Measurement and analysis on failure lifetime of serpentine interconnects for e-textiles under cyclic large deformation

Abstract

Abstract Serpentine interconnects are promising for electronic textiles (e-textiles), because they can maintain low electrical resistance even under cyclic large deformation. However, previous studies have not deeply discussed their failure lifetime, and the relationship between the elongation (engineering strain applied to entire structure) and cycle number to failure remains unclear. This clarification will contribute to the lifetime prediction. Therefore, this study investigates the relationship using interconnects having the same conductive material and geometric parameters but with different laminated structures: copper adhered to a polyurethane laminated knit textile (type A); copper adhered directly to a knit textile (type B); and polyethylene-naphthalate-laminated copper adhered to a knit textile only at both ends of the interconnect (type C). An elongation of 7%–70% was applied to the prepared samples with a tensile testing machine. The measurement and analytical calculation show that the type-C interconnects have the highest lifetime, and the relationship between the elongation ϵ appl and cycle number to failure N f is given by ϵ appl = A′N f −c/2, where A′ is the coefficient determined by the material properties, geometric parameters, and laminated structures, and c is the fatigue ductility exponent of copper. Moreover, this paper demonstrates the washing durability of an e-textile device using type-C interconnects. A cloth-face-mask-type device that monitors facial skin temperatures was fabricated and repeatedly washed with a washing machine. The demonstration confirms that the temperature variations were stably monitored before and after the first washing, and the multiple failures occurred in the interconnects after ninth washing.