Flexible printed circuit 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 of the product only encountered in the assembly process, or the bending or folding that rarely occurs during use. Like single-sided, double-sided and multilayer circuit boards, folding static design can be successfully realized. Generally, for most double-sided and multi substrate designs, the minimum bending radius of folding should be ten times the thickness of the whole circuit. Circuits with more layers (eight or more) will become very hard and difficult to bend, so there will be no problems. Therefore, for double-sided circuits requiring strict bending radius, all copper wires should be set on the same side of the substrate film in the folding area. By removing the coating on the opposite surface, the folded area is similar to a single-sided circuit.
2. Dynamic design
Dynamic circuits are designed for repeated bending throughout the product life cycle, such as the cables of printing machines and disk drives. In order to achieve the longest bending life cycle of the dynamic circuit, 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 central layer of the material constituting the circuit. By using the same thickness of substrate film and coating on both sides of copper, the copper foil will be accurately placed in the center, and the pressure will be minimized during bending or bending.
Multilayer complex designs requiring high dynamic bending cycles and high density can now be achieved by connecting double-sided or multilayer circuits to single-sided circuits using various anisotropic (Z-axis) adhesives. Bending only occurs at the place of single-sided assembly. It belongs to multi-layer independent area outside the dynamic bending area. Here, it is not endangered by bending, and complex wiring and required components can be installed.
Although flexible printed circuits are expected to meet all applications requiring bending, bending and some special circuits, a large part of bending or bending fails in these applications. Flexible materials are used in the manufacture of printed circuit boards, but the flexible materials themselves can not guarantee the reliability of circuit functions when they are bent or bent, especially in dynamic applications. Many factors can improve the reliability of forming or repeated bending of printed flexible printed circuit board. In order to ensure the reliable operation of the finished circuit, all these factors must be taken into account in 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 minimum number of bends.
3) The conductors shall be staggered to avoid type I micro agglomeration effect, and the conductor path shall be orthogonal to facilitate bending.
4) Do not place pads or through holes in curved areas.
5) Do not place ceramic devices near any bending area, so as to avoid coating discontinuity, electroplating discontinuity or other stress concentration. It should be ensured that there is no distortion in the completed assembly. Distortion may cause undue stress at the outer edge of the circuit. Any burr or irregularity during blanking may cause the circuit board to crack.
6) Factory forming shall be preferred.
7) In the bending area, the conductor thickness and width shall remain unchanged. Changes should be made in the plating or other coating to avoid necking shrinkage of the conductor.
8) Make a long and narrow cut in the flexible printed circuit to allow different wooden supports to bend in different directions. Although this is an effective means to maximize efficiency, it is easy to cause tearing and crack extension at the incision. This problem can be prevented by making a hole at the end of the incision, and strengthening these areas with a rigid plate or a thick piece of flexible material or polytetrafluoroethylene (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 1i2in from the end of the notch.