PCB Tech - Basic knowledge of PCB soft board

Multi-layer FPC soft boards 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 sides of many single-sided or double-sided microstrip flexible FPCs together, but the center The parts are not glued 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 FPC soft board 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 z-planes between the flexible circuit layers to achieve the required interconnection. This multilayer flexible PCB is most suitable for designs that require flexibility, high reliability, and high density.

2) Multi-layer PCB is formed on a flexible insulating substrate, and the finished product is not specified to be flexible: This type of multilayer FPC soft board is made of flexible insulating materials, such as polyimide film, laminated to make a multilayer board. The inherent flexibility is lost after lamination. This type of flexible PCB is used when the design requires maximum use of the insulating properties of the film, such as low dielectric constant, uniform thickness of the medium, lighter weight, and continuous processing. For example, a multilayer PCB made of polyimide film insulating material is about one-third lighter than a rigid PCB with epoxy glass cloth.

3) A multilayer PCB is formed on a flexible insulating substrate, and the finished product must be shapeable, not continuously flexible: this type of multilayer FPC soft board is made of soft insulating materials. Although it is made of soft materials, it is limited by electrical design. For example, for the required conductor resistance, a thicker conductor is required, or for the required impedance or capacitance, a thicker conductor is required between the signal layer and the ground layer. The insulation is isolated, so it is already formed in the finished application. The term "formable" is defined as: a multilayer flexible PCB component has the ability to be shaped into the required shape and cannot be flexed in the application. Used in the internal wiring of avionics units. At this time, it is required that the conductor resistance of the strip line or three-dimensional space design is low, the capacitive coupling or circuit noise is extremely small, and the interconnection end can be smoothly bent to 90°. Multi-layer flexible PCB made of polyimide film material achieves this wiring task. Because the polyimide film is resistant to high temperatures, flexible, and has good overall electrical and mechanical properties. In order to achieve all the interconnections of this component section, the wiring part can be further divided into a plurality of multilayer flexible circuit components, which are combined with adhesive tape to form a printed circuit bundle.

1.4 Rigid-flexible multilayer PCB

This type is usually on one or two rigid PCBs, and contains the soft PCB that is necessary to form a whole. The flexible PCB layer is laminated in a rigid multi-layer PCB. This is to have special electrical requirements or to extend outside the rigid circuit to dynamize the Z-plane circuit mounting capability. This type of product has been widely used in electronic equipment that takes compression of weight and volume as the key, and must ensure high reliability, high density assembly and excellent electrical characteristics.

Rigid-flexible multi-layer PCBs can also bond and press the ends of many single-sided or double-sided flexible PCBs together to form a rigid part, while the middle is not bonded to form a soft part. The Z-side of the rigid part is interconnected with metallized holes. even. The flexible circuit can be laminated into the rigid multi-layer board. This type of PCB is increasingly used in applications that require ultra-high packaging density, excellent electrical characteristics, high reliability, and strict volume restrictions.

There have been a series of mixed multilayer flexible PCB components designed for use in military avionics. In these applications, weight and volume are critical. In order to meet the specified weight and volume limits, the internal packaging density must be extremely high. In addition to the high circuit density, in order to minimize crosstalk and noise, all signal transmission lines must be shielded. If you want to use shielded separate wires, it is practically impossible to economically package them into the system. In this way, a mixed multi-layer

Flexible PCB to realize its interconnection. This component contains the shielded signal line in a flat stripline flexible PCB, which in turn is an essential part of a rigid PCB. In relatively high-level operating situations, after the manufacturing is completed, the PCB forms a 90° S-shaped bend, thereby providing a way for z-plane interconnection, and under the action of vibration stress in the x, y and z planes, it can be used in the solder joints. To eliminate stress-strain.

2. advantage

2.1 Flexibility
A significant advantage of using FPC soft board is that it can be more convenient to wire and install in three-dimensional space, and it can also be rolled 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.

2.2 Reduce the size

In the assembly and connection of components, the conductor cross-section of the flexible PCB is thin and flat compared with the use of conductive cables, which reduces the size of the conductor and can be formed along the casing, making the structure of the device more compact and reasonable, and reducing the size of the assembly. volume. Compared with rigid PCB, space can be saved by 60~90%.

2.3 Reduce weight

In the same volume, the weight of a flexible PCB can be reduced by about 70% compared with a wire and cable at the same current carrying capacity. Compared with a rigid PCB, the weight can be reduced by about 90%.

2.4 Consistency of assembly and connection

Use FPC soft board to install the connection, eliminating the error when wiring with wires and cables. As long as the processing drawings are proofread and passed, all the winding circuits produced later will be the same. There will be no wrong connection when installing the cable.

2.5 Increased reliability

When a flexible PCB is used for assembly and connection, because it can be routed on the three planes of X, Y, and Z, the transfer interconnection is reduced, the reliability of the entire system is increased, and the inspection of faults is convenient.

2.6 Design controllability of electrical parameters

According to application requirements, designers can control capacitance, inductance, characteristic impedance, delay and attenuation when designing flexible PCBs. Can be designed to have the characteristics of a transmission line. Because these parameters are related to wire width, thickness, spacing, insulating layer thickness, dielectric constant, loss tangent, etc., it is difficult to do this when using wire cables.

2.7 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, devices, and plugs. Dip soldering or wave soldering can be used to replace the manual soldering of each wire.

2.8 Material use is optional

The FPC soft board can be manufactured with different base materials according to different usage requirements. For example, polyester film can be used in assembly applications that require low cost. In demanding applications, excellent performance is required, and polyimide film can be used.

2.9 Low cost

Using FPC soft board to install the connection can reduce the total cost. This is because:

1) Due to the consistency of the various parameters of the flexible PCB wires; the overall termination is implemented, which eliminates the errors and rework that often occur when the cable wires are installed and connected, and the replacement of the flexible PCB is more convenient.
2) The application of flexible PCB simplifies the structural design, it can be directly adhered to the component, reducing the clamp and its fixing parts.
3) For wires that need to be shielded, flexible PCBs are cheaper.

2.10 Continuity of processing
Since flexible foil-clad boards can be continuously supplied in rolls, continuous production of flexible PCBs can be realized. This also helps reduce costs.
3. shortcoming

3.1 One-time high initial cost
Since flexible PCBs are designed and manufactured for special applications, the initial circuit design, wiring and photographic masters require higher costs. Unless there is a special need to apply a flexible PCB, it is usually best not to use it for a small number of applications.

3.2 It is difficult to change and repair soft 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 repair and restored after repair, which is a relatively difficult task.

3.3 Size is restricted
Flexible PCBs are usually manufactured by batch process when they are not yet common, so they cannot be made very long and wide due to the limitation of the size of the production equipment.

3.4 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 trained