Flexibility and Reliability Design of Printed Circuit Board
Flexible printed circuit boards can be classified according to the types of bending encountered during assembly and use. There are two types of design, which are discussed as follows:
1. Static design
Static design refers to the bending or folding that product only meets in the process of assembly, or the bending or folding that seldom occurs during use. Like single-sided, double-sided and multi-layer circuit boards, folding static design can be successfully realized. Usually, for most double-sided and multi-substrate designs, the minimum bending radius of folding should be 10 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 problem. Therefore, for double-sided circuits with strict bending radius, all copper wires should be placed on the same surface of the substrate film in the folding area. By removing the film on the relative surface, the folded area is similar to a one-sided circuit.
2. Dynamic Design
Dynamic circuits are designed for repeated bending throughout the life cycle of a product, such as cables for printers and disk drives. In order to achieve the longest bending life cycle of dynamic circuits, the relevant part should be designed as a one-sided circuit with copper on the central axis. The central axis refers to a theoretical plane, which is 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 position and will be subjected to the least pressure during bending or bending.
Multilayer complexity designs requiring high dynamic bending cycles and high density can now be realized by using anisotropic (z-axis) binders to connect double-sided or multi-layer circuits to single-sided circuits. Bending only occurs in one-sided assembly area. Beyond the dynamic bending area, it belongs to a multi-layer independent area, which is not endangered by bending. It can install complex wiring and required components.
Although flexible printed circuits are expected to satisfy all applications requiring bending, bending and some special circuits, in these applications, a large part of bending or bending fails. 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 boards. 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 techniques for increasing flexibility:
1) In order to improve dynamic flexibility, electroplating board should be selected for circuits with two or more layers.
2) It is recommended to keep the minimum number of bends.
3) The conductors should be staggered in order to avoid the type I micro-agglomeration effect, and the conductor paths should be orthogonal to facilitate bending.
4) Do not place pads or holes in the bending area.
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 guaranteed that there is no distortion in the finished assembly. Distortion may cause undue stress on the outer edge of the circuit. Any burr or irregularity in the blanking process may lead to circuit board rupture.
6) Forming process should be the first choice.
7) In the bending region, the thickness and width of the conductor should remain unchanged. Changes should be made in plating or other coatings to avoid necking shrinkage of conductors.
8) Making a narrow notch in the flexible printed circuit allows different wooden supports to bend in different directions. Although this is an effective way to maximize effectiveness, the incision is prone to tear and crack extension. This problem can be prevented by making a drill hole at the end of the incision and strengthening these areas with rigid plates or a thick flexible material or polytetrafluoroethylene (Finstad, 2001). Another method is to make the incision as wide as possible and to 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 notch.
