IEI Integration ICE-DB-9S - Manual (Page 102)

Page 89
ICE Module
Figure 5-2: Six Layers Stack
NOTE:
All high-speed signals should reference solid ground planes through
the length of their routing and should not cross plane splits. To
guarantee this, both planes surrounding strip-lines should be GND.
IEI recommends that high-speed signal routing be done on internal,
strip-line layers.
For high-speed signals transitioning between layers next to the
component, the signal pins should be accounted for by the GND
stitching vias that would stitch all the GND plane layers in that area of
the board.