Control in Flexible PCB
When circuits are not impedance matched, energy will be reflected and can interfere with signal transmission. This distortion can bleed over into neighboring circuits and cause other problems, such as electromagnetic interference (EMI). The goal of impedance control is to maintain specific impedance values throughout the board to ensure accurate signal flow. Impedance control is essential for high-powered digital and RF designs where large amounts of energy are transferred through the PCB.
Impedance control is a challenge in flexible pcb because of the layer structure of flex circuits. Unlike rigid PCBs, which have a single copper plane that can be used to carry signals, flex circuits have multiple layers of conductive material. This makes it hard to ensure that each layer has the right impedance. However, careful design can overcome this problem.
The most important step in achieving impedance control is to use the correct dielectric materials. Different insulating layers have different dielectric constants, or Dk. The lower the Dk, the better for impedance control. For example, FR-4 has a low Dk of about 3. This means that signals pass through the insulating layer with little energy loss. In contrast, other materials with higher Dk values, such as polyimide and PTFE, have higher dielectric losses and are not as good for controlled impedance.
Another way to improve impedance control is to keep the thickness of insulating layers to a minimum. Thicker insulating layers lead to larger differences in impedance. The resulting differences can disrupt signal flow and cause jitter, which is undesirable. To minimize the effects of varying layer thicknesses, it is best to stick with the recommended thicknesses for each layer in the stackup.
Impedance Control in Flexible PCB
It is also important to keep the trace widths consistent across all layers of the PCB. This helps to reduce timing skew problems caused by traces with different lengths from the same source. In addition, it is a good idea to place capacitors symmetrically between related traces to avoid impedance discontinuities that can interfere with signal integrity.
Lastly, it is essential to make sure the PCB manufacturer understands exactly which layers are meant to be controlled. If a specification document calls for a different trace width on each layer of the PCB, this will cause confusion with the fabricator and may lead to delays in delivery.
To make sure the PCB manufacturing process is achieving the desired impedance, the designer should provide detailed notes to the fabrication engineer. The notes should specify the desired layer-by-layer impedance target, track widths, and spacing. In addition, the note should explain why certain requirements are more critical than others.
In order to ensure the PCB will be able to perform as designed, the flex PCB must be thoroughly tested. This testing will help to catch any errors or flaws that could impact performance. This is particularly important for high-speed applications where any irregularities can be fatal. This testing can be done with test coupons, which are small boards that are manufactured on the same panel as the final product and subjected to the same manufacturing conditions.