Does layer order matter in a four layer PCB?

Question

The PCB doesn't have blind vias. In a four layer PCB the inner layers will often be a ground and a power plane. Does it matter which one you place closer to the component side?

Answer 1

In an ideal world \$V_{CC}\$ is the same as ground for AC signals, and then the order doesn't matter. In practice ground and power net impedances aren't zero and there are noise signals between them.

The picture shows how signals on the top layer are coupled to the \$V_{CC}\$ layer, not to ground. Especially HF signals you'll want on the layer closest to ground, here layer 4.

For low power, LF designs the order will not make much difference.

Picture from this paper

Answer 2

It does. And not only does the order of your layers matter, you can also influence your results by specifying the thickness of the FR4 cores or prepregs and the copper traces.

As a start, you can think of the innermost FR4 prepregs as a dielectric for a capacitor between VCC and GND or you can use the cores between 1-2 or 3-4 for impedance controlled striplines between your signals and VCC or GND. The capacitor will not replace all the capacitors you still put onto your board, but it will greatly improve their performance.

It really depends on what you wish to achieve. As an example, you could route all your fast signals on Layer 4, have them laid out with a controlled impedance with regard to GND in layer 3 (using a defined FR4 thickness), build a bypass capacitor with (Layer 2) - (thin FR4) - (Layer 3) between VCC and GND, and route the less critical stuff on layer 1.