When Trace Resistance Trumps Trace Impedance
Unless you have a photographic memory, you probably don’t have every single design formula memorized. It’s understandable; looking throughout the compendium of knowledge in physics and engineering, there are far too many formulas for any one person to memorize. Thankfully, some folks have created useful calculators that help you save time when designing a new system.
The resistance and impedance of traces in your PCB have major impacts on power distribution and signal integrity throughout your device. Understanding the differences, limitations, and applicability of each calculator is important for preventing design mistakes that can be overlooked, even by experienced designers.
Resistance vs. Reactance vs. Impedance
Some folks only tend to pay attention to resistance when designing their power/ground planes and traces in their PCBs. This is appropriate in some cases, like in purely DC circuits or in very low frequency AC circuits. In other cases, where frequencies or switching speeds are higher, the reactance of a circuit becomes important.
A real PCB has some equivalent capacitance and inductance that depends on the arrangement of traces and power/ground planes. If you remember your Intro to Electronics courses, you should know that these elements produce reactance, which depends on the signal frequency. In a DC circuit, a capacitor acts like an open circuit once it charges, and an inductor doesn’t do anything, so you only need to worry about the resistance in closed circuits formed by your traces.
The resistance and reactance add together in quadrature to give you the impedance, and the impedance depends on the frequency and switching speed of signals in your circuits. Even if you are working with a high frequency or high speed device, the DC resistance of various elements on your PCB are still important and will affect the performance of your device.
Trace Resistance Calculators
Calculating the resistance of a trace is actually very easy when you assume that the temperature of your trace does not change during operation. This is actually a dubious assumption and is almost never realized in a real device without a complicated temperature regulation system.
Some trace resistance calculators don’t include temperature effects, giving you a baseline resistance value at a certain reference temperature. Those calculators that do account for temperature require that you set a specific operating temperature for your traces. This operating temperature represents a design goal in your device. If you are designing your PCB for a specific temperature, you’ll need to take steps to ensure that your PCB doesn’t exceed this temperature.
Time to put your PCB on ice
If your PCB will be operating in an environment below room temperatures, most calculators can actually return a negative value for the resistance of your traces, which is obviously incorrect. You’ll need to do more than just search the internet for a basic trace resistance calculator. Some application notes and research publications provide a very thorough view of trace resistance at very low temperatures.
The difference between an impedance calculator and a resistance calculator is that the trace impedance depends on the surrounding board material and trace arrangement with respect to nearby conductive planes or other traces. The board material also affects temperature rise, but calculating the actual temperature rise for a given power level is a much more involved problem.
Trace Resistance and Power Distribution
With all of this frequency dependence going on, why should the resistance in a DC circuit matter? We’re dealing with conductors, which already have small resistance. The resistance of a typical trace is about 0.03 Ohms, which is extremely small. This means that the current in some typical traces can be quite large, resulting in a large temperature change during operation.
The first reason the trace resistance is important has to do with IR losses in various elements throughout a PCB beyond just traces. Features like vias and solder points can have high resistance, and a significant amount of power can be lost as it is routed to critical components. These losses accumulate if your power lines are routed through a large number of vias or solder points, and downstream components might not receive the power they need to work properly.
As power is dissipated in various parts of your circuits, the temperature of these resistance elements increases, which then increases the resistance. Eventually, the thermal gradient between these resistive elements, the board substrate, and air causes heat to dissipate, and the temperature will saturate at its operating temperature.
The increased resistance at high temperature then affects power distribution, resulting in greater power loss as power is routed to downstream components. In order to ensure that the proper amount of power reaches downstream components, you’ll want to remove any unnecessary high resistance elements like vias from your power connections.
Heat sink on PCB component
Keeping signal integrity, power distribution, and the temperature of your board within specifications is much easier when you use powerful PCB design software with the best design and analysis tools. The rules-driven engine in Altium Designer 18.1 gives you all the tools you need to select the right trace geometry for your application. Talk to an Altium expert today if you want to learn more about how Altium Designer can help you reach your design goals.