Mechanical Cycling to Facilitate Accelerated Degradation of TIMs

(14:48 + Q&A) Pranay Nagrani, Purdue University
From the 2024 IEEE Symposium on Reliability for Electronics and Photonics Packaging
Summary: Due to advances in the 3D integration and miniaturization of chips, the power density and hotspots within electronic packages have increased exponentially. Therefore, a good thermal management solution is essential to ensure the reliability of electronic packages. With recent advances in heat sinks of electronic packages in the form of single- and two-phase forced convection cooling, one of the major thermal resistance bottlenecks is the interfacial resistance between solid-solid contacts within electronic packages. Hence, it is essential to ensure good reliability of thermal interface materials (TIMs) such as thermal greases which are composite materials comprising of high thermal conductivity fillers within a polymer matrix. TIMs often degrade with time due to pumpout (TIM moves out of interface) and dryout (phase separation of composite) phenomena due to repeated heating and cooling of the die. Traditionally, the reliability of TIMs is assessed by thermal cycling which has a long testing period of the order of days or months. Hence, in this study, we propose mechanical cycling at a fixed power level to facilitate accelerated degradation of such materials wherein we conduct reliability tests spanning 6 hours. Specifically, we test three commercial TIM materials, by first characterizing their thermal conductivity (k) and rheological properties such as storage modulus and viscosity using small amplitude oscillatory shear (SAOS) tests. Next, we subject the TIMs to accelerated degradation using mechanical cycling and investigate the reliability at different oscillation amplitudes and squeezing pressures using a state-of-the-art in-house designed and machined experimental rig. We leverage high-resolution infrared (IR) imaging of TIMs under mechanical oscillations to capture steady-state 2D temperature maps, from which we calculate metrics such as void fraction, area-normalized end-of-life thermal resistance, and the ratio of area-normalized EOL to beginning-of-life (BOL) thermal resistance. Our results uncover that high modulus and viscosity TIMs have lower thermal resistance ratio at different oscillation amplitudes and are more prone to pumpout resistance. Further, we investigate the EOL and reliability performance of TIMs by comparing their thermo-rheological properties and introduce a novel thermo-rheological coefficient, which serves as a global metric to assess TIM reliability.
Bio: Pranay Nagrani works under the guidance of Prof. Amy Marconnet and Prof. Ivan Christov and his research concerns analyzing and mitigation of thermal interface material (TIM) pump-out degradation phenomenon to improve the reliability of electronic packages. He received his MSME degree from Purdue University in 2021 during which his research concerned studying particle migration under the influence of shear and thermal gradients and its application to mitigate junction level temperature rise. He received his B.Tech degree from National Institute of Technology Karnataka (NITK) in 2015.

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