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Flexible Thinking: Designing Flex Circuits for Dynamic Reliability

Flex circuits flex. No surprises there. However, they are also very commonly designed into products because they are thin and offer consistent thickness and dielectric properties, attributes highly prized by present-day product designers of personal electronics. This would include smartphones and, increasingly, wearable electronics for medical monitoring and even fashion.

Bendability is perhaps most important for these applications, but designers routinely tap into dynamic flexibility as a hallmark feature of flexible circuits for certain types of products where they anticipate or require repeated dynamic bending over the life of the product.

I’ll examine some of the high-level requirements for reliably designing flex circuits for dynamic applications—conditions where you might anticipate continuous or intermittent bending, twisting, or motion of the circuit or elements of the service. Designing for such conditions is an area of electronic circuit design where small oversights can quietly turn into field failures.

With that in mind, I’ll include concerns that designers should consider, ordered by how often they impact and drive reliability issues. Because the subject is often overshadowed by dynamic flexing, shock, and vibration, these effects are explicitly noted and linked to mitigation strategies.

Bend Reliability and Strain Management
Designers get the most bang for their buck here. The simple reality is that most dynamic flex failures are caused by copper fatigue from cyclic strain. My good friend, Werner Englemaier, wrote extensively on this topic before his untimely passing, and his great work in the area still holds true. Key concerns include:

• Bend radius too tight for copper thickness
• Repeated bending occurring at a single hinge point
• Copper grain orientation not optimized for flexing
• Excessive copper thickness or stacked copper layers
• Copper not kept in the neutral axis

What can you do? First, follow the rules for dynamic bend radius, not static bend radius, by providing the largest possible radius based on the copper thickness through the bend area. The nature of the copper foil is also very important. The columnar grain structure of the electrodeposited copper (ED) foils of old was not ideal because it facilitated crack propagation perpendicular to the bend. Rolled annealed copper (RA) is now recommended; however, modern electroplated foils with improved chemistry and additive are more equiaxed than ED foils and perform much better.

The rules of thumb vary in terms of the multiplier and depend on the experiences of the purveyor. A bend radius of 100x the copper thickness or greater has been suggested for high-cycle applications. This ties to the concept of spreading the bend area, which forces a greater length of the flex circuit element involved to improve operational longevity. If possible, use a single copper metal layer through the bend zones. Avoid creasing of the circuit in its bend areas.

Keep copper in the neural axis. In practice, the “sandwich” of flexible materials that cover and support the copper traces should be equal in thickness and material properties, i.e., consistency, on both sides. In theory, when something is cyclically bent back and forth, the center is unaffected. The center, more or less, goes along for the ride. However, reality is more nuanced. The copper foil, too, has its center, which means that the strains of cycling ultimately affect it (Figure 1).

Finally, for materials, use adhesive-less flexible laminates, when possible, which are much better for dynamic flex applications. Do not ignore them.

Shock and Vibration Loading of the Circuit
This level of concern is too often underappreciated and underestimated. Shock and vibration don’t typically cause immediate damage; instead, they accumulate and accelerate fatigue in the copper through-bend areas. The forces also affect flex-to-rigid transition areas, such as where stiffeners might be attached or integrated, as well as plated vias, solder joints, and connectors.

Shock concerns are typically infrequent and result from physical/mechanical impacts on the assembly. This sudden acceleration causes high strain at locations such as stiffener edges, connector interfaces, and flex-to-rigid transitions. The mass of the components will affect the potential for damage.

On the other hand, vibration concerns are less obvious. Compared to traditional flexing, which tends to be high amplitude and relatively low frequency, vibrational “flexing” is low amplitude and high frequency. The vibrational resonance leads to subtle but amplified cyclic strain, which can be rather insidious as the copper imperceptibly work hardens from the minuscule excitation associated with vibration.

Suggestions for mitigating vibrational damage:

• Avoid placing connectors or heavy components on areas of “free-floating” flex. This is what stiffeners are for.
• Strain relief loops or service loops should be provided to decouple any motion.
• Taper the stiffener edges gradually and provide fillets to improve the transition and mitigate strain.
• When possible, use an adhesive-backed stiffener, which tends to be softer and more pliable, rather than hard epoxy.

When testing your design, validate it with random vibration profiles rather than a simple sine sweep. Explore ways to shift natural frequencies by changing the flex length, adding dampening materials, and modifying the mounting method.

Conclusion
I’ve provided a brief review of some of the more important, first-order concerns a flex circuit designer should consider when designing circuits for reliable dynamic flexing. There’s more to know and understand, so I recommend IPC-2223, Sectional Design Standard for Flexible Printed Boards, and IPC-4202/4203, Flexible Base Materials, as guides. Take advantage of your supplier’s engineering team; they can be a valuable resource. They have seen much over their careers and can serve you well based on both their positive and negative experiences.

Enjoy your journey and stay flexible.

Joe Fjelstad is founder and CEO of Verdant Electronics and an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 185 patents issued or pending. To read past columns or contact Fjelstad, click here. Download your free copy of Fjelstad’s book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series “Flexible Circuit Technology.”

This column originally appeared in the March 2026 issue of I-Connect007 Magazine.