Flexible PCB bend radius

The bend radius of a flexible PCB is a key parameter that reflects how tightly the printed circuit board can bend without causing damage. The design and installation stages must consider the bending radius. They are vital for the flexible PCB's integrity and performance.

What is the flexible PCB bend radius?

The bend radius is the minimum radius to bend a flexible PCB. Bending it more can cause failures like cracked circuits or a broken substrate. If the flex PCB bending radius is too small, it can cause the following problems:

Copper circuit cracking: Bending too much can crack or break the copper circuit, causing it to fail.

Layering: The layers of flexible PCB can start to separate. This can cause a loss of strength and may lead to electrical failure.

Material fatigue: Bending the PCB beyond the recommended limit will wear it out over time, shortening its service life.

How do you calculate the smallest bend radius for a flexible PCB?

It's crucial to calculate the flex PCB's minimum bending radius. It avoids damage, like cracking or delamination, during installation or use. Factors like material properties, the number of layers, and the flex PCB thickness also affect the bending radius.

General Formula for Minimum Bending Radius

The basic formula to estimate the minimum bending radius of a flex PCB is R = t × (10 × N).

Note: t: Total thickness of the flex PCB.            N: Factor based on the type of flex PCB.

• For single-layer flex PCB: N ≈ 6 to 10

The bend radius should be at least 6 to 10 times the thickness of the PCB.

For example, if the thickness of the printed circuit board is 0.2 mm, the minimum bend radius should be between 1.2 mm and 2 mm.

• For double-layer flex PCB: N ≈ 10 to 15

The bend radius should be at least 10 to 15 times the thickness, due to the presence of copper on both sides. This increases the risk of trace cracking if bent too tightly.

• For multilayer flex PCBs: N ≈ 20 or more.

The bend radius should be 20 times or more the thickness of the PCB, depending on the number of layers and materials.

Step-by-Step Calculation

Determine the total thickness (t):

Calculate the flex PCB's total thickness. It includes the copper, dielectric, adhesive, and coverlay layers.

Select the Bending Factor (N):

Choose the appropriate factor N based on the number of layers and the specific design of the flex PCB. Single-layer flex PCBs typically have a smaller minimum bending radius than multilayer flex PCBs.

Calculate the flex pcb minimum bend radius:

Use the formula R = t × (10 × N).

Example Calculation

Assume a double-layer flex PCB with a total thickness of 0.2 mm: t = 0.2.

For a double-layer flex, N = 12 (common factor).

Minimum bending radius, R = 0.2 × (10 × 12) = 24 mm.

So, the minimum bending radius for this double-layer flex PCB would be 24 mm.

3 main types of bending in flexible PCBs:

1. Static Bending: Here, the flexible circuit board is bent once during installation. It then remains in that bent shape. It is usually used where fixed wiring is needed in a small space, like display connections or those inside a mobile phone.

2. Dynamic Bending: The flex PCB needs to be bent repeatedly during use. It must endure constant movement or bending. This is common in folding smartphones, printers, wearables, and other devices that need to repeat movements.

3. Compound Bending: This is the need for the flexible circuit board to bend or twist in multiple directions at once during installation or use. Compound bending is usually more complex than single-direction bending. Design must ensure no damage to the circuit board.

When designing and making flexible circuit boards, consider the bending types. Also, account for the use environment and needed mechanical properties. This will ensure the boards are reliable and perform well throughout their service life.

Important Considerations

• Material Properties: The type of materials used (e.g., polyimide, copper thickness) can affect flexibility.

• Bend Type: Static bends can tolerate smaller radii compared to dynamic bends.

• Stack-Up Design: If the flex PCB includes stiffeners or is part of a rigid-flex design, these will affect the bending radius.

Factors Affecting the Bend Radius

Several factors influence the bend radius of a flexible PCB:

• Material Thickness: Thinner flex PCBs can be bent more tightly than thicker ones. Using thinner copper and polyimide layers can help achieve a smaller bend radius.

• Type of Copper: RA copper is more flexible than ED copper. It is also less prone to cracking. So, it is better for applications requiring tight bends.

• Layer Count: As the number of layers in a flex PCB increases, the flexibility decreases, necessitating a larger bend radius.

• Flex Cycles: If the flex PCB is for dynamic use, it needs a larger bend radius. This reduces stress and prolongs the board's lifespan.

• Bend Location: Avoid placing vias, pads, or components in the bend area to reduce the risk of mechanical failure. Traces should also be routed perpendicular to the bend direction where possible to minimize stress.

Best Practices for Designing Flex PCBs with Bends

• Gradual Bends: Design bends to be as gradual as possible to distribute stress evenly along the length of the bend.

• No Sharp Corners: Avoid sharp corners or angles in the design, as they can concentrate stress and lead to cracks.

• Support Structures: Use stiffeners or added supports to reinforce areas around the bend, especially near connectors or solder joints.

• Test Prototypes: Always test prototypes under expected bending conditions. This ensures reliability before full-scale production.

Conclusion

The bend radius is a vital consideration in the design and application of flexible PCBs. Following the recommended bend radius prevents circuit damage. It ensures optimal performance and longevity. Designers can create reliable, durable flexible PCBs. They should consider material thickness, copper type, and application-specific requirements.

For info on flex PCB design guides, bend radius, or innovative circuitry, visit Flex Plus FPC. We are committed to providing high-quality flex PCBs and rigid-flex PCBs to meet the different needs of various industries.