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PCBs, copper pours, ground planes, and you

(lcamtuf.substack.com)
284 points surprisetalk | 4 comments | 30 Jan 25 04:33 UTC | HN request time: 0s | source
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hex4def6 ◴[31 Jan 25 00:53 UTC] No.42883763[source]▶
> The return current on the back is free to spread out, but in practice, it will prefer the path of least resistance — i.e., the shortest line between the two vias.

This is a bit misleading. It does preferentially flow along the shortest path, but not exclusively. It will indeed spread out. It takes all paths with a current proportional to their resistance, not just the shortest path. The percentage of total current that isn't flowing directly along the shortest path is still very significant.

Think of it as a bunch of resistors in parallel. The shortest path might be 0.1ohm, and the longest path 10x longer at 1ohm, but current will be flowing along both of those paths. If 1A is flowing down the shortest path, you will still have 100mA down the 10x length path.

replies(1): >>42884175 #
1. avsteele ◴[31 Jan 25 02:20 UTC] No.42884175[source]▶
At DC this is a good model. At higher frequencies no so much. At high freq the 'return current' is all basically right under your trace.
replies(1): >>42884270 #
2. ChadNauseam ◴[31 Jan 25 02:42 UTC] No.42884270[source]▶
I don't know anything about electricity so this intrigued me. Is it related to the "proximity effect" [0]? Is it caused because of the fact that the magnetic field created by high-frequency current is always changing, so this induces a voltage in any nearby conductor?

[0]: https://www.sciencedirect.com/topics/computer-science/proxim...

replies(1): >>42886369 #
3. londons_explore ◴[31 Jan 25 10:25 UTC] No.42886369[source]▶
When the frequency of AC divided by the speed of light gets close to the length of your wire (ie. around 1Ghz), the regular rules of electricity no longer apply.

In the high frequency rules, you should consider that the energy you're sending down a wire is instead transmitted in the insulator - ie. it is effectively a radio wave guided by the conductors on either side.

If one of your conductors is not a wire but a ground plane, then the radio wave will travel in the insulator between the other wire and the closest bit of ground plane. Further away bits of ground plane, the signal gets effectively cancelled out because all the alternate paths the signal could take all have different path lengths and therefore phase shifts.

replies(1): >>42888673 #
4. H8crilA ◴[31 Jan 25 15:35 UTC] No.42888673{3}[source]▶
Energy is always transported in the insulator, but the bigger point is valid - at high enough frequencies you have to actually think about it.