3 ms·

The default today is likely signal/ground/ground/signal. Or really, the default today is a 6 layer board because 4 is still kinda bad. Today's engineers know

The default today is likely signal/ground/ground/signal. Or really, the default today is a 6 layer board because 4 is still kinda bad. Today's engineers know that signal-top has a return path through ground-top. But if you ever were to via a wire from signal-top to signal-bottom, the return path gets lost (aka: return path is now through the board or worse, through the air and radiating off of your board). To prevent this erratic behavior, you must continue to think about the return path and tie a via from ground-top to ground-bottom as close as possible to the via between signal-top and signal-bottom. -------- 6 layer can do signal/ground/signal -core- signal/ground/signal Where core is the FR4 material (keeping the middle signals far enough apart that they likely don't interfere with each other). This allows vias between layer#1 and #3 without needing a secondary return via. (But if you need layer#1 to layer#6 via, then the previous advice still applies where ground-top needs a secondary return via to ground-bottom).

neltnerb 2y ago
For anyone else that also wants a much more in depth explanation, this hour long talk is a gem. https://resources.altium.com/p/2-the-extreme-importance-of-pc-board-stack-up https://resources.altium.com/p/2-the-extreme-importance-of-p... That AC signal energy is contained in the field between the trace and it's return is such a powerful model compared to taking the DC model of "going the lowest impedance path" and just assuming it applies at AC. Very, very, very good talk. This model also automatically makes clear why you'd want signal-GND-GND-signal instead of a power plane because that side's signals will have coupling into the power plane. It is mentioned explicitly in the video as well.
Dylan16807 2y ago
> 6 layer can do signal/ground/signal -core- signal/ground/signal So preferring a fourth signal layer before adding a power layer?
- dragontamer 2y ago
  Just route power as a signal. Maybe a really fat trace / signal but it's still a signal. Power planes are overrated especially for 100mA designs. If you are running a 500Amp GPU obviously you should consider a power plane but what kind of designs are people doing here? EDIT: 10 Amp on 1oz according to first trace width calculator is supported by 20mm or 300 mil trace width. There's no reason to have a full power plane on amperage grounds. And if you need board capacitance for your design to work then yeah, Power Planes can be useful for that. But only the highest of frequency circuits (think GHz+) seem to require those. And if you do use Power Planes, then return currents may be induced on the Power plane instead of the ground plane, which is bad and hard to think about. (Power Planes cannot be shorted to Ground, meaning your return current needs to go across a high inductance via, into a capacitor, through to the ground plane where the return current should have been in the first place). --------- Basically, power planes are one of those things that work at 50MHz just fine (enough random capacitors that magically transfer signals to the ground plane). But much above that frequency and those capacitors look more like a parasitic inductor. 500MHz signals will NOT go through a 100nF 0402 cap, that thing is practically an indicator by those frequencies. So it's not like power planes are easy to use. The people who 'Need' a power plane are doing far more complex designs than the vast majority of board designers. And if they really need it, then they are using complex FEA simulations and calculations of PDNs that far exceed the scope of the level of advice in this topic.
  - dragontamer 2y ago
    > EDIT: 10 Amp on 1oz according to first trace width calculator is supported by 20mm or 300 mil trace width. There's no reason to have a full power plane on amperage grounds. I mixed up internal vs external trace width here. 700mil or 20mm internal trace. 300mil or 8mm external trace.

neltnerb 2y ago

For anyone else that also wants a much more in depth explanation, this hour long talk is a gem. https://resources.altium.com/p/2-the-extreme-importance-of-pc-board-stack-up https://resources.altium.com/p/2-the-extreme-importance-of-p... That AC signal energy is contained in the field between the trace and it's return is such a powerful model compared to taking the DC model of "going the lowest impedance path" and just assuming it applies at AC. Very, very, very good talk. This model also automatically makes clear why you'd want signal-GND-GND-signal instead of a power plane because that side's signals will have coupling into the power plane. It is mentioned explicitly in the video as well.

Dylan16807 2y ago

> 6 layer can do signal/ground/signal -core- signal/ground/signal So preferring a fourth signal layer before adding a power layer?

dragontamer 2y ago
Just route power as a signal. Maybe a really fat trace / signal but it's still a signal. Power planes are overrated especially for 100mA designs. If you are running a 500Amp GPU obviously you should consider a power plane but what kind of designs are people doing here? EDIT: 10 Amp on 1oz according to first trace width calculator is supported by 20mm or 300 mil trace width. There's no reason to have a full power plane on amperage grounds. And if you need board capacitance for your design to work then yeah, Power Planes can be useful for that. But only the highest of frequency circuits (think GHz+) seem to require those. And if you do use Power Planes, then return currents may be induced on the Power plane instead of the ground plane, which is bad and hard to think about. (Power Planes cannot be shorted to Ground, meaning your return current needs to go across a high inductance via, into a capacitor, through to the ground plane where the return current should have been in the first place). --------- Basically, power planes are one of those things that work at 50MHz just fine (enough random capacitors that magically transfer signals to the ground plane). But much above that frequency and those capacitors look more like a parasitic inductor. 500MHz signals will NOT go through a 100nF 0402 cap, that thing is practically an indicator by those frequencies. So it's not like power planes are easy to use. The people who 'Need' a power plane are doing far more complex designs than the vast majority of board designers. And if they really need it, then they are using complex FEA simulations and calculations of PDNs that far exceed the scope of the level of advice in this topic.
- dragontamer 2y ago
  > EDIT: 10 Amp on 1oz according to first trace width calculator is supported by 20mm or 300 mil trace width. There's no reason to have a full power plane on amperage grounds. I mixed up internal vs external trace width here. 700mil or 20mm internal trace. 300mil or 8mm external trace.