Why should there be no slots in ground plane?

Thread Starter

terabaaphoonmein

Joined Jul 19, 2020
90
I am learning about distribution of signal and return in routing signal traces. And one point it says there should be no slots like shown here in ground plane. But why? I could not understand.

What i already know-:
1) Return current follows the path of least inductance at high speed and least resistance at low speed.
Also what other factors do I need to consider while routing signals and return in PCB?


1635328285515.png
 

MrAl

Joined Jun 17, 2014
8,872
Note sure what you need exactly, but one of the function of slots in a PC board is to reduce mechanical stress on parts mounted on the board. As the board flexes, it puts stress on the solder joints and also on the part itself and some parts are sensitive to bending forces. By placing slots near the component, as the entire board flexes the part has it's own little floating PC board area that does not bend because that part of the board is somewhat separate from the rest of the board that is flexing. The part therefore gets reduced stress and so continues to function properly and also lasts longer.
 

dcbingaman

Joined Jun 30, 2021
559
Has to do with inductance. The inductance of a given path is greater with greater loop area. When the return current can follow the signal under the ground plane the enclosed loop area is significantly less. With the slot, the return current has to go out and around the slot and back again. This is for high frequency signals. If the trace is just carrying a DC voltage it would not matter. With higher inductance the probability of inductively coupled noise on the board increases.

You can have 'slots' in the ground plane. But do not route high frequency traces over the top of such slots.
 
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