Refer to the PDF data sheet for device specific package drawings
The major advantage of a multilayer PCB with ground-plane is the ground return path below each and every signal or power trace.
As shown in Figure 8-12 the field lines of the signal return to PCB ground as long as an infinite ground is available.
Traces near the PCB-edges do not have this infinite ground and therefore may radiate more than the others. Thus, signals (clocks) or power traces (core power) identified to be critical must not be routed in the vicinity of PCB edges, or, if not avoidable, must be accompanied by a guard ring on the PCB edge.
The intention of the guard ring is that HF-energy, that otherwise would have been emitted from the PCB edge, is reflected back into the board where it partially will be absorbed. For this purpose ground traces on the borders of all layers (including power layer) must be applied as shown in Figure 8-13.
As these traces must have the same (HF–) potential as the ground plane they must be connected to the ground plane at least every 10 mm.