Bending Setups for Reliability Investigation of Flexible Electronics

Abstract

Flexible electronics is a rapidly growing technology for a multitude of applications. Wearables and flexible displays are some application examples. Various technologies and processes are used to produce flexible electronics. An important aspect to be considered when developing these systems is their reliability, especially with regard to repeated bending. In this paper, the frequently used methods for investigating the bending reliability of flexible electronics are presented. This is done to provide an overview of the types of tests that can be performed to investigate the bending reliability. Furthermore, it is shown which devices are developed and optimized to gain more knowledge about the behavior of flexible systems under bending. Both static and dynamic bending test methods are presented.

Keywords: bending; bending apparatus; bending reliability; dynamic bending; flexible electronics; four-point bending; mechanical characterization; push to flex; roll to flex; static bending; three-point bending.

Figure 1

Flexible electronics and some of their applications in daily life [11].

Figure 2

Monitoring of some physiological signals using flexible sensors and diagnosis and evaluation per remote medicine. Adapted from [41]. The abbreviations EEG, HR, ECG, and EMG stand for electroencephalogram, heart rate, electrocardiogram, and electromyogram, respectively.

Figure 3

Examples for smart textiles and their applications [32]. (a) Temperature sensing yarns in a textile for health monitoring [45]. (b) Optoelectronic near-infrared spectroscopy smart textile for measuring the blood oxygenation levels in health care [46]. (c) A temperature-sensing sock for fitness and health care [47]. (d) Embedded photodiodes in textile for health-monitoring [48]. (e) Tactile-sensing fabric for applications in human-machine interfaces (HMIs), smartphones, and Internet of Things (IoT) devices [49].

Figure 4

Handling of flexible devices and displays. Adapted from [54].

Figure 5

Applications for bio-monitoring, diagnosis, and hazards prevention [9]. (a) Flexible electronics for regenerative neuronal cuff implants [62]. (b) Flexible graphene wearable electrodes for dynamic ECG sensing [63]. (c) Flexible polymer transistors for application in electronic skin and health monitoring [64]. (d) Contact lens with ocular diagnostics of detecting the glucose levels in tears in diabetes patients [65]. (e) Ambulatory and wearable EEG sensor around the ear [66]. (f) Electronic nose on a flexible substrate for detecting hazardous gases [67].

Figure 6

When bending a structure built on a substrate with a given radius, the outer plane will be under tension and the inner plane under compression strain.

Figure 7

Schematic view of three-point bending setup.

Figure 8

Diagram of a four-point bending setup before and after the bending.

Figure 9

Principle sketch shows a system bent over a defined bending radius. With Equation (10), the bending stress ${\sigma }_{str}$ at the top of the structure can be calculated.

Figure 10

(a) 3D printed structures for static bending test, (b) convex (tensile) and (c) concave (compressive) bending setup of thin chip on flexible foil. Adapted from [92].

Figure 11

Experimental setup in three-point bending. (a) Perspective view; (b) when bending with large deflection; (c) diagram of the sample in plain position and (d) with a deformed position of the sample. Adapted from [78].

Figure 12

Dynamic bending setup with full reverse loading. Adapted from [74,100].

Figure 13

Schematic illustration of cyclic bending test system. Adapted from [37,74,91,96].

Figure 14

Schematic view of dynamic push to flex bending setup: (a) In a flat position and (b) under bending. Modified and freely sketched after [95,101].

Figure 15

(a) Dynamic push to flex bend-testing setup; (b) components on the top are in tensile and (c) in compression (reproduced with permission) [102].

Figure 16

Illustration of a roll to flex bending machine with attached sample; (a) in a flat and (b) in a bent position. Modified and freely sketched after [94].

Figure 17

A cyclic bending tester with roll to flex method. Modified and freely sketched after [103].

Figure 18

Dynamic bending machine. Modified and freely sketched after [93].

Figure 19

Examples of two bending test setups using the roll to flex bending measurement method with decoupled influence of tensile forces on the bent samples; (a) on-belt flex bending setups (modified and freely sketched after [6]) and (b) bending test device (reproduced with permission) [14].