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When FPC FPCs are bent, the stress types on both sides of the centerline are different. The inside of the curved surface is pressure and the outside is tension. The stress is related to the thickness and bending radius of FPC FPC. Excessive stress will cause FPC FPC to delaminate, copper foil to break, etc. Therefore, the lamination structure of FPC FPC should be reasonably arranged in the design to make the lamination at both ends of the bending surface centerline as symmetrical as possible. At the same time, the minimum bending radius should be calculated according to different applications.

Case 1. The minimum bending of single sided flexible circuit board is shown in the figure below:

Its minimum bending radius can be calculated by the following formula: R=(c/2) [(100 Eb)/Eb] – D

Where: R=minimum bending radius (in µ m), c=thickness of copper sheet (in µ m), D=thickness of coating film (in µ m), EB=allowable deformation of copper sheet (in percentage)

The deformation of copper sheet is different for different types of copper.

A. The maximum copper sheet deformation of rolled copper is ≤ 16%

B. The maximum copper sheet deformation of electrolytic copper is ≤ 11%.

In addition, the copper skin deformation value of the same material is different in different applications. For one-time bending occasions, use the limit value of breaking critical state (for rolled copper, the value is 16%). For bending installation design, use the minimum deformation value specified in IPC-MF-150 (10% for rolled copper). For dynamic flexible applications, the copper sheet deformation is 0.3%. For magnetic head applications, the copper sheet deformation is 0.1%. By setting the allowable deformation of the copper sheet, the minimum bending radius can be calculated.

Dynamic flexibility: This copper sheet application scenario realizes its functions through deformation. For example, the phosphor copper spring in the IC card holder is the position where the IC card contacts the chip after insertion. The spring continuously deforms during insertion. This application scenario is flexible and dynamic.

Case 2. Double sided board