After we show how to calculate bending radius for 1 layer flex PCB, here explain how to do it for double-sided FPC:

In the picture:
R=minimum bending radius, unit µm, c=copper skin thickness, unit µm, D=covering film thickness, unit µm, EB=copper skin deformation, measured in percentage.
The value of EB is the same as above.
D=thickness of interlayer dielectric, unit µm
The minimum bending radius and flexural strength of FPC
type minimum bending radius:
1. Single panel 3~6 times the thickness of the wire 4 [1 Z- C2 n9 g
2. Double-sided board 6~10 times the thickness of the wire 4 c) q* ~( e5 x
3. Multilayer flexible board 10~15 times the thickness of the wire. p Y, E+ M9 Z/ H
4. Dynamic single panel 20-40 times the thickness of the wire