PCB Track width

Chris W · Mar 25, 2005

I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

--
Chris W

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Rene Tschaggelar · Mar 25, 2005

Chris W wrote:

I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

for the individual LEDs I'd use 20mil tracks and for the combined
perhaps a 60mil one.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Dwayne · Mar 25, 2005

"Chris W" <1qazse4@cox.net> wrote in message
news:FD_0e.7065$Z37.1722@lakeread06...

I am working on the layout of board to control 32 LEDs using 2 16 bit LED
drivers. There will be up to 25mA going through each LED, which means the
ground pin of each driver will sink the current for all 16 LEDs, 400mA, and
the ground connection for the whole board will sink 800mA. Can you tell me
what trace width I will need to for the individual LED tracks and for the
common track? I understand a solder mask will improve the current handling
ability, so I plan on taking advantage of that.

--
Chris W

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First you need to find the maximum current density that the board can carry.
Then find out the thickness of the copper. Then you can (trivially)
calculate the required width.

Dwayne

Scott Stephens · Mar 25, 2005

Chris W wrote:

Can you tell me what trace width I will need to for the
individual LED tracks and for the common track?

No, but try google:

Trace Width Javascript calculator:
http://www.geocities.com/CapeCanaveral/Lab/9643/TraceWidth.htm
http://www.desmith.com/NMdS/Electronics/TraceWidth.html
....
http://www.google.com/search?q=pcb%20trace%20width%20javascript

Which is the best?

Scott

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John Fields · Mar 25, 2005

On Fri, 25 Mar 2005 14:35:54 -0600, Chris W <1qazse4@cox.net> wrote:

I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

---
Somone has that info posted on the web somewhere, and there may even
be a calculator for it, but in case you can't find them, I posted a
graph for you on abse under the same heading as this thread.

A solder mask _won't_ improve the current handling capability because
it'll provide a thermal barrier between the trace and ambient air,
which will restrict heat removal by convection, causing it to run
hotter than if it was uncovered. However, it's not a good idea not to
use a soldermask since it will protect the traces from solder bridges
and oxidation. Besides, looking at the chart you can see that if you
use 1oz copper and you specify 0.015" wide traces you can expect a
temperature rise of about 10°C over ambient with about 1A going
through the trace. For power traces you should use something like at
least 0.06" wide traces, so that puts the temp rise down in the noise.
Specify a double sided board with plated-through holes, nice wide
traces, Solder Mask Over Bare Copper, (SMOBC) and 2oz copper if you're
not very experienced with PCB fabrication and you expect to be doing a
little rework, and you'll wind up with a nice looking board which you
can take some liberties with and not beat to death if you make a
mistake or two.

--
John Fields

Ian · Mar 26, 2005

"Rene Tschaggelar" <none@none.net> wrote in message
news:42447de8$0$1150$5402220f@news.sunrise.ch...

Chris W wrote:
I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

for the individual LEDs I'd use 20mil tracks and for the combined
perhaps a 60mil one.

Rene

google on: "temperature pcb" in the All words field, and "track width"
in the exact phrase field.

Regards
Ian

Rene Tschaggelar · Mar 26, 2005

Ian wrote:

"Rene Tschaggelar" <none@none.net> wrote in message
news:42447de8$0$1150$5402220f@news.sunrise.ch...

Chris W wrote:

I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

for the individual LEDs I'd use 20mil tracks and for the combined
perhaps a 60mil one.

Rene

google on: "temperature pcb" in the All words field, and "track width"
in the exact phrase field.

Thanks Ian,
I was aware of these calculations.
Why should a track be made as thin as possible to have
maximum temperature rise ? From my selfetching days, I still
try to avoid tracks and distances below 15mils when the
space is there.
Actually since doing precision analog, I calculate the
microVolts lost over a few inches of tracks when passing
milliAmps.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Pooh Bear · Mar 26, 2005

Rene Tschaggelar wrote:

Pooh Bear wrote:
Rene Tschaggelar wrote:

Thanks Ian,
I was aware of these calculations.
Why should a track be made as thin as possible to have
maximum temperature rise ? From my selfetching days, I still
try to avoid tracks and distances below 15mils when the
space is there.
Actually since doing precision analog, I calculate the
microVolts lost over a few inches of tracks when passing
milliAmps.

Now do the calculation when you have peak currents of over 50 Amps such as in
high power audio ! 20A rms isn't uncommon on a pcb and it's nice to know you
don't need a track 20mm wide ( and what the temp rise will be ).

I have a similar guide somewhere based on a British Standard IIRC. Thankfully
it's in metric ( none of that US 'square mils' nonsence ).

True.
That would be a case for 3 to 10 ounce copper then.
Equivalent to 100 to 300um thickness.

In practice 2 oz is widely used ( 70 um ). The 20A rms is not a continuous figure
thankfully with audio.

Graham

John Fields · Mar 26, 2005

On Fri, 25 Mar 2005 22:08:59 +0100, Rene Tschaggelar <none@none.net>
wrote:

Chris W wrote:
I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

---
That's not true. It increases the thermal resistance between the trace
and the ambient air and causes the trace to run hotter than if it were
bare.
---

--
John Fields

budgie · Mar 27, 2005

On Sat, 26 Mar 2005 16:55:53 -0600, John Fields <jfields@austininstruments.com>
wrote:

On Fri, 25 Mar 2005 22:08:59 +0100, Rene Tschaggelar <none@none.net
wrote:

Chris W wrote:
I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

---
That's not true. It increases the thermal resistance between the trace
and the ambient air and causes the trace to run hotter than if it were
bare.

and by precluding plating, it delivers a higher resistance_per_unit_length than
unmasked/plated tracks.

Ian · Mar 27, 2005

"Rene Tschaggelar" <none@none.net> wrote in message
news:4245b711$0$1151$5402220f@news.sunrise.ch...

Ian wrote:

"Rene Tschaggelar" <none@none.net> wrote in message
news:42447de8$0$1150$5402220f@news.sunrise.ch...

Chris W wrote:

I am working on the layout of board to control 32 LEDs using 2 16 bit
LED drivers. There will be up to 25mA going through each LED, which
means the ground pin of each driver will sink the current for all 16
LEDs, 400mA, and the ground connection for the whole board will sink
800mA. Can you tell me what trace width I will need to for the
individual LED tracks and for the common track? I understand a solder
mask will improve the current handling ability, so I plan on taking
advantage of that.

A soldermask is some plastic to cover the copper tracks
such that there won't be any tin. It has no influence on the
current handling capability.

for the individual LEDs I'd use 20mil tracks and for the combined
perhaps a 60mil one.

Rene

google on: "temperature pcb" in the All words field, and "track width"
in the exact phrase field.

Thanks Ian,
I was aware of these calculations.
Why should a track be made as thin as possible to have
maximum temperature rise ? From my selfetching days, I still
try to avoid tracks and distances below 15mils when the
space is there.
Actually since doing precision analog, I calculate the
microVolts lost over a few inches of tracks when passing
milliAmps.

Rene

Hi Rene:

I was sure you would know these resources, I was giving a
pointer to the OP, and wanted to keep your response as the
alternative engineering "this works" approach.

Regards
Ian

PCB Track width

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