Mechanism-Based Study of the Performance of Printed RF Elements under Stretching and Bending
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Zhou, Yi
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Abstract
Flexible hybrid electronics (FHE) has attracted substantial interest by combining the high performance of silicon integrated circuits (ICs) and mechanical versatility of flexible printed circuits. FHE systems are being increasingly used in health, automotive, and Internet-of-Thing (IoT) applications, where Radio Frequency (RF) communication is an important element for the wireless operation of the FHE devices. The interconnect is a key component in the RF module of the FHE, as it connects the ICs, antennas, and other passive components together. Thus, understanding the RF performance of printed interconnects under various extents of deformation in their operating conditions, such as bending and stretching, is crucial to the reliable design of FHE devices. A more accurate evaluation of RF performance requires the characterization of scattering parameters (S-parameters). The scarcity of experimental data and numerical models for FHE relevant components in the RF regime remains a major obstacle to improving the reliability of FHE devices. This dissertation explores the multi-physics characterization and mechanism-based modeling of RF performance changes in printed interconnects subjected to mechanical deformation. Through a combination of empirical testing, computational simulations, and modeling improvements, this work addresses critical gaps in understanding how stretching and bending impact the S-parameters of screen-printed coplanar waveguides (CPWs) and inkjet-printed microstrip lines.
The study begins by thoroughly characterizing the material sets, analyzing their mechanical and electrical properties to establish a foundation for design and fabrication of high-quality printed interconnects. Once the fabrication is completed, it then investigates changes in S-parameters through stretching and bending experiments, defining failure criteria and benchmarking results for simulation validation. This includes developing testing methodologies compatible with S-parameter measurements. Numerical models are then first developed in CSTTM with the assumptions of changed geometry and conductivity degradation due to mechanical strain in three cases: in-situ stretching of CPWs, in-situ bending of microstrip lines, and post bending up to 12500 cycles of microstrip lines. With the omission of other potential strain-induced mechanisms, the predictive models consistently underestimate the extent of RF degradation observed in measurements. To address these shortcomings, other experimentally observed mechanisms, including strain-induced changes in dielectric properties and surface roughness of printed conductors are systematically integrated into refined CSTTM simulation models for all three cases. By incorporating all deformation-dependent material parameters, these models demonstrate significantly improved prediction accuracy across a range of strain and frequency levels. Validation against measured S-parameters confirms the necessity of incorporating multiphysics-informed adjustments of dielectric constant of and conductor surface roughness, beyond traditional geometry and conductivity assumptions.
Ultimately, by integrating material characterization, experimental data, and simulation insights, this dissertation establishes a mechanism-based modeling strategy that bridges empirical characterization and predictive simulation, contributing critical insights into the design and reliability of FHE. The methodology developed here serves as a valuable tool for future research and device development in FHE for wearable, automotive, and IoT applications requiring mechanically robust RF performance.
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Date
2025-12
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Dissertation (PhD)