I have been reading up on the EMI issues in Electromagnetic Compatibility
Engineering by Henry Ott. (wonderful book btw).
One of the topics "PCB Layout and Stackup" (aka Ch 16) there is section about ground fill (16.3.6).
Basically what it states, that to minimize the "return current path" filling the areas between connector pads with ground fill should be done. Quite understandable, however in the same section at the end it states "Although often used with analog circuits on double-sided boards, copper fill is not recommended for high-speed digital circuits, because it can cause impedance discontinuities, which can lead to possible functional problems.".
That last part confused me a bit, since I would expect that for high frequency signals (that try and follow the signal trace) a longer path would be decremental.
Can anyone explain why this remark is made?
Best Answer
Sure, lets take the common case of a microstrip. It's impedance is a combination of itself and it's return path (and the dielectric but lets keep it simple). In a microstrip's case this will be the reference plane underneath.
Now if you go and throw a piece of grounded copper right next to that microstrip, it's impedance is now a combination of itself, it's reference plane and that grounded copper next to it. You usually can't get a 100% symmetrical fill around the microstrip, because of vias, other lines or just going into a pin on a package. So in short anywhere you have this copper fill changing your impedance you are going to get discontinuities or changes in impedance.
For example in the image below there'd be a discontinuity for the main trace where the flood is interrupted by a via.
To be fair though there is a type of transmission line we sometimes use called a co-planar wave guide which essentially looks like a trace with two wide copper fills along it's sides (symmetrically along it's sides).