Each trace line is composed of the flat narrow parts of the residual on -carved copper foil. The resistance determined by its width, thickness, and length must be low enough to be carried by the conductor carrier. Power and ground wiring may need to be wider than signal wiring. In the multi -layer board, the entire layer can be mainly solid copper, for ground planes for shielding and power returned. For microwave circuits, transmission cables can be arranged in a plane, such as band lines or micro bands, and their size is carefully controlled to ensure consistent impedance. In radio frequency and fast -switch circuits, the inductance and capacitors of the printing circuit board conductor become an important circuit component, which is usually unwilling; on the contrary, they can be regarded as a conscious part of the circuit design without additional separate components.