Your next flexible PCB design may roll up like a newspaper
Whether you like it or not, flexible and rigid – flexible printed circuit boards will continue to exist, and electronic products that are gradually becoming more flexible are stepping out of the research phase and entering the industrial field. Flex PCBs are now not only used to power motors in magnetic hard drives. Any device containing foldable or translational components may contain dynamic flexible PCBs.
With the growth of the flexible PCB market reaching tens of billions, PCB designers can learn as much as possible from both. Static and dynamic flexible PCB design. Your company’s next product or redesign of existing products may depend on it.
Static and Dynamic Flex PCB
Any flexible PCB is similar to a rigid flexible PCB because they use the same material as the flexible layer. The flexible PCB is completely composed of flexible materials, rather than a rigid part of a polyimide core surrounded by copper and prepreg. Polyimide is commonly used because it is easy to adapt to rigid flexible manufacturing processes and relatively cheap, although polymer materials such as polyethylene naphthalate (PEN), polytetrafluoroethylene (PTFE) and aramid can also be used for flexible belts.
Flexible PCBs can be designed as dynamic or static PCBs. Designing any type of flexible PCB is a mechanical exercise, not an electric exercise. PCB design software with ECAD/MCAD collaboration function is very suitable for designing any type of flexible PCB. When planning for stacking and placing flexible PCB traces, the curved part of the board should be considered as a curved rectangular board during mechanical analysis. This is crucial for determining the correct wire thickness to prevent cracking and failure.
Whether you are designing static or dynamic flexible PCBs, thicker overall flexible layers require a larger bending radius. This reduces the amount of tensile and compressive stress along the bending concentration, while shaping the PCB into the desired angle. Placing a smaller bending radius in a thicker PCB can cause the covering layer to gather on the surface layer inside the bending. Then, this will apply greater compressive shear stress on the trace located within the neutral bending axis. If you want to reduce the overall thickness, you can use a flexible covering layer that does not require adhesive.
Static flexible PCB: manufacturing considerations
Manufacturing static flexible PCBs, which are usually bent to the required curvature radius and bending angle during assembly with compression forming tools. The function of this tool is similar to a vise and can use customized forming tools to simultaneously place multiple bends in a single flexible belt.
Static flexible PCBs typically exceed the yield point, which means they bend beyond the expected bending radius to ensure some plastic deformation occurs during the forming process. This can prevent the flexible PCB from loosening back to its original shape after being removed from the molding tool. When specifying the static bending radius and angle in a static PCB, it should actually be planned to have a safe edge on the trace thickness to prevent wire cracking and failure during the over forming process.
Static flexible ribbon for medical devices, as described in “Electronic Weekly”.
Intuition may indicate that the traces should be thicker to withstand the pressure required during the overmolding process, but intuition is not always correct. In short, the thicker the circuit, the less bending and no damage. Thicker flexible PCBs will require greater over forming to meet the expected bending radius and angle, and they will also require greater over forming. This will exert greater pressure on the marks during the forming process.
Just like a curved rectangular plate, there is a neutral bending axis on the entire curved plate, which defines a curve along which there are no longitudinal tensile or compressive stresses. Thin traces can withstand greater compressive stress than tensile stress, so finer traces can be placed within the neutral bending axis. The offset of the neutral bending axis will depend on the bending radius. A good rule of thumb for minimum bending radius is to use the following equation:
If the stacking and wiring thickness are properly selected, following this rule will ensure that the neutral bending axis does not significantly deviate from the centerline of the PCB. As the number of layers increases, this will ensure that you meet the bending ratio (bending radius divided by thickness) of the IPC 2223C standard in a flexible PCB.
Dynamic flexible PCB: durability
Many of the same design considerations for static flexible PCBs also apply to dynamic flexible PCBs. A key problem in dynamic flexible PCB is work hardening during repeated bending. Copper will harden under repeated cycles, eventually becoming brittle and prone to fracture. Durability can be extended by simply allowing for a larger bending radius. It is usually recommended that dynamic flexible PCBs not exceed a 90 ° bending angle.
When the PCB is bent, the neutral bending axis will move towards the inside of the bending. This is very important in dynamic flexible PCBs because it limits the allowed number of copper layers to a lower number, usually only a single layer that coincides with the neutral bending axis. Although copper has ductility, it hardens when subjected to repeated pressure. If you choose to use multiple layers in a flexible PCB, the traces should be staggered, that is, they should not overlap in adjacent layers to avoid excessive stress on traces farther from the neutral bending axis.
To prevent excessive pressure on the marks, it is important to leave sufficient safety limits and pay attention to the minimum bending radius. Ensure that the minimum bending radius is less than the expected bending radius to prevent stress at the edges of the plate. This will slow down work hardening and help extend the life of the PCB.
Static flexible tape hard drive
As flexible PCB design continues to make progress in more devices, designers need flexible PCB software to simplify stack design, production planning, and more. Altium Designer provides these important design tools, MCAD tools, and more in a single unified design interface. The intuitive design interface and rule-driven design engine can easily adapt to rigid, flexible, and fully flexible PCB designs.
