The board we first saw was called the substrate. Simply speaking, it is an insulating material, bakelite, and plastic are all possible. A piece of copper foil is then applied to the substrate. The curved line we saw above is copper foil. Find a circuit board and you can see it with a knife. But in the initial state, it is a whole piece covered on the substrate. Then etch according to the fabricated circuit diagram. Simply put, the part that needs to be preserved, the other parts are completely etched away with chemical potions, and the rest are those that are curved.