Thanks for the tips, Abdul. I particularly like the idea of the power and
ground planes forming the plates of a capacitor.... that never occurred to
me.

Bob Stephens


A stackup that I particularly like, if I want to keep layer count down to 
6, is one with the power and ground layers in the middle, fairly close 
together. Then, on either side of them, symmetrically, are signal trace 
layers, X and Y as described. And above those signal layers are the top and 
bottom layers. To match impedance with the buried layers, traces on the top 
and bottom need to be relatively fat, because they are well above the 
reference planes.

There are a number of advantages to this stackup: the power and ground 
planes are close together (it might be 5 mils of prepreg or core between 
them). This creates a relatively high interplane capacitance which not only 
does yeoman work bypassing power -- perhaps even more effectively than 
scattered bypass caps -- but it also allows return currents to switch 
planes easily, through the interplane capacitance. Because the power and 
ground plane are so intimately connected at high frequency, they can act 
like a single plane for return purposes. The outside of the board serves to 
mount the parts and can be used for power traces or other low-speed 
signals, or, with the right trace widths, a few high-speed signals.




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