PCB Tech - Flexible Printed Circuit (FPC) Start Basics

With the continuous increase in the output ratio of Flexible Printed Circuit (FPC) and the application and promotion of rigid-flex PCB, it is now more common to add soft, rigid or rigid-flex when talking about PCBs, and say that it is a PCB with several layers. Generally, a PCB made of a flexible insulating substrate is called a Flexible Printed Circuit (FPC) or a Flexible Printed Circuit (FPC) , and a rigid-flex composite PCB is called a rigid-flex PCB. It meets the needs of today's electronic products in the direction of high density and high reliability, miniaturization, and light weight. It also meets the strict economic requirements and the needs of market and technology competition.

1. Flexible Printed Circuit (FPC) classification

Flexible Printed Circuit (FPC) are usually classified as follows according to the number and structure of conductors:

1.1 Single-sided Flexible Printed Circuit (FPC)

Single-sided Flexible Printed Circuit (FPC) 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.

Single-sided Flexible Printed Circuit (FPC)s can be further divided into the following four categories:

1) Single side connection without covering layer

The conductor pattern of this kind of Flexible Printed Circuit (FPC) is on the insulating substrate, and the conductor surface has no covering layer. Like the usual single-sided rigid PCB. This type of product is the cheapest one, usually used in non-critical and environmentally friendly applications. The interconnection is realized by soldering, welding or pressure welding. It is commonly used in early telephones.

2) One-sided connection with cover layer

Compared with the previous type, this type only has an extra layer of covering on the surface of the wire according to customer requirements. The pads need to be exposed when covering, and it can simply be left uncovered in the end area. If precision is required, the form of clearance hole can be adopted. It is the most widely used and widely used single-sided Flexible Printed Circuit (FPC), and is widely used in automotive instruments and electronic instruments.

3) Double-sided connection without covering layer

This type of connection pad interface can be connected on the front and back of the wire. In order to achieve this, a via hole is opened in the insulating substrate at the pad. This via hole can be punched, etched or made by other mechanical methods at the required position of the insulating substrate. It is used for two-sided mounting of components, devices and occasions where soldering is required. There is no insulating substrate in the pad area of the via. Such pad area is usually removed by chemical methods.

4) With cover layer connected on both sides

The difference between this type and the previous type is that there is a covering layer on the surface. However, the cover layer has via holes, which allows termination on both sides and still maintain the cover layer. This kind of Flexible Printed Circuit (FPC) is made of two layers of insulating materials and a layer of metal conductors. It is used in the occasions where the covering layer and the surrounding devices need to be insulated from each other, and the ends need to be connected to both the front and back sides.

1.2 Double-sided Flexible Printed Circuit (FPC)

Double-sided Flexible Printed Circuit (FPC) with two layers of conductors. The application and advantages of this type of double-sided Flexible Printed Circuit (FPC) are the same as those of a single-sided Flexible Printed Circuit (FPC), 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 Printed Circuit (FPC) without covering layer is rarely used.

1.3 Multilayer Flexible Printed Circuit (FPC)

Flexible multi-layer PCB, like rigid multi-layer PCB, adopts multi-layer lamination technology to make multi-layer Flexible Printed Circuit (FPC). The simplest multilayer Flexible Printed Circuit (FPC) is a three-layer Flexible Printed Circuit (FPC) formed by covering two copper shielding layers on both sides of a single-sided PCB. This three-layer Flexible Printed Circuit (FPC) is equivalent to a coaxial wire or a shielded wire in electrical characteristics. The most commonly used multilayer Flexible Printed Circuit (FPC) 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 Printed Circuit (FPC) is that the base film is light in weight and has excellent electrical properties, such as low dielectric constant. The multi-layer Flexible Printed Circuit (FPC) 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 Printed Circuit (FPC). Most of these products do not require flexibility.

Flexible Printed Circuit (FPC)

Multilayer Flexible Printed Circuit (FPC) 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 Printed Circuit (FPC)s 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 Printed Circuit (FPC) 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 Printed Circuit (FPC) is most suitable for designs that require flexibility, high reliability, and high density.

2) A multilayer PCB is formed on a flexible insulating base material, and the finished product can be flexible: this kind of multilayer Flexible Printed Circuit (FPC) 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 Printed Circuit (FPC) 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 Printed Circuit (FPC) 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 Printed Circuit (FPC) 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 Printed Circuit (FPC) 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 PCBthat is necessary to form a whole. The Flexible Printed Circuit (FPC) 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 PCB can also bond and press the ends of many single-sided or double-sided Flexible Printed Circuit (FPC)s 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 Printed Circuit (FPC) 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 multilayer Flexible Printed Circuit (FPC) is used to realize its interconnection. This component contains the shielded signal line in a flat stripline Flexible Printed Circuit (FPC), 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.