PCB News - Knowledge of flexible circuit boards

With the continuous increase in the yield ratio of flexible PCBs and the application and promotion of rigid-flex PCBs, it is now more common to add softness, rigidity or rigid-flexibility when talking about PCBs, and say that it is a few-layer FPC. Generally, FPC made of soft insulating substrate is called soft FPC or flexible FPC, and rigid-flex composite PCB is called rigid-flex PCB. It meets the needs of today's electronic products developing towards high density, high reliability, miniaturization and light weight.

1. Flexible PCB classification and its advantages and disadvantages

1. Flexible PCB classification

Flexible PCBs are usually classified as follows according to the number and structure of conductors:

1.1 Single-sided flexible PCB

Single-sided flexible PCB, with 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 PCB can be further divided into the following four categories:

1) Single side connection without covering layer

The conductor pattern of this kind of flexible PCB is on the insulating substrate, and the conductor surface has no covering layer. Like the usual single-sided rigid FPC. 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 PCB, and is widely used in automotive instruments and electronic instruments.

3) Double-sided connection without covering layer

This kind of connection board interface can be connected on the front and back of the wire. In order to achieve this, a via hole is opened on 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 PCB 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 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 FPC flexible circuit board. 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 flexiblePCB componentmust 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 is not specified to be flexible: This type of multilayer flexible PCB is laminated with a flexible insulating material, such as a 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: the 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.