Introduction When engineers discuss dry film photoresist for printed circuit board manufacturing, attention usually goes first to the photosensitive resin. However, another material plays a critical supporting role throughout coating, handling and exposure: the PET base film. In dry film resist systems, a photosensitive resin layer is laminated onto a light-transmitting polyester base film, while a polyethylene cover film is typically used on the opposite side for protection. During PCB processing, the cover film is removed, the photosensitive layer is laminated onto the circuit-board substrate, and UV exposure is carried out through the PET base film before the carrier is