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Development of Screen-Printable Flexible Circuits on Fabric Using Commercially Available Conductive Inks

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Author: Amini, Arvin
Over the past decades, electronics and silicon manufacturing have grown rapidly. Alongside this, the capability of integrated circuits (ICs) has significantly increased; modern electronics are becoming smaller and faster while also consuming less energy. Though there have been some developments with wearable devices and sensors, most consumer electronics still use printed circuit board (PCB) fabrication methods, making them inflexible or limited in flexibility, and thereby difficult to integrate into the fabric of clothing.  This thesis outlines a screen-printing method that utilizes inexpensive, commercially available conductive ink laid over fabric to make an entire functional and flexible printed circuit which will be referred to as Screen Printed Circuit (SPC). To achieve this goal, all designed are modeled in electronic Computer Aided Design (CAD) software. This task presents several challenges, including figuring out the physical properties of traces such as their resistance, clearance limit, resolution, ink penetration depth, and stretchability. Knowing these parameters, one can model and make useful and relevant electronic circuits. This thesis validates performance of this methodology by constructing and testing an array of passive low-pass and high-pass filters. The method is evaluated by several qualities: accuracy; solderability of the inked traces; ability to make custom resistors and capacitors; and durability. These measured quantities are compared with their theoretical counterparts.  Each developed SPC functions as intended and performs to within reasonable range of theoretical values across all important metrics. Further, the circuits are very flexible and perform as expected after being normally handled. This demonstrates that one can use these printed circuits in most low-speed applications where traditional (inflexible) PCBs are currently used, with minimal changes to design flow. Importantly, these printed circuits also demonstrate their ability to easily integrate into clothing, where PCBs are not typically appropriate.  It should be noted that the benefits of these flexible circuits do not come without some drawbacks. The conductive inks used in the traces have markedly higher resistivity than copper PCB traces, which decreases the power efficiency of the circuit. As well, the traces are brittle and show cracking after even moderate stretching, which further increased resistance. Future research should focus on potentially combining the thesis results with more complex and expensive methods that show improved reliability and deformability. For example, the developed methods could be combined with conducting threads or customized conductive polymer composites. Finally, developing additional custom resistors and capacitors would increase the range of circuits one can build with this method.
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