Flexible PCB stands for  Flexible Printed Circuit Board (FPCB)  or sometimes we just called it FPC which has the characteristics of high wiring density, light weight and thin thickness and extensively used in mobile phones, notebook computers, PDAs, digital cameras, LCM and many other products.

Flex pcb is made of a flexible plastic substrate, copper foil, and adhesive laminate. When the flexible plastic substrate can be divided into polyimide or polyester film. 

According to the combination of substrate and copper foil, flexible circuit boards can be divided into two categories: PI and PI without an adhesive. Among them, the price of the PI without an adhesive is much higher than that of PI, but its flexibility, the bonding force of copper foil and substrate, the flatness of pad are also better than those of the PI flexible board. Therefore, it is only used in those occasions with high requirements, such as: COF (CHIP ON FLEX)and so on.

Due to its high price, most of the flexible boards currently used in the market are still flexible boards with PI. Since the flexible board is mainly used in the occasions that need to be bent, if the design or process is unreasonable, it is easy to produce defects such as micro-cracks and open welding. The following is about the structure of the flexible circuit board and its special requirements in design and process.

The structure of the flexible board

According to the number of layers of conductive copper foil, it is divided into single-layer board, double-layer board, multi-layer board, double-sided board, etc. 

Structure of single-layer board: The flexible board of this structure is the simplest. The base material + transparent glue + copper foil is a set of purchased raw materials, and the protective film + transparent glue is another purchased raw material. First, the copper foil needs to be etched and other processes to obtain the required circuits, and the protective film needs to be drilled to expose the corresponding pads. After cleaning, use the rolling method to combine the two. Then, gold or tin is plated on the exposed pad part for protection. In this way, the flex board is ready. Usually will punch into small size circuit boards. There is also a method of directly printing a solder mask on the copper foil without a protective film, so the cost will be lower, but the mechanical strength of the circuit board will be worse.

Unless the strength requirement is not high but the price needs to be as low as possible, it is best to apply a protective film. 

Structure of double-layer board: When the circuit is too complicated, the single-layer board cannot be wired, or copper foil is required for grounding shielding, it is necessary to use a double-layer board or even a multi-layer board. 

The most typical difference between a multi-layer board and a single-layer board is the addition of a via structure to connect each layer of copper foil. The first machining process is to make vias. First drill holes on the substrate and copper foil, after cleaning, plated with a certain thickness of copper, through holes, and you are done. The subsequent production process is almost the same as that of the single-layer board. 

Double-sided structure: There are pads on both sides of the double-sided board, which are mainly used for connection with other circuit boards. Although it is similar in structure to a single-layer board, the manufacturing process is very different. Its raw materials are copper foil, protective film + transparent glue. First, drill holes on the protective film according to the requirements of the pad position, then paste the copper foil, corrode the pads and leads, and then paste another protective film with the drilled holes.

3.FPC product features:

It can be bent, folded and rolled freely, and can be moved and stretched freely in three-dimensional space.

Good heat dissipation performance, can use FPCB to reduce the size.

Realize light weight, miniaturization and thinning, so as to achieve the integration of component device and wire connection.

4. Application field

FPC application fields MP3, MP4 players, portable CD players, home VCD, DVD, digital cameras, mobile phones and mobile phone batteries, medical, automotive and aerospace fields FPC has become an important variety of epoxy copper clad laminates The flexible copper clad laminate (FPC) of the substrate is more and more widely used due to its special functions, and is becoming an important variety of epoxy resin-based copper clad laminates.