There are pads on both sides of the dual panel, which are mainly used for connections to other circuit boards.
From the bottom to the above: protective film, adhesives, copper foil, adhesives, substrates, adhesives, copper foil, adhesives, protective film.
Double-layer, multi-layer board when the circuit line is more complex, single-layer board can not be wired, you need to use double-layer board or multi-layer board. The difference between multilayer plate and single layer plate is that the multilayer plate adds the perforation structure to connect the copper foil of each layer. From bottom to upper in turn: substrate, adhesives, copper foil, adhesives ... Protective film.