Using an IPC-2152 Calculator: Designing to Standards
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If you look at modern EDA programs, there are many calculators and simulators built into these applications. But one of the areas of simulation that has lagged behind everything else is thermal simulations. Thermal calculations are important particularly in power electronics, and in high-reliability electronics, even if these systems are running lower overall power. There are other instances where we can determine the potential need for an estimate of trace heating given the current supplied to a trace.
The industry has stepped up in a long-standing effort to develop standards that cover best practices for thermal management. The results have been a bit underwhelming, leading to a set of empirical formulas defined in IPC-2152 and IPC-2221. These formulas can be used to estimate the relationship between current in a trace, the trace width, and the expected temperature rise above ambient, assuming the design is on an FR4-grade substrate.
First some background: if you are familiar with the evolution of IPC standards, you may remember that the original trace design standards were based on 50-year-old experimental results on polyimide boards. The relevant FR4 material parameters are only different from polyimide by about 2%, so the IPC-2152 standards are equally applicable to PCBs on FR4. The relationship between temperature rise in a PCB, the current in the traces, and the cross-sectional area of the traces were summarized in a series of charts and an empirical formula in the IPC-2221B standard.
Since 2009, the IPC-2152 standard became the prominent standard for sizing conductors on a PCB. While the standard is important for thermal management in PCBs, it wasn’t until recently that everyone agreed on the correct formula to use to size traces. Given the number of layout possibilities in any given PCB, the original charts in IPC-2221B are known to not be applicable to every design. The new IPC-2152 standard presents results that summarize the relationship between the following quantities:
- Thermal conductivity
- PCB thickness
- Distance to a nearby plane and the plane area
- Copper weight
- Trace width
- Expected or required temperature rise above ambient
- Internal vs. external traces
The results where summarized in a set of charts for internal and external traces, but there is no explicit formula that can be used to calculate the expected temperature rise in a PCB trace. However, it is possible to pick data from the chart and develop a mixed power law model; this was done by the folks at SMPS.us, and I've reproduced their inteprolation formula below. The resulting master formula is used to calculate the cross-sectional area of the trace for a desired temperature rise above ambient (∆T) and current (I):
This formula is implemented in the following calculator application for determining PCB trace widths.
The application below provides a simple way to calculate the required trace width (in mil) for a given input current and temperature. Simply enter your required temperature rise limits and operating current (RMS). In the case where there is a plane present, a correction factor is applied to determine the required copper area and width.
IPC-2152 calculators are typically only valid when traces are spaced by more than 1 inch. Anyone who has designed a real PCB knows that this is not practical for signal traces, especially when placed on the same layer as a large power rail (e.g., the SIG+PWR/GND/GND/SIG+PWR stackup). The temperature of closely-spaced parallel traces running at the same current could be higher than that of a single trace. One way to address closely spaced traces is to treat them as a single trace, where the combined current is used to determine the combined cross-sectional area and temperature rise.
Next, the calculator does not account for any of the standard thermal management techniques used in a PCB, such as the use of:
- Heat sinks mounted on components
- Heat dissipation or generation in active components
- Convection cooling over the surface of the board
- Conduction cooling into the device enclosure
I recently had the privilege to discuss the accuracy and applicability of IPC-2152 with Mike Jouppi, an expert in performing thermal measurements on circuit boards. From Mike's experience, it was found that the estimates produced by the IPC-2152 nomograpghs and equations tend to overestimate the PCB trace width or polygon width required to keep temperature rise within some particular limit. I discussed this with Mike on a recent episode of the Altium OnTrack Podcast.
An overestimate of the PCB trace width is not always a bad thing. For example, if you are evaluating whether a large polygon used as a power rail will stay cool, and the polygon width is much wider than the returned result from IPC-2152, then you can rest assured that your polygon will not have any heating issues. However, keep these points surrounding IPC-2152 in mind as they can cause a designer to over-design a circuit board when it may not be necessary.
In closing, it's important to note that the IPC-2152 results are based on the original data and methods in IPC-2221, and the standard even includes internal and external trace width figures from IPC-2221. If you want to develop your own calculator that attempts to summarize these data into a similar kind of master equation, then these standards are a good place to start.
If you don't have access to a simulator, specifically a 3D thermal solver, then you will have to use charts and calculators to estimate the equilibrium temperature for a given average current. In the list of links below, we've provided some resources for manually calculating trace heating limits, current limits, and temperature rise using thermal resistance/thermal conductivity:
- Trace width vs. current table under IPC-2152 (this shows an internal trace table)
- IPC-2152 data for internal and external traces
- Power plane current limit calculations
- How to use thermal resistance and conductivity to estimate temperature
- A guide to thermal analysis for circuit boards
I'll continue to update these resources on thermal capabilities as we develop more web apps and as they become available for Altium Designer users.
One of the most recent updates in Altium Designer® includes an autoamted trace heating calculator that estimates the current limit under the IPC-2221 standard. While there is still contention between IPC-2221 and IPC-2152, you can use both tools to get a PCB trace width estimate and ensure your design is reliable. 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. Come see the newest feature releases in Altium Designer.
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