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Reliability of screen- printed conductors and resistors on flexible substrates
Keywords: Reliability, screen printed, conductor and resistor
Scalable printing of conductor and resistor components has revolutionized the field of flexible electronics by enabling a myriad of low cost highly conformable devices. The flexible electronic devices need to exhibit a reliable performance under strenuous mechanical deformations to be adopted in application such as human and asset monitoring. The reliability of the devices is in turn affected by the microstructure of the materials, manufacturing processes and conditions of use. In this research the mechanical behavior and microstructural properties of the stretchable silver conductor and stretchable carbon conductor ink on two different flexible substrates are studied. The test vehicles (such as 4- point probe structures) were screen printed on polyethylene terephthalate (PET) and thermoplastic polyurethane (TPU) and cured in a reflow oven. The quality of the printed traces including the resolution, and thickness profile are measured by Confocal Laser Scanning Microscope. The microstructure of the sample including particle/nanoparticles morphology and interparticle porosity is studied by Scanning electron microscopy (SEM). The electrical resistance is measured by 4 -point probes method and the sheet resistance of the printed samples is calculated. The mechanical and electrical reliability of the samples are investigated by fatigue- cycling and stretching tests. The test vehicles are exposed to 150 fatigue cycles with 0.05 and 0.1 mm/s extension rates. The samples are stretched to 1.5% and 3% of initial gauge length and in-situ electrical resistance is measured. The resistance at peak of each cycle is considered for analysis Stability of the mechanical and electrical properties of printed samples in the humid environment is examined via putting the samples in the humidity chamber for the extended period of the time. In terms of electrical conductivity, the silver printed traces show different behavior compared to the carbon printed samples when exposed to fatigue cycling. The resistance of the silver samples increases after each cycle and after 150 cycles the final resistance of the silver printed samples increases up to 70 % (120 Ω) of the initial value. However, the electrical resistance of the carbon samples decreases after 150 cycles and the final resistance is 23% lower than the initial resistance. The decrease in the electrical resistance is important in applications such as strain sensors. The effect of viscoelastic strain of the substrate on the mechanical/electrical behavior as well as stability in the humid environment will be discussed.
Behnam Garakani, PhD candidate- Research assistant
Binghamton University- Center for Advanced Microelectronics Manufacturing (CAMM), General Electric Global Research Center
Binghamton, New York
United States

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