MBReview has posted an article covering the topic of the motherboard design process.

The layering layout of the motherboard is also very important, especially when it comes to servicing the various system busses and other high speed transmission lines. Each of these transmission lines requires something called a return path. As current, a signal, travels down one of the high speed interconnect wires, a return current must flow in the opposite direction of the signal on the copper plane that is located directly under the interconnect. An easier way to understand this would be to think of the return current as a way to complete the circuit full circle. The key to this is for the designer to try to achieve the most efficient return path for this return current.

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