PCB Flex Design

As the name implies, pcb flex are circuit boards that can bend and fold. They’re used in mobile devices, wearable technology and medical equipment to provide a flexible form factor, and they offer a variety of benefits over rigid-flex boards. These advantages include thinner designs, faster assembly, and greater connectivity. However, bending a flex PCB requires careful consideration to ensure the board’s integrity. The minimum bending radius for a flex PCB is determined by the manufacturer, and it must be kept below this value to avoid damage to traces or the substrate material.

The maximum bending radius for a flex PCB depends on a number of factors, including the design and materials. For example, the thickness of the board and copper foil thickness can impact the minimum bending radius. A flex PCB with thicker layers may require a larger bending radius to prevent cracks in the copper traces or delamination of the dielectric layers. The type of etching used to remove the copper from the surface of the substrate can also affect the bending limit.

Using a lower-quality etching process can increase the minimum bending radius. This is because it can weaken the substrate, increasing the stress applied to the pcb flex when bent. A flex PCB’s intended application and environmental conditions can also impact the minimum bending radius requirement. For example, repeated bending cycles and exposure to harsh environments can cause the copper to fatigue and break.

The Role of Bending Radius in PCB Flex Design

In addition to determining the bending radius, a flex PCB design must also consider component placement to maintain its flexibility and reliability. To minimize stress points, designers should avoid overlapping traces. They should also ensure that the flex area does not contain any tight corners or sharp bends. A good technique is to use a gradual transition from wide to narrow traces, rather than an abrupt change in width near a connector or a via.

One of the most common failure modes for a flex circuit is copper trace fracture. Excessive bending can cause the copper traces to crack and break, resulting in open circuits and short-circuiting. The bending stress can also cause the dielectric layers (e.g. polyimide or liquid crystal polymer) to delaminate from the copper, causing serious problems with the flex circuit’s functionality.

To avoid these issues, the designer must choose a suitable bending radius and carefully consult the manufacturer’s guidelines to ensure that it is not exceeded. A bending radius calculator can be helpful to calculate the minimum bending radius for a specific PCB. This can be done by dividing the bending radius by the PCB’s thickness and multiplying it by two. In general, a bending radius should not be less than 10 times the thickness of the PCB. The bending radius can be calculated for a single-time bending or for a dynamic flex PCB that will undergo regular re-bending. In either case, the bending radius should be shown in the fabrication drawing for the flex circuit.