routing on power or gnd layer in 6layer pcb

[robertferanec]

I always try to have some solid GND planes in PCB. I would not cut it down - the ground loops are travelling under the whole circuits, I am not really sure how helpful it would be to leave solid ground under memory layout and remove it in other places.

I would try to find a way how to do it with solid GND planes.

[kamikazi]

hi
i want to design one 6layer pcb for AM3358 TI processor about stack up layer i set in this case :
layer 1 :signal
layer 2 : gnd
layer 3 : power
layer4 : signal
layer 5 : gnd
layer 6 : signal
size of this board is very small (40x40mm) and routing in 3 layer is very hard i want to know in layer5(gnd) i can set one plane under high speed signal (such as under ddr3) and i rout other track that no need controlling impedance on empty space (such as gpio) on layer 5 pls guide me thanks