4 Layer stackup : L4 mixed SIG and PWR ? - Page 1
Forum section: electronics / projects

author: niclatrique
author_rank: Contributor
timestamp: June 13, 2025, 12:45:08 pm
content: Hi,I'm working on a 4 layer stackup design. Not a heavily populated board nor a terribly high speed design. Only a crystal at 20MHz and 3 slaves on the I2C bus at 100kHz. So far I've deisgned it like so :1 - SIG2- REF3- REF4- SIGIt is a single supply system at 5V. As of right now, POWER has been routed both on L1 or L4 using thick traces and large polygons.Is this a good or bad idea to make a "power pour" on L4 ? This would become this :1- SIG2- REF3- REF4- SIG / PWRYou can look at the pictures for a better understanding if my description is not clear enough (english not being my primary language).Thanks for your advices.Regards,

author: ajb
author_rank: Super Contributor
timestamp: June 13, 2025, 02:38:56 pm
content: The L4 power pour doesn't look like it would be appreciably better than the polygons in this case, due to how it's cut up by the L4 traces. You could rework a lot of the tracks to move most of the routing to the top side and end up with a nearly complete pour on the bottom, but you've already done a good job of creating a clear path for the power distribution through the board, so I don't think that would make much difference. However, if you did move most of the signal routing to the top layer, I think you could make L4 a ground pour, and do this board in only two layers instead of four.

author: jeduffy
author_rank: Contributor
timestamp: June 13, 2025, 03:23:10 pm
content: Depends on the goal, but almost certainly best to do polygons. You usually want to keep the power a little ways away from signals to avoid coupling either way. Going 4-layer typically means you get much tighter coupling to ground on everything, so a little distance isolates signals very effectively.Your power routing looks just fine, at least at first glance. Just make sure there's enough ground vias near everywhere it's used.Often Ill also put ground pours on empty areas of layers 1 and 4, with via stitching to make sure there's nothing dangling. Having ground between signals like that *can* improve isolation, though there can be some gotchas to that approach.

author: jeduffy
author_rank: Contributor
timestamp: June 13, 2025, 03:34:26 pm
content: Looking closer, I'm a bit confused about whats going on with the little traces going to the big grid on vias in the lower-right - is the cap only attached through that thin little trace? Might be best to remove that and directly (or with thermals) attach the cap to the polygon. Same for the thin trace going up on the top layer.Where is power coming in? Through the connector in the top-left? If so I'd connect that with a MUCH fatter trace to the polygon, and with more vias, and put the big cap close to that.Also looks like power is getting to the can through a pretty thin trace going through all the stuff in the top-left. I'd run a polygon, probably on the bottom layer, over there, either from the big via array in the lower right (if powers coming in near there), or the (new) via array by the connector, if you change that area.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 13, 2025, 04:33:46 pm
content: For a single supply I would by default make layer 3 power. The only reason not to is if you have critical analog signals that need to run on layer 4, or you have a lot of high speed analog or digital signals that need to go through vias between 1 and 4.If you had multiple supplies, then it is more tricky. You can break layer 3 up into polygons for the different supply rails, but if you do so you need to watch out for signals on layer 4 that cross the boundary. Ideally you would avoid that entirely by keeping routing on the top whenever possible.For a mostly analog board, I would probably do what you have suggested with 2 ground planes. Then I would route power with polygons and thick traces. I wouldn't do a power pour. If anything I would do a ground pour if I was worried about signal crosstalk. Usually that is not necessary in a 4 layer board. That's more of an issue in a two layer board where the ground plane is usually 1.6 mm away.

author: niclatrique
author_rank: Contributor
timestamp: June 13, 2025, 06:00:24 pm
content: Thanks to everyone for replying this quickly.The cap on the bottom right is a supercap for the RTC. a few nA will only will ever going in or out this cap. It was not finished routing. I rushed to put in some pictures only to picture support my explanation/question.Not being a professional in any kind, I watched lots of videos from Rick Hartley and Eric Bogatin in the last few months. This made me realizing a lots of things. This is why I went with L2-L3 as reference. You are right, with some traces rework, I could probably ending up putting every signals on L1. I will keep the 4 layers though. On L4, after rework, I ended up with an RS232 Rx/Tx on L4 and an analog signal. This is the current sense for the 2 power load.As you'll see with the main blocks of the pcb1: Power2: Current sense resistors and ICs (INA226)3: Load's power mosfet (whitched back and forth at 0.5Hz)4: User interface (only a few LEDs)5: MCU6: Indicator to the Rx/Tx so the maintenance guy can see if data has been received and transmitted.7: Radio power bank (4 x 47uF) and itt load switch8: RTC and its supercap9: radio module connector (Mesh radio from Digi)I could have put current sense chips closer to the current sense resistor but wanted to keep the I2C lines short. Also, I removed the stitched guard ring around the radio module. Digi has products using this exact radio module and they do not put a guard ring around it. Anyway, that radio module is already capped with a metal lid.So, knowing all that, Could I safely go with the following stackup :1 - SIG2 - REF3- PWR4 - SIGThanks,