Thermo-mechanical properties and bending reliability of polymeric materials for flexible electronics

Abstract

Flexible electronics have received great research effort and interest under increasing demand on the next generation smart devices. Various types of portable displays, flexible sensors, flexible batteries, and wearable health-care devices are being actively developed for practical usage in daily life. In order to realize the flexible devices, polymeric materials serve as essential elements basically due to their intrinsic flexibility and ductility that can accommodate mechanical deformation. Polymers also obtain many attractive features for their applications to flexible electronics such as light weight, easy processability, large-area fabrication, and controllable functionality. These merits enable the polymers to be utilized in various key components in a flexible device. Due to inevitable exposure to severe thermal loadings and large bending deformations, mechanical reliability is the critical bottle neck for commercialization of flexible devices. In mechanical aspects, it is essential to fundamentally understand the mechanical behaviors of the polymer-based key components. Specifically, material properties and failure analysis are necessary to optimize material design, structure, and fabrication process of the flexible device. Nevertheless, it has been difficult to investigate the mechanical response of polymeric materials in flexible devices using conventional mechanical testing methods. In this dissertation, mechanical behaviors of the polymeric materials in flexible devices are investigated regarding two main contributions: Thermo-mechanical properties and bending reliability. For the first part, novel experimental methods are developed for accurate evaluation of the material properties. This part focuses on unveiling the thermo-mechanical properties of flexible polymers that have not been investigated systematically. Firstly, coefficient of thermal expansion (CTE) of wearable platform materials are investigated using digital image correlation (DIC) method for elastomeric substrates and composite laminate substrates (DIC-CTE method). The coefficient of thermal expansion, glass transition temperature, and tensile properties of free-standing ultra-thin polymer films are examined using a hot liquid platform (Film on glycerol (FOG) method). For the second part, a novel method is developed that directly visualize and quantify the full-field strain under bending. Using the method, dynamic and static bending reliability of chip-in-flex package is studied and various enhancing methods are suggested. Finally, direct and full-field verification is conducted for multiple neutral plane strategy using a laminate structure composed of plastic film substrates and very soft interlayer.