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 Understanding and Calculating Bend Radius in Flexible PCB Design

Understanding and Calculating Bend Radius in Flexible PCB Design

Introduction:

Flexible PCBs, often referred to as FPCBs or flex circuits. have become increasingly popular in various industries. Due to their versatility and space-saving advantages. One critical aspect of designing and working with flexible PCBs. That is understanding and calculating the bend radius. In this blog post, we’ll dive into the importance of bend radius in flexible PCB design. And guide you through the calculation process.

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Why is Bend Radius Important in Flexible PCB Design?

Bend radius is a crucial factor in flexible PCB design. because it directly affects the board’s performance, reliability, and longevity. Here’s why it matters:

※ Preventing Damage:

Excessive bending can lead to mechanical stress, cracks. or damage to the flexible PCB traces and components. Calculating the proper bend radius helps avoid these issues.

※ Electrical Integrity:

Maintaining the integrity of signal paths. and connections is essential in flexible PCBs. Incorrect bend radii can cause signal degradation or open circuits.

※ Mechanical Durability:

In applications where the flex circuit is subject to repeated bending or flexing. choosing the right bend radius is vital for ensuring the PCB’s mechanical durability.

※ Calculating the Bend Radius:

  • Determine Material Properties: Start by gathering information about the flexible PCB material. including its mechanical properties such as tensile strength, yield strength, and elastic modulus.
  • Identify the Bend Angle: Define the angle at which the flexible PCB will be bent. This angle is typically measured in degrees.
  • Measure the Thickness: Measure the thickness of the flexible PCB material in millimeters (mm) or inches (in).
  • Use the Formula: Depending on the bending method, use one of the following formulas to calculate the bend radius.
  • For air bending:

    For air bending

  • For press bending: 

    For press bending

    Remember to convert the bend angle from degrees to radians using the formula: θ (radians) = θ (degrees) * (π / 180).
  • Consult Material Specifications: Ensure that the calculated bend radius is within the allowable limits. specified by the flexible PCB material manufacturer.
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Calculation of Bending Radius of Single-Sided Flexible PCB

Its minimum ending radius calculated by the following formula:

R = (c/2)[(100-EB)/EB]-D

Explain:

  • R = Minimum bending radius in um
  • C = Copper skin thickness in um
  • D = Cover film thickness in um
  • EB = Copper skin deformation, measured in percent
single-sided FPC calculating-the-bend-radius

The amount of skin deformation varies for different types of copper. The maximum value of copper skin deformation is ≤16% for calendered copper and ≤11% for electrolytic copper. Moreover, in different applications. The values of copper skin deformation for the same material are also different. The value of the deformation of the same material is also different in different applications. For one-time bending, the limit value of the critical state of fracture used (for rolled copper, this value is 16%).

Bending installations, the minimum deformation values specified in IPC_MF-150 used (for rolled copper, 10%). For dynamic flexible applications, use 0.3% for copper skin deformation. And for magnetic head applications, use 0.1% for copper skin deformation. By setting the allowable copper skin deflection, the minimum radius for bending can be calculated.

Example:

  • 50um polyimide, 25um adhesive, 35um copper
  • Therefore, D = 75um, c = 35um
  • Total thickness of the flexible sheet T = 185um
  • For one-time bending, use 16% R = 16.9um or R/T = 0.09
  • Bend mounting with 10% R = 0.08um or R/T = 0.45
  • Dynamic bending with 0.3% R = 5.74um or R/T = 31
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Calculation of Bending Radius of Double-Sided Flexible PCB

The minimum bending radius calculated by the following formula:

The minimum bending radius calculated by the following formula:

R = (d/2+c)[(100-EB)/EB]-D

Explain:

  • R = minimum bending radius in um
  • C = copper skin thickness in um
  • D = cover film thickness in um
  • EB = Copper skin deformation, measured in percent
  • d = thickness of interlayer medium in um
double-sided flexible FPC calculating the bend radius

Example:

  • Substrate thickness: 50um polyimide, 225um adhesive, 235um copper
  • then d = 100um, c = 35um
  • Cover film thickness: 25um polyimide, 50um adhesive
  • then D = 75um
  • Total thickness of flexible sheet T = 2D+d+2C = 320um
  • Substitute in the above equation:
  • For one-time bending, EB = 16% R = 0.371um or R/T = 1.16
  • Bending installation, EB = 10% R = 0.690um or R/T = 2.15
  • For dynamic bending, EB = 0.3% R = 28.17um or R/T = 88
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Considerations for Flex PCB Designers

Minimum Bend Radius:

Determine the minimum bend radius for your specific flex PCB material and application.

Copper Trace Design:

Pay attention to trace and pad placement to minimize stress on conductive elements during bending.

Layer Stacking:

Consider the number of layers and their arrangement when designing for flexibility.

Conclusion:

Calculating the bend radius is a critical step in ensuring the reliability. And functionality of flexible PCBs in various applications. By understanding the importance of bend radius and following the calculation process. you can design flex circuits that meet your specific requirements. and deliver exceptional performance.

If you have any questions or need assistance with your flexible PCB design. Don’t hesitate to reach out to our team of experts at GESP Technology. We’re here to help you achieve your flexible PCB design goals.

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