# Sizing a trace on a PCB to carry 2.5 amps

I need a trace on my PCB to carry up to 2.5 amps (average) current, with 5-6 amp spikes (it's going to a switch mode power supply.) How wide should the traces be? I've got a trade off between reliability and size, as the product is space constrained. Any tips would be appreciated.

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Copper thickness/weight? –  tyblu Dec 30 '10 at 4:02

After doing a quick google of "PCB Current Calculator", I found a PCB Current Calculator. It bases the width of the track on how much of a temperature rise the trace is allowed to have. It's nice in that it shows how much power you waste through your trace. I would design for your worst-case RMS current, since it's going to be a periodic signal.

If you use 2 oz/ft2 copper instead of the standard 1 oz/ft2 copper, you won't need as wide of a trace to achieve the same resistance. For example, allowing for a 10 oC rise, you can get away with these numbers at 3 A:

• 280 thou (7.11 mm) on 1/2 oz/ft2 copper
• 140 thou (3.56 mm) on 1 oz/ft2 (35µm) copper
• 70 thou (1.78 mm) on 2 oz/ft2 (70µm) copper
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140/1000 inch is often referred to as 140 mils. I've never seen it as thou before. –  stevenvh Dec 30 '10 at 8:44
Well, I am considered about skin effect during these pulses :-) –  BarsMonster Dec 30 '10 at 9:43
@stevenh, May be an old term or from across the ocean. A few old techs I've worked with use it. Hear it often in machine shops, too. Helps the uninitiated to not get mixed up with mm; I run across those who use mil as short for mm in speech often. Makes my brain skip a beat. –  tyblu Dec 30 '10 at 10:06
@ThomasO, That refers to internal or external traces, referring to their contact with ambient air. On a 2-layer, both layers are external. –  tyblu Dec 31 '10 at 0:42
Thou for .001 inch is commonly used in the UK, although mil tends to be used much more often for PCB work, as in the USA. –  Leon Heller Dec 31 '10 at 15:22

I remember having seen this nomogram in another answer:

Select 2.5A on the vertical axis of the top graph. Move to the line indicating the allowed temperature rise. Move downward to the PCB's copper thickness in the bottom graph. This intersection gives you the required width on the vertical axis.

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You probably mean this answer: electronics.stackexchange.com/a/5405/4950 –  PetPaulsen Apr 13 '12 at 8:17
@PetPaulsen - that's the one. Thanks, I'll update my answer. –  stevenvh Apr 13 '12 at 8:21

Another option would be to use PCB Busbar soldered into the PCB - they would also add some nice rigidity to your PCB should it need it

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A 1.6mm FR4 PCB is many times stiffer than a Busbar! –  stevenvh Apr 13 '12 at 7:45
@stevenvh - What? No they're not. Plus, a buss-bar is generally perpendicular to a PCB, so it is far more rigid. In fact, they're actually sold specifically as PCB stiffeners. –  Connor Wolf Apr 13 '12 at 9:02
@Fake - Yes, I've seen them sold as stiffeners, but I'm not sure that's a good idea: you want as little force as possible exerted on soldering connections. –  stevenvh Apr 13 '12 at 9:08
@Fake - just asked this question about it. –  stevenvh Apr 13 '12 at 9:22