Power planes, i.e. a continuous copper layer except for clearance areas around any holes e.g. through hole component pads, vias or mounting holes serves the purpose of screening tracks and providing a return path for signal and power connections for improved EMC performance, plus they can allow for the increased placement density of components.
Traditionally this is the inner two layers of a 4 layer PCB with one layer assigned to the supply voltage and the other the supply ground.
As always there can be many other considerations such as analog and digital grounds, multiple supplies and required gaps in the copper area for aerials especially for IoT designs.
Within DesignSpark PCB when producing a 4 layer PCB you have the option of choosing the internal layers as "power planes" or "signal layers", so which do you choose?
Select a "power plane" and assign to a net only if you wish to produce a 'pure' continuous copper layer without any other tracking routes on those layers.
Our recommendation for flexibility and for any future design changes is to create the inner layers as "normal signal layers", then add copper pour areas as required. These can each have different pour settings and boundaries to allow flexibility in the layout. This allows split ground areas to be created as used by mixed signal designs. Plus you can also add some nets to the layer if it does not reduce the copper area efficiency.
Note. a "keep out" area for areas requiring low capacitance and for areas adjacent to aerial tracks which has been a feature of copper pours is also now available for power planes as of V8.1.
Using the PCB wizard for either option select the 4 layer option i.e. two signal and 2 planes.
The layers can now be configured at the next step in the wizard.
Select "Define Layers" and click the "4 Layer Board" option.
Now configure Layer 2 and Layer 3 as required.
With Powerplane check boxes ticked each layer should be assigned to a net.
With the Powerplane boxes unchecked as shown below, the layers will be normal signal layers providing complete flexibility of how the layers are used.
While specifying the layers we also recommend selecting the Paste and solder masks for both board sides. These will be automatically generated whether selected or not at the Manufacturing Output stage as Gerber layers, however if you select them at this step they will be visible as layers in your design using the interaction bar, short cut F9 and selecting the "Layers" tab.
Here they can be selected as visible or not, however with the layer selectable it provides the opportunity to edit them in the design.