PCB clearance and creepage: spacing for higher voltages
For mains or other higher voltages, the spacing between copper areas is a safety matter, not only a manufacturing limit.
Clearance and creepage
Clearance is the shortest distance between two conductors through the air. Creepage is the shortest distance along the surface of the insulating material between them. Creepage is always equal to or longer than clearance, because a path along the surface cannot be shorter than the straight line. A board must satisfy both.
What sets the distance
- The voltage between the two conductors: a higher voltage needs more distance.
- Pollution degree: dust, humidity or condensation in the device lower the insulation along the surface, so a dirty environment needs more creepage.
- Material group (CTI): laminates differ in how well they resist surface tracking; a better material allows a shorter creepage.
- Type of insulation: basic, supplementary or reinforced insulation each have their own required values.
- Altitude and overvoltage category of the equipment, for clearance.
The numbers come from the standard of your product, for example IEC 60664-1 or the product standard such as IEC 62368-1, and from the spacing table of IPC-2221. Take the values from them; this guide does not replace the standard, and a certification lab has the final word.
The manufacturing minimum is not a safety value
The smallest spacing of the process, 0.15 mm, only says what can be etched reliably (design rules). It says nothing about the voltage it can withstand. For a higher voltage, draw the spacing from the standard and use the minimum only for low-voltage signals.
How to gain creepage
- Increase the spacing between the two copper areas, including the parts of the layout that are not obvious, such as pads of two parts or the two sides of an opto-coupler.
- Cut a slot in the board between the two areas: the surface path has to go around the slot. Draw the slot in the outline layer like any internal cut-out (DFM checklist) and check the minimum slot width with us.
- Use a better material (higher CTI) where the standard allows a shorter distance for it.
- Coat the board: a suitable conformal coating can change the pollution degree considered, but only according to the standard.
Do not count on the solder mask alone as insulation unless the standard accepts it, and keep copper and mask openings away from the board edge.
Inner layers and thickness
The distance between layers is given by the thickness of the laminate between them (stack-up guide); for higher voltages a thicker dielectric is the usual solution. Vias and holes with copper must also keep the required distance to conductors of another net (vias and hole sizes).
Order your boards
Mark the slots clearly in the files, write the voltage class in the notes of the request, and upload the archive in the price calculator.
Frequently asked questions
What is the difference between clearance and creepage?
Clearance is the shortest distance through the air between two conductors; creepage is the shortest distance along the surface of the board between them.
Is 0.15 mm enough spacing for mains voltage?
No. 0.15 mm is only the manufacturing minimum. For mains voltage the distance must come from the product standard and is many times larger.
How can I increase creepage without a larger board?
Cut a slot in the board between the two copper areas, so the path along the surface becomes longer, or choose a laminate with a better CTI where the standard allows it.
Does the solder mask count as insulation?
Do not rely on it alone. Whether it can be counted depends on the standard of your product, so check it before you shorten a distance.
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