PCB Tech - Advantages and disadvantages of flexible PCB

The advantage of a multilayer flexible PCB is that the substrate film is light in weight and has excellent electrical properties, such as a low dielectric constant. The multi-layer flexible PCB board made of polyimide film as the base material is about 1/3 lighter than the rigid epoxy glass cloth multi-layer PCB board, but it loses the excellent single-sided and double-sided flexible PCB. Most of these products do not require flexibility.

Multilayer flexible PCB can be further divided into the following types:

1. A multilayer PCB is formed on a flexible insulating substrate, and the finished product is specified to be flexible. This structure usually bonds the two-sided ends of many single-sided or double-sided microstrip flexible PCBs together, but the central part of the PCB is not bonded together, so it has a high degree of flexibility. In order to have the desired electrical characteristics, such as the characteristic impedance performance and the rigid PCB to which it is interconnected, each circuit layer of the multilayer flexible PCB component must be designed with signal lines on the ground plane. In order to have a high degree of flexibility, a thin, suitable coating, such as polyimide, can be used on the wire layer instead of a thicker laminated cover layer. The metallized holes enable the required interconnection between the flexible circuit layers. This multilayer flexible PCB is most suitable for designs that require flexibility, high reliability, and high density.

2. A multilayer PCB is formed on a flexible insulating substrate, and the finished product is not specified to be flexible. This type of multilayer flexible PCB is made of flexible insulating materials, such as polyimide film, laminated to make a multilayer board, which loses its inherent flexibility after lamination. This type of flexible PCB is used when the design requires the greatest use of the insulating properties of the film, such as low dielectric constant, uniform thickness, light weight, and continuous processing., A multilayer PCB is formed on a flexible insulating substrate. Although it is made of soft materials, due to electrical design constraints, such as the required conductor resistance, a thick conductor is required, or the required impedance or capacitance requires a thick layer between the signal layer and the ground layer. The insulation is isolated, so it has been formed in the finished application and cannot be flexed in the application. Generally used in the internal wiring of avionics units.

4. Advantages:

1) Flexibility. A significant advantage of the application of flexible PCB is that it can be more convenient to route and install in three-dimensional space, and it can also be crimped or folded for use. As long as it is curled within the allowable radius of curvature, it can withstand thousands to tens of thousands of times without being damaged.

2) Reduce the volume. In the assembly and connection of components, the conductor cross-section of the flexible PCB is thin and flat compared with the use of wire and cable, which reduces the size of the wire and can be formed along the casing, making the structure of the device more compact and reasonable, and reducing the number of connections. Volume: Compared with rigid PCB, space can be saved by 60% to 90%.

3) Reduce quality. In the same volume, compared with a wire cable, a flexible PCB can reduce its mass by about 70% at the same current carrying capacity; compared with a rigid PCB, its mass can be reduced by about 0%.

4) Consistency of installation and connection. The flexible PCB is used to install the connection, which eliminates errors when wiring with wires and cables. As long as the processing drawings are verified and passed, all the flexible circuits produced are the same, and there will be no misconnections when installing the connecting wires.

5) Increased reliability. When a flexible PCB is used for assembly and connection, it can be wired on the three planes of X, Y, and Z, which reduces the transfer interconnection, increases the reliability of the entire system, and facilitates fault inspection.

6) Controllability of electrical parameter design. According to the requirements of use, the designer can control the capacitance, inductance, characteristic impedance, delay and attenuation when designing the flexible PCB, so that it can be designed to have the characteristics of a transmission line. These parameters are related to wire width, thickness, spacing, insulating layer thickness, dielectric constant, loss tangent, etc., which are difficult to achieve when using wire and cable.

7) The end can be soldered as a whole. Flexible PCB, like rigid PCB, has terminal pads, which can eliminate wire stripping and tinning, thereby saving costs. The terminal pads are connected to the components and plugs. Dip soldering or wave soldering can be used instead of manual soldering of each wire.

8) The use of materials is optional. Flexible PCB can be manufactured by using different base materials according to different usage requirements. For example, polyester film can be used in assembly applications requiring low cost; polyimide film can be used if excellent performance is required.

9) Low cost. With flexible PCB assembly, the total cost can be reduced. Because of the consistency of the wire parameters of the flexible PCB and the implementation of overall termination, errors and rework that often occur during cable wire assembly are eliminated, and the replacement of the flexible PCB is more convenient. The application of flexible PCB simplifies the structural design, it can be directly adhered to the component, reducing the clamp and its fixing parts. For wires that need to be shielded, flexible PCBs are cheaper.

10) Continuity of processing. Since the flexible foil-clad laminate can be continuously supplied in rolls, the continuous production of flexible PCBs can be realized, which is also conducive to reducing costs.

5. Disadvantages:

1) The one-time initial cost is high. Since flexible PCBs are designed and manufactured for special applications, the initial circuit design, wiring and photographic masters require higher costs.

2) It is difficult to change and repair the flexible PCB. Once the flexible PCB is made, it must be changed from the base map or the programmed light drawing program, so it is not easy to change. The surface is covered with a protective film, which must be removed before repairing and restored after repairing, which is more difficult.

3) The size is restricted. Flexible PCBs are usually manufactured by batch process. Due to the limitation of the size of production equipment, they cannot be made very long and very wide.

4) Improper operation is easy to damage. Improper operation of the assembly and connection personnel can easily cause damage to the flexible circuit, and its soldering and rework need to be operated by trained personnel.