Laser-Assisted Bonding of Flexible Polymer Substrates and Electrodes Using Carbon Nanotubes

To enhance the mechanical reliability of next-generation electronic devices, including flexible and wearable devices, a high level of mechanical reliability is required at various flexible joints. Organic adhesive materials, such as epoxy, used for bonding polymer substrates inevitably increase joint thickness and cause issues such as delamination and cracking at the bonding interface due to repeated deformation. Therefore, developing a bonding process that minimizes joint thickness is crucial to achieving both flexibility and robustness. Additionally, a low-temperature bonding process is required to prevent heat damage to thin polymer substrates. In this study, a laser-assisted bonding method using carbon nanotubes (CNTs) is developed to bond flexible polymer substrates and electrodes while ensuring high flexibility, robustness, and minimal thermal damage. For substrate-to-substrate bonding, laser transmission welding (LTW) using a CNT coating and an expanded laser beam is introduced. For CNT electrode-to-substrate bonding, size-controllable laser heating is applied to achieve robust bonding. The bonding process is designed to induce localized melting of the polymer substrate surface by releasing thermal energy from the CNT coating layer through a dispersed laser beam. The laser process conditions were optimized to minimize bonding time and thermal damage to the substrate, and the mechanism of CNT bond formation between polymer substrates was analyzed. To evaluate the stiffness and flexibility of the joint, shear tests, tensile tests, and three-point bending tests were conducted. The mechanical and electrical durability of the flexible bonded interfaces was assessed through electrode rubbing tests, repeated bending tests, and water immersion tests.