The structure of FPC double-layer flexible board: When the circuit is too complex, single-layer board can not be wired, or need to copper foil for ground shielding, it is necessary to choose double-layer board or even multi-layer board. The most typical difference between multi-layer boards and single-layer boards is the addition of a perforated structure to connect the layers of copper foil. Generally, the first step in the process of substrate+transparent adhesive+copper foil is the creation of perforations. The holes are drilled in the substrate and the copper foil, cleaned and plated with a certain thickness of copper, and the perforations are ready. After that, the production process is almost the same as the single-layer board.