Strain Localization and Electrical Resistance Evolution in Ag Nanoflake-Based Conductive Inks under Monotonic and Cyclic Stretching

Flexible hybrid electronics (FHE) devices are an emerging class of electronics devices that are flexible and conformal to non-planar surfaces. Interconnect materials for FHE devices known as conductive inks consist of submicron Ag flakes embedded in polymer binder materials and screen printed on polymer substrates. For these conductors, strain localization plays a dominant role in the electrical resistance increase with uniaxial strain. The current work seeks to investigate the origins of ink strain localization, the important material and structural dimensional factors affecting strain localization, and the relationship between strain localization mechanisms and resistance increase with both monotonic and cyclic uniaxial strain. A secondary research objective is the investigation of key empirical parameters for characterizing ink resistance evolution with cyclic strain in order to model ink resistance evolution with cycling.

The primary method for investigating strain localization was uniaxial stretch testing with synchronous resistance measurement. In particular, experiments with in situ scanning electron microscope (SEM) imaging were used to simultaneously monitor the evolution of strain localization (primarily cracking) and electrical resistance with monotonic or cyclic stretching. Different ink and substrate materials were tested using the in situ SEM technique to observe differences in cracking mechanisms. The in situ images were analyzed using digital image correlation (DIC) to generate strain maps, leading to the numerical modeling of resistance based on crack statistics. Experiments with in situ confocal microscope (CM) imaging were used to help understand the effects of ink trace line width and thickness. Finite element modeling was used to investigate the origins of strain localization, which were hypothesized to be surface roughness and local variations in Ag flake concentration. For the second research objective, a series of cyclic experiments helped identify the key empirical parameters, leading to a model for predicting resistance assuming a constant rate of resistance increase with cycling.