bending test standards for rigid flex boards
Rigid flex boards offer the benefits of a rigid PCB’s mechanical stability with the flexibility to bend, fold and flex. As such, they are used in a wide range of commercial products from portable electronics to wearable fitness and health monitors to automotive controls. However, these flexible circuits are a complex design challenge that requires careful consideration to ensure the ribbon-like structure does not break under mechanical stress. This includes choosing the right minimum bending radius, layer stackup and material constraints.
The minimum bending radius for a rigid flex board is influenced by the circuit’s materials and construction, with various industry standards and formulas available to help determine the appropriate value. For example, polyimide-based flex circuits have a smaller minimum bending radius than glass-based circuits.
In addition, the use of different dielectric layers can impact the required minimum bending radius. Choosing the best material for the flexible sections of your rigid flex board can also affect signal integrity and electromagnetic interference (EMI). For example, selecting the right thermal expansion coefficients between different dielectric layers is vital to maintaining electrical performance throughout the flex section.
What are the bending test standards for rigid flex boards?
It’s also important to take into account how the flex circuit will be connected to other rigid parts of your product when designing for a bending zone. For example, selecting the wrong connector can result in mechanical stress on the rigid-flex section and lead to failure over time.
To avoid stress concentrations, dummy traces are a good way to increase the mechanical strength of your flex circuit. It’s also a good idea to avoid sharp corners in the flex region and opt for curved corners instead. It’s also beneficial to avoid plated through holes in the flex area, and opt for anchors and teardrops for connecting traces, as well as maintaining straight and perpendicular traces across bending zones. Lastly, consider using cross-hatched ground planes in the flex area to reduce stress on the copper and maintain flexibility.
To test your flex board’s ability to resist a bending zone, you can run a flexural fatigue test or an IPC-D-556B flex test. For the flexural fatigue test, you place the sample on a support span and then apply a load to produce three points of bending at a rate specified by your chosen test standard (ASTM D790 or ISO 178). The test stops when the specimen reaches 5% deflection or breaks.
You can then repeat the test with a new sample to confirm the results. A rigid flex board is capable of withstanding much greater deflection than a traditional rigid-board, but it’s still important to follow strict testing guidelines for your design. By taking these steps, you can be confident your rigid flex board will not fail during its intended application. For more information on design guidelines for rigid-flex PCBs, download our free PCB Design for Manufacturing handbook. You can also utilize PCB design software like OrCAD X to get the most out of your rigid-flex design, with features like zone-specific stack-ups and real-time DRC updates.
