IBASE Technology COM EXPRESS ET976 Design Manual page 87

4.2.3 Eight Layer Stack-up
Figure 38: Eight-Layer Stack-up
The figure above is an example of an eight layer stack-up. Layers L1, L3, L6 and L8 are used
for signal-routing. Layers L2 and L7 are solid ground planes, while L4 and L5 are used for
power.
Microstrip Layers 1 and 8 reference solid ground planes on Layers 2 and 7 respectively.
Inner signal Layers 3 and 6 are asymmetric striplines that route differential signals.
These signals are referenced to Layers 2 and 7 to meet the characteristic impedance
target for these traces. To reduce coupling to Layers 4 and 5, specify thicker prepreg to
increase layer separation.
COM Express® Carrier Board Design Guide 81