Trace Length in PCB

  • Trace on a PCB
    • True transmission line
    • It has both significant inductance and capacitance per unit length.
    • Lumped inductance and capacitance are proportion to the length.
  • Contributing Parameters for Delay
    • Trace length. Usually, the rest of the parameters are preset.
    • Trace width, thickness, shape
    • Distance of trace from its reference plane
    • Dielectric constant of the board material and/or coating
  • Trace Length Effect on Rise/Fall Time
    • Impedance looking into the trace
      • Characteristic impedance Z0 (for an infinitely long trace)
      • Load of receiver (for an infinitesimally short trace), usually much larger than the Z0
    • For a finite length trace, the load that the driver sees will be somewhere between those two.
    • The amount of driver loading will affect the rise/fall time.