To make layer 2 a full-board GND plane in Flux:
- Open the PCB view.
- In the right panel, open Ruleset / stackup settings for the layout.
- Set the board stackup to a 4-layer stackup if it is not already.
- Change layer 2 from a signal layer to a Ground Plane layer. Flux supports changing individual copper layer types in the stackup, including assigning an inner layer as a power or ground plane. The platform guidance explicitly distinguishes signal layers from power/ground plane layer assignments.
- Make sure your schematic/layout has a valid GND net. Ground fills in Flux are net-connected automatically to ground when enabled, and ground fills are enabled by default on layers unless you disable them.
- Verify that the layer spans the entire board outline. A plane layer follows the full layout shape, so if layer 2 is configured as a ground plane, it becomes a full-board inner GND plane unless local keep-outs or exclusions interrupt it.
For the top and bottom layer ground pours:
- Flux documentation indicates ground fills are enabled on every layer by default, while power fills are disabled by default. So if your top and bottom layers have unused copper area and are allowed to fill, they should already behave as ground fills tied to GND unless disabled.
If you want, I can also give you a short click-by-click checklist for finding the exact stackup controls in the right-side PCB panel.