PCB Guide 6 - Fill zones, decoration and production

IN PROGRESS
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Otherwise, please have much caution trusting any info on this page.

Now that the routing is finished, let's tune the PCB up.

Step 11. Ground fills

You may remember the mention about how the crystal zone is sensitive.
We can protect it, along with other components, from external electrical noise through the use of ground fills.

Select create fill zones to begin.

We will draw two zones: One around the MCU, and one for the USB connector.

Then click on the first corner of the zone to surround to bring up a menu.

Select the ground net for the fill net, and high resolution for prettier edges.

Click at the corners, and double click at the last corner to end.
Then, press B to fill the zone.

The ground fill question is always an interesting one.
On one hand, it reduces EMI interference both entering and exiting the board.
However, it adds a significant amount of copper, making it more difficult to solder due to the necessity to raise temperatures further for ground-connected components.
It also makes the PCB opaque. This can be a major disadvantage if you are wishing for the translucent PCB aesthetic.

I will say two things regarding this topic:

If doing full ground fill, make sure there is no ground current interference (i.e. Ground current from LED doesn't take a path that cuts across MCU/Crystal zone).
If doing minimal ground fill, at least do the following:
- Around the crystal
- Under the MCU, possibly a bit around
- Below the USB connector
Also, in this case, it is critical that the USB lines are routed properly: Not close and parallel to pulsing matrix and LED lines, crossing perpendicularly to other traces as much as possible.

Whether you choose aesthetic or function is up to you. Such is a design decision left for the designer.
My personal recommendation is to use fill around the entire MCU zone or entire board until you are used to PCB design and have a few functional prototypes.