In situ SEM analysis of cracking and its quantitative link to resistance evolution in uniaxially stretched nanocomposite inks

Abstract

Nanocomposite conductive inks consisting of metal flakes embedded in a polymer binder are employed as interconnect materials in flexible hybrid electronics (FHE) devices. Crack formation has been hypothesized to play a key role in the ink's electrical degradation (increase of resistance), but the two have not been directly correlated. To address this gap, two classes of inks are studied using uniaxial stretch experiments, with in situ SEM imaging, and synchronous electrical resistance measurement. In plane strain maps obtained from digital image correlation (DIC) analysis of wide-field SEM images (∼300 μm in width) identify crack patterns at various applied strains. From these strain maps, crack length measurements are obtained. A finite-element based numerical model adapted for non-uniform crack lengths is used to predict normalized resistance increase from the crack lengths. The model predictions are compared against the experimentally measured resistance changes. There is a remarkable correlation between the predicted and experimentally measured normalized resistance values. The linear crack density was also used to help understand the relation between the effective crack length and resistance increase. Closeup images of the ink top surface and the ink cross-section during the in situ SEM stretch experiment are used to explain differences between predicted and measured normalized resistance.