6-layer board stackup?

[allanvv]

Never got a reply in the professional hardware forum, so I'll ask here:

I'm making a board with a Spartan-6 and DDR2 RAM. The 256 ball BGA escape can be done with only 4 layers, but the price difference between 4 and 6 layers from PCBcart isn't that high. I saw this six-layer stackup:

Milkymist One RC2 PCB Specification - Qi

Should I use this stack-up, having 3 signal layers, but better coupled ground/power planes? Or go with the standard sig/gnd/sig/sig/vcc/sig configuration?

 

[loosemoose]

Question:

Are you able to route all critical signal on one inner layer alone?

You don't want any high speed signals on the outer layers because they can radiate.
If you can the more GND planes you have the quieter the design.

 

[allanvv]

I'm not sure yet. I'm deciding my board stackup before doing the actual routing. I've seen boards with DDR2 and FPGA's routed on two signal layers alone, so I'm sure it'd be possible to route it with three signal layers. I'm just not sure if getting power and ground close together is that important.

I've never seen this three signal layer stack-up mentioned before in the books I have on high speed digital design, so I'm wondering if it's actually good.

 

[loosemoose]

In that 3 layer stackup you will need to balance the copper. Maybe ask the board vendor to add copper thieving on the signal layers.
I would use Top | GND | Signal | PWR | GND | Bottom or Top | GND | PWR | Signal | GND | Bottom.

 

[grid]

I recommend to use the standard sig/gnd/sig/sig/vcc/sig configuration. You can route high-speed bus by using layers 3 and 4 between gnd and vcc (one for horizontal, other for vertical routes). Both layers have reference plane beside and when step eg. from layer 3 to 4 you do not need to go through reference layer (gnd or vcc).