(25:32 + Q&A) Michael Vecchio, Ph.D., Nokia — The reliability of negative temperature coefficient (NTC) chip-style thermistors acquired from two suppliers was determined via aging over a broad temperature range, 85°C -200°C, and times, 3000hrs – 8600hrs. Both suppliers advertise their respective thermistors as high stability devices. Stability was determined by periodically pulling the devices from the aging chamber and characterizing the change in resistance (drift) per unit time (deltaR(t)). In this study, thermistors from both suppliers display decreasing ?R(t) at shorter time scales and lower temperatures (t 85°C) that gradually approaches an inflection point wherein deltaR(t) begins to continuously increase for the remainer of the aging study independent of temperature. This is strikingly unlike other NTC thermistor reliability reports as the short-term decreasing resistance combined with ?R(t) inflection point is not typically modeled, analyzed and discussed. The change in resistance versus time was mathematically modeled by a piece-wise equation where decreasing resistance was modeled with a stretched exponential function (DeBast-Gillard and Williams-Watts type relaxation) and increasing resistance by a linear function. Both the decrease and increase regions of resistance drift were found to be well represented by Arrhenius expressions indicating both were thermally activated and characterized by Ea=0.43-0.64eV, growth by Ea=0.44-0.57eV, respectively. Further, the time interval that ?R(t)’s inflection point occurred was also found to be well represented by a thermally activated process with an activation energy of Ea=0.45-0.52eV. Using mathematical fitting functions and activation energies, the implication of decreasing followed by increasing resistance change behavior with respect to device lifetime is described in detail. Finally, a phenomenological defect-chemistry-based model is used to describe the observed ?R(t) behavior that combines a localized field-induced oxygen vacancy migration mechanism with the classical NTC thermistor degradation mechanism, poisoning, of transition metal oxidation on tetrahedral and octahedral sites that inhibits polaron transport through the ceramic.
Michael A. Vecchio, Ph.D. received a B.S. in Physics from Dickinson College (2014) and Ph.D. in Materials Science and Engineering from Penn State University (2019). His Ph.D. thesis addresses challenges associated with using high-permittivity dielectrics for high-energy density applications, with a focus on charge injection, space-charge migration, and subsequent high-field failure mechanisms of PVDF based dielectrics. Michael joined Infinera Corporation in 2019 as a Sr. Process Development engineer. He was promoted to Staff optical-electronics packaging and hardware reliability engineer 2023 and presently continues this position at Nokia after Nokia’s acquisition of Infinera. Michael works in a small, multidisciplinary team whose mission is to develop technological building blocks deemed critical to meet projected optical engine data rate and power management demands. His professional interests span general packaging sciences, data analytics, reliability testing and lifetime analysis (PIC/MEMS/thermal control HW/active and passive electrical components), and applying material characterization techniques to model hardware failure mechanisms. Ultimately, he gains most satisfaction by using his technical skill to guide decision making in high-stakes business situations.

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