The basic characteristics of FPC flexible board depend on the performance of the base material. Therefore, to improve the technical performance of FPC flexible circuit board, the performance of the base material must be improved first. Then, let’s introduce the development of soft board materials!
- Performance comparison of common film substrates
The function of the film substrate is to provide the carrier of the conductor and the insulating medium between lines, and it can be bent and curled at the same time. PI polyimide film and PET polyester film are commonly used as FPC soft board substrate. Flexible copper clad laminates and rigid copper clad laminates have the same environmental requirements for halogen-free. Green and environment-friendly materials are an inevitable trend. Polyester (PET) resin has good mechanical and electrical properties, but its biggest disadvantage is poor heat resistance, which is not suitable for direct welding assembly. The adhesive combines the copper foil with the substrate film. The commonly used adhesives are PI resin, PET resin, modified epoxy resin and acrylic resin.
- PI substrate of two-layer structure
Flexible copper clad laminate (FCCL) usually has a three-layer structure, namely polyimide, adhesive and copper foil. Because the adhesive will affect the performance of the flexible board, especially the electrical performance and dimensional stability, a two-layer flexible copper clad laminate without adhesive has been developed. At present, there are many methods to deposit electroplating metal layer on polyimide film, and two-layer structure copper clad laminate is preferred for flexible boards with high performance requirements.
- LCP base material
Liquid crystal polymer (LCP) copper-clad sheet, a thermoplastic liquid crystal polymer film is continuously hot pressed by covering copper foil to obtain single-sided or double-sided copper-clad sheet. Its water absorption is only 0.04%, and the dielectric constant is 2.85 at 1GHz, which is suitable for the requirements of high-frequency plate. The polymer is in liquid crystal state, and the one that will melt when heated is “TLCP hot melt liquid crystal polymer”. TLCP has the advantages of injection molding, extrusion processing into thin films to become the base material of PCB and FPC, and secondary processing to achieve recycling. Due to its low moisture absorption, high frequency suitability and thermal dimensional stability, TLCP films are applied on FPC-COF multilayer substrates.
- Halogen free flexible substrate meeting environmental requirements
Halogen free (bromine) substrates have been developed and applied in both rigid plates and flexible plates. For flexible substrates, including FCCL, covering film, adhesive sheet and solder resist, as well as reinforcement plates, both flame retardancy and halogen free are required.
- New copper foil
The conductive materials of FPC soft board are mainly copper copper foil, and some of them are aluminum, nickel, gold, silver and other alloys. In addition to conducting electricity, the conductor layer must also be resistant to bending. Copper foils can be divided into electrolytic copper foils and calendered copper foils according to different production methods. The difference between RA copper foil and ED copper foil is that the crystal shape is different. RA copper foil is columnar arrangement, with uniform and flat structure, easy to coarsen or etch; ED copper foil is a fish scale sheet stack. After rolling, the copper foil is smooth and has good toughness, but roughening or etching becomes difficult.
- Conductive silver paste
In FPC soft board manufacturing, conductive ink is used to print on the insulating film to form a conductor or shielding layer. Most of the conductive ink is silver paste conductive paste. The conductive layer formed by printing is required to have low resistance, firm bonding, flexible, and easy printing operation and curing. Using conductive ink to make graphics is also an environment-friendly, low-cost technology.