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64 I-CONNECT007 MAGAZINE I MARCH 2026 tronic 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 ex- plicitly 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 rec- ommended; 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 appli- cations. 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 cycli- F L E X I B L E T H I N K I N G Figure 1: The center of the copper foil.