Cover film is a key protective material for flexible PCBs (FPC), responsible for insulating protection, circuit anti-oxidation, bending resistance, and environmental isolation of internal conductive traces. PI (Polyimide) and PET (Polyethylene Terephthalate) are the two most widely used FPC cover film materials. Due to differences in molecular structure and physical and chemical properties, the two materials show significant gaps in heat resistance, bending performance, dimensional stability, and environmental adaptability, adapting to different FPC application scenarios ranging from low-end consumer electronics to high-end industrial and automotive fields.

PI cover film is a high-performance special polymer material, the mainstream choice for high-end FPC protection. Its most prominent advantage is excellent high-temperature resistance, with a long-term working temperature of -200℃ to 260℃ and instantaneous temperature resistance up to 400℃, fully adapting to high-temperature reflow soldering, wave soldering, and long-term high-temperature working environments. PI has ultra-high mechanical flexibility and fatigue resistance, capable of withstanding more than 100,000 repeated bending cycles without cracking or deformation, making it suitable for FPC products requiring frequent folding and dynamic bending. In addition, PI features stable dimensional change rate, excellent chemical corrosion resistance, and low water absorption, which can effectively resist the erosion of moisture, flux, and chemical solvents, ensuring long-term reliability of FPC circuits in complex environments.

PET cover film is a common general-purpose polymer material with outstanding cost advantages and basic protective performance. Its mechanical strength is stable, with good tensile resistance and scratch resistance, which can meet the static protection and insulation requirements of conventional FPC circuits. The production process of PET cover film is mature, with low raw material and processing costs, and it has high cost performance in low-end FPC mass production. However, PET has obvious performance shortcomings: its heat resistance is poor, with a long-term working temperature not exceeding 120℃, and it is prone to thermal deformation, brittleness, and yellowing in high-temperature soldering and high-temperature working environments, unable to adapt to multi-time reflow processes. Meanwhile, its bending fatigue resistance is far lower than PI, and long-term repeated bending will cause material cracking and insulation failure.

The application scenarios of the two cover film materials are clearly differentiated. PET cover film is mainly used in low-end, static, and low-cost FPC products, including ordinary wearable device accessories, simple consumer electronic connection lines, and low-frequency static circuit boards that do not require high temperature resistance and frequent bending. PI cover film is applied to high-end high-reliability FPC products, such as folding screen mobile phones, automotive electronic wiring, medical flexible circuits, and aerospace precision FPCs. These scenarios require high temperature resistance, frequent dynamic bending, and harsh environmental adaptability, where PI’s comprehensive performance advantages are irreplaceable. In FPC design and material selection, cost priority scenarios choose PET, while performance and reliability priority scenarios must adopt PI cover film.