PCB Tech - Classification of FPC circuit board multilayer soft board

Multilayer FPC soft board

Multi-layer FPC soft boards, like rigid multi-layer PCBs, can be made into multi-layer FPC soft boards by using multi-layer lamination technology. The simplest multi-layer FPC soft board is a three-layer flexible PCB formed by covering two copper shielding layers on both sides of a single-sided PCB. This three-layer flexible PCB is equivalent to a coaxial wire or a shielded wire in electrical characteristics. The most commonly used multi-layer FPC soft board structure is a four-layer structure, which uses metallized holes to realize interlayer interconnection. The middle two layers are generally the power layer and the ground layer.

The advantages of multi-layer FPC flexible boards are that the base film is light in weight and has excellent electrical properties, such as low dielectric constant. The multi-layer FPC soft 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 FPC soft board. Flexibility, most of these products do not require flexibility. Today, FPC manufacturers will take everyone to understand the classification of multilayer soft boards:

Multi-layer FPC soft boards can be further divided into the following types:

1) A multilayer FPC soft board 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 PCBs together, but The central part is 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 kind of multilayer FPC soft board is most suitable for the design that requires flexibility, high reliability and high density.

2) A multilayer PCB is formed on a flexible insulating substrate, and the finished product can 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 FPC soft board is used when the design requires the 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 multi-layer 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: the multi-layer FPC soft board 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°. The multi-layer FPC soft board 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 board.

FPC multilayer soft and hard combined circuit board (soft and hard combined board)

This type is usually on one or two rigid PCBs, including the FPC soft board that is necessary to form the whole. The FPC soft board layer is laminated in a rigid multi-layer PCB. This is to have special electrical requirements or to extend to the outside of the rigid circuit, in order to replace 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.

FPC multilayer flexible and rigid board can also bond and press the ends of many single-sided or double-sided FPC flexible boards together to form a rigid part, while the middle is not bonded to become 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 hybrid multi-layer FPC soft board components designed for 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 FPC soft board is used to realize its interconnection. This component contains the shielded signal line in the flat stripline FPC soft board, which in turn is an essential part of the 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.