Split power plane guidance please

[Smillsey]

Hi all

I have a general question about split power planes.

let’s look at an example of a split power plane on layer 5 of an 8 layer PCB.

There will be high speed signals on layer 3 and layer 6. Impedance calculations have been made for 100Ohm differential signal traces on layer 3, and other signal layers, the ground plane is closer (6mil) to the signal layers than this split power plane(15mil)

the power plane will provide power to ICs on the top and bottom layer, but these are power islands at the moment and I only have 1x 3.3V regulator and 1x 1.8V regulator to supply all islands


The 3.3v islands will become 1 plane.

can I simply daisy chain the 1.8V islands together, using vias to another layer and connecting with big traces through a via and running a thick trace to the next island?

or am missing something very obvious?

 

[barry]

Why don’t you just have two continuous planes instead of all those islands? For one thing, You’ll get better signal integrity. Without knowing more about your layout, currents, etc., it’s hard to say much more.

or, consider changing one of your ground planes to a power plane.

 

[HasHx]

If layers 3 and 6 have sensitive signals, you may want to consider making it a 10 layer board and make inserting 2 power planes between 4 and 5 making L-5 GND. this is the safest approach.

If you want to keep it as a 8-layer board, then maybe making L-2 a power plane would be safer. just fill any gaps with GND on top and bottom.

 

[Smillsey]

If layers 3 and 6 have sensitive signals, you may want to consider making it a 10 layer board and make inserting 2 power planes between 4 and 5 making L-5 GND. this is the safest approach.

If you want to keep it as a 8-layer board, then maybe making L-2 a power plane would be safer. just fill any gaps with GND on top and bottom.

Hey HasHx

thanks for that

I can go to 10 layers, no problem.
Do you mean;

SIGNAL
GROUND
SIGNAL
POWER
GROUND
GROUND
POWER
SIGNAL
GROUND
SIGNAL

OR;


SIGNAL
GROUND
SIGNAL
POWER
GROUND
POWER
POWER
SIGNAL
GROUND
SIGNAL

 

[HasHx]

SIGNAL
GROUND
SIGNAL
GROUND
POWER
POWER
GROUND
SIGNAL
GROUND
SIGNAL

that should do.

 

[Smillsey]

Ok I’ll calculate the impedances for my LVDS lines and go from there

 

[Owais Jamil]

SIGNAL
GROUND
SIGNAL
GROUND
POWER
POWER
GROUND
SIGNAL
GROUND
SIGNAL

This is a better technique.