Mechanical and electrical characteristics of screen printed stretchable circuits
Tang, Reuben Wei Liang
Date of Issue2017-05-09
School of Mechanical and Aerospace Engineering
The interest in flexible stretchable printed electronics has increased in recent times due to the wide applications they can be used for, especially in terms of wearables devices. Many of these researches uses conventional lithography, etching method or vapour deposition to sinter a continuous thin film of metal in the form of meanders or wavy-like structure to provide flexibility and stretchability. These methods are time consuming and expensive, which led to exploring the use of emulsion inks to develop a stretchable interconnect using screen printing. In this research, meander patterns of varying radii of curvature and the meander angles were designed and screen printed on thermoplastic polyurethane substrate. Silver inks and Carbon inks were used to print the interconnects which were 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 interconnect and set the conditions for a cyclical test of up to 10000 cycles. For the silver inks, it is found that the maximum strain at break is 47% and the best design for stretchability is at 0° meander angle. Detailed failure analysis shows that the ink failed, similar to bulk metal where dislocations propagate and crack through beyond the elastic limit. Various curing conditions and cooling rates were experimented to obtain different microstructures in the printing. It was found that a curing temperature of 130°C and time of 30 minutes provides optimum stretchability. Carbon ink on the other hand, has a higher initial resistance but the maximum strain at break is 125%. The best design for carbon ink is 0° meander angle at 130°C for 30 minutes with oven cooling. It was found that the increase in resistance follows a consistent power law progression and the behavior is similar to diffusion based failure mechanisms. Detailed failure analysis shows that ink adhesion plays an important role in stretchability. We have thus concluded that conductive inks operates based on bulk and percolation mechanism, and found that percolation brings about a large increase in the stretchability in the circuits.
Final Year Project (FYP)
Nanyang Technological University