Mechanical and electrical characteristics of screen printed stretchable circuits on thermoplastic polyurethane

Abstract

In this research, we designed serpentine patterns of varying radii of curvature and the meander angle and screen printed the test coupons on thermoplastic polyurethane. The inks chosen were silver flake ink and carbon black ink. The test coupons are then subjected to destructive tensile testing while monitoring the change in resistance. The electrical characteristics are then used to determine the elastic limit of the test coupons and set the conditions for a cyclical test. The coupons were then tested to 1000 cycles. For the silver inks, it is found that the maximum strain at break is 42% and the best design for stretchability is at 22.5 degree meander angle. Detailed failure analysis shows the mechanical deformation is similar to bulk metal where dislocations propagate and crack through beyond the elastic limit in silver ink. Carbon ink, on the other hand, has a higher initial resistance but the maximum strain at break is 125.84%. It was found that for carbon ink the increase in resistance follows a consistent power law progression and the behavior is similar to diffusion based failure mechanisms We have thus provided an insight into the printing of stretchable circuits with conductive inks based on bulk mechanism and percolation mechanism and found that percolation brings about a big increase in the stretchability in the circuits.