So far, what we’ve presented about electrical signals and routing between components focuses on a specific type of signal and trace arrangement. In particular, we only looked at an individual trace that gets routed on its own; the only other important element needed to define the voltage carried by a signal is the ground plane. Together the trace geometry and the ground plane define a very important electrical quantity of PCB traces: the impedance seen by a traveling signal.

In this lesson, we’ll explore the various factors that affect the impedance of a PCB trace. Once you’ve determined the impedance you need for your traces, we can apply the impedance you’ve determined as a design rule. Once you’ve set up these design rules, you can start routing some of the more complicated computing interfaces on a PCB, such as USB or HDMI. We’ll also look at setting up design rules for length matching in differential pairs, and how to apply length matching in a PCB layout.

Trace Geometry and Impedance

In Lesson 1 of this unit, we briefly looked at the concept of impedance in PCB traces. Single-ended traces have a characteristic impedance that is used in signaling standards, and it needs to be calculated to ensure signal integrity at the receiving end of the trace. When designing a PCB that requires specific impedance on some traces, there are some important factors that determine characteristic impedance for traces in your PCB. 

A specific distance from a trace to a ground plane will require a specific trace width in order for the trace to have a target impedance. This is why you want to set a trace width only after you decide on a stackup. If you go back and change the layer thickness in the PCB stackup, you may need to change some trace widths in order to make sure you hit your impedance targets on certain nets.

The geometric variables needed to hit a specific characteristic impedance or differential impedance depend on the stackup. The main parameter is the thickness of the dielectric.

Microstrips are shown above, but the same idea applies to striplines. Now, we want to look at how you can actually calculate the trace width you need to hit a specific characteristic impedance value (for single-ended traces), or the width and spacing you need to hit a specific differential impedance (for differential pairs).