[_Microft]

> Above is the final DDR3 routing on all the PCB layers. Layers 2 and 6 are ground, 5 is supply voltage, and others are reserved for signals. Two grounds are needed for correct impedances on the top, middle, and bottom traces of the PCB. With only one ground plane, the distance from the signal to ground would be too large on either the top or bottom layer.

Is the textual description in the article correct? To me the images make it look like signals are on 1 (red), 3 (orange) and 6 (blue), with ground on 2 (green) and 5 (pink) and supply voltage on 4 (teal). If you match some vias, you will find that 2 and 5 are definitely connected.

[henrikf]

That's a mistake in the text. You're correct that layers 2 and 5 are ground planes.

[nick__m]

thanks, layer 2 and 5 would fit the rest of the description and the reason would then be in the text :

  Two grounds are needed for correct impedances on the top, middle, and bottom traces of the PCB

[_Microft]

Here are some resources on PCB layer stack-up by the way. While I'm an amateur, they didn't sound unreasonable. Chapter 10[0] has a list of links to different layer stack-ups for boards with 4 to 10 layers.

[0] https://web.archive.org/web/20200124214936/http://www.hottco...

Edit: removed wall of links