What cases to use slots/cutouts in PCB and based on which parameters we deduce their dimentions!?

Thread Starter

Younes Thabet

Joined Jan 9, 2019
Hello all,

I am designing a PCB that contains some relays, so I decided to use cutouts to separate relay's contacts (depicted by green in image below) and maybe Neutral/line based on what I have seen in some PCB designs...
my questions are: what is the purpose of cutouts/slots other than separating high voltage from low voltage!? I mean what other cases that cutouts might be useful? and how to determine the shape and size of cutouts in each case?

Also I decided to use cutouts to make a line that separates power supply from digital circuit (depicted with blue)? is this really necessary and if this might affect board strength!?

Screenshot 2021-02-08 152635_LI.jpg



Joined Aug 24, 2018
The FR4 that the board is made from is actually a far better insulator than air, so from a strict breakdown voltage perspective, you would be able to support a far higher voltage without the slot. However, the slot makes it much more difficult for surface contamination of the board to cause a bridge between two copper areas or traces. This could be anything from solder flux to dust.

You'll often see cutouts used in power converter circuits to reduce the chance of contamination causing a short between +Vdc and GND, where +Vdc may be hundreds of volts. You'll also often see slots underneath isolated gate drive and measurement ICs to prevent contamination from causing a short from the high voltage area to the low voltage control circuits.

Personally, I've used slots even where I didn't really need to just to provide a very visual separation between the "bits that kill you" and the "bits you don't mind plugging your laptop into".