Impedance Control PCB

Impedance is the sum of the resistance and reactance of an electrical circuit expressed in Ohms. Resistance is the opposition to current flow present in all materials. Reactance is the opposition to current flow resulting from the effect of the inherent capacitance and inductance of the conductor interacting with changes in voltage and current. There is no reactance and the resistance of copper conductors is typically insignificant. However in high-speed AC circuits (those with sharp changes in voltage and/or current) the reactance and thus the impedance can become very significant. This can become critical to a design’s functionality because of the effects that changes in the impedance along the signal path from transmitter to receiver will have on the efficiency of power transfer as well as signal integrity. While a circuit’s speed is often expressed as the frequency of the waveform: the critical concern is the speed at which the voltage and/or current is required to change. This Gold Phoenix PCB Knowledge Center article explains the principles of impedance control in PCB design and presents a 4-layer FR-4 impedance-controlled board example with key stackup and impedance parameters.

Controlled Dielectric

The typical design considerations involved in the determination of the requirement for controlling the impedance are the strength of the signals involved, the susceptibility of the circuit to noise and signal distortion, the criticality of signal timing and the speed at which the signal’s source is attempting to force a change in voltage and/or current.

The design considerations for setting the value of the impedance for the conductor is typically the output impedance of the transmitter and the input impedance of the receiver. The impedance of other conductors (i.e. coaxial cables) in the circuit path will also need to be considered. The acceptable range (tolerance) for the impedance will need to be determined and taken into account during the design phase as well as when specifying the PCB parameters. In many cases merely by using software models to determine the anticipated impedance with particular dielectric materials and spacing, followed by requesting these parameters be followed when the PCBs are fabricated will suffice. This is what is commonly called “controlled dielectric”. For more critical applications you will need to specify “controlled impedance” and supply the actual impedance requirements for the conductors, and the dielectric layers and conductors can then be fine-tuned to meet these requirements. The impedance requirements will have to be specified based on the layer and the conductor widths.

4-Layer Controlled-Impedance PCB Example

General Description

An illustrative 4-layer impedance-controlled PCB can be built on an FR-4 substrate with Tg 150°C. The example uses a 1.6 mm board thickness and 1 oz copper on each layer, with impedance requirements defined for the controlled traces.

Example Parameters

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