Electronic – Controlled impedance in presence of vias and through-hole components (PTHs)

characteristic-impedancecontrolled-impedanceimpedancelayoutpcb

We have some controlled impedance traces on layer 4 of a board. Layer 3 is a GND plane. Layer 5 is a 3.3V plane. Both planes are unbroken (they occupy the entire layer), with the exception of vias and holes.

There are a lot of holes on this PCB, because we have a lot of through-hole connectors. See the not-so-pretty picture below:

traces going through holes

The white circles are the holes in the PCB. My question is, how do all these holes affect the impedance of the traces? Is there a minimum distance that should be maintained from the holes to ensure that the impedance is within specified tolerances (100ohms +- %5-10 for differential lines for example)

Another somewhat similar question: Consider the picture below:

traces above respective planes

Let's assume that layer 3, the GND plane layer is now split into 2, one AGND and one DGND section. Do the traces running entirely on a single plane layer (like in the picture) maintain the controlled impedance value? Is there a limit to how close they can get to the edges of the planes before starting to show deviations from the target characteristic impedance?

Best Answer

If the height between the signal trace and the ground plane is h, a fair rule of thumb is to keep all potential perturbing features at least 3h away from your traces. If you can manage more separation, that's even better.

Also, if the trace length is less than 1/10 wavelength at your frequencies of interest, determined by the rise and fall times of your digital signals, remember that it probably doesn't matter much what you do. That's a trace length of 1.4 meters at 10 MHz or 14 cm at 100 MHz. If your sketch is showing through holes spaced at 0.1 inches, it looks like your board is less than 1 inch square and you could get away with well over 100 MHz signals without worrying excessively about controlled impedance and careful terminations.

Edit

This is not to say you should totally ignore good design practice and get rid of your ground plane or run traces across slots in the ground plane, as indicated in comments below. Also, the distance values above (1.4 m and 14 cm) are corrected from my initial answer.

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