A nominal CAD clearance is only the starting point. The manufactured gap can change with board routing, drilling, registration, component placement and part dimensions. Review the tolerances that affect the same interface together, especially where the board must fit an enclosure or maintain an electrical isolation boundary.
Identify the function of each clearance
Separate fabrication spacing, assembly access, mechanical fit and electrical insulation. Each has a different acceptance basis. For electrical isolation, use the applicable product and environmental requirements; do not substitute the fabricator’s minimum copper spacing for a safety-related distance.
Build a dimensional chain
Choose a datum and list the dimensions between the features that must remain separated. Include the board outline, hole position, component body and placement variation as applicable. Record which tolerances are independent and which share a datum or process relationship.
Evaluate the relevant extreme conditions
A worst-case stack can reveal whether the nominal design has enough margin, while a statistical approach needs justified distributions and process evidence. State the method used. Do not claim a statistical yield from nominal CAD dimensions without data supporting the variation assumptions.
Make critical interfaces inspectable
Identify the dimensions that receiving or assembly inspection must verify. Provide an understandable drawing view and acceptance limit. Where a go/no-go fixture is useful, define the fixture’s own tolerance and relationship to the product requirement.
Example: a component beside a routed edge
The minimum remaining gap depends on the routed-edge location, placement variation and component-body tolerance. List all three with their directions. If the enclosure also approaches that component, extend the chain to the enclosure datum rather than checking the PCB in isolation.
Design review checklist
- Classify each clearance by its actual function.
- Use consistent datums and units.
- Review the full tolerance chain and the chosen analysis method.
- Define how critical dimensions will be verified.