The role of strain localization on the electrical behavior of flexible and stretchable screen printed silver inks on polymer substrates

This work explores conductivity under applied uniaxial strain of two silver-based inks with similar flake volume fractions of ~50% that are screen-printed with a single pass (thickness: 10 µm) onto three different polymer substrates. The normalized resistance increases more rapidly with applied strain for the “flexible” ink (5025 with an acrylic binder), and has three times greater resistance at 35% strain when compared to the “stretchable” ink (PE874 with polyurethane binder). While resistance increase is qualitatively consistent with percolation theory, the in-situ strain map analysis and post-mortem fractography reveal drastic differences in the root causes of the inks’ electrical behavior. Both inks form strain localization bands with similar spacing. For the flexible ink (5025), strain localization is accompanied by local necking and flake fraction reduction. For the stretchable ink (PE874), strain localization is associated with surface cracking initiated by pre-existing voids, with minimal changes in the flake fraction. A model incorporating strain localization through a Gaussian distribution of flakes evolving with applied strain more closely accounts for the 5025 ink's normalized resistance increase compared to models that assume uniform strain and a uniform flake distribution. Overall, local necking and reduction of the flake fraction appear to be more detrimental to the resistance than the formation of surface cracks.