Fabricate a Robust Flex Circuit
When fabricating a flexible circuit, it is important to pay special attention to the design of the component mounting and connectors. The resulting assemblies should be strong and durable to ensure that they can be easily flexed to accommodate the required positioning of the components in the assembly, yet also to allow for repeated bending. There are many considerations in a flex circuit design that need to be taken into account in the layout process to meet these requirements, including material type/availability, bend capability/flexibility and impact on standard level signal lines.
The flex circuit manufacturing process usually involves a combination of rigid and flex board fabrication techniques. In high volume production, the flex circuit manufacturer is cut to shape using a hydraulic punch and die set. This method provides the highest quality for high-speed flex circuits. Prototype and low-volume flex PCBs are often cut manually using a blanking knife. This method is labor intensive, and the resulting boards are of less quality than the high-volume production methods.
A flex circuit can be designed to withstand either static or dynamic flexing. Static flex circuits are only flexed to fit into an assembly, and will not flex more than 100 times over the life of the product. Dynamic flex circuits, on the other hand, are likely to be flexed many more times than this. For these designs, the flex circuit needs to be more rugged.
How to Fabricate a Robust Flex Circuit
One of the key factors in determining a flex circuit’s robustness is copper thickness. A flex circuit’s copper thickness must be sufficient to withstand the required current levels. However, this can conflict with the desire for high flexibility and bend capabilities in the design. The thicker cores necessary to achieve high current levels can substantially reduce a flex circuit’s bend capabilities.
Another key factor in a flex circuit’s robustness, is the location of the flex area in the assembly. It is important for the flex circuit designer to make sure that trace routing and pad placement are kept away from these areas, especially when the design is intended for a dynamic flex application.
The final element of a flex circuit that contributes to its robustness is the material and construction of the core. Depending on the flex circuit application, the core can be adhesive-less or adhesive-based. For a high-performance design with stripped line controlled impedance or radio frequency (RF) shielding, the flex core may be manufactured in house to ensure that it meets specific performance requirements.
In order to mitigate signal attenuation, a flex circuit manufacturer will typically add solder tabs and/or anchors on the component pads to prevent them from lifting during flexing. In addition, they will avoid placing any vias in the flex area. Vias placed in the flex area are more susceptible to peeling, and it is best to avoid placing them at all when possible. Moreover, it is important for the flex circuit manufacturer to ensure that all bend locations are properly aligned with their respective stiffeners and that all of the layers are correctly stacked and staggered.