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FPC boards can be classified according to the type of bending encountered during assembly and use. There are two design types, which are discussed as follows:
1. Static design
Static design refers to the bending or folding that the product encounters only during the assembly process, or the bending or folding that rarely occurs during use. Single-sided, double-sided, and multi-layer circuit boards can successfully achieve a folded static design. Generally, for most double-sided and multi-substrate designs, the small folding radius should be ten times the thickness of the entire circuit. Circuits with more layers (eight layers or more) will become very rigid, and it is difficult to bend them, so there will be no problems. Therefore, for double-sided circuits that require a strict bending radius, all copper traces should be placed on the same surface of the substrate film in the folding area. By removing the film on the opposite side, the folded area approximates a single-sided circuit.
2. Dynamic design
The design of dynamic circuits is aimed at the repeated bending during the entire life cycle of the product, such as the cables of printers and disk drives. In order to make the dynamic circuit reach a long bending life cycle, the relevant part should be designed as a single-sided circuit with copper on the central axis. The central axis refers to a theoretical plane, which is in the center layer of the material that constitutes the circuit. By using the same thickness of the substrate film and coating on both sides of the copper, the copper foil will be accurately placed in the center and the pressure during bending or bending is small.
Multi-layer complex designs that require high dynamic bending cycles and high density can now be achieved by using anisotropic (z-axis) adhesives to connect double-sided or multilayer circuits to single-sided circuits. Bending occurs in single-sided assembly, and the dynamic bending area belongs to the multi-layer area. It is not endangered by bending and can install complex wiring and required components.
Although it is expected that flexible printed circuits can meet all applications that require bending, bending, and some special circuits, in these applications, a large part of bending or bending fails. Flexible materials are used in the PCB manufacture, but the flexible materials themselves cannot guarantee the reliability of the circuit function when being bent or bent, especially in dynamic applications. Many factors can improve the reliability of the molding or repeated bending of the printed flexible printed circuit board. In order to ensure the reliable operation of the finished circuit, all these factors must be considered during the design process. Here are some tips for increasing flexibility:
1) In order to improve dynamic flexibility, electroplated boards should be selected for circuits with two or more layers.
2) It is recommended to keep the number of bends small.
3) The wires should be arranged staggered to avoid the I-type micro-clustering effect, and the wire paths should be orthogonal to facilitate bending.
4) In the bent area, do not place pads or through holes.
5) Do not place ceramic devices near any bending area, so as to avoid discontinuous coating, discontinuous plating or other stress concentration. It should be ensured that there are no distortions in the finished assembly. Twisting may cause undesirable stress on the outer edge of the circuit. Any burrs or irregularities in the blanking process may cause the circuit board to crack.
6) PCB factory forming processing should be.
7) In the bending area, the thickness and width of the conductor should remain unchanged. There should be changes in electroplating or other coatings to avoid neck-like shrinkage of the wires.
8) Make a long and narrow cut in the flexible printed circuit, allowing different wooden brackets to bend in different directions. Although this is an effective means of chemical efficiency, it is easy to cause tearing and extension of the slit at the incision. This problem can be prevented by making a hole at the end of the incision, using a rigid plate or a thick flexible material or PTFE To strengthen these areas (Finstad, 2001). Another method is to make the incision as wide as possible and make a complete semicircle at the end of the incision. If it cannot be reinforced, the circuit cannot be bent at a distance of 1I2in from the end of the cut.
