Abstract
As the technology of flexible electronics has remarkably advanced, the long-term reliability of flexible devices has attracted much attention, as it is an important factor for such devices in reaching real commercial viability. To guarantee the bending fatigue lifetime, the exact evaluation of bending strain and the change in electrical resistance is required. In this study, we investigated the bending strains of Cu thin films on flexible polyimide substrates with different thicknesses using monolayer and bilayer bending models and monitored the electrical resistance of the metal electrode during a bending fatigue test. For a thin metal electrode, the bending strain and fatigue lifetime were similar regardless of substrate thickness, but for a thick metal film, the fatigue lifetime was changed by different bending strains in the metal electrode according to substrate thickness. To obtain the exact bending strain distribution, we conducted a finite-element simulation and compared the bending strains of thin and thick metal structures. For thick metal electrodes, the real bending strain obtained from a bilayer model or simulation showed values much different from those from a simple monolayer model. This study can provide useful guidelines for developing highly reliable flexible electronics.
Highlights
Electronic devices have been progressing towards flexible electronics, such as flexible displays, batteries, solar cells, and sensors [1], to increase performance and portability and to reduce weight
Flexible electronics are operated with repeated mechanical deformations, including bending, rolling, and twisting, so their mechanical reliability during repeated deformations is a critical hurdle to reaching real commercial viability [2,3,4]
When an external mechanical stress is applied to electronic devices, several mechanical reliability problems can occur in the metal layer, which is an essential part for electrical connections
Summary
Electronic devices have been progressing towards flexible electronics, such as flexible displays, batteries, solar cells, and sensors [1], to increase performance and portability and to reduce weight. The effect of the thickness of metal films on fatigue lifetime was reported in polymer substrate. We investigated bendingand strain and long-term fatigue lifetime of of different a metal repeated bending. We measured the electrical resistance of flexible metal electrodes thicknesses of metal film and polymer substrate based on the bending strain obtained from monolayer in situ during repeated bending deformations. WeWe compared and analyzed method the mechanical reliability and bilayer models for calculating bending strain. We used a finite-element analyzed the difference between the monolayer and bilayer strain calculations and discuss themethod proper (FEM) simulation to determine the precise bending strain and compared it with those from analytic calculations. We analyzed the difference between the monolayer and bilayer strain calculations and discuss the proper method that depends on the sample structure. This study can provide helpful information to predict the exact mechanical reliability of flexible electronics
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