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In the Digital Isolator Design Guide from Ti, there are some PCB Design Guidelines regarding Creepage.

In addition to wide isolator packaging, techniques such as grooves can be used to attain a desired creepage distance (see Figure 4-2).

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They further advise:

Also, on all layers keep the space under the isolator free from traces, vias, and pads to maintain maximum creepage distance (see Figure 4-1).

enter image description here

So far so good. But in the image, they also mention to "Keep this space free from planes" and actually illustrate it that way.

I don't see what the planes have to do with creepage. And I really don't like to split planes if there isn't a good reason to. Any idea why they would explicitly point that out?

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    \$\begingroup\$ But if you want isolation, why would you even want a ground plane to be continuous between two isolated areas? You could even have a slot under the chip instead of just a groove. \$\endgroup\$
    – Justme
    Mar 3, 2023 at 8:30

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Like Justme points out there are not so many good reasons to let the planes continue below the isolator, because the isolated part most of the time has it own plane or ground potential which also needs to be isolated :)

Creepage might not be the best word related to ground planes, but the hint nevertheless is Ok.

To achieve a high isolation you have to

  • not only fight surface resistance by a large horizontal distance (which increases with grooves and slits)
  • but also volume resistance by accepting that 10mil prepreg is often not considered enough to withstand a few kV

So the resistance between the planes and the surface-pads on the isolated side (just 10 mil in this example) might not be enough for regulatory purposes.

You need also keep in mind the real dimensions, which are strongly distorted in figure 4-1: compare pin row distance (often 4-6 mm) to board thickness and prepreg-thickness etc.

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