스트레처블 전극용 실버입자 크기 및 기판필름의 최적화 연구

Abstract

In recent years, the research and development of stretchable electronic devices, an evolution beyond flexible electronics, have been actively pursued. These next-generation devices can operate without losing their characteristics even when their substrate is stretched or severely bent, and maintain their performance once the external force is removed. Flexible electronic components based on substrates such as glass, PET (polyethylene terephthalate), and PI(polyimide) are already ahead of their rigid counterparts. To create wearable devices and stretchable printed circuit boards(PCBs), considerable research has been done on various stretchable substrates(films) and electrodes. Stretchable substrates are broadly classified into two categories: urethane-based and silicon-based. Clear Flex and TPU(thermoplastic polyurethane) are representative of urethane-based stretchable films. However, Clearflex is not suitable for contact printing methods like screen printing due to its excessively sticky surface. To apply Clearflex to screen printing, a primer layer must be coated onto the film to reduce its stickiness. Hence, TPU film is considered preferable in the urethane category. PDMS(polydimethylsiloxane) and EcoFlex are typical silicon-based materials. However, when a film is made using silicone-based resin, the adhesion is weak when a non-silicone-based electrode binder is used. Therefore, a special surface treatment, like plasma treatment, must be applied, or the resin used in film production must be used as a binder for the conductive paste. For this reason, it's best to print the PDMS film with a silver paste that can form a stretchable electrode. Many methods for creating stretchable substrates and electrodes have been studied, but most of these studies are for research purposes rather than product commercialization. Hence, for commercialization and mass production, there's a need for simpler, cost-effective stretchable substrates and an easy method to form electrode patterns. In this research, we aimed to resolve these challenges by optimizing screen printing conditions and silver particle size for TPU and PDMS films currently in mass production. We used a 150 μm TPU film from Matai, Japan. PDMS films are commercially available, but for this study, we manufactured our own film to use as a substrate. The most important aspect was to create a silver paste optimal for PDMS films, which led to our development of a silver paste suitable for screen printing on PDMS film and featuring excellent storage stability. We used three types of flake silver powder 2μm, 7μm, and a 50:50 weight ratio mix of 2μm and 7μm to produce the silver paste, minimizing resistance changes due to strain after screen printing and curing on the TPU or PDMS film. We evaluated the dispersibility and screen printability by measuring the viscoelasticity, or rheological properties, of the produced silver paste. In the case of stretchable PCB electrodes, the thicker the coating, the less the resistance changes due to stretching. Our studies worked towards optimizing the particle size of silver powder best suited for stretchable TPU and PDMS films.