# PCB track thickness

Discussion in 'Electronic Design' started by [email protected], Aug 21, 2007.

1. ### Guest

What would the recommended PCB track thickness be for minimal
temperature rise when carrying a current of 12A rms continuous?

(105um) track to using a 2oz (70um) track except cost?

I am trying to fit tracks into a very small transformer core and I
need the optimum track size and thickness.

2. ### EeyoreGuest

Define minimal !

2/3 the DC resistance.

A planar core ?

Graham

3. ### Guest

It seems that the windings I calculated will fit into the core.

Yes Graham, it is a planar transformer. I want to attach a picture of
my design but I can't seem to figure out how!

12A running through 2oz (70um) of copper tracks with a temp rise of
10degrees C requires 1.2um trace width? I got that info from
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/.
It doesn't really match the chart of my PCB manufacturing company.

According to their chart 12A with a temp rise of 100degrees C I
require a 2.5mm track width at 70um?

4. ### RST Engineering \(jw\)Guest

Why don't you lay the track down any thickness/width you find reasonable and
solder a #10 wire over the top of it?

Jim

5. ### SteveGuest

A width chart from Mil 275 appears here:
http://www.boarddesignnow.com/index_pg3.html

Steve

6. ### Heinz LiebhartGuest

Hmmm. Sounds pretty much^2 too small...

Lets calc (sorry for the european units ;-)...

Cu has a rho=1.72E-8Ohm/m.

Your profile has 105E-6 x 70E-6 m^2 = ~7E-9 m^2

The resistance of a 1m piece would have 2.34Ohm, one inch has approx.
0.06Ohm. (R = rho * length/section; 1m == 39,37")

With a current of 12amps rms this would give a power dissipation of
approx. 8W per inch. (P=I^2*R)

This simply can't fit!

Heinz

7. ### Rich GriseGuest

Get the thickest copper plating you can get, within reason. I
once bought a board with 4 oz copper - I don't know if it comes
any thicker than that.

Good Luck!
Rich

8. ### Harry DellamanoGuest

When designing power electronics you should worry more about power loss and
voltage drop. If this is done the temperature rise will not be significant.
The PCB trace width calculators are designed for single layer boards with
only trace area surface convection cooling. In multi layer PCB much of the
trace heat is conducted to other layers and in shorter traces to their end
points. Those calculators also do not include trace length which is absurd!
I normally allow 12.0mV per trace voltage drop. This yields a A*12mW power
loss and a 12.5mV/amp trace resistance. If this is done then the temp rise
in any trace of any length will be less than 2.5C. In any trace you know how
long it must be so you need only calculate its width. Use this formula:

W=L*A/25*oz where W +L = trace width + length in inches.
A = amps oz = cu thickness 1oz =1.4m"

So for your situation you have 12A and 2oz and I will assign a 1.50" trace
length and not worry about temperature rise, so:
W= 1.5*12/25*2 = 0.36" = 9.14mm Contrary to popular belief, In PCBs,
length is important!
Cheers,
Harry

9. ### EeyoreGuest

Well ...... a planar transformer usually has a multilayer construction and the
typical calculators don't do this. They also tend to assume ready radiation to
free air and the core's going to get in the way here.

I suspect 2.5mm of 70um (2 oz) copper should be OK though.

Graham

10. ### EeyoreGuest

" #10 wire " almost certainly won't fit inside a planar core.

Graham

11. ### EeyoreGuest

You got the 'profile' wrong. It's not 105 x 70 um.

Graham

12. ### Guest

Graham,

Can I ask you a few question via email?

13. ### Heinz LiebhartGuest

Oops

Loos like I've read too fast thru the posting, sorry.
105um is the 3oz Cu layer thickness... got it now

Heinz