Hi Joe.
I have done PWB's which isolated ground planes from the
heatsink.
These boards had both signal ground planes and chassis ground planes.
The two types of ground planes are isolated from each other and
referenced to each other at only a few points around the perimeter.
The chassis ground layer is tied to the heatsink electrically, also at only a
few points, and again around the perimeter.
This setup was devised by some EMI engineers after much trial and
error, to reduce EMI.

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