Flexible circuits come in single-sides, double side or multi layered forms each of which requires a variety of materials to function. The FPC (Flexible Printed Circuit) uses several layers of polymer. In fact, most of the layers of an FPC are made of polymer materials these days:
[ 1. Composition ]
Substrate layer (Insulators): This is the base layer of the FPC, made of a flexible polymer material like polyimide or polyester. The substrate provides the mechanical support for the FPC.and determines the overall flexibility and durability of the circuit.
Adhesives: Adhesive can be select from the range between Acrylics, Epoxies, PSA and more. It is often determined by factors including the end-users requirement of the application to the dimension of the conductor etc.te. Adhesives used for FPCs are typically heat-resistant and can withstand the bending and flexing of the FPC.
Surface Finish: can be selected from Carbon, Gold, Hard Nickel, Tin, Silver etc.
Conductive layer: This layer consists of thin, flexible strips of copper or other conductive metals that are printed onto the substrate. The conductive layer forms the electrical connections between components on the FPC.
Coverlay layer (Insulators): This layer is applied over the conductive layer to protect it from damage and to provide insulation. The coverlay is typically made of a polymer material like polyimide, and is applied in a liquid form and cured using heat.
Surface finish layer: This layer is used to protect the exposed copper traces on the FPC from oxidation and to improve their solderability. The surface finish layer is typically made of a thin layer of gold, silver, or other metals.
Some of the most common materials use as an insulator include: screen printed dielectric insulator, solder mask, Polyester; PI (Polyimid), PET ( Polyethylene terephthalate) to PEN (Polyester Naphthalene) in particular.
# For more information on comparing PI, PET, PEN on FPC Application, please refers to the following articles:
These layers are laminated together to form the FPC. The number and thickness of the layers can vary depending on the specific application and requirements of the FPC.
[ 2. Evaluating the Polymer Materials used as the Insulator in FPC ]
Customers typically look for several key mechanical properties when evaluating the polymer, including:
Flexibility: FPCs are designed to be flexible, so the polymer material used must be able to bend and flex without cracking or breaking. The ability to flex and bend repeatedly without damage is critical to the performance and durability of the FPC.
Tensile strength: The polymer material used in FPCs must have sufficient tensile strength to withstand the forces exerted on it during the manufacturing process and during use. The tensile strength of the material is also important in determining the maximum allowable bend radius of the FPC.
Tear resistance: FPCs are subject to tearing if they are subjected to excessive stress or strain. The polymer material used in FPCs should have high tear resistance to minimize the risk of tearing.
Dimensional stability: The polymer material used in FPCs must have good dimensional stability to maintain the desired shape and size of the FPC during manufacturing and use.
Moisture resistance: FPCs are often used in environments where they may be exposed to moisture. The polymer material used in FPCs should have low moisture absorption to maintain the electrical performance and reliability of the FPC.
Chemical resistance: FPCs may be exposed to chemicals or solvents in certain applications. The polymer material used in FPCs should have good chemical resistance to maintain the performance and reliability of the FPC.
Heat resistance: The heat resistance of the polymer material used in FPCs is critical to ensuring the longevity and reliability of the FPC in high-temperature applications. Customers typically look for polymer materials that can withstand the specific temperatures to which the FPC will be exposed, and that can maintain their mechanical and electrical properties even at require temperatures.
Electronic Insulation: FPCs are often used in electronic applications where it is critical to maintain electrical insulation between different circuits or components. The polymer material used in FPCs must have good dielectric properties to provide effective electrical insulation. The dielectric properties of the polymer material used in FPCs are typically measured by its dielectric constant (Dk) and its dissipation factor (Df). The dielectric constant measures the material's ability to store electrical energy, while the dissipation factor measures the amount of energy lost as heat when an electrical field is applied to the material.
Cost: In some cases, cost may be a primary concern, and a lower-cost polymer such as polyester may be the most suitable choice. For example, if the FPC is being used in a low-temperature, low-stress application where high heat resistance and dimensional stability are not critical, then polyester may be a good choice due to its low cost. In other cases, the specific requirements of the application may dictate the use of a more expensive polymer material, such as polyimide, which has better heat resistance and dimensional stability than polyester. For example, if the FPC is being used in a high-temperature or high-stress application where durability and reliability are critical, then the extra cost of polyimide may be justified.
Overall, the choice of polymer material for an FPC will depend on the specific requirements of the application, including the temperature range, frequency range, and mechanical stresses to which the FPC will be subjected. Polyester is a good choice for low-cost, low-temperature applications, while polyimide is preferred for high-temperature and high-frequency applications. PEN may be suitable for some applications where a balance of mechanical and electrical properties is required.
# For more information on comparing PI, PET, PEN on FPC Application, please refers to the following articles:
References Zhang, Y., & Huang, G. (2020). Flexible Printed Circuit Board Based on Polyimide. In Flexible Printed Circuit Board (pp. 83-103). Springer, Singapore. https://doi.org/10.1007/978-981-15-2848-2_5
Stankova, M., & Kostadinova, T. (2020). Investigation of the mechanical properties of flexible printed circuit boards. In 2020 International Conference on Electronics, Communications and Networks (CECNet) (pp. 297-301). IEEE. https://doi.org/10.1109/CECNet49855.2020.9161505
Wang, X., Zhan, W., Liu, F., Yu, L., & Guo, L. (2017). Electrical and Mechanical Properties of Flexible Printed Circuit Board Based on PEN Substrate. In 2017 International Conference on Circuit, Power and Computing Technologies (ICCPCT) (pp. 1-5). IEEE. https://doi.org/10.1109/ICCPCT.2017.8074356
Kim, D. Y., Cho, B. J., Kang, J. G., & Lee, J. H. (2018). Influence of polymer materials on the electrical properties of flexible printed circuit boards. Journal of Materials Science: Materials in Electronics, 29(9), 7238-7247. https://doi.org/10.1007/s10854-018-8886-9
Li, T., Li, C., Li, X., Li, Y., Wang, B., & Hu, N. (2019). Effects of mechanical and electrical properties of polymer dielectric on flexible printed circuit board (FPCB). Materials Science and Engineering: B, 241, 39-45. https://doi.org/10.1016/j.mseb.2018.12.016
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