Conductor annular ring design and filleting
All pads, on both through-hole and surface mounted pads should be filleted to reduce stress points. This helps eliminate breaking during flexing.
Tie-downs and radiused traces
Tie-downs are captured by the coverlay to anchor the copper to prevent separation between the copper and the base material. Radiused traces help to reduce breaking during folding and bending.
I-beam design and stiffeners
I-Beam constructions occur when the inductors on both layers lie directly on top of each other, increasing the stiffness of the circuit through fold areas. A better alternative is stagger conductors, alternating their location to retain the maximum flexibility of the circuit.
Stiffeners design reference
Stiffener and coverlay termination line should overlap a minimum of 12mil to avoid stress points. Eliminating the stress points reduce the chance of traces breaking.