There are several reasons for laying copper in general.
1.EMC.For large areas of power supply laying copper , will play a shielding role, such as PGND play a protective role.
2.PCB process requirements. Generally, in order to ensure the plating effect, or the lamination is not deformed, the PCB board with less wiring is coated with copper.
3.Signal integrity requirements.Give high frequency digital signal a complete backflow path, and reduce DC network wiring. Of course, there are also heat dissipation, special device installation require to lay copper and other reasons.
First, a great advantage of laying copper is to reduce the ground wire impedance (a large part of the anti-interference is caused by the reduction of the ground wire impedance). There are a lot of peak current in the digital circuit, so it is more necessary to reduce the ground wire impedance.
Second, the significance of laying copper ：
1. Laying copper and ground wire are connected, which can reduce the return circuit area.
2, a large area of laying copper is equivalent to reducing the resistance of the ground wire, reducing the pressure drop. From these two points, both digital and analog circuits should be copper-plated to increase the ability to resist interference.