Rigid Flex Circuits
Rigid-flex circuits are ideal for dynamic applications, such as wearable electronics, automotive infotainment systems, medical imaging equipment, or industrial control panels. The ability to incorporate both rigid and flexible sections in a single design significantly reduces package size, assembly time, and weight. In addition, the reduction in interconnections also improves signal integrity and decreases noise and electromagnetic interference (EMI).
Mechanical stability is one of the most important aspects to consider when designing a rigid flex circuits PCB. If you are not careful, the board may experience damage or failure during use. The good news is that, with the right tools and techniques, it is possible to create a robust and reliable rigid-flex circuit.
To achieve mechanical stability, it is necessary to ensure proper material selection and placement as well as an optimal layer stack. Staggering traces and eliminating copper overlaps will also help to mitigate mechanical stress during bending. Additionally, it is critical to avoid placing pads, holes, and vias near bending areas, as these can apply mechanical stresses that can weaken or tear the structure of plated holes. To prevent this, you should use a symmetrical and even material layup, and place your flex components in the center of the board. You should also keep your flex-to-rigid interface as close to the rigid section as possible, and keep your vias at least 20 mils away from the rigid-to-flex interface.
How to Achieve Mechanical Stability in Rigid Flex Circuits
The thickness of the rigid-flex substrate is also a major factor in achieving mechanical stability. For this reason, it is critical to choose a high-quality material that will provide adequate stiffness, flexibility, and conductivity. In particular, FR4 and polyimide films should be used, as they offer the best combination of strength and conductivity for rigid-flex PCBs. These materials are also low-cost and have excellent thermal, chemical, and mechanical properties.
Aside from the thickness of the substrate, another important consideration is its dielectric material. Choosing the right dielectric can affect the electrical performance of your rigid-flex PCB, so be sure to select an appropriate material that will meet your application’s requirements.
For instance, if you are using a dynamic rigid-flex PCB that will be subjected to repetitive bending and torsional stresses, you should choose a material with low dielectric constants and a high-frequency cutoff frequency. This will ensure that the flex-to-rigid interface is matched to the specific flex-to-rigid transition point.
In addition, it is recommended to choose a conductive material with low dissipation loss. Alternatively, you can choose to use a non-conductive material such as a non-woven fiberglass or a polyimide film. Lastly, it is important to consider the abrasion resistance of the rigid-flex substrate when selecting your finish. A good finish will protect the surface of the PCB from scratches and other contaminants, while still allowing you to solder components onto it. A topcoat with an anti-corrosion coating is a great option for this purpose.
