[SI-LIST] power planes (4-layer-board)

Hi, 

the standard layout for an 4-layer PCB is, using outer layers for the signals 
and the two inner layers for one ground and one power plane. The power plane 
seems to have a few advantages as well as disadvantages (using an 4-layer 
board). 

If a trace running at the top layer has to change to the bottom layer - also 
the return current has to change the layer (using stitched capacitors). 

What do you think about the proposal to use the third layer for ground as well 
as for power? On every position where an signal trace is running at the bottom 
layer, the third layer will be a ground plane, connected to the ground plane on 
the second layer.... and when there`s no trace on the bottom layer I will use 
an power plane on the third layer.  From this it follows that I will get some 
power islands on the third layer which were connected with traces on the bottom 
or top layer.  The complete return current should use the ground plane (which 
has an lower impedance) and not one of the small power islands. 

Would there be an impedance change for the signal traces changing the layer 
(top to bottom or vica versa) when both inner layers will use an ground plane 
where the signal is running? 


x----------|                      (top layer - signal layer)
    GND   |   GND            (second layer is a complete ground plane)
Power    |   GND            (ground and power islands)
              |---------x         (bottom layer - signal layer)

The line will show a signal trace changing from the top to the bottom layer. 
Both ground planes using at the inner layers, will be connected together by 
many vias. Could you tell me if this would be a good decision improving signal 
integrity? Are there any kind of disadvantages?


best regards
Daniel  
    


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