Rigid-flex design — bend rules that prevent cracked traces.

Bend radius — the one rule to never break

When a flex bends, the outer surface stretches and the inner compresses; the strain on the copper scales inversely with bend radius. Too tight and the copper work-hardens and cracks. The standard minimums:

For a 0.2 mm flex, that’s a ≥ 2 mm static / ≥ 20 mm dynamic radius. Dynamic applications (hinges, moving heads) also force single-layer flex with the copper on the neutral axis — multilayer copper in a repeatedly-flexed zone will eventually fail.

Routing rules in the flex region

• Perpendicular to the bend — traces cross the bend line at 90°, never run along it.
• Curved, not cornered — use arcs; sharp corners concentrate strain and etch unevenly.
• Stagger between layers — on 2-layer flex, offset top and bottom traces (not stacked) so the cross-section bends evenly. This is the essence of bookbinder construction for multilayer flex: layers are cut to different lengths so each sits at its natural radius.
• Cross-hatch the reference — solid copper planes stiffen the flex and crack; use a hatched (mesh) pour for flexibility, accepting the impedance/return-path tradeoff.
• No vias or plated holes in the bend — ever. Keep them in the rigid sections.

The rigid-to-flex transition

The interface between rigid and flex is the highest-stress region. Rules:

1. Keep the coverlay/stiffener edge back from the bend, and don’t end it on a sharp line across traces — stagger or radius it.
2. No plated through-holes within ~50 mil of the transition.
3. Add teardrops where traces meet pads/vias near the transition for strain relief.
4. Add stiffeners under connectors and component areas so flexing stress never reaches solder joints.
5. Anchor the coverlay into the rigid section so it can’t peel.

Rigid-flex fails at two places: a too-tight bend and a sharp rigid-to-flex edge. Design both deliberately and the board lasts.

Rigid-flex checklist

• ☐ Bend radius ≥ 10× (static) / 100× (dynamic) flex thickness
• ☐ Dynamic bends: single copper layer on the neutral axis
• ☐ Traces perpendicular to the bend, curved, staggered between layers
• ☐ Hatched (not solid) reference copper in the flex
• ☐ No vias/plated holes in the bend or within ~50 mil of the transition
• ☐ Teardrops + staggered coverlay edge at the transition
• ☐ Stiffeners under connectors/components
• ☐ Fab-reviewed stackup (rigid-flex is fab-specific — confirm early)

Need it designed?

Rigid-flex (2–8 layer, bookbinder, dynamic) is in our wheelhouse — see services or scope a project. We confirm the stackup with your fab before routing, because rigid-flex tolerances are fab-specific.