Dual supply PCB layer stackup

Question

My PCB is powered from USB port and includes Recom RKZ0505D for powering dual-supply OPAMPs. Recom gives +5V and -5V outputs. So I have:

1. Top signal & components
2. Bottom signal & components
3. GND
4. 3.3V
5. +5V
6. -5V

What should be the stackup of layers in a 4-layer PCB? There are about 30 ICs on the PCB. Some of them powered from 3.3V and some of them +5/-5V. If the layers used are:

1. Signal
2. 3.3V
3. GND
4. Signal

Then the power lines +5/-5V will be long. How can I optimize layers and right order?

Answer 1

I would suggest something more like this:

1. Signal
2. GND
3. +3.3V/+5V/-5V plane shapes
4. Signal

There's no reason why you can't have your power supply layer have a bunch of smaller plane shapes where they're needed.

As with any four-layer PCB, you'll need to follow some high speed guidelines if you have signals with content >100MHz or so (many square waves even at low frequencies contain frequency content this high). This is especially important for signals on layer 4 since you'll have plane breaks on Layer 3:

• http://www.ti.com/lit/an/scaa082a/scaa082a.pdf (especially the parts about return paths and loop areas)
• https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/an/an315.pdf (the part about "Discontinuities related to a transmission path")