Calculation of the Flex PCB Bend Radius

Flexible circuit boards, also known as “flex boards”, are printed circuits made from a flexible, insulating substrate. Flexible circuits provide excellent electrical performance, meet the design needs of smaller and higher density installations, and help reduce assembly processes and increase reliability.
Flexible circuit boards are the only solution to meet the miniaturization and mobility requirements of electronic products. Can be freely bent, coiled, folded, can withstand millions of times the dynamic bending without damage to the wire, according to the requirements of the spatial layout of any arrangement, and in three-dimensional space, arbitrary movement and expansion, so as to achieve the integration of component assembly and wire connection; flexible circuit boards can greatly reduce the size and weight of the electronic products for electronic products to the direction of high-density, miniaturization, the development of high-reliability needs.

Advantages

(1) can be freely bent, coiled, folded, according to the requirements of the spatial layout of any arrangement, and in the three-dimensional space of any movement and expansion, so as to achieve the integration of component assembly and wire connection;
(2) The use of FPC can greatly reduce the size and weight of electronic products;
(3) FPC also has good heat dissipation and weldability, as well as easy to install and connect, the advantages of lower overall cost, the combination of hard and soft design also to a certain extent to make up for the flexible substrate in the component carrying capacity of the slight shortage.

Flex PCB Bend Radius

There is, however, a limit to the amount of strain flex PCBs can be subjected to. When they are bent out of shape, the internal bend experiences compressive forces while the external bend experiences tensile forces. Knowing the limits of these forces that the circuit board can withstand helps with the continued functionality and enhanced performance of an electrical device.

The bend radius is a measure of how much you can bend a flex circuit board without causing any damage or shortening its lifespan. The smaller the bending radius of a circuit is, the more flexible it will be. There are three types of design standards for flex circuit boards:

1. Flex to Install
This is also called the stable flex and involves the flex layer to be bent into shape to fit into a design. The bend is introduced in the beginning, and the layer is not subjected to further stress. For one or two layers, the minimum bend radius can be 6X, while for multiple layers, it can be up to 12X.

2. Dynamic Flex
This design involves repetitive bending of the design; therefore, limiting it to two layers is recommended. The copper should be allowed to sit on the neutral axis, which is the point that experiences minimal strain or stress. The minimum bending radius is roughly around 100X.

3. One Time Crease
Minimum bend radius is irrelevant in this design since the flex layer is creased before being installed into the design. Very thin layers and copper weights are recommended. The copper should be placed as near the neutral axis as possible.

How to Calculate Flex PCB Bend Radius

When an FPC flexible circuit board is bent, the type of stresses applied to the two sides of its centerline are different. The inner side of the bending surface is under pressure and the outer side is under tension. The magnitude of the stress applied is related to the thickness and bending radius of the FPC flexible circuit board. Excessive stress will make the FPC flexible circuit board delamination, copper foil fracture and so on. Therefore, in the design should be reasonable arrangement of FPC flexible circuit board lamination structure, so that the bending surface of the center line at both ends of the lamination as symmetrical as possible. Also according to different applications to calculate the small bending radius.

Situation 1, the small bending of a single-sided flexible circuit board is shown in the figure below:

Its small bending radius can be calculated by [sensitive word] formula: R = (c / 2) [(100-Eb)/Eb]-D
Where: R = small bending radius (in µm), c = copper skin thickness (in µm), D = cover film thickness (in µm), EB = allowable deformation of the copper skin (measured in percent)

Different types of copper have different amounts of copper skin deformation.
A, the large value of the copper skin deformation of the copper crush is ≤ 16%
B, electrolytic copper copper skin deformation is ≤ 11%.

And in different occasions, the same material copper skin deformation value is not the same. For one-time bending, the limit value of the critical state of fracture is used (16% for copper lapping). For bending installation designs, the small deformation values specified in IPC-MF-150 are used (10% for milled copper). For dynamic flexible applications, use 0.3% copper skin deflection. For magnetic head applications, the copper skin deformation is 0.1%. By setting the permissible deformation of the copper skin, it is possible to calculate the small radius of the bend.

Dynamic flexibility: this copper skin application scenario is through the deformation to achieve functionality, let’s say: IC card holder within the phosphor bronze shrapnel, is the IC card [sensitive word] after the contact with the chip parts, inserted in the process of the shrapnel constantly deformation, this application scenario is flexible and dynamic.

Situation 2, double-sided board

FPC bending radius where: R = small bending radius, unit & micro; m, c = copper skin thickness, unit & micro; m, D = cover film thickness, unit & micro; m, EB = copper skin deformation, measured in percent.

The value of EB is the same as above.
d=Interlayer media thickness, in µm