Part of the rigid-flex circuit board resource.
A ratio does not predict a cycle count
Bend-radius screening is useful for early geometry, but it cannot by itself predict fatigue life. The conductor geometry, foil, plating, dielectric, adhesive, coverlay, motion, and restraints all influence the result. Define the intended operating duty before selecting a test method or declaring a sample representative.
Reproduce the important geometry
Use the intended flex thickness, trace orientation, transition, rigid support, and strain relief. Define radius convention, angle, travel, movement frequency, and how the fixture holds the assembly. An unrestrained hand bend can apply a very different strain distribution from the product enclosure.
Define what failure means
Specify the electrical measurement, allowable change, inspection observations, and stopping criteria. Decide whether continuity is monitored during motion or only between intervals. Intermittent faults may not appear in a simple end-of-test check. Keep the measurement method and equipment settings in the test record.
Control the environment and sample history
Record temperature, relevant exposure, assembly conditioning, prior bends, and any rework. Trace each sample to its material and fabrication revision. Do not pool results from different constructions without identifying the differences and their effect on the intended conclusion.
Use the result within its scope
A successful test supports the tested construction, restraints, and conditions. It does not establish unlimited motion life or automatically qualify a changed material. Share failures and observations with the manufacturing partner and update the qualification plan when the design or process changes.
Continue with the design guidelines, assembly review, prototype plan, or project RFQ.