Are the heat sinks (Lids) on the Flip-Chip (FF) packages floating or grounded?
The lid can be thought of as floating. Prior to package assembly, the heat sinks are floating from all ground ball sites.
There is no direct connection from balls/ball sites to the heat sink. When fully assembled (as seen by the user) the metal top (Lid) in any Flip-Chip package is thermally connected to the die with a high-resistivity, low thermal conductivity paste, but it is considered electrically isolated.
This applies to all Flip-Chip packages across several architectures.