Flex and Rigid-Flex Bend Capabilities in PCB Design

Certain applications require flex or rigid-flex printed circuit boards to function. These are devices that are particularly small or unusually shaped, so that you must bend the printed circuit board for it to fit inside the device. Naturally, there are certain parameters you will want to know about when it comes to your flex or rigid-flex circuit boards.

What You Need to Know About Flex Bend Radius

Naturally, you need to know the bend radius of your flex or rigid-flex boards. What you need to know before determining the adequate minimum bend radius is the type of flex printed board design standard you will be working with. These design standards are Flex to Install, Dynamic Flex and One Time Crease.

How to Calculate Flex and Rigid Bend Flex Radius

In a Flex to Install printed circuit board design, you bend the flex circuit to a specific shape to install it in the assembly. This one bend should be sufficient, and once you have installed the board, you will not need to bend it any further. In a Flex to Install design, calculate the minimum bend capabilities as six times the finished flex thickness. For designs with three or more layers, increase this to 12 times the finished flex thickness.

In a Dynamic Flex printed circuit design, the flex circuit will be repeatedly bent as a function of the final assembly operation an indefinite number of times. These will typically be only one-layer designs, although sometimes you can find two-layer dynamic flex designs. These designs allow the copper to sit on the neutral axis of the bend radius and will have a minimum bend radius of about 100 times the finished flex thickness.

On a One Time Crease design, the flex circuit is bent to a zero-bend radius and installed. It is not designed to be unfolded. Bend radius is thus irrelevant. In this one-layer or sometimes two-layer design, the copper sits on the neutral axis of the bend radius as well. You need very thin flex materials as well as copper weights to make this type of design work, and you will usually add pressure-sensitive adhesive near the creased area to keep it folded in place.

Flex and Flex-Rigid Board Capabilities

It’s important to note that different features of flex and rigid-flex boards can thicken the design and thereby WHERE limit the minimum bend capabilities of the circuit. For example, if you have exact controlled impedance requirements, you may need a thicker core to ensure proper dielectric spacing between layers. If you require EMI/RF shielding, this will necessitate additional layers which can interfere with flexing.