Bending Radius For Rigid Flex Circuit Boards

Rigid-flex circuit boards revolutionized the electronics industry by allowing electronic devices to bend and fold into tight spaces or conform to irregular shapes. However, there is a limit to how much rigid-flex PCBs can be bent without damaging the circuitry or suffering from mechanical stress and delamination. To avoid this, it is important to follow manufacturer guidelines and industry standards for minimum bending radius. Careful design considerations can also reduce the stress and strain on the flexible circuits, potentially enabling smaller minimum bending radius requirements.

In multilayer rigid flex circuit, the number of layers directly affects the maximum bending radius required. The higher the layer count, the lower the flex board’s flexibility will be. However, reducing copper trace thickness can increase the flexibility of a multilayer flex circuit board by decreasing resistance and increasing conductor width.

The rigid-flex board’s material type and construction methods will also impact the bending radius. For example, the use of more flexible dielectric materials such as polyimide can increase a PCB’s flexibility and allow it to be bent to a narrower angle. Additionally, using specialized construction techniques like embedding reinforcements can decrease the stiffness of the flex circuit and enable it to be bent with a smaller radius.

Maximum Bending Radius For Rigid Flex Circuit Boards

It is also important to consider the amount of flexing and bending the rigid-flex circuit will undergo in its final application. Static flex circuits are typically only bent once and never unfolded, whereas dynamic flex circuits may undergo repeated bending and flexing over time. The bending radius requirements will vary depending on the application, requiring designers to take both factors into consideration.

While the bending radius will vary based on the specific flex circuit board, the basic formula for calculating the optimum bend radius is to calculate the flex circuit’s thickness multiplied by a factor. This will help ensure that the flex circuit can withstand the anticipated bending and flexing stresses in its final application.

In addition to the minimum bending radius, the location of components on a flex circuit will influence its bending capability. For example, if the flex circuit is near the transition between the rigid and flex sections of the PCB, it will need to be designed with greater tolerances to prevent damage at this point. It is also recommended that a rigid-flex PCB have adequate space between the rigid section and the flex circuit in order to accommodate thermal expansion and contraction, which can occur due to environmental factors such as temperature and humidity.

In addition, the rigid-flex PCB must be designed to withstand a range of bending and flexing forces, including the tensile stress created when the flex circuit is creased to form a 90° angle. This can result in stress concentrations along the bending axis, which could cause damage to the copper traces or through holes. To minimize these stresses, a rigid-flex PCB should be manufactured using a pad-only plating technique such as button plating, in which the copper is only deposited on the pads/vias. This can significantly reduce the risk of stress cracking and peeling at the interface between the rigid and flex sections of the circuit board.