PCB stackup for Mixed-Signal IC

Question

I need to design high-density board with MCU, external RAM, GPS, GSM, Bluetooth, CAN, RGB interface on small size(5x10cm) and got into thinking which stackup should I use.

Currently I have almost half bigger PCB(8x15cm) on 4 layer board(SIG/GND/PWR/SIG) and everything is running well. However I am not happy about how I route PWR plane. Next HW version should meet CE/FCC requirements.

I was thinking about using this stackup(6 layers):

1. Sig./Loc. ground/Pwr
2. GND
3. Sig
4. Sig
5. GND
6. Sig./Loc. ground/Pwr

or this one(4 layers)

1. Sig./Pwr
2. Local ground/Sig.
3. Full ground plane
4. Sig./Local ground/Pwr

I have only 2 voltage levels(5V/4V/3V3) but I don't think I need power plane.

The problem with 6 layer stackup I think I would have is that my traces would be mainly horizontal so I cannot use vertical/horizontal routing technique while having GND layer in the middle.

Since I don't have experience with custom stackups, how much spacing between layers is required for solution 2 or solution 1? Do you suggest using this stackups? Would it be better to have full power plane for 3V3 and use 5V/4V as local PWR nets since there is only one IC per voltage?

Answer 1

An advantage of a local power plane is that you can leave all the power routing out of your signal layers and in stead focus on the coupling, routing and impedance control of your signals.

Other than that the best advice is always based on your complete and exact design, so I'll tell you some of my preferences and their reasons, and leave them for you to consider.

For reasons of know-variables I prefer to keep no other layers between the GND and important signals, so in complex designs I try to make as many Signal layers directly next to a ground that fits my stack-budget (of course I'm not spending the money for 16 layers on each design I make!). And if I can only get 1 reliable layer like that, I make sure that layer has only signals and hosts at least the signals that are most important or highest frequency.

For the distances of the stack-up you best call the fab you are having the PCB made at, they know what they can do and what they stock. Once you have those numbers you can use them for your impedance control if you need to.

They can also tell you how accurate their PrePreg procedure is. If it's not very accurate or the layer it is spread on has a lot of copper areas and a lot of gaps as well (this makes PrePreg harder to get uniform) sometimes you will want your Signal and GND on either side of a normal plate, to be able to perform good impedance control. If that is a demand you might want to go for your first choice, but swap the "SIG" and "Sig/Pwr/Gnd" layers.

Another thing you put in your title is Analogue, if you have high-fidelity requirements of analogue signals you are not going to regret splitting your Analogue and Digital power domains completely, including the ground planes and only connecting them at the power-input of your board. You'll be thanking yourself for the extra effort once you find you measure very little digital noise in your analogue signals.