Checking Feature Clearances at a Rigid-Flex Transition

The boundary between a rigid section and a flexible section is not just a visual line on the board outline. Several materials and manufacturing features meet there. A clearance measured to the wrong boundary can leave a via, pad, or exposed conductor in a location the selected construction does not support.

Agree on the transition reference

Würth Elektronik’s rigid-flex construction rules distinguish clearances for different copper layers, exposed features, drill pads, and coverlay arrangements at the transition. The numerical limits belong to that construction. Their useful general lesson is that one universal setback does not describe every feature.

Ask the fabricator to identify the finished rigid edge, coverlay extension, bonding region, and relevant registration tolerances. Show these references on a section and a plan view. The CAD boundary used for rigid-flex regions should map clearly to the manufactured structure.

Audit each feature type

• Locate via pads and drilled structures on every affected layer.
• Check outer and inner copper clearances separately.
• Review exposed lands and coverlay openings near the boundary.
• Include profile recesses, corners, stiffeners, and adhesive edges.
• Confirm the location of the actual bend relative to the transition.

For example, moving a via away from the nominal rigid edge may still leave it too close to a different process boundary. Resolve the supplier’s reference dimensions before accepting the movement as a completed fix. A DRC rule is useful only when it measures the intended geometry.

Coordinate manufacturing and mechanical review

The transition can be affected by both fabrication tolerances and the installed motion. Check whether enclosure supports or forming tools force bending immediately beside a feature that was assumed to remain mechanically quiet. Include assembly access and the order in which rigid sections are positioned.

Review proposed supplier edits on an annotated drawing. If a material boundary moves, inspect all nearby features rather than only the one that triggered the change. A local concession can affect another layer or a coverlay opening that is invisible in the current view.

Release the accepted construction, transition references, feature-specific clearances, and supplier interpretation as one record. Where CAD cannot represent a requirement directly, retain a documented manual review. Revisit the audit after stackup or profile changes. This makes the rigid-flex interface reproducible and avoids treating a collection of different material boundaries as a single idealized line.

Keep representative section photographs or other agreed inspection evidence linked to the construction revision when the interface is important to qualification.