Impedance Control

What is the impedance of PCBs?

Impedance is the sum of resistance and reactance of a circuit when operating at high frequencies, and measured in Ohms. Impedance is an alternating current (AC) characteristic, meaning that it is frequency-dependent. The longer the trace, or the higher the frequency, the greater the need to control the impedance of the trace.     

Unless you carefully design the circuits and its surroundings, the impedance is usually uncontrolled, which means that the impedance varies from point to point along the trace. The function of the PCB trace is to transmit the signal power from the driver device to the receiving device. Power needs to propagate over the entire length of the trace. However, maximum signal power can only be transferred when the impedance is matched with the PCB.

The importance of impedance control

A well-controlled impedance means that the trace impedance is constant at every point on the PCB path, which indicates that wherever the trace goes, even if the trace changes layers, the impedance of the entire part should be the same from source to destination. There is no reflection or attenuation of the signal during the transmission.

Impedance control of the circuit is vital for signal integrity when high-frequency signals transport on the PCB transmission lines, and can ensure the propagation of signals without distortion. Impedance control technologies are significant in high-speed digital circuit design, and effective methods must be adopted to make sure the excellent performance of high-speed PCBs.

The factors determine the impedance of the PCB

The characteristic impedance of a PCB trace usually depends on its inductance and capacitance reactance, resistance, and conductance. The impedance of the PCB trace ranges from 25 to 125 Ohms; the impedance value generated by the PCB structure will be determined by the following factors.

l The width and the thickness of the copper signal trace at the top and the bottom of the board

 The impedance trace width is inversely proportional to the impedance value. The larger the trace is, the smaller the impedance value is. And for the outer layer, the difference in copper thickness will affect the impedance within 2 Ohms, and the difference in etching line width caused by the difference in copper thickness has a high impact on the impedance consistency, the thicker the copper is, the smaller the impedance value is.

l The thickness of the core or prepreg material on each side of the copper trace

The characteristic impedance is proportional to the natural logarithm of the thickness of the prepreg (insulating) material. The larger the thickness of the insulating material becomes, the larger the impedance will be.

l The dielectric thickness and the dielectric constant(DK)

The dielectric thickness is the thickness between the shielding layer and the impedance trace. The thicker the dielectric is, the larger the impedance value is. And the larger the dielectric constant is, the smaller the impedance value is.

l Distance of signal layer and potential

Usually, the larger the distance becomes, the larger the impedance will be.

Impedance control has been needed for the PCBs used in fast digital applications: telecommunications, signal processing, high-quality analog video, RF communication, and household appliances such as phones, radar, video games, and satellite TV, etc. As the use of high-speed equipment continues to increase, PCB designers need to consider the factors that may affect PCB performance. One of these is impedance control, which is important for signal integrity and board operation. Experienced engineers in NextPCB will help customers to achieve better impedance control by proper multilayer PCB stack-up design and PCB laminate materials.

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