Flex circuits trade rigidity for geometry: the same copper and dielectric, rolled thin enough to bend. The material is rigid flex grade polyimide — Dk 3.4–3.8, stable at temperatures where epoxy laminates give up, and thin enough that the copper, not the substrate, dominates the mechanical behaviour.

Bend radius is the whole design

A flex survives bending when copper sits near the neutral axis — the plane inside the bend that neither stretches nor compresses. Push copper away from the neutral axis and every bend work-hardens the foil until it cracks. The rules of thumb:

  • Static bends (bent once at installation): keep the bend radius at least 10× the flex thickness.
  • Dynamic bends (hinged, repeatedly flexed): radius 100× or more, single copper layer centred on the neutral axis, and no plated through-holes in the bend zone.
  • Teardrops at every pad-to-trace junction in bend regions; traces through the bend run perpendicular to the bend line, never along it.

Coverlay replaces solder mask

Flex zones use punched polyimide coverlay with adhesive, not liquid solder mask — the mask cracks on the first bend. The coverlay opening is slightly smaller than the pad, which means the pad geometry must leave room for the opening tolerance. This is the single most common first-round flex revision.

When rigid-flex beats a cable-and-connector

Rigid-flex laminates rigid sections and flex sections into one board. It costs more per board and repays it by deleting the things that fail: the cable, the two connectors, and their assembly tolerances. It wins when:

  • Reliability matters more than BOM cost (medical, aerospace, industrial sensors).
  • Space is tight enough that connector stacks dominate the volume.
  • Assembly count matters — one board replaces cable harness, connectors and two assembly steps.

What we need to quote it

A bend-region drawing: which zones flex, how often, and in which direction. From that we pick adhesive or adhesiveless polyimide, layer count and copper weights, and check that annular rings and vias sit outside the bend zones. Flex stackups are quoted individually — the material combinations multiply quickly.

Questions we get about this

Can I put vias in a flex bend area?
No. Plated through-holes in a bend zone concentrate stress on the barrel and crack. Keep vias and vias-in-pads outside the bend regions.
Why does my flex board have no solder mask?
Flex zones use polyimide coverlay instead of liquid solder mask, because mask cracks at the first bend. Rigid sections of a rigid-flex can carry normal mask.
Is rigid-flex worth it over FFC cable and connectors?
When reliability, volume or assembly count dominates — yes. When the design is cost-driven and the linkage is long-lived, cable plus connectors usually wins.