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Beyond Design: Interconnect Impedance

Arguably, the most critical factor in high-speed PCB design is the impedance of the interconnect. We know that transmission line drivers must be matched to the impedance of the line for the perfect transfer of energy. Energy is never lost but rather transforms into other forms of energy. Specifically, in the case of an unmatched transmission line, energy can be transferred into heat, coupled into adjacent elements, reflected, or radiated. In this month’s column, I will look at why interconnect impedance is so important to the correct performance of the system.

Impedance is an extension of the definition of resistance to alternating currents (AC). Impedance includes both resistance (the opposition of the electric current) and reactance (the measure of opposition as the current alternates). Reactance also includes the effects that vary with frequency due to distributed parasitic inductance and capacitance of the transmission line.

Impedance is at the core of the methodology that is used to solve signal integrity issues:

1. Signal quality issues arise because voltage signals reflect and are distorted whenever the impedance changes along a transmission line.

2. Crosstalk arises from the coupling of electric and magnetic fields between adjacent traces or coupling between traces and return paths. The inductance and capacitance between the traces establish an impedance, which determines the amount of coupling.

3. Differential mode propagation can be converted to common mode by parasitic capacitance or any imbalance caused by impedance variation, signal skew, rise/fall time mismatch, or asymmetry in the channel. Common mode currents are the main source of electromagnetic radiation.

Not only are the problems associated with the signal integrity best described by the use of impedance, but the solutions and design methodology for good signal integrity are also based on the use of impedance. The two key processes—modeling and simulation—are based on converting electrical properties into an impedance and then analyzing the impact of that impedance on the signals.

The iCD Stackup Planner in Figure 1 illustrates the three most common transmission line structures of a multilayer PCB. For embedded microstrip (solder mask coated microstrip), the electromagnetic field propagates partially in the dielectric material, solder mask, and air. Whereas in both stripline structures, the electromagnetic field propagates in the dielectric material sandwiched between the planes.

To read this entire column, which appeared in the January 2020 issue of Design007 Magazine, click here.