PCB Impedance Control

Key Takeaways

• Trace width and thickness are prominent in creating impedance to the flow of current.

• Through PCB impedance control, the impedance offered by the trace to an alternating current signal can be predetermined.

• The PCB trace impedance increases as the width decreases. 

Traditional measurements of PCB impedance use a Smith chart 
 

Trace impedance is detrimental to high-frequency signals, especially when the frequency is above 100MHz. The trace impedance or PCB impedance damages the integrity of both analog and digital signals. PCB impedance control is an important design constraint when working on high-frequency circuits. By controlling the PCB impedance, unexpected damages or errors can be limited to some extent. Let’s discuss the need for impedance control and various factors influencing PCB impedance control in this article.

What Is PCB Impedance?

As electrical engineers, we are familiar with the term impedance, which is the ratio of alternating voltage to alternating current. Impedance can be resistive, inductive, or capacitive, depending on the nature of the circuit. The unit of impedance is Ohm.

In PCBs, traces are routed to transmit signals between two points. These copper traces are sources of impedance in PCBs. The trace width and thickness are prominent in creating impedance to the flow of current. Other factors that induce trace impedance are:

•  Vias
•  The thickness of the prepreg material on the two sides of the trace
•  The dielectric constant of the core and prepreg material 
•  Separation from the reference copper plane
•  The presence of solder resist

The Importance of PCB Impedance Control

We are looking forward to achieving 100% signal integrity in PCBs. The presence of trace impedance limits this objective by reflecting and attenuating signals. There is no signal deterioration if the signal passes from the trace of a given impedance to another of the same impedance. But this is rarely true unless we are deliberately designing this to be so.

It is hard to find a trace with the same characteristic impedance throughout its length. If a signal is passed through such a trace, the signal will be the same as the original with zero loss or deterioration. Such traces or transmission lines are called uniform transmission lines or traces. However, in most cases, signals encounter reflection, attenuation, and distortion due to the variations of impedance in the traces.

To maintain good signal integrity, conscious efforts need to be taken to control the PCB trace impedance. The traces should be designed with uniform characteristic impedance for achieving signal integrity, which leads to PCB impedance control.

PCB impedance control is the method through which reasonably uniform impedance is achieved in PCB traces by taking into account manufacturing aspects. The manufacturing parameters include the width, length, thickness, etc. of the copper traces. The impedance achieved through this method is called controlled impedance. Through PCB impedance control, we predetermine the impedance offered by the trace to an alternating current signal.

Is PCB Impedance Control Inevitable?

Generally, PCB impedance control is preferred in circuits handling high-frequency signals. In high-frequency signals, the clarity and integrity of the signal are maintained by controlling the trace impedances. As the frequency increases or the length of the trace increases, PCB impedance control becomes mandatory to protect the signal from degradation.

PCBs incorporated in the following products are always designed with impedance control:

• Computers
• Mobile phones and tablets
• Analog and digital communication circuits (RF circuits)
• Signal processors
• Digital cameras
• Motor control modules   

Types of PCB Traces and PCB Impedance Control

In PCBs, impedance plays a significant role in defining the trace parameters and selecting the PCB substrate material, especially when handling high-frequency signals. We know that traces routed in the PCB are of different types. Striplines and microstrip lines are the most common types of traces seen in high-speed and high-frequency circuits. The controlled impedance method applies to any trace that handles high-speed or high-frequency signals. The types of traces where impedance control is applied are:

1. Single-ended and differential pairs of microstrip lines

2. Single-ended and differential pairs of strip lines

3. Single-ended and differential pairs of coplanar waveguides

4. Embedded microstrip lines

Factors Influencing PCB Impedance 

The following table explains how various factors influence the impedance of traces.