My apologies to the group if this question is off-topic. Just looking for some “rules of thumb” from the experts on the list. If I should post this elsewhere, please let me know a good list to use.
My question is, when should I use a fat trace and when should I use a copper plane to move large currents (1-5 A) around a board (6 in x 6 in)?
I would think that the switching speed of the current signal would be important. Take three option - no switching, slow switching (100 msec to seconds), and fast switching (less than 100 msec), if those divisions make sense. Also, assume there is a ground plane on the multi layer board.
5A isn’t a large current.
How sensitive is your circuit to voltage drops? That’s really the deciding factor. If you have a lot of components that use the same voltage, than a plane makes sense. If you only need to ship the current from point A to point B, then use a fat trace.
Can I extrapolate from your comments that if you have several components at point A and several at point B, that a fat trace in between the groups is OK, but the groups are better served by a plane of copper? Or, should the plane contain all of them?
Also, is there any disadvantage to using a plane over a fat trace?
One could also say that configuration in my example is a bad design from the get go, but it is also not always possible to have a perfect layout!
Thanks for your insightful comments. They really help!
There are no set rules for this. You just draw whatever fits the task at hand. Tracks are easier and quicker to draw, while copper zones can make better use of the available area.
5A is also a quite moderate current. Below an example of KiCad’s own calculator tools:
So, with 5A and 100mm length, a track width of 2.7mm (on standard 35um thick copper) has a voltage drop of less then 90mV. This may be perfectly all right for some applications, and completely unacceptable for other situations. It all depends on the circumstances. Sometimes it’s acceptable as long as the thing does not get too hot, other times, you can’t tolerate 10uV of voltage drop.
In a perfect world, all powers and ground would get their own plane. But, that would make the board very expensive. So, most designers use a single layer for ground, and then another layer for all the powers. I often have 1.8v, 3.3v, 5v, and 12v sharing a single layer. Sometimes the board isn’t routable unless that layer also is used for signals. Good parts placement can make routing power easier - group components that share a power source close together if possible.
