Image Plance for USB 2.0

An image plane is a layer of copper (voltage plane or ground plane), physically adjacent to a signal routing plane. Use of image planes provides a low impedance, shortest possible return path for RF currents. For a USB board, the best image plane is the ground plane because it can be used for both analog and digital circuits.

1. Do not route traces so they cross from one plane to the other. This can cause a broken RF return path resulting in an EMI radiating loop as shown below. This is important for higher frequency or repetitive signals. Therefore, on a multi-layer board, it is best to run all clock signals on the signal plane above a solid ground plane.
2. Avoid crossing the image power or ground plane boundaries with high-speed clock signal traces immediately above or below the separated planes. This also holds true for the twisted pair signals (DP,DM). Any unused area of the top and bottom signal layers of the PCB can be filled with copper that is connected to the ground plane through vias.

Do not overlap planes that do not reference each other. For example, do not overlap a digital power plane with an analog power plane as this produces a capacitance between the overlapping areas that could pass RF emissions from one plane to the other, as shown in below.



Avoid image plane violations. Traces that route over a slot in an image plane results in a possible RF return loop, as shown in below.