Introduction to the process requirements of flexible circuit boards!
As more and more mobile phones adopt the clamshell structure, flexible circuit boards are also used more and more. According to the combination of the base material and the copper foil, the flexible circuit board can be divided into two types: the flexible board with glue and the flexible board without glue. Among them, the price of the non-glue flexible board is much higher than that of the glued flexible board, but its flexibility, the bonding force between the copper foil and the substrate, and the flatness of the pad are also better than those of the glued flexible board. Therefore, it is generally only used for occasions with high requirements, such as: COF (CHIP ON FLEX, a bare chip mounted on a flexible board, which requires a high level of pad flatness) and so on.
Due to its high price, most of the flexible boards currently used in the market are still flexible boards with glue. What we want to introduce and discuss below is also a flexible board with glue. 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 is divided into single-layer board, double-layer board, multi-layer board, double-sided board, etc. according to the number of layers of conductive copper foil. The structure of single-layer board: The flexible board of this structure is the flexible board with the simplest structure. Usually 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 big board is ready. Generally, small circuit boards of corresponding shapes are also punched. 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 requirements are not high but the price needs to be as low as possible, it is best to apply a protective film method.