SDG #116 Creepage and Clearances - Adding mechanical routing to your PCB

Spacing on a mains or high voltage PCB is governed by two separate numbers that are easy to confuse: clearance, the shortest path through air between two conductors, and creepage, the distance current has to travel across the surface of the board material. Creepage is usually the harder constraint because contamination, dust, humidity and carbonised tracking all make the laminate surface progressively more conductive over time. This video works through why those figures come out where they do, and what happens when your enclosure or connector layout simply will not let you open the gap far enough.

The practical fix covered here is mechanical routing: adding a milled slot or cutout in the board between the primary and secondary sides so the surface path is interrupted entirely, forcing the leakage route around the edges of the slot. That effectively buys extra creepage distance without growing the outline of the board, which matters when a design has to fit an existing case or when isolation barriers cross under an optocoupler or transformer. There is also discussion of how routing interacts with board strength, panelisation, and what the fab actually needs to see in the Gerber or mechanical layer so the slot gets cut where you intended rather than being interpreted as a scoring line or an outline error.

The useful takeaway for anyone designing an offline supply, a solid state relay or an isolated interface board: decide on the isolation barrier early, keep it as a clean straight line across the layout, and treat slots as a tool for tight layouts rather than a substitute for getting the spacing right where space allows. Standards tables, pollution degree and working voltage all feed into the number you are actually required to hit.