Abstract
Understanding the mechanical behavior and failure mechanisms of stretchable electronics is key in developing reliable and long-lasting devices. In this work a micron-scale stretchable system consisting of an aluminum serpentine patterned interconnect adhered to a polyimide substrate is studied. In-situ experiments are performed where the stretchable sample is elongated, while the surface topography is measured using a confocal microscope. From the resulting height profiles the microscopic three-dimensional deformations are extracted using an adaptive isogeometric digital height correlation algorithm. The displacement information is compared to realistic numerical simulations, in which the interface behavior is described by cohesive zone elements. It is concluded that despite fitting the traction separation law parameters, the model fails to correctly capture the distinct out-of-plane buckling (with magnitude of a few micron) of the interconnect. The model is updated with residual stresses resulting from processing and crystal plasticity induced behavior (decreased yield strength) in the aluminum layer, but both measures are not resulting in the experimentally observed deformations. Finally, mixed-mode cohesive zones are implemented, in which the properties are different in the shear and normal direction. After fitting the corresponding parameters to the experimental data, the model shows realistic in-plane and out-of-plane deformations. Also a predictive simulation for a different geometry results in the correct experimentally measured behavior. It is concluded that the aluminum-polyimide interface mode-angle dependency explains the observed microscopic failure mode of local delamination and buckle formation.
Highlights
A relatively new and evolving direction for electronic applications is the development of highly deformable systems, i.e., stretchable electronics
This work aims at the experimental-numerical characterization of interface delamination in stretchable electronics, where the numerical model is a true representation of the real experiment at the micron-scale
The 3D displacement field is calculated both for the aluminum interconnect and the adjacent polyimide substrate, to account for all deformation modes occurring in the sample, including delamination between the interconnect and substrate
Summary
A relatively new and evolving direction for electronic applications is the development of highly deformable systems, i.e., stretchable electronics. Flexible and stretchable electronic devices are mainly used in biomedical applications, in order to bridge the gap between traditionally rigid, flat electronics and soft, curved biological tissue (such as skin and organs). Examples include patches that are adhered to human skin for health monitoring (Kim et al, 2011; Koh et al, 2016), flexible devices for cardiac diagnostics (Sterken et al, 2011; Gutbrod et al, 2014), smart contact lenses (Quintereo et al, 2017) and stretchable surgical tools, such as a balloon catheter with sensors for blood flow monitoring (Kim et al, 2011; Klinker et al, 2015). ⇑ Corresponding author at: Eindhoven University of Technology, Gemini-Zuid.
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