# PCB track current capacity

• posted

What is the rule of thumb for the current carrying capacity of a tinned copper PCB track on standard substrate?

For example, what width would be required for 1 amp at 100Hz 50% duty cycle?

Is there any process by which the track can be made less wide, ie. a thicker deposition of copper on the board prior to etching.

Thank you,

Claus Jensen

• posted

Yes, you can make the track thicker -- in the US this is expressed as ounces of copper per square foot, with one ounce being pretty standard fare, and Really Serious Power boards going up to 4 oz.

Dunno about trace widths, though -- someone will chime in.

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• posted

0.5oz (per square foot) is 18um 1 ounce copper per square foot is 35 um thick and so on. 4 ounce copper would be 0.14mm thick, and really closed-spaced narrow tracks become impracticable - the gaps between the tracks need to be wider than they are deep.

-- Bill Sloman, Nijmegen

• posted

Type "pcb trace calculator" into Google and you'll get plenty of calculators. Usual rule of thumb is to calculate width for a 10degC rise in temperature, but your electrical requirements will always come first of course. It's not an exact science though, calculations are based on old extrapolated IPC graphs and can be quite error prone. It also varies based on solder mask, and whether it's an inner or outer layer etc. Good info is here:

Yes, that is common. Normal boards have either "0.5oz" or "1oz" copper thickness (weight actually), but you can specify up to say 4oz copper from your PCB manufacturer. That allows greater current capacity for a given width.

Dave.

• posted

Thank you for your advice. The calculators appear to assume DC current. How would one adjust the final figure for a lesser duty cycle? In practice, is the relationship purely linear? For example, would a 50% average duty cycle mean you can simply use a 50% thinner trace?

Claus Jensen

• posted

SORRY, I left out the key problem in my last post. I need to draw two spirals, one within the other, with equal spacing between the traces. IOW a double trace spiral. I can draw one spiral, but simply rescaling the second one won't make it fit. Any ideas?

Claus

• posted

Arithmetic spiral or geometric spiral?

For an arithmetic spiral (radius is proportional to angle), adding a fixed offset to the angle (i.e. rotation) is equivalent to adding a fixed offset to the radius.

For a geometric spiral, (radius is exponential to angle), rotation is equivalent to scaling.

You state that rescaling doesn't work, so I'd assume that it's arithmetic, in which case you should just need to rotate it.

• posted

yeah, that, or plating extra copper on after etching

even a thick layer of solder will help a bit.

if it's localised to a small part of thr board you can "surface mount" some copper wire on the track.

• posted

Here's a neat online tool:

Of course, while I don't know of any problem with that thing, you shouldn't trust your career solely to a link you find online.

As always, all applicable disclaimers apply. :)

• posted

Sure: look up ampacity of a given cross-sectional area of copper, and find out how thick the copper is (is that spec easily available?) multiply by the width, and that's the area.

Cheers! Rich

• posted

On Sat, 21 Feb 2009 21:44:07 +0000, C;aus Jensen wrote: [someone wrote:]

Draw a trace that's the width of the sum of the two plus the gap; then "un-draw" the gap out of the middle. ;-)

Hope This Helps! Rich

• posted

Keep in mind that even though a trace may be capable of carrying X amps, the voltage drop may cause other problems. Often, the drop is a bigger problem than the heating.

John

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