Mechanical Reliability and Failure Mechanisms of Printed Flexible & Stretchable Electronics

Electronics has enabled the advancement of technology and industry profoundly, especially in the current Internet of Things era. However, conventional electronic devices are commonly non-deformable due to the rigidity of the materials used, which has limited their application to surfaces where mechanical deformation is required, such as the human body and robot joints. Flexible and stretchable electronics address this challenge by introducing mechanical flexibility and stretchability into electronic materials. Although there have been advances in improving their flexibility and stretchability through material and fabrication process development and device architecture design, their reliability under mechanical deformation and the associated failure mechanisms have been rarely understood due to limited knowledge of their mechanical behavior and the lack of reliability test standards.

This dissertation addresses this challenge by (1) developing cyclic mechanical test methods; (2) investigating the reliability and failure mechanisms of printed flexible and stretchable electronics under the tests (i.e., cyclic rolling, bending, and wear tests) together with electrical characterization, surface characterization and imaging for failure analysis; and (3) investigating the impact of the fabrication process on the printing quality and interface adhesion.

The research found that, under repeated mechanical impact, the printed flexible and stretchable electronics mainly failed due to interface sliding and delamination (adhesive failures), adhesive and conductor cracking, particle flaking (cohesive failures), and two-body and three-body abrasive wear against rubbing. The failures were commonly caused by the local strain concentration due to mechanical impacts. Overall, these failures are highly related to aspects of material property, fabrication process, and device architecture.

Therefore, to improve the mechanical reliability of printed flexible and stretchable electronics, these approaches are promising to explore: (1) Material development through the design and synthesis of functional nanomaterials, which are expected to improve the material’s mechanical properties, printability, and cohesion; (2) Surface engineering to modify the material surface for improved interface adhesion; (3) Development of the printing process: the complexity is that the optimized printing process varies from material to material, and the high printability also depends on the compatibility of contacting materials. By optimizing the fabrication parameters, the print quality and reliability of the fabricated device can be improved. (4) Device architecture design: a good design can minimize the possible artifacts occurring in the fabrication, avoiding or migrating the structural weakness for improved robustness. This can be achieved using some computerassisted tools like modeling and simulation.

The mechanical reliability of both rigid and flexible electronics depends on the properties of the materials used. Usually, the required reliability depends on the application, which is the main difference between them. Because of this difference, their dominant failure mechanisms also differ, and their reliability cannot be compared straightforwardly.

Overall, flexible and stretchable electronics, as a branch of electronics, demonstrate a promising future with their advantage in mechanical deformability. Printing provides cost-efficient and energy-efficient methods for their fabrication. Their rapid progress in improving flexibility and stretchability has been achieved, whereas their reliability under mechanical impacts and the associated failure mechanisms have not been understood sufficiently. By addressing this challenge, this dissertation has improved the understanding of their mechanical reliability and failure mechanisms and demonstrated the possibility and potential of improving their reliability. By revealing the dominant failure mechanisms and factors causing the failure, this dissertation also provides guidance on approaches for improving the reliability of printed flexible and stretchable electronics.