EMC Question of the Week: May 5, 2025
If a multi-layer printed circuit board has a solid ground plane on Layer 2, then a ground fill on Layer 1 is usually not recommended. This is because filling empty areas of Layer 1 with ground often increases the
- magnitude of the eddy currents
- isolation between signal traces
- inductance of connections to ground
- cost of the board
Answer
The best answer is “c.” Connection from a component's ground pin to the board's ground plane should be made through a via at the pin location. Often, if there is a ground fill nearby, PCB layout professionals (and many automated routers) will connect to the component pin to the ground fill instead of connecting directly to the ground plane. The current in this pin must then find its way to nearby ground vias before reaching the plane and returning to the source.
The figure shows a section of a circuit board where three ground pins of an IC share the same via connection to the ground plane (indicated by the yellow arrow). This can increase the noise on the IC due to "ground bounce" and increase the coupling to other circuits that share the same via connection.
If every component pin that connects to ground already has its own via connection to the ground plane, then adding ground fill to Layer 1 does not increase the inductance of any of the connections. However, once a ground fill is in place, it becomes the preferred connection point for most auto-routers and PCB layout software.
Ground fill does NOT significantly reduce coupling between signal traces when there is already a ground plane 0.25 mm or less below those traces. In fact, there is no EMC-related reason to have a ground fill one-layer removed from a ground plane.
Sometimes ground fills are used for copper balance or to reduce the manufacturing cost of the board (by requiring less copper to be removed). If the board has a ground fill, it is generally necessary to manually inspect the board in order to ensure that all high-speed connections go directly to the plane through dedicated vias rather than through the ground fill.
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