Direct Answer
PCB creepage and clearance rules protect users and equipment when dangerous voltages are present. Clearance controls arcing through air. Creepage controls tracking along an insulating surface. The correct values come from the applicable product safety standard, not from a generic PCB manufacturer capability chart.
Use layout DRC to enforce preliminary spacing, but treat DRC as a guardrail. The PCB Creepage and Clearance Calculator can screen early layout margin, but final approval requires checking working voltage, transient voltage, insulation category, pollution degree, material group, altitude, coating assumptions, and enclosure design. For manufacturing context, pair this page with Common DFM Issues and How to Avoid Them in PCB Design.
Engineering Principles
The first principle is failure path. Air can break down when electric field strength is high enough, so clearance is measured as the shortest direct path through air. Contaminated surfaces can conduct over time, so creepage is measured along the board or insulation surface. Dust, moisture, flux residue, conductive particles, and ionic contamination all make creepage more important.
The second principle is certification scope. A spacing that works in a clean lab may fail a safety review if the product is used in a polluted environment or at altitude. A spacing that is valid for functional insulation may be invalid for reinforced insulation. Do not copy spacing from another board unless the voltage, environment, standard, and insulation role are the same.
Design Formula
Use these definitions during layout review:
clearance = shortest distance through air between two conductive parts
creepage = shortest distance along the insulating surface between two conductive parts
A useful worked example is a primary-to-secondary boundary in an isolated power supply. If the initial PCB placement cannot meet the required creepage along the board surface, a milled slot may lengthen the surface path. But the slot must keep enough copper-to-edge clearance, must not trap contamination, and must survive routing tolerance. The slot is part of the insulation design, not a decorative cutout.
Comparison Table
| Item | Clearance | Creepage |
|---|---|---|
| Path measured | Through air | Along an insulating surface |
| Main failure mode | Arc or flashover | Tracking and surface leakage |
| Strongly affected by | Voltage transient, altitude, air gap | Pollution, material group, moisture, residues |
| PCB design lever | Component spacing, barriers, keepouts | Surface distance, slots, coating, routing path |
| DFM risk | Copper too close to board edge or slot | Contaminated slots, mask assumptions, flux residue |
DFM and Assembly Details
High-voltage spacing is easy to damage late in the layout. Silkscreen, test pads, mounting hardware, connector bodies, heatsinks, and conformal coating keepouts can all reduce the practical insulation path. If an assembler leaves flux residue near a high-voltage boundary, a mathematically correct creepage distance may still underperform. This is why cleaning process and inspection plan belong in the same conversation as layout spacing.
Slots need special attention. A narrow slot can be difficult to route cleanly. A slot too close to copper creates fabrication risk. A slot under a tall component may collect residue. If the design uses isolation slots, call them out in fabrication notes and include them in the board outline or mechanical layer unambiguously.
Release Checklist
Before Gerber release, mark high-voltage nets and isolation boundaries. Confirm the governing safety standard with the product team. Define whether each boundary is functional, basic, supplementary, or reinforced insulation. Verify creepage and clearance in the PCB tool and again in a 3D mechanical context with connectors and enclosure parts installed.
If the board also has controlled impedance or dense routing, isolate those decisions early. High-speed convenience cannot override safety spacing. See Controlled Impedance PCB Design Rules and PCB manufacturing capabilities for the manufacturing side of those tradeoffs.
Related Omini Engineering Notes
- Common PCB Defects and How to Identify Them
- PCB Stack-Up and Signal Integrity for High-Speed Boards
- Custom PCB: The Most Important Part in Your Electronic Device