SN74CB3Q3345: Thermal pad connection to GND

See the Switches and muxes: Can I leave unused pins floating? video, or [FAQ] Where do I connect the thermal pad of the logic QFN devices?

Franziskus,

It's recommended to do so but not entirely necessary. If the device isn't expected to get hot or deal with signals that push the upper ends of the electrical characteristics it isn't entirely necessary. Typically the pad is electrically connected to the lowest polarity (sometime Vss, in this case GND). If the datasheet doesn't recommend connecting the pad to a pin then you can leave it floating. It still must be soldered down to the board though. 

Here's a good video that Clemens also linked for you : Switches and muxes: Can I leave unused pins floating? 

Hope this helps,
Rami