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66 I-CONNECT007 MAGAZINE I MARCH 2026 cally 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 underappreciat- ed 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, con- nector 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 fre- quency, 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 compo- nents 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 ran- dom 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 de- signer should consider when designing circuits for reliable dynamic flexing. There's more to know and understand, so I recommend IPC-2223, Sec- tional Design Standard for Flexible Printed Boards, and IPC-4202/4203, Flexible Base Materials, as guides. Take advantage of your supplier's engi- neering 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. I-CONNECT007 Joe Fjelstad is founder and CEO of Verdant Electronics and an international author- ity and innovator in the field of electronic interconnec- tion and packaging technol- ogies with more than 185 patents issued or pending. To read past columns or contact Fjelstad, click here. F L E X I B L E T H I N K I N G Download your free copy of Fjelstad's book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series "Flexible Circuit Technology." " Explore ways to shift natural frequencies by changing the flex length, adding dampening materials, and modifying the mounting method."