proper grounding in rigid flex
Rigid flex is used in applications that require the best of both worlds: flexibility and rigidity. Its unique combination of structural integrity and mechanical stability makes it a popular choice for high-speed, high-density circuitry that needs to withstand harsh conditions. But like any PCB, rigid flex can be subjected to stresses and deformations that may lead to failure if not adequately addressed during the design process.
One such issue is proper grounding in rigid flex. This requires careful consideration of the copper trace thickness, stress distribution in bending locations, and pad and hole placement. Inadequate consideration can lead to signal integrity issues, conductive fracture of copper conductors, and solder joint reliability concerns. To address these challenges, it is crucial to incorporate stiffeners into your PCB design. These supplementary stiffeners will add resistance in areas of your circuit that are susceptible to the most severe stresses during fabrication and assembly.
When it comes to plated holes or vias, placing them close to the flex transition area can apply mechanical stress that could tear or damage the structure of a flex circuit. To avoid this, it is a good idea to keep pads and vias away from the flex region and place them in areas that do not experience bending or movement, such as the rigid section of the board.
How do you ensure proper grounding in rigid flex?
Likewise, it is recommended that you stagger your traces instead of stacking them on top of each other. Staggering will help to distribute the mechanical stress more evenly and prevent copper trace thinning around the bend radius. This will also improve your circuit’s overall durability and longevity.
The use of hatched ground planes will also help to improve impedance control and increase the flexibility of your flex circuit. However, these types of ground planes are not as effective for high-speed routing because they can produce interference due to their mesh geometry. Regardless of whether you choose to use solid or hatched copper, it is vital that the design rules in your PCB software set the correct impedance values for each.
Inadequate hole-to-flex spacing can also result in stress concentration, which could aggravate the mechanical stress on the traces or cause them to break. This is especially critical for the holes located in the flex-to-rigid transition region. To prevent this, it is recommended to use a gap of 50 mil or more between the hole and the flex-to-rigid interface.
In addition, the gap should be filled with a rigid dielectric material such as FR-4 to ensure that the gap does not become an electrical short. This will prevent EMI from entering the rigid-flex circuit from adjacent layers and potentially causing interference with signal propagation. In some cases, it may be necessary to add stiffeners in these regions as well. This will be dependent on the overall mechanical design of your rigid-flex circuit and the manufacturing capabilities of your supplier.
