Analysis of Repetitive Bending on Flexible Wireless Power Transfer (WPT) PCB Coils for Flexible Wearable Devices

In this article, we first designed, measured, and analyzed flexible wireless power transfer (WPT) coils with repetitive bending cycles. To analyze any changes in electrical characteristics through repetitive bending cycles, 3\times3 cm flexible printed circuit board (PCB) coils were fabricated using polyimide substrates. Different coil configurations are designed to find a mechanically robust, flexible PCB coil design. First, we analyzed single-layer and double-layer coils. Generally, via is the mechanically weakest part of the PCB, hence the diameter of the via varies to observe the effect of repetitive bending cycle. Ten thousand bending cycles with a bending radius of 3 mm and a bending cycle frequency of 1 Hz were used to analyze the electrical changes. Moreover, the bending position of the coil is varied to observe the bending effect. The flexible coil's direct current (dc) resistance is measured during the bending cycle. After the repetitive bending test is completed, the coil impedance is analyzed in a frequency domain. As a result, we conclude that the flexible WPT coil is robust from mechanical stress when the layer of the coil is single a layer, the bending line is applied vertically, and the diameter of the via is small.