Rigid Flex Architecture against Two Boards and a Connector
Rigid-flex architecture eliminates discrete connector failures and signal discontinuities while reducing assembly labor, justifying higher bare-board costs.
Dynamic Flexure Fatigue
Copper Degradation
Material failure occurs when a flexible circuit is subjected to repeated bending cycles until the copper traces or the dielectric substrate crack. Dynamic flexure fatigue measures the endurance of a circuit board designed for motion, such as those found in laptop hinges or robotic arms. The failure begins as microscopic fractures in the crystalline structure of the rolled-annealed copper.
Stress Concentration
Areas where the trace width changes or where a component is soldered to the flexible material are the most susceptible to damage. In a dynamic flexure fatigue scenario, these points of stiffness create a pivot that focuses all the mechanical energy into a small area. Engineers use teardrop shapes at pad entries and avoid placing vias in the bend area to distribute this energy more evenly.
Durability Testing
Specialized equipment bends the sample over a fixed radius at a controlled speed until an electrical open is detected. The number of cycles until failure provides the data needed to guarantee the service life of the device. Designing for a high dynamic flexure fatigue resistance requires selecting materials with high ductility and keeping the traces on the neutral axis of the bend.
Thinner copper foils generally provide a higher cycle life than thicker alternatives. Proper material selection prevents premature circuit failure in moving parts.