This disclosure discloses a flexible printed circuit, a chip on film, and a bonding method and a display device using this flexible printed circuit and chip on film. The flexible printed circuit/chip on film of this disclosure a humidity detection layer located between a flexible base film and a metal foil, wherein the humidity detection layer and the flexible base film, and/or the humidity detection layer and the metal foil, are optionally bonded by an adhesive layer. The humidity detection layer can change resistance and/or color according to the humidity, so that intuitive and rapid localization of coating badness is performed directly (by color change) or by means of a detecting lead and an impedance/voltage detecting circuit, and finally the object of reducing the ratio of defective products is achieved.