Flexible printed circuit boards are becoming increasingly popular in electronic devices due to their ability to conform to non-planar surfaces.
However, their flexibility also makes them more susceptible to damage, particularly trace fractures. These fractures can result in device failure and compromise the reliability of the circuit.
In this blog, we will discuss the common causes of trace fractures in flexible PCB designs and provide tips on how to avoid them.
By following these guidelines, designers can improve the durability and reliability of their flexible PCBs, ensuring optimal performance of their electronic devices.
What is Fracturing Traces?
Fracturing traces refer to the damage or breakage of the conducting paths on a printed circuit board (PCB).
In a flexible PCB design, these fractures can occur due to repeated bending or flexing, causing the conductive traces to crack or separate.
Fractured traces can lead to a variety of issues, such as signal loss, intermittent connections, and device failure.
To avoid these problems, it’s essential to design flexible PCBs with proper materials and techniques that can withstand the stress of bending and flexing without compromising the reliability and durability of the circuit.
Fracturing Traces
Bend Capabilities of Flex PCBs
Flexible printed circuit boards offer unique bend capabilities, allowing them to conform to non-planar surfaces and fit into tight spaces that rigid PCBs cannot.
Flex PCBs can bend and flex repeatedly without compromising the reliability or functionality of the circuit. These bend capabilities make them ideal for use in a wide range of applications, including wearable technology, medical devices, aerospace, and military equipment, and more.
Understanding the bend capabilities of flex PCBs is essential for designing reliable and durable circuits that can withstand repeated bending and flexing without compromising performance.
By taking advantage of the bend capabilities of flex PCBs, designers can create innovative electronic devices that are more versatile, durable, and reliable.
Bend Radius Calculation
The formula for the flex PCB’s bend radius is:
Min. bending radius = (r/h of single layer dynamic flex) x application thickness.
where r = bending radius, and h = overall height of the flexible part.
Bend Capabilities of Flex PCB
IPC 2223 Minimum Bend Guidelines
The following table shows the calculation of the bend radius of flexible printed circuit boards according to IPC 2223:
| STATIC | DYNAMIC | |
| Single Layer Flex PCB | 10:1 | 100:1 |
| Double Layer Flex PCB | 10:1 | 150:1 |
| Multilayer Flex PCB | 20:1 | Not recommended |
Types of Flex PCBs Based on Bend Radius
Flexible printed circuit boards (PCBs) come in different types based on their bend radius, which can be either static or dynamic.
Static Bend
Static flex PCBs are designed to bend only once, and they have a larger bend radius. They are suitable for applications that require a fixed bend angle, such as those found in most handheld electronic devices.
Static Bend
Dynamic Bend
Dynamic flex PCBs can bend and flex repeatedly and have a smaller bend radius. They are ideal for applications that require continuous bending, such as those found in medical devices, robotics, and aerospace equipment.
Dynamic Bend
Factors that May Cause Fracturing Traces in Flex PCBs
Flexible printed circuit boards (PCBs) are highly susceptible to trace fractures due to their thin and flexible nature. Several factors can contribute to the fracturing of traces in flex PCBs.
High-Stress Points
These stress points occur in areas where the flex PCB experiences the highest levels of stress and strain, such as at the bend points or corners. When the PCB is flexed repeatedly, the stress in these areas can cause the conductive traces to crack or separate, leading to trace fractures and device failure.
To prevent these issues, designers must identify high-stress points during the design phase and employ appropriate techniques to reinforce those areas.
Techniques such as adding extra copper or placing additional support structures can help distribute the stress evenly across the board and prevent stress concentration in critical areas.
By taking steps to mitigate high-stress points, designers can improve the reliability and durability of their flex PCB designs, ensuring optimal performance of their electronic devices.
Solder Joints/Vias Too Close to Bend Points
When the PCB is bent or flexed repeatedly, the stress on the board can cause the solder joints and vias to crack or break, leading to trace fractures and device failure.
To avoid this issue, designers must ensure that solder joints and vias are placed in areas with minimal stress and strain, such as the neutral plane of the PCB.
Additionally, they should avoid placing components or other objects near bend points that could contribute to stress concentration. By taking these steps, designers can reduce the risk of trace fractures in their flex PCB designs, ensuring the reliability and durability of their electronic devices.
Unbalanced Dielectric Thickness
Unbalanced dielectric thickness is another factor that may cause fracturing traces in flexible printed circuit boards (PCBs). When the dielectric layers are not evenly distributed, it can lead to uneven bending and flexing of the PCB, resulting in areas of higher stress and strain. Over time, this can cause the conductive traces to crack or separate, leading to trace fractures and device failure.
To prevent this issue, designers must ensure that the dielectric layers are evenly distributed across the PCB and that the layer sequencing is appropriate for the intended application.
Additionally, designers should select materials with matching coefficients of thermal expansion (CTE) to minimize stress and strain during thermal cycling. By taking these steps, designers can minimize the risk of trace fractures in their flex PCB designs, ensuring the reliability and durability of their electronic devices.
Copper Selection
Poor copper quality, such as low ductility or high levels of impurities, can make the traces more susceptible to fracture when the PCB is bent or flexed repeatedly.
Additionally, the inappropriate copper thickness can cause uneven bending and flexing, leading to areas of higher stress and strain that may result in trace fractures.
To prevent this issue, designers must select high-quality copper with appropriate ductility and purity levels for their intended application.
Additionally, they should ensure that the copper thickness is adequate for the intended application and that the copper plating process is consistent and uniform.
Tighter Bends
When a PCB is bent too tightly, the conductive traces can stretch and deform beyond their limits, resulting in fractures or breaks.
To prevent this issue, designers must ensure that the bend radius is appropriate for the intended application and that the PCB is not bent beyond its maximum allowable limit.
Additionally, designers should use appropriate materials and construction techniques to reinforce critical areas of the board and distribute stress evenly across the PCB.
How to Prevent Fracturing Traces
Fracturing traces in flexible printed circuit boards (PCBs) can be prevented by following specific design guidelines and construction techniques. Here are some methods to prevent fracturing traces:
Follow Bend Capability Requirements
One of the best ways to prevent fracturing traces is to follow the bend capability requirements provided by the manufacturer. The bend radius, thickness, and the number of layers all have specific limitations that must be considered to ensure that the board can withstand the intended application’s stress and strain.
Thinner Flex PCBs are Better
Thin and flexible PCBs are less susceptible to fracturing traces. Designers can use thinner copper and substrate layers to increase flexibility and reduce stress concentration, resulting in improved bend reliability.
Use ENIG Surface Finish
Using ENIG (Electroless Nickel Immersion Gold) as the surface finish for your flexible PCB can help prevent fracturing traces. ENIG provides a smooth and uniform surface that is ideal for flexible PCBs.
Use Mechanical Stress Concentrators
Mechanical stress concentrators are added to a flexible PCB design to help redistribute the stress caused by bending. They work by creating small notches or grooves in specific areas of the PCB, which helps to prevent cracks from forming.
Utilize Epoxy Bead at Transition
Using an epoxy bead at the transition point between the flex PCB and the rigid board can help prevent fracturing traces. The bead acts as a buffer, reducing the amount of stress and strain at the transition point.
Use Coverlay
Coverlay is a protective layer applied to the top of the PCB to protect the conductive traces from damage. It can help reduce stress and strain by distributing the force more evenly across the board.
Rounded or Pointed Edge Stiffeners
Rounded or pointed-edge stiffeners can be added to a flexible PCB design to reinforce areas that are more likely to experience stress. These stiffeners help distribute the stress evenly, reducing the risk of fracturing traces.
Avoid PTH and Via in Flex Area
Plated through holes (PTH) and vias in the flex area can weaken the board and make it more susceptible to fracturing traces. Designers should avoid placing them in areas that may be subjected to stress and strain.
Flex PCB in Neutral Axis
Flex PCBs should be bent or flexed in the neutral axis, where the stress and strain are the lowest, to minimize the risk of fracturing traces.
Use RA (rolled annealed) Copper Type
Using RA (rolled annealed) copper type is another way to prevent fracturing traces in flexible PCBs. RA copper is more ductile and flexible than other copper types, making it ideal for flexible PCB designs.
Check the Grain Direction
Checking the grain direction of the copper foil can help minimize the risk of fracturing traces. Designers should make sure that the grain direction is perpendicular to the bend line to reduce the risk of cracking.
Use I-Beam Design
An I-beam design can help distribute stress and strain evenly across the board, reducing the risk of fracturing traces. The I-beam design consists of several small beams connected to the PCB’s top and bottom surfaces, improving the board’s strength and flexibility.
Avoid Sharp Bending/Folding
Avoiding sharp bending or folding of the flexible PCB can also help prevent fracturing traces. Bending the PCB too sharply can cause stress concentration, leading to fractures in the trace.
Use Curved Traces
Using curved traces instead of straight traces in the flexible PCB design can also help prevent fracturing traces. Curved traces provide more flexibility, reducing the stress concentration at sharp corners.
Fracture Traces Prevention Process
Final Words
In conclusion, flexible printed circuit boards (PCBs) offer many advantages, such as compactness and flexibility, but they can be prone to fracturing traces due to the stress caused by bending. Fracturing traces can cause connectivity issues, short circuits, and ultimately, failure of the device.
By taking these measures mentioned above, you can help ensure that your flexible PCB design is robust and reliable, with minimal risk of fracturing traces.
It is essential to carefully consider the requirements of your project and select the appropriate design techniques and materials to ensure the long-term performance of your flexible PCBs.
