Another PCB trace width question.. But quick I promise.

Hello everybody,

So I started looking at trace widths and looking at the currents I draw which are less than 100ma.

I have always just been using Egales default of .40 but I’m starting to understand that its way over what I need. Thats at 1oz, it looks like with that kind of thickness I would be good for up to 1.5 amps.

I was reading some of what you guys recommend and it looks like .25 might be perfect for me. A all round good number unless I’m driving something power hungry I would never need to go over it.

Then on my power and ground sides I can run .40 just to be extra safe even though I draw around 100ma through them.

Does this sound right.

Using the attached I was able to see that at 1oz with a 1amp current draw that it still says .04 is fine for track width. But the lowest the fab house I use goes to is .1524.

So using the same table I see that .1524 would be good for up to 2amps…

Am I reading all this correctly if so then I have been way way over doing it on the traces.

Do your traces fit your design without being SMALLEST POSSIBLE?

Don't make them smaller.

Some fab houses have rules about how small traces can be and how much distance is required between traces. If you just think about your current capacity requirements, you might fail to be able to have the board manufactured.

Find a fab house like OSHPARK and read their rules and then decide.

For power and ground the resistance of the traces is not the only issue (in fact its a minor issue), the stray inductance should be nice and low (meaning wide traces between chip power/ground pins and the nearest decoupling capacitors - well all the decoupling capacitors relevant to that chip really).

"I have always just been using Egales default of .40 " What's the units on that? I think the default for a from-scratch design is usually 6 mil, which is way too small for a power trace.

I thought it defaulted to 16 mil?

I usually use 16 for signals, 12 or 10 when things are getting tight.

24, 32, or more for power traces. I don't calculate out whether the trace widths are sufficient unless I'm pressed for space - else if I'm close enough that I feel like I should check, I just make them wider.

I use a trace width calculator when I’m not sure.
But 1Amp is nothing to worry about.
Even 7mil would do if you are pressed for space according to this calculator.

http://www.4pcb.com/trace-width-calculator.html

Use the “external layer” box.
Leo…

You have sometimes to worry about the voltage drop along the trace, not just whether it will melt the board!

And as mentioned above thin traces have lots of stray inductance, sometimes this is an issue.