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Joel R. , 10-07-2025, 11:38 PM

Hello, I’m designing a 6-layer mixed-signal PCB and would appreciate feedback on two stackup options.

My power delivery:
A. Input power from USB -> filter -> BQ24296MRGER (charger) - Charger output = VSYS → feeds two LDOs → PWR1 (Digital) and PWR2 (Analog) rails.

These are the stackups I was thinking about:
Option A
1. Signal + PWR0 + GND (Remaining area is filled with GND for copper balance)
2. GND (solid)
3. Signal + PWR1 (Digital) + PWR2 (Analog) pours
4. GND (solid)
5. GND (solid)
6. Signal (with GND fill for balance)
Question:
1. Is it acceptable to pour PWR1 and PWR2 on Layer 3 like this?
2. Is there any risk of imbalance or other issues from having two adjacent GND planes (Layers 4 and 5)?
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Option B

1. Signal + PWR0 + GND (Remaining area is filled with GND for copper balance)
2. GND (solid)
3. Signal + PWR1 + PWR2
4. GND (solid)
5. Signal (with GND fill for balance)
6. GND (solid)

Question:
1. My concern about this last one is that I have some components on the bottom layer (TVS, resistors, capacitors; no ICs). With Layer 5 as Signal instead of pure GND, their return path won’t be entirely ground. However, if I route them so that signals on Layer 6 reference the GND on Layer 5, there shouldn’t be major issues, right?

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Other questions I have:
1. Which option provides better return paths and noise isolation for mixed-signal designs?

2. Are there alternative 6-layer stackups you’d recommend for this power tree (USB -> charger -> LDOs -> digital/analog)?

3. On Layer 3 (Signal + PWR1 Digital + PWR2 Analog), is it acceptable to fill the remaining area with GND for the sake of density balance?, resulting in:
Layer 3 (Signal + PWR1 Digital + PWR2 Analog + GND)
More broadly, is it good practice to have GND and multiple power pours (PWRx) on the same layer? I’ve seen some videos where designers separate digital and analog by using GND between them, does that actually help?

Thank you for any suggestions.

QDrives , 10-08-2025, 07:07 PM

Do you need layers for power (power planes)? https://www.youtube.com/watch?v=kdCJxdR7L_I
Balancing copper is needed for 2 different things:
a) the amount of copper on the layers symmetrical to the board center (vertical). Your will bow/twist if you do not (speaking from experience).
b) the amount of copper spread over the layers that are plated (often only the outer layers) as the plating thickness may vary if you do not.

As for your questions:
1.1 - "*if I route them so that signals on Layer 6 reference the GND on Layer 5*" -- that contradicts "*...their return path won’t be entirely ground...*"
2.1 - I do not see much issue directly in either, given the previous point 'signals reference a Gnd'.
2.2 - You could go for a 2x3 layer stack-up where layer 2 and 5 are Gnd. The cores are 0.2mm (layer 2-3 and 4-5). The outer prepreg is 2x1080 (~0.16mm, layers 1-2 and 5-6). The inner prepreg (+ optionally 'fake' core) is thick (>= 0.6mm, i.e. 4x7268).
2.3 - See the video mentioned on power planes.