Effect of a Metal Thin Film's Residual Stress and Manufacturing Process on Thin Film Micro-Crack

In recent automobile trends, the functions of opening and closing the vehicle door are accomplished by touch sensor and smart-phone NFC (Near Field Communication) systems. These convenience features are incorporated into the outdoor handle. However, this function can’t be used when chrome plating is applied to this part for design purposes. To solve the problem of chrome plating, we studied a metal sputtering deposition process technology, which can preserve the metal feeling without interfering with NFC and touch sensor operation. To achieve this interface communication and sensing performance, we developed a surface treatment that can generate micro-cracks in the thin film layer. We also investigated how the door handle manufacturing process affected the shape of the micro-cracks in the thin film. Results showed that the thickness of the thin film and the target power played a crucial role in controlling the residual tensile stress in the thin film, which was one of major factors responsible for generating micro-cracks in the thin film layer. The shape of the micro-cracks in the thin film was affected by the adjacent layers of the thin film, the primer paint and UV top coat. The surface energy of the primer paint and the shear stress produced by the hardening of the UV top coat were found to affect the shape of the micro-cracks. In addition, we found that there was no change in the shape of the micro-cracks with additional heat treatment, if the residual tensile stress was sufficiently relieved by the micro-cracks formed in the thin film. The slits between the micro-cracks in the outdoor handle cover allowed the Capacitance Sensor and NFC to perform.