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

Daniel, as you propose this will work very well.  It avoids injecting 
return current into a high impedance power cavity which can become an 
EMI / SI / PI nightmare.  Just be sure to stitch the GND shapes on layer 
3 well to layer 2 and manage the impedances and isolation as needed of 
your power islands.

Steve.
Daniel Bauer wrote:
> 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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