Creepage and clearance: turning isolation requirements into PCB spacing
Illustrative image: Unsplash contributor on Pexels
A wide-bodied isolator does not establish the insulation spacing for an entire PCB. Its package provides specified distances between particular pins. Copper, vias, solder joints and neighbouring metal can create a shorter path elsewhere in the assembled product.
Clearance is the shortest distance through air between conductive parts. Creepage follows the surface of an insulating material. A routed slot can lengthen a surface path without increasing every air path by the same amount, so the two measurements must be assessed separately.
The required distances depend on the insulation function and the applicable equipment standard. Working voltage, transient overvoltage, pollution degree, material group and altitude are inputs to that assessment. A component's isolation withstand test voltage is not a substitute for them.
From the standard to the assembled board
Identify the circuits separated by the barrier and whether the product requires functional, basic or reinforced insulation. Then record the actual voltage across that boundary, the supply environment and the relevant overvoltage category.
IEC 60664-1 provides an insulation-coordination framework for low-voltage supply systems, including equipment rated up to 1,000VAC or 1,500VDC. The current consolidated publication includes the 2025 amendment. Its stated frequency scope extends to 30kHz; IEC 60664-4 addresses periodic voltage stress above 30kHz and up to 10MHz when used with the specified companion requirements.
Pollution degree describes the environment affecting the insulation, rather than simply whether the enclosure looks clean. Material group relates to resistance to tracking. Altitude matters because reduced air density changes the clearance needed for an air gap. None of these can be inferred from the isolator's package width alone.
Check the distance between copper features on both PCB faces, including exposed pads, plated holes and test points. The assessment must also include the final component leads, solder fillets, conductive heatsinks and mounting hardware where they cross or approach the barrier.
An isolation slot is useful only if its width and geometry qualify under the applicable spacing rules. Narrow grooves and slots are not automatically counted as their full surface length. Likewise, ordinary solder mask should not be treated as qualified solid insulation without the evidence and construction required by the relevant standard.
Manufacturing tolerance belongs in the drawing. A layout positioned exactly at the minimum distance can fall below it when routing, copper registration or assembly tolerances are applied. The controlled requirement should survive those tolerances in the finished assembly.
TI's isolation guidance works through clearance and creepage separately, including voltage, overvoltage category, pollution degree, altitude and insulation level. Its example dimensions are tied to those stated conditions. A spacing taken from that example remains valid only when the design's conditions and governing standard support it.
For the assembly drawing, the minimum spacing must refer to the nearest conductive features after manufacturing and assembly tolerances are applied. An isolator package with adequate pin spacing can still sit in a layout whose solder fillet, test point or mounting screw creates the limiting path.
