Back to Basics: PCB Clearance and Creepages

Last week a client was working on some high-voltage PCBs and I thought it’s a nice time to address those here as well. How do you design for High-voltage PCBs? Let’s at least discuss a major aspect of design, for that you need to understand two key components, Creepage and Clearance.

Clearance is the minimum spacing between 2 items in a PCB through air or Line of Sight. These could be track-to-track spacing, track-to-components, or component-to-components. Now creepage is the spacing between 2 items along the surface of the PCB. Check images for clarification. These differ in cases where there is a slot on the PCBs between 2 items, Clearance distance will be the straight line path between them, but creepage would be all the way around the slot. So it will be much higher.

Now for high-voltage PCBs, these terms are important because high-voltage sections can always arc over from one section to another if the distance is too small. Hence you must give some sort of clearance or slots between. It depends on the environmental conditions(Humidity, dust), Altitude(Air pressure reduces with height so does the breakdown voltage of air) where your PCB is used, and the coatings you provide on the PCBs(Conformal or Soldermasks). The standards which govern these are mentioned in the guideline IPC 2221B document. Check the image for a table that tells you the minimum spacing needed between conductors for different use cases. For cases above 500V, multiply the voltage difference after 500V with the multiplication factor and add it to the row above.

Use the table from IEC 60950-1 Device Safety standard for Creepage values. It contains a table for minimum creepage distances for different voltages and degrees of pollution the PCB might be subject to.

Now next time you do high voltage designs keep the distances in mind. What are your favourite high-voltage design tips?

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