Begin with the enclosure
Map rigid board positions, the flexible spans between them, assembly sequence and the final folded condition. Include clearance to fasteners and sharp enclosure edges. A flat outline alone does not communicate whether a circuit can be assembled without excessive strain.
Keep stress away from vulnerable features
Review vias, pads, neckdowns and abrupt geometry changes near bends and rigid-to-flex transitions. The necessary separation depends on the construction and loading, so have the fabricator review the mechanical drawing. Avoid treating a generic clearance value as a qualified production limit.
Route for both mechanics and signals
Trace direction, copper balance, shielding and reference planes affect flexibility and signal behavior. A solid plane may improve electrical continuity while increasing stiffness. Evaluate those tradeoffs together; impedance simulations must use the actual rigid and flex constructions.
Use a controlled release package
Assign one revision to the stackup, drill data, netlist and mechanical drawings. Mark controlled impedance nets, flex layers, bend zones and material requirements. Resolve DFM questions in a tracked change record before releasing fabrication.
Validate the real motion
Build representative samples and test the actual bend geometry, motion, temperature and installed constraints. Passing continuity on a flat board does not establish repeated-bend life in the final assembly.