When Trace Resistance Trumps Trace Impedance
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Trace resistance, trace impedance, plane resistance, and plane impedace: when do you need to worry about all of these quantities?
Resistance and impedance both are important in small traces and fat rails on a PCB, both internally and externally. Sometimes a new designer entering the world of impedance controlled buses might think they have a trace resistance target, when really they have a trace impedance target. While in power electronics, we care much more about resistance due to power loss and Joule heating in conductors on the PCB.
Resistance vs. Reactance vs. Impedance
If you're a new designer with a bit of engineering background, then you should know the difference between trace resistance, reactance, and the total impednace. Impedance is a complex quantity that includes the trace resistance and reactance.
For a trace on a PCB, the trace can be considered a reactive element that has some DC resistance. In some cases, we only care about the resistance, such as in DC power electronics. In other cases, we care much more about the impedance, such as in RF or digital desings.
Trace resistance matters in DC power delivery. When a trace is carrying a DC current, then the resistance will determine how much power could be lost along that current path. The DC resistance is determined by the trace's conductivity and the cross-sectional area. While every trace has an impedance, we don't care about the trace reactance if the trace is only carrying DC current.
DC power being carried by a trace determines the temperature rise in the trace, which should be limited in general. To overcome excessive heating, larger traces or rails are used when currents are larger. There are two industry standards used to calculate trace cross-sections required to keep temperature rise limited:
- IPC-2221 - A general-purpose design standard that gives conservative estimates of trace resistance. Access our free IPC-2221 calculator.
- IPC-2152 - A more accurate standard that incorporates planes into models for trace heating. Access our free IPC-2152 calculator.
Some CAD tools, including Altium Designer, incorporate an IPC-2221 calculator into routing tools, so you can see a conservative estimate of the current limit given a specified 20 °C temperature rise above ambient. The video snippet below shows where this is accessible inside Altium's routing tool.
As I mentioned above, all traces have an impedance value, it's just that the trace impedance does not matter if the trace is only carrying DC current. When the trace is carrying an AC signal or a switching digital signal, then the trace impedance becomes important. The reason for this is that the AC signal interacts with the reactive elements along the path of the trace. These reactive elements are:
- Trace inductance that includes the current path through the ground net
- Trace capacitance with respect to the ground net
Together, these values define the transmission line impedance, which we design to a specific target (usually 50 Ohms for a single-ended signal). Note that only interfaces with a specific impedance value require you to design to a trace impedance value; for other interfaces, designing to a specific trace impedance is optional. For most high-speed signals, you are actually working with differential pairs, so you are designing to an odd-mode/differential impedance specification.
The impedance of a trace is calculated based on the placement of plane and signal layers in the PCB stackup. Make sure you understand the factors that influence transmission line impedance, and if needed, use a calculator to get an initial estimate of impedance before programming the required trace width as a design rule. The video snippet below shows where to access Altium's impedance calculator inside the Layer Stack Manager so that you can quickly create an impedance profile for any of your signal layers.
Because impedance matters for AC circuits, and resistance matters for power distribution, what happens when we have a high-speed digital design that must pull AC power into a PDN? This is the main consideration of designing PCBs to have good power integrity.
Resistance and Impedance Meet in High-Speed PCB Design
In high-speed PCB design, the goal in system design and power routing in the PCB is to ensure stable power reaches fast digital components. We also want to ensure that there is minimal resistive power loss along the current path in the PCB. High-speed digital systems, where components are pulling power at edge rates reaching below 1 ns, can have very high average currents moving through the PDN. Larger boards that include a processor, multiple ASICs, and daughtercards/modules an have currents reaching dozens of Amps. Once you look at backplanes with a full load of daughtercards, the board might need to support ~100 A!
In these systems we care about both the resistance and total impedance of the PDN. In particular, we want to satisfy the following two design goals:
- Low rail resistance to ensure minimal power loss
- High capacitance back to ground to maintain stable power at midrange frequencies (~100 MHz)
- Low inductance to prevent high impedance up to high frequencies
To address both design goals, we use large rails or planes in a high-speed PCB. As currents and edge rates scale up, so does the size of the rails. It is common to also see multiple rails in these systems operating at multiple voltages, or providing power through their own regulators at the same voltage. The image below shows an example with multiple power rails on the back layer of a large PCB providing different voltages to a large FPGA.
To make the resistance side of things simple, designers just need to focus on using large rails for each voltage level required in a component. If you're building something like a backplane, where the design needs to supply many dozens of Amps, then multiple power plane layers would be needed. Make sure you understand how to set a current limit for a single plane layer, and then add in additional power/ground plane pairs to provide the required power.
Whenever you need to determine trace resistance or trace impedance, use the complete set of CAD tools in Altium Designer®. When you need to access advanced DC power integrity simulation capabilities for your board, you can use the new Power Analyzer extension, powered by Keysight. When you’ve finished your design, and you want to release files to your manufacturer, the Altium 365™ platform makes it easy to collaborate and share your projects.
We have only scratched the surface of what’s possible with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.