Neil:
Because you are using stripline constructions for layers 3 and 8, the impact
should not be massive, but will certainly reduce your noise margins as a
minimum. Assuming you are targeting ~62 mils PCB thickness and 1-ounce
copper, I recommend you consider asymmetrical layering of layers 3 and 8
closer to the ground planes (than the power planes) to concentrate their
respective signal return currents in the continuous ground planes. This
technique will minimize the impact of cuts in the adjacent power planes.
Note that this is a workaround to the use of continuous planes and will keep
costs in check; however, there WILL be some non-ideal performance.
To minimize the effect of the creation of common-mode noise (caused by the
cuts in the power planes), I recommend you plate the ground planes up to
2-ounce. You may also have more accurate impedance control (and cut costs)
by using only 0.5-ounce copper for the signal traces...provides more accurate
etching.
Mike Conn
Owner/Principal Consultant
Mikon Consulting
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