Current limits for printed circuit board traces

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Rodney
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Current limits for printed circuit board traces

Post by Rodney »

I used to have a chart that indicated what the maximum current carrying capacity of printed circuit traces was when using 1 oz. and 2 oz. copper plating based on the width of the traces. I can not locate it. Can anyone refer me to such a source please? Thanks Rodney
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CeaSaR
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Re: Current limits for printed circuit board traces

Post by CeaSaR »

Search so far shows this old Mil-Spec pdf >here< with a chart on page 34.

Another that is MUCH more readable >here<.

Still another (with formula) >here<.

An on-line calculator>here<.

And my search >here<.

Hope it helps.

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MrAl
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Re: Current limits for printed circuit board trace

Post by MrAl »

Hi there,


Usually we know the PC board copper thickness based on the ounces per square foot surface area
which is usually only specified by the word "ounce" as in "1 oz copper". This formula takes
advantage of this as we only have to know the 'ounces', the width, and the temperature rise:

I=0.025*dT^0.45*W^0.79*(Oz*1.4)^0.53

where
I is the max allowable current for the temperature rise dT,
dT is the allowable temperature rise in degrees C,
W is the conductor width in thousandths of an inch,
Oz is the 'ounce' spec of the copper on the board.

For example, for a 10 deg C max rise with a width of 50 thousandths of an inch (50 mils)
using one ounce copper pc board:

dT=10
W=50
Oz=1
so the current I is:
I=0.025*(10)^0.45*(50)0.79*((1)*1.4)^0.53
so
I=1.85 amps max

Some additional thought has to go into these calculations however as newer designs
sometimes depend on the copper clad itself to act as a heatsink as with various
diodes and transistors. The max power with a diode that has to pass significant
current is usually only realized with the copper acting as the heat sink so the
copper temperature naturally rises because of that also.
LEDs vs Bulbs, LEDs are winning.
Rodney
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Re: Current limits for printed circuit board traces

Post by Rodney »

Thanks guys, your inputs were very helpful and just what I needed. Rodney
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philba
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Re: Current limits for printed circuit board traces

Post by philba »

You can do all the math but why bother - use a calculator like this one http://circuitcalculator.com/wordpress/ ... alculator/
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CeaSaR
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Re: Current limits for printed circuit board traces

Post by CeaSaR »

Hey Philba,

You found the same calculator I did. Search engines are great, aren't they?
CeaSaR wrote:An on-line calculator>here<.
CeaSaR
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Robert Reed
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Re: Current limits for printed circuit board trace

Post by Robert Reed »

MrAl
Something doesn't sound right here -

"Usually we know the PC board copper thickness based on the ounces per square inch surface area
which is usually only specified by the word "ounce" as in "1 oz copper".

Or maybe it is just the way it is worded. When first read, it sounds like 1 0z/sq. inch which would make a 12" x 12" board weigh in at roughly 10 pounds. I was under the impression that the Oz. ratimg was per sq. foot. Howvever, I was never sure about that since some terms are carryovers from ancient times - Gauge being a good example. Maybe its just the way I misinterpeted your post.
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philba
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Re: Current limits for printed circuit board traces

Post by philba »

CeaSaR wrote:Hey Philba,

You found the same calculator I did. Search engines are great, aren't they?
CeaSaR wrote:An on-line calculator>here<.
CeaSaR
You got me there. I have a hard time seeing links without rolling over them. It would be nice if the BB SW automatically set up links to have a different color. I didn't see an option to change that on my view though even if there was one, it should default to a distinguishing color.
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CeaSaR
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Re: Current limits for printed circuit board traces

Post by CeaSaR »

Philba wrote:You got me there. I have a hard time seeing links without rolling over them. It would be nice if the BB SW automatically set up links to have a different color. I didn't see an option to change that on my view though even if there was one, it should default to a distinguishing color.
I'm with you there Philba. Since we have no real control over that, I have taken to emphasizing
or highlighting my links by putting them between reversed brackets:

><, usually with the word "here" between, typically shown as >here<.

I have even used the scheme of typing:

Link: Some Name Here

As most of us are used to seeing links underlined and usually in a much different color than
surrounding text, the current way that the board presents links does leave us little choice
but to be somewhat visually creative in the way we write our links.

Hmm, maybe I'll have to resort to placing links in columnar fashion, ie.:

Links below:
Link1
Link2
etc.

Ponderingly,
CeaSaR
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Lenp
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Re: Current limits for printed circuit board traces

Post by Lenp »

Copper Weight: it defines the number of ounce copper on 1 sq. ft. area, such as 0.5, 1.0, 2.0, 3.0 oz, etc. Usually this parameter is used to specify the thickness of the copper on each layer of the board. It is very easy to work out the thickness of 1.0 oz copper on 1 sq. ft. board based on the copper density (8.9 g/cm3), which is 34.29 µm and rounded up to 35 µm. So we have the following list.

0.5 oz means 17.5 µm copper thichness
1.0 oz means 35 µm copper thichness
2.0 oz means 70 µm copper thichness

Reference:->> http://www.speedypcb.com/pcb-knowledgeB ... Weight.htm

Len
Len

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MrAl
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Re: Current limits for printed circuit board trace

Post by MrAl »

Robert Reed wrote:MrAl
Something doesn't sound right here -

"Usually we know the PC board copper thickness based on the ounces per square inch surface area
which is usually only specified by the word "ounce" as in "1 oz copper".

Or maybe it is just the way it is worded. When first read, it sounds like 1 0z/sq. inch which would make a 12" x 12" board weigh in at roughly 10 pounds. I was under the impression that the Oz. ratimg was per sq. foot. Howvever, I was never sure about that since some terms are carryovers from ancient times - Gauge being a good example. Maybe its just the way I misinterpeted your post.
Hi Robert,

Almost forgot to come back to this thread to make corrections.

Oh you mean you have boards that weigh less than 10 pounds? I was wondering why the postage
for those pc boards i ordered was so low :smile:
Thanks for bringing that up. I mistyped that, and typed 'inch' instead of 'foot'. I am so used to calculating
power dissipation based on square inch board area so it was habit to think in square inches.
The formula i posted is correct however.

I have since corrected the 'inch' error.

Len:
Yes, the formula i posted takes care of the thickness so we dont have to keep referring back to
a table of oz per sq foot to thickness...

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I=0.025*dT^0.45*W^0.79*(Oz*1.4)^0.53
LEDs vs Bulbs, LEDs are winning.
Bygar
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Re: Current limits for printed circuit board traces

Post by Bygar »

PCB trace width rule of thumb, 1/16", 1oz., 5 Amp., 20 degree C rise.
Double for 2 oz.
Half for 1/2 oz.
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