SI is often a deciding factor for proper functionality and is one of the main concerns in today’s high-speed designs. At the speeds where the electrical delay exceeds or becomes a significant portion of the signal edge-times, interconnects must be considered as transmission lines with distributed properties, similar as in microwave circuits. The transmission lines must be designed such that they minimize distortion of the signals at the receivers, which mostly come from:
- Shifts in reference-levels (power and ground bounce)
- Switching noise due to currents across impedance of the PCB power distribution system
- Impedance-discontinuity due to non-ideal current paths
Therefore, the characteristic impedance, impedance matching, propagation delay and timing, crosstalk and current path are some of the areas of design consideration. Selection of PCB material, board stackup and assignment of the layers, as well as power decoupling and bypassing are the areas of additional concern. Often used rules of thumb may not work anymore, and the fundamentals of the high-speed EM propagation must be understood and used in design.