Electrical – PCB Current Draw vs Temperature Rise Interpretation and Question

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.

My response is going to be the opposite to that of @NickAleeev.

I had similar questions in the past about trace current limits and here are my findings. I haven't done real world tests to confirm this (yet).

The standard for current capacity in a trace was established in a document (IPC-2221) which is what alot of online calculators use. The document is old and outdated. The new standard is the IPC-2152. The IPC-2221 is conservative, and if you can go that route it might be best too. If however, you are limited in space, then IPC-2152 would give you better results. Different calculators will give you different answers, depending on what standard they are basing it off. I have only found two calculators that use IPC-2152 and when I asked my fab house, they said that they can't tell me.

Also internal layers while they are sandwiched (in FR4) have a greater thermal conductivity than air. (The link below goes a bit more into it). They will dissipate the heat more to the surface, and if you have a solid plane between your internal layer and the outer layer, you have a pretty good sink for all that heat.

Have a look at a question I had asked in the past [what is the current limit through a trace? ] you can probably just jump to the end of the question and read the answer.

Some advice I got from this, was just build a dummy board, inject your current into a trace, and see how it reacts. All the, this table says this, that table says that, can't beat an experiment that you can test yourself.