Reliability Studies on Printed Conductors on Flexible Substrates under Cyclic Bending

This thesis investigates the reliability of printed conductors on flexible substrates under cyclic bending. The topic is approached by studying the effects of several key design parameters on the bending lifetime of printed conductors under dynamic loading. In this thesis, the test specimens with various cross-sectional geometries were printed on different plastic and paper substrates. The test samples were fabricated with two printing methods, silk screen and roll-to-roll printing, by utilizing three different silver pastes as the conductive material. To evaluate the long-term performance of the printed traces, the test specimens were exposed to cyclic bending and their electrical behaviour during the test was monitored with resistance measurements. The bending test results were analysed by utilizing the Weibull analysis, with a 20% increase in resistance as the failure criterion. This method yielded a characteristic lifetime for a test population, including 10 individual test specimens. When comparing the characteristic lifetimes of all of the fabricated test populations, it was observed that all of the varied design parameters had an effect on the bending reliability of the printed traces. However, within the studied geometries and materials, the substrate was found to have the highest impact on the long-term electrical performance of printed conductors under cyclic bending. The study proves that the bending reliability of printed conductors can be enhanced by proper design choices. This is done by minimizing the strain when the printed trace is subjected to bending.