Is trace impedance/resistance related to its length ?

Discussion in 'General Electronics Chat' started by shengwuei, Oct 20, 2011.

  1. shengwuei

    Thread Starter Member

    Aug 22, 2008

    There are a lot of PCB trance impedance calculation tool available on internet, for example :

    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.
  2. crutschow


    Mar 14, 2008
    1. Yes, that calculates the "characteristic impedance" of the trace, used for high frequency design where the trace is viewed as a transmission line.

    2. The maximum length of the trace versus clock speed is more determined by the clock rise and fall times rather than the frequency. You generally don't have to worry until the propagation time of the trace length approaches 1/6th of the rise/fall times. Above that you can start having transmission line effects (significant overshoot, ringing, etc.) of the waveform.

    Here's a trace resistance calculator.
  3. gootee

    Senior Member

    Apr 24, 2007
    You might also want to worry about the inductance and capacitance of traces.