PCB Technical - FPC classification

1.1 Single-sidedflexible PCB

Single-sided flexible PCB has only one layer of conductor, and the surface can be covered or not covered. The insulating base material used varies with the application of the product. Commonly used insulating materials include polyester, polyimide, polytetrafluoroethylene, and soft epoxy-glass cloth.

1.2 Double-sided flexible PCB

Double-sided flexible PCB with two layers of conductors. The application and advantages of this type of double-sided flexible PCB are the same as those of a single-sided flexible PCB, and its main advantage is to increase the wiring density per unit area. It can be divided into with or without metallized holes and with or without covering layer: a without metallized holes, without covering layer; b without metallized holes, with covering layer; c with metallized holes, without covering layer ; D There are metallized holes and covering layers. The double-sided flexible PCB without covering layer is rarely used.

1.3 Multilayer flexible PCB

Flexible multi-layer PCB, like rigid multi-layer PCB, adopts multi-layer lamination technology to make multi-layer flexible PCB. The simplest multilayer flexible PCB 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 multilayer flexible PCB 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 advantage of multilayer flexible PCB is that the base film is light in weight and has excellent electrical properties, such as 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 sides of many single-sided or double-sided microstrip flexible PCBs together, but the center The parts are not glued together, thus having 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 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) A multilayer PCB is formed on a flexible insulating substrate, and the finished product can be flexed: this kind of multilayer flexible PCB is laminated with a flexible insulating material, such as polyimide film, 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 flexible PCB 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 the circuit noise is extremely small, and the interconnection end can be smoothly bent to 90 Å S Mg. Hong Nao CB has realized this kind of 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-bend-shaped bend, thereby providing a way for the z-plane interconnection, and under the action of the x, y, and z plane vibration stress, it can be placed on the solder joints Eliminate stress-strain.