1.Performance comparison of common film substrates
The function of the film substrate is to provide the conductor carrier and the insulating medium between the lines and can be bent and curled at the same time. FPC soft board substrates commonly used PI polyimide film and PET polyester film. Flexible copper clad laminates and rigid copper clad laminates also have halogen-free environmental protection requirements, and green materials are an inevitable trend. The mechanical and electrical properties of polyester (PET) resin are acceptable, but the biggest disadvantage is poor heat resistance and is not suitable for direct welding assembly. The adhesive combines the copper foil with the base film. Commonly used are PI resin, PET resin, modified epoxy resin, and acrylic resin.
Flexible copper-clad laminates (FCCL) usually have a three-layer structure, namely polyimide, adhesive and copper foil. Since the adhesive will affect the performance of the flexible board, especially the electrical performance and dimensional stability, a two-layer structure flexible copper clad laminate without an adhesive has been developed. At present, there are many methods of depositing electroplated metal layers on polyimide films, and the two-layer structure copper clad laminates tend to be used for flexible boards with high-performance requirements.
Liquid crystal polymer (LCP) copper clad laminates are continuously hot-pressed from a thermoplastic liquid crystal polymer film covered with copper foil to obtain single-sided or double-sided copper-clad laminates. The water absorption rate is only 0.04%, and the dielectric constant is 2.85 at 1GHz. It is suitable for high frequency boards. Requirements. The polymer is in a liquid crystal state and will melt when heated as a “TLCP hot-melt liquid crystal polymer”. The advantages of TLCP can be injection-molded, can be extruded into a film to become the substrate of PCB and FPC, and can be processed twice for recycling. The low hygroscopicity, high frequency suitability, and thermal dimensional stability of TLCP film make TLCP film enter the application on FPC-COF multilayer substrates.
Halogen-free (bromine) substrates have been developed and applied in both rigid and flexible boards. For flexible substrates, including FCCL, cover films, adhesive sheets and solder resist, as well as reinforced boards, they must be flame-retardant and halogen-free.
The conductive material of the FPC soft board is mainly copper-copper foil, and some alloys such as aluminum, nickel, gold, and silver are also used. In addition to conducting electricity, the conductor layer must be resistant to bending. According to different production methods, there are two types of copper foil: electrolytic copper foil and rolled copper foil. The difference between RA and ED copper foil is that the crystal shape is different. RA copper foil is arranged in a columnar shape, with uniform and flat structure, easy to roughen or etch treatment; ED copper foil is a stack of fish-scale sheets, and the rolled copper foil is smooth and tough Good, but roughening or etching becomes difficult.
In the manufacture of FPC flexible boards, conductive inks are used to print conductive inks on insulating films to form wires or shielding layers. Most of these conductive inks are silver paste conductive pastes. The conductive layer formed by printing is required to have low resistance, strong bonding, flexibility, and printing operations. Easy to cure. The use of conductive ink to make graphics is also environmentally friendly and low-cost technology.
Below are some flex PCBs with Dupont PI material:
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