Although some customers believe otherwise, PCB Trace suggests that flex circuits are not indestructible. As with any product containing a metal part that is handled or bent beyond its mechanical capabilities, it is likely to break. Primarily, there are three main causes that lead to fractures or cracks in flex circuits, and we will review the methods for preventing such damages. In the design of flexible circuit boards, it is very important to determine its minimum bend capabilities.
Minimum Bend Capabilities of Flex Circuits
Materials comprising flex circuits can fail primarily when the bending exceeds the minimum bend radius. This is specifically true for the layers containing the copper circuits. However, the application defines the minimum bend radius.
Flex Circuit with Static Bend
The most common application of a flex circuit is the static bend. This is a one-time bend, where the user bends the flex circuit into position one time for fitting it into the assembly. They do not significantly move the circuit again unless for servicing.
The most common issue of a trace crack or fracture in a flex circuit with a static bend happens when the bend formation is too tight, and it exceeds the capabilities of the material. The minimum bend radius is critical for a static bend application, even when the bending happens only once. Also, there are applications where the flex circuit requires bending multiple times for facilitating the assembly process or for performing. The primary root cause of traces fracturing is basically due to bending the flex several times in an excessively tight bend radius.
Flex Circuit with Dynamic Bend
Dynamic bend is another common application for flex circuits. The application requires the flex circuit to bend and unbend many times as a function. The inkjet printer is a prime example of such an application. The head in the inkjet printer cycles back and forth as it operates. A flex cable connects the printer head to the main electronics, and it bends and unbends as the head cycles.
For dynamic bend applications, the hardening of the copper traces is a common scenario after a fixed number of cycles. In combination with many operational cycles, failure can happen if the flex circuit exceeds the minimum bend by only a small amount. Copper is ductile, but the traces harden when they are driven beyond the point of ductility. With each cycle, the copper traces harden slightly and start to become brittle. After a fixed number of cycles, the copper circuits start to crack.
Pads/Traces with ENIG Surface Finish
Another failure mechanism is bending the flex circuit immediately after an unsupported pad or trace with an ENIG surface finish. The ENIG finish comprises two metals on the exposed copper, an initial layer of nickel followed by a layer of gold.
Nickel, being a brittle metal inherently, cannot withstand bending to any significant degree. Copper features representing a pad or a PTH typically have a trace connecting it to an external layer. A small portion of the trace usually remains exposed, starting from the edge of the pad to the opening of the coverlay. Typically, the ENIG surface finish also covers this exposed trace section.
If there is no stiffener supporting this trace, or there is insufficient support, and the flex is made to bend immediately adjacent to the pad, there is a high probability that the trace segment with ENIG surface finish will crack. This is due to a high mechanical stress on the ENIG surface concentrating on the underlying copper that results in the crack propagating from the ENIG to the copper trace.
This is readily seen in flex circuits with a ZIF connector. Typically, the crack occurs at the edge of the coverlay where it finishes and the ZIF contact fingers begin. Mishandling of the ZIF connector during installation, such as bending it, causes a crack to develop on the ENIG surface finish, ultimately cracking the copper layer below.
Mechanical Stress Concentrators
The next failure mechanism is because of mechanical stress concentrators existing within a substandard design. The design may be such as to kink the flex circuit sharply. This may lead to a situation where the bend radius is tighter than the specifications for the material, with the design intent allowing for a smooth arc with the largest possible radius.
The layout of the circuitry may create mechanical stress concentrators in several places. This may be in the bent areas of the flex circuit, in the flex circuit outline, near the location of stiffeners, in the flex area containing via or PTH holes, or in the configuration of the coverlay layers.
Rigid-flex scenarios can also create a similar situation. This mechanical stress concentration typically occurs in the area very close to the transition from the rigid area to the flex area. Bends at this transition create high stress, causing the flex to kink sharply rather than follow a smooth curve and stay within its minimum bend specifications.
Preventing Trace Fractures
It is very important to understand the requirements of the project and design the flex accordingly to prevent fractures or cracks of traces. This is because it is not possible to alter material properties during the manufacturing process. The designer must consider a few parameters for a successful design:
Minimum Bend Capability
It is necessary to define the bend requirements of the application and include it in the design of the flex circuit. This will ensure the material stack-up meets the bend requirements. It is also necessary to include assembly requirements, so that multiple bends that the assembly process requires are taken care. A significant factor is the education of assembly technicians. They must understand the limitations of flex circuits so that they do not inadvertently mishandle the flex circuit during assembly.
ENIG Surface Finish
The IPC 2223 design standard recommends supporting all component areas with a rigidizing stiffener. This prevents bending the flex circuit in the area where it has soldered components. Not only does this eliminate the opportunity for cracked circuits in the ENIG-related areas, but also avoids potential reliability issues occurring in solder joints.
Manufacturers make rigidizing stiffeners from FR4 materials, with thicknesses ranging from 10 mil to 59 mils. If adequate space is available, fabricators may also use stainless-steel stiffeners.
It is necessary to educate assembly technicians to avoid ENIG-related fractures near ZIF contact fingers. They should ensure that the latching mechanism on the ZIF connector are fully open prior to inserting the circuit. They must understand that it requires zero insertion force (hence the acronym) to insert a flex circuit into the ZIF connector. Once they have fully inserted the flex, they will encounter a resistance. At this point, closing the ZIF latch will lock the flex circuit in the connector.
If the flex is partly inserted or the ZIF lock is partially open, the operator will experience excessive force when engaging the flex. This may result in kinking of the flex, thereby creating cracks in the traces and contacts.
Mechanical Stress Concentrators
Rounded/Radius Corners
When the flex area requires a change in direction, it is necessary for the designer to use rounded corners for traces. They must use as relaxed a radius as the design allows. They must not use ninety-degree corners, and preferably use corners with a radius rather than 45-degree corners.
Rigid-Flex Design
Fabricators must use a flexible epoxy bead at the area where there is a tight bend requirement, close to the transition from the rigid to the flex area. The presence of the bead will ensure the flex will bend in a smooth radius without kinking at the transition.
Coverlays and Stiffeners
The recommendation is to not stop or start the coverlays and stiffeners at the same location. Stopping at the same location allows the thickness of the flex to change significantly and abruptly, thereby creating a strong mechanical stress concentrator.
It is also possible to use rounded or pointed edges on stiffeners rather than straight edges, as the former help mitigate stress concentration. Moreover, the designer must avoid placing vias or PTH in flex areas that will undergo bending, and prefer placing them in areas supported by stiffeners. Using vias and PTH for creating an EMI Faraday cage will significantly reduce the minimum bend capability of the design.
Minimum Bend Radius Guidelines
Materials making up the flex circuit and its thickness define its minimum bend radius. The IPC 2223 design standard offers calculations based on material properties for both static and dynamic applications. In most cases, the industry uses the following rule of thumb for deciding the minimum bend radius:
| Minimum Bend Radius | |
| Static Bend Applications | 10X Flex Thickness (1-2-Layer Design) |
| 15-20X Flex Thickness (3-Layer Design) | |
| 25-30X Flex Thickness (4-Layer Design) | |
| Dynamic Bend Applications | 100X Flex Thickness (1-2-Layer Design) |
Conclusion
According to PCB HERO, the root cause of trace failures in flexible circuits lies in the design and materials. In most cases, individual root causes are also dependent on each other, accumulating to generate potential reliability issues. For more information, please visit our contact us page or contact our experts to ensure higher reliability in your flex designs.