When fpc is bending, the stress borne by both sides of center line is different. Pressure is inside of bending face, while tension is outside. The stress has something to do with thickness and bending radius of fpc. Excessive stress will lead to delamination of fpc, fracture of copper foil and so on.Therefore, the laminated structure of fpc should be arranged reasonably in the design, so that the lamination at both ends of the center line of the curved surface should be symmetrical as far as possible. At the same time, the minimum bending radius of fpc should be calculated according to different applications.
Situation: the minimum bending radius for single-sided fpc:
The calculation method：R=（c/2）[（100-Eb）/Eb]-D
Wherein：R=Minimum Bending Radius(unit:µm); c=Copper Thickness(unit: µm); D=Thickness of Coverlay(unit:µm); EB=Copper Elongation Desired(Measured as a Percentage)
Copper elongation desired also differs among various types of copper.
A.The maximum copper elongation desired for RA Copper ≤16％
B.The maximum copper elongation desired for ED Copper ≤11％
Moreover, value for copper elongation desired for the same material will be different in different applications. For one-time bending, the limit of the critical state of fracture is used (for RA Copper, the value is 16%). For bending installation design, use the minimum deformation value specified by IPC-MF-150 (10% for RA Copper).For dynamic flexible applications, copper elongation desired is 0.3%. For magnetic head applications, copper elongation desired is 0.1%. By setting the copper elongation desired, the minimum bending radius of fpc can be calculated.
Dynamic flexibility: In these applications, copper functions through deformation. For example, the phosphorus and copper metal dome in the IC card seat, i.e. the part where the IC card contacts with the chip after being inserted, and the metal dome is constantly deformed in the process of inserting. This kind of application is flexible and dynamic.
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