Development of a new Stretchable and Screen Printable Conductive Ink

Abstract

Stretchable conductive ink is a key enabler for stretchable electronics. This thesis research focuses on the development of a new stretchable and screen printable conductive ink. After print and cure, this ink would be capable of being stretched by at least 500 cycles at 20% strain without increasing its resistance by more than 30 times the original resistance, while maintaining electrical and mechanical integrity. For a stretchable and screen-printable conductive ink, the correct morphology of the metal powder selected and the ability of the binder to be stretched after the sintering process, are both indispensable. This research has shown that a bi-modal mixture of fine and large-diameter silver flakes will improve stretchability. While the smaller flakes increase the conductivity and lower the sintering temperature, the larger flake particles promote ohmic connectivity during stretching. The bi-modal flake distribution increases connection points while enhancing packing density and lowering the thermal activation barrier. The polymer binder phase plays a crucial role in offering stretchability to the stretchable conductive inks. The silver flakes by themselves are not stretchable but they are contained within a stretchable binder system. The research demonstrates that commonly used printable ink binder when combined with large-chain polymers through a process known as ‘elastomeric chain polymerization’ will enable the conductive ink to become more stretchable. This research has shown that the new stretchable and screen printable silver conductive ink developed based upon the two insights mentioned above; (1) bi modal flakes to improve ohmic connectivity during stretching and (2) elastomeric chain polymerized binder system which could stretch even after the ink is sintered to the substrate, can exhibit an ink stretchability of at least 500 cycles at 20% strain while increasing the resistance by less than 30 times the original resistance. Wavy print patterns can enhance the stretchability of stretchable conductors. The research also demonstrates that FEA modeling, simulating the total principal strain on the printed patterns, can be used to estimate the comparative resistance changes caused by stretching and these changes can be explained by some basic equations from Percolation Theory.