I work with a hardware startup that is working on new light sources for microscopic imaging. The visionaries abound and several talented people are working to make the devices into promising prototypes. But even in this early stage, there has been little talk of designing the product according to industry standards.
While it may seem like a simple requirement, knowing the temperature rise and current carrying capacity in your PCB are outlined in the IPC standards for electronic devices. 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.
Charts vs. Calculators
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 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 in the IPC-2221B standard.
Given the number of layout possibilities in any given PCB, the original charts in IPC-2221B are not applicable to every design. The new IPC-2152 standard presents results that summarize how thermal conductivity, vias, board material, and thickness affect the relationship between current, trace cross-section, copper weight, and temperature. The standards also account for the distance between traces and copper planes, as well as buried and surface traces.
If you’ve ever tried to extract numerical results from a chart, you know that you will inevitably be forced to estimate. Imaging software can be used to get an accurate value using pixel values between tick marks on a chart, but this time-consuming as you will need to measure pixel distances for every ordered pair on a chart.
This is where an online calculator becomes a real time saver. Rather than peeling values off of a chart with imaging software or a ruler, you can directly calculate the appropriate trace area and current capacity for a given temperature rise. Anyone can obtain the IPC-2152 data and determine these formulas on their own. This is a great way to support the PCB design community and preserve your own sanity in the process.
Online calculators vs handheld calculators
Using an IPC-2152 Calculator For Design
I can’t count the number of times I have transposed or omitted digits when working with a handheld calculator. Working with a calculator can be a real time saver and can also prevent errors that can be made during manual calculations. If you search online, several designers have taken the time to summarize much of the IPC-2152 information into compact formulas and have built calculators that work with these formulas.
For simplicity, IPC-2152 formulas don’t account for the effect of the board material. Although the IPC-2221B results were originally defined based on data for polyimide boards, the results are still accurate for FR4 boards. You cannot insert an arbitrary thermal conductivity value in the formula to determine the temperature rise for a given current capacity.
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. When designing a PCB according to industry standards, it is best to allow for some safety margin in your design. Designing with an allowance of higher temperature rise than your application requires ensures that you can cover the 2% difference between polyimide and FR4.
PCB on green FR4 substrate
Materials manufacturers can modify the important material properties of FR4, giving designers some flexibility. In general, if the thermal conductivity of your board is lower, the temperature rise will be larger when all other design parameters are held constant, and vice versa. The IPC-2152 standards can still be used when these changes are taken into account as long as you keep your estimates conservative and allow for the appropriate safety margin.
Most calculators are only valid when traces are spaced by more than 1 inch. Anyone who has designed a real PCB knows that this is not practical. The temperature of closely spaced parallel traces will be higher. 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.
Working according to industry standards requires PCB design software with powerful CAD tools and the right material specifications. It doesn’t hurt to have power delivery and thermal analysis tools so that you can verify your layout will meet industry standards.
Thankfully, any circuit board can be designed to have a proper trace width with an accurate trace width calculator. No more worrying about copper thickness and carrying capacity with Altium Designer®’s trustworthiness ensuring your printed circuit board, in fact, becomes a printed circuit. Altium Designer gives you all of this and more. If you want to learn more about how Altium Designer can help you reach your design goals, talk to an Altium expert today.
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