External or Internal Traces?

Hi there!
I am trying to calculate the necessary trace width for certain traces on my pcb with online calculators.
From googling around I found out that fritzing/aisler produces pcbs with a copper thickness of 35 um which is one variable needed for the calculation. ( I hope the 35 um are correct).
The online calculators always calculate a trace width for internal and external traces.
I think that my traces are external but I am not sure. I am unsure because the traces on the pcbs are enclosed by the surface-finish-layer. Does this surface-finish-layer make all traces internal traces? Probably not, but I am not sure, so hopefully someone of you knows an answer, so I can be sure which calculated trace width is the correct one for me:-)

Btw: My pcb is a one-layer-pcb. So I have the electronic parts going in on the top side and all the traces are located on the bottom side. I am not using any vias or so.

Thanks, jagup

I’m not quite sure, (but read about it in past) I think internal layers means, traces between pcbs…
Like, when it’s about multi layer pcb, some traces goes between two layers of PCB.

I think by “surface-finish-layer” you are talking about “solder mask”…
No, It still makes the traces external…

Hi Blue,
thanks for your reply. Yes, I have read a little bit more about it online and I think that it’s still external traces regardless of the solder-mask :slight_smile:

The issue here is resistive heating in the traces. It will be worse for the internal traces (because of the extra layer of fiberglass the heat has to get through to dissipate.) Most of the calculators I have used want an ambient temperature and an acceptable rise in temperature to do the calculation.


Hi Peter,
thanks for your reply! Do you know exactly what the ambient temperature and the temperature rise are?
Regarding the ambient temperature I assume that it is the temperature inside the box where my pcb sits in?!

All the best, Jagub

Correct, that should be the expected temp of the board when it is unpowered. The temp raise is how much of a temperature rise is acceptable when the board is powered (calculated by how much heat the current draw will cause in the traces by resistive heating in the traces.) You probably want to choose an ambient temp that is the highest you expect your box to be exposed to rather than normal room temperature so if you hit the worst case the temp rise isn’t unacceptable. This is of course hard to guess :slight_smile: