PCB Trace length vs Impedance

Question

There are a lot of PCB trance impedance calculation tool available on internet, for example : http://www.eeweb.com/toolbox/microstrip-impedance

Trace impedance can be calculated by entering parameters "Trace Thickness", "Substrate Height", "Trace Width",and "Substrate Dielectric", but not the length of the trace.

My questions :

1. Does the above calculation calculate "characteristic impedance", not the real trace resistance ?
2. Do I need to concern trace resistance during PCB design ? For example, is there any performance concern to send a 100MHz clock through a trace as long as 10cm, compare to only 1cm ? If so, is there any calculation tool to for the trace resistance ?

Thanks a lot.

Answer 1

What Neil said is true, that said, you do get RF losses in PCB materials, but they stem from different mechanisms and vary in importance with the frequency of operation. For a 100MHz signal running a few centimeters, the most likely loss is actually in the dielectric material (the run-of-the mill fiberglass-epoxy material often used acts as a somewhat lossy capacitor.

Whether it's important depends on the nature of the signal and what your operating margins are. If it's a 100MHz sine wave you only have to deal with effects at 100MHz, but if you are working with a digital signal with fast rise and fall times (say to prevent metastable states, etc.) then you have to look to see how the board materials behave at much higher frequencies.

From your example the following is likely not applicable but just in case; with trace resistance effects at high frequencies you're looking at skin effect as well as surface roughness, but those really only start coming into play at GHz frequencies and then only with certain designs such as millimeter wave LNAs. Surface roughness depends on the substrate and at those frequencies you are starting to get into pretty exotic stuff, like alumina, quartz to lower the dielectric losses and reduce trace lengths, right down to having entire RF systems on a silicon substrate.

Answer 2

Trace impedance and trace resistance are different things, important in different situations.

For RF work, and for high speed digital, the characteristic impedance of the trace is important, as it needs to be driven and terminated in a way that minimises reflections. This characterstic impedance is independent of length and trace material, depends on substrate thickness and trace width, and is usually in the 50 to 100 ohm range. This is the ratio of voltage to current as a wave propagates down the line.

When you are distributing power, DC and low frequency, the trace resistance becomes important. This is proportional to length, trace material, thickness and width, but not to substrate thickness. It is usually in the milli-ohms range. This is the ratio of the voltage dropped over the length of the line to the current flowing through it.