Ensure Proper Grounding in a Flex Circuit PCB
The complexities of designing and manufacturing flex circuit PCBs are many. From layer stack design and parts placement to cutouts, vias, and copper thicknesses, there are a number of material quirks that impact flex performance and reliability.
The high-speed signal integrity that a flex circuit depends on depends on the proper function of all of these elements. Using simulation tools early in the design process can help to mitigate these issues and prevent costly and time-consuming physical prototyping iterations.
To ensure proper grounding in a flex circuit pcb, all of the traces should be routed to a common ground plane. This helps to reduce crosstalk between different layers of the circuit and external EMI interference from other components in the system. Ideally, the ground plane should have low impedance, but this isn’t always possible.
In some cases, it may be necessary to add a separate grounding channel for specific components that generate large amounts of EMI. This can be done by adding a shielded connector to the package or by routing all of the signal and power signals to a single grounding point on the flex circuit.
Another important consideration for grounding in a flex circuit is the sensitivity of the copper to bending. Flex circuit materials have lower tolerances than rigid boards, which can cause stresses on copper traces that can lead to breakage. To avoid this, it is important to use a larger diameter hole for all of the pad and lands. Ideally, the diameter of the pads and lands should be twice as large as their hole size.
How to Ensure Proper Grounding in a Flex Circuit PCB
To help avoid stress, it is also a good idea to stagger traces across the board instead of running them all in parallel. This minimizes the stress points that can cause traces to break during bending. It is also important to avoid discontinuities, like vias, slits, and holes in the area of the bend, as they can cause stress cracks in the copper.
Lastly, it is crucial to center the neutral bend axis as close to the center of the layers of the circuit as possible. This helps to minimize the distances that are needed for the circuit to stretch and maintain its structural integrity.
A good way to do this is by using balanced construction and by making sure that all of the layers have the same thickness. This will help to keep costs down and also improve the quality of the finished product.
It is also essential to use a solid ground plane rather than a mesh grounding plane. Mesh grounding can create radiated EMI from its open cavities, which can interfere with the traces on a flex circuit and introduce interference into the system.
Finally, it is important to consider the impact of varying copper sizes on impedance and signal integrity. A flex circuit with a consistent copper thickness, width, and spacing will provide a better overall function. One way to achieve this is through button plating, where the copper is only deposited on the drilled pads and lands. This can improve etch yields and allow for controlled impedance at higher speeds.
