Impedance in Flexible Printed Circuits
In high-speed electronic design, controlled impedance is an absolute requirement. But how do you achieve it in flexible printed circuits? This blog article delves into the intricacies of achieving controlled impedance in both flex and rigid-flex PCBs, examining the factors that influence it and how it can be achieved in various configurations without impacting flexibility or cost.
Impedance control is essential for high-speed flex and rigid-flexible printed circuit designs. This is because signal traces have an impedance that varies from point to point as the signals propagate along them. If the characteristics of the trace vary, this can cause reflected signals to interfere with the original signal, which can lead to slower rise times and electromagnetic interference (EMI).
For a flex or rigid-flex PCB to function correctly at high frequencies, its characteristic impedance must be consistent across the entire board. This means that the trace width, dielectric thickness, and bending of the board must be carefully controlled to ensure that the signal has the same impedance at every point on its path.
Controlled Impedance in Flexible Printed Circuits
Achieving controlled impedance is a complex task, but there are a few strategies that can be used to help. One method is to use a PCB material with a specified dielectric constant (Dk) and flex core thickness. This can improve impedance matching by ensuring that the signal layer has the same Dk as the reference plane, reducing the amount of signal reflection.
Another strategy is to use a flex PCB with a specified trace width and spacing. This can also improve impedance matching by ensuring that all of the traces have a similar length. This can reduce timing skew issues that occur when traces are longer or shorter than other traces, which can have a significant effect on signal integrity.
Other options for improving impedance match include using a symmetrical placement of capacitors between related traces. This can help maintain consistent capacitive loading and equal signal propagation, which also contributes to better impedance matching. Finally, a PCB manufacturer can also use special routing techniques to ensure that all of the signal traces on the board have the same impedance.
While a flex PCB fabricator can usually achieve impedance match with these methods, it can be difficult to get the exact impedance tolerances that are required for high-speed designs. This can require multiple rounds of questioning between the PCB designer and the fab to determine the best way to achieve the desired impedance tolerance.
To save time and ensure that the final flex or rigid-flex PCB meets the design requirements, it is important to carefully specify all of the impedance control requirements. PCB designers can do this by including the impedance target, copper weight, and layer thickness in the documentation they send to their fabricators. This helps to avoid wasting valuable project time on multiple rounds of back and forth questions between the PCB designer and the fabricator. This information can also be included in a chart to make it easier for the fabricator to understand the specifications that have been provided.
