Using Kirchhoff’s Law to Determine Sufficient PCB Trace Thickness

For as long as I can remember, I've been a fan of peanut butter. Back in elementary school, my mom limited my peanut butter intake to a thin spread on a slice of bread. This didn’t come close to satisfying my cravings so I’d sneakily add a generous inch of peanut butter on sandwiches without my mom’s knowledge. My peanut butter thickness preference, unfortunately, did not help with PCB trace thickness in earlier days of hardware design.

It wasn’t until a PCB trace dramatically broke off that I finally started to pay attention to trace thickness. Whether I’m looking at a particularly demanding routing array, or if I’m testing around to maximize my cost efficiency during a prototyping stage, being able to take care of a circuit board with voltage source, dielectric constant, impedance, resistance, and other voltage law into account is paramount.

How does circuit board design software help here? With smart rule checking and an intuitive layout program, you can ensure that signal integrity is maintained and traces are optimized to avoid those detrimental printed circuit conditions like voltage drop.

What is Kirchhoff’s Law?

Prototyping without concern for trace thickness can become a costly endeavor for you. To understand why trace thickness can make or break a PCB prototype, you need to revisit Kirchhoff’s Law. If you were paying attention in your college days and haven’t forgotten one of the fundamental laws of electronics, you may recall that Kirchhoff’s Law defines how energy is conserved in a closed electrical circuit.

Discussion on Kirchhoff’s first current law

There are two Kirchhoff’s Laws that form the fundamentals of circuit analysis. However, you should pay special attention to Kirchhoff’s Current Law, which states that the total amount of current entering a junction is equivalent to the total current leaving it.

Kirchhoff’s Law and Current Paths in a PCB

In PCB design, it’s understandable to be worried about electromagnetic interference or signal integrity issues. But sometimes, a seemingly simple problem, like a current overload on a trace, can doom your design. Current overloads can be even more perplexing if you’ve already accounted for all high current components.

This is where the Kirchhoff’s Current Law comes into play. The current calculation should not be limited to high current components and the traces immediately connected to them. Current travels in a complete loop and for a trace that consists of several branches, the total current equals the sum of the current of the branches.

For example, an individual trace for a single LED only has an average of 10 mA passing through it. Typically, there are no issues as a single LED isn’t considered a high current component. But when you have the returning path of 500 LEDs converging into one, the total current on the merged section is 5 A, which may cause the track to heat up, especially if the resistance of the track is relatively high.

How to Ensure a PCB Trace is Sufficient for Current Flow

To ensure that a PCB trace can safely withstand the amount of current that flows through, a few parameters need to be considered, namely the current flowing through, copper width, copper thickness, and the maximum allowable temperature. The correlations between these factors are specified by the IPC-2221 standards.

You can download these standards and start determining the adequate cross-sections of the copper trace from the graph. Alternatively, you can use the PCB trace width calculators available online or within the latest version of Altium Designer^®.

The general rule is having lower trace resistance for a higher amount of current flowing through. This is usually done by increasing the trace width. Of course, there are circumstances where increasing trace width is impossible due to size constraints. In such cases, you should consider using thicker traces to increase the copper cross-section.

Ensure PCB traces are sufficient to handle high current by increasing the cross-section.

Most PCB manufacturing suppliers provide a few choices of copper thickness. The standard copper thickness is 1 oz, and if you require thicker copper for your PCB, you’ll need to specify otherwise. Remember that Kirchhoff’s Law applies to the entire circuit loop. If you’re increasing the copper width for the incoming trace, ensure the outgoing trace is sufficient to handle the current.

By keeping copper trace thickness in mind in your initial prototyping, you can make it so that your small batch prototyping doesn’t turn into multiple runs of small batch prototyping. Furthermore, you can save yourself the headaches when you realize you’ve overlooked such a simple problem to fix. Using a PCB editor which truly understands the needs of a will ensure you can make the most out of the time you spend designing.

Altium has a PDN Analyzer^™ that allows you to analyze and detect current density issues before submitting your manufacturing files. With Altium’s high-performance, user-friendly solutions, you can ensure that your design adheres to Kirchhoff’s Law so your PCB traces are protected from the current flow. For more tips on current handling capacity for PCB traces, talk to an expert at Altium.