Re: [SI-LIST] : 0.5 oz Cu's current density?

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From: Abd ul-Rahman Lomax (marjan@vom.com)
Date: Tue Oct 26 1999 - 20:26:38 PDT


At 09:43 AM 10/27/99 +0800, Hanyz@gsta.com wrote:
> I am routing a power supply board, but I don't know what trace width I
>should to use to feed a 10A current to load. Can somebody here tell me how
>calculate this? Thanks for any help HanYiZhou

I'm not sure this belongs in the SI list, here is what I can write on the
subject:

There are two basic aspects to the issue:

(1) temperature rise in conductor
(2) DC voltage drop in conductor

Without knowing more about the power supply, there is not much I could say
about item (2); if the drop is large enough to be really significant, in
most cases, item (1) would also be a problem -- or the conductor is too long.

Temperature rise allowable for the conductor should be specified, and, of
course, the thickness of copper, including copper plating.

There is a calculator for trace width vs. current, copper thickness, and
temperature rise at http://www.aracnet.com/~gpatrick/

It does take into account whether or not a layer is an inner or outer
layer, as it should. From my old copy of Preben Lund's book, I get for a
10A trace, 20 degree rise, 100 mils wide with 2 oz copper. One common
practice is to split that into two traces, one on the top of the board and
the other one the bottom; each trace could then be 50 mils wide.

The web calculator comes up with 89 mils....

*But* Lund advises derating by 30% for a tin/lead surface and 15% if there
is a solder mask. If I combine these, I would treat the 10A current as if
it were 17A, and the chart tells me that this trace should be almost 200
mils wide if it is on one side only. I don't know exactly why each of these
deratings is as it is; on the face of it one could improve matters by using
SMOBC; the derated current of 12A would only require, as I read the chart,
about 130 mils.

Anyway, a 100 mil trace, 2 oz copper total thickness, would be roughly 1.2
milliohms per cm;, from which the voltage drop can be calculated.

Since, in general, most of us would prefer to not to generate even 20
degrees C. of heat in the traces, I would assume that one would throw about
as much copper at this trace as practical.... I'd probably go for 100 mils,
2 sides, if possible.

marjan@vom.com
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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