Editor's Note: This column originally appeared in Inside Design, our weekly design newsletter.

In this column I will continue my discussion on coplanar structures and unique reference plane scenarios as they relate to PCB fabrication. In the first part, I will talk about coplanar type structures and the effect of unintentional coplanar coupling on impedances.

In the second part, I will talk about the use of unique reference plane scenarios for multiple impedances--what benefits they may have versus any drawbacks.

What is a "coplanar" structure anyway?

Coplanarity, as it relates to the signal on a PCB, exists when a portion of a metal plane exists on the same layer as the impedance signal and induces some specific amount of coupling due to the proximity of the adjacent plane/copper pour.

Often in conjunction with ref planes on adjacent layers, these structures offer mechanical and heat sink capabilities greater than those in "free space" or, in the case of the PCB, unused non-metal areas.

On the other hand, multiple sequential coplanar structures increase the possibility of crosstalk and other unwanted resonance effects, resulting in reduced transmission line performance.

One example of unintentional coplanar coupling is when a design is near completion and all details about the outer layer impedances have been worked out. At the last moment, the engineer asks for more thermal dispersion or heat sink capabilities and asks the layout person to perform a copper pour on the surface layers, to create better thermal capacity and minimize emissions. 

Having done so, the fabricator now attempts to model the impedances based on coplanar structures. In some cases, if the impedances are not revisited after final layout prior to release to the potential fabricator to within 10% of the original intended impedance, you may receive a call from your fabricator regarding subtle re-line sizing to get closer to the intended impedances.