pcb design - What is a suitable temperature rise for PCB traces?

I'm designing a PCB for a 10A relay and am trying to determine what the trace width should be.

According to this trace width calculator, the biggest factors in the width are copper thickness, maximum current, and temperature rise. I would like to use 1oz for the copper thickness since that makes the PCB cheaper, so that leaves temperature rise as the most desirable variable to change.

Using the above calculator with these settings:

Current: 10 amps
Thickness: 1 oz/ft^2

Temperature Rise: 10 C
Trace Length: 1 in

The calculated trace width is 283 mils, which is too wide.

If I change the Temperature Rise to 100 C, the calculated trace width is 70 mils, which isn't bad at all. However, I don't know what kind of temperature rise is acceptable. How do I determine this?

Answer

Temperature rise is something you have to consider, but usually the resistance and the resulting voltage drop at full current have been the limiting factors when I've gone through this. That said, 100°C is a large temperature rise. That's not enough to be a problem for a copper trace on a FR4 board by itself, but that's going to affect the apparent ambient temperature for nearby components.

If you have that much temperature rise, you're dissipating significant power in the trace, which means power loss in your system. Again, the first concern should be how much voltage drop you can tolerate. Once you get that to acceptable levels, the temperature rise is usually low enough.

Also consider that 2 oz copper and more is widely available. The extra cost of specifying 2 oz copper for outer layers may be less than making the board larger or dealing with the heat or voltage drop. 2 oz on outer layers doesn't usually add that much cost. If you stitch together a trace on both outer layers, you have 4x the copper cross section than for a single trace of 1 oz thickness. If it's only one or two traces in a otherwise low current design, you can leave the soldermask off the trace and have a copper wire soldered over the trace. There are actually bus bars meant for this. However, consider the manufacturing cost. 2 oz copper may start to look like the cheap option when you consider the total cost of alternatives.

Again, look at all the options and all the criteria for deciding on trace width. Don't just focus on temperature rise, or assume that thicker copper is more expensive once the whole system is considered.