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I am working on designing a pcb that will have a copper pour. I have selected to use thermal reliefs to connect the pour to pads, but I am a little confused on the thickness. Should each spoke of the pour be as thick as a trace (assuming current will take a direct path, using only one spoke) or should I make it so the sum of the spokes equals the track thickness (assuming the current will only flow to max of a trace, then find another route)?

My current at max will be around 1 amp (and I am using traces of .035" to be safe). So with this example, should he spokes each be .035" or should each spoke be .00875"?

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  • \$\begingroup\$ The difference is often negligible, but usually each spoke has the track width. What widths/currents/temps are we talking about? \$\endgroup\$ – Wesley Lee Mar 12 '17 at 5:43
  • \$\begingroup\$ At the highest point, around 1 amp. I will update the question and I will add some images later. \$\endgroup\$ – user173724 Mar 13 '17 at 16:57
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You use the term 'trace width' as if it's some sort of universal constant. It's not. Unless a trace has to be a specific width (for impedance control), it's generally as narrow as it can be limited by the necessity to carry current, not drop too much voltage, or be printable within the PCB fab tolerances that you've chosen to afford.

The requirements for the spokes of a thermal relief are different, but they too will be limited at the small feature end by the pcb fab tolerances.

... (assuming current will take a direct path, using only one spoke) ... (assuming the current will only flow to max of a trace, then find another route)?

Neither. You would need to be at high microwave frequencies for for 'direct path, other path' geometry to be significant. The current doesn't 'know' what the max of a trace is, it doesn't use one up to the max and then find another route.

What the current will do is use all parallel paths from pad to ground pour in the ratio of their impedances. The resistive part will tend to be more or less balanced by geometry, and at modest frequencies, say below 3GHz, the inductive difference will be negligible as well. That means that, as a first approximation, the current will split more or less equally through all spokes.

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  • \$\begingroup\$ +1 for well written "The resistive part will tend to be more or less balanced by geometry, and at modest frequencies, say below 3GHz, the inductive difference will be negligible as well. That means that, as a first approximation, the current will split more or less equally through all spokes." \$\endgroup\$ – Michael Karas Mar 12 '17 at 6:53
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Here are a couple useful facts about copper foil: 1) squares of copper foil have 70 degree centigrade thermal resistance/watt per square of copper, whether 1mm square or 10mm square or 100mm square 2) squares of copper foil have 0.0005 (+- some% with temperature), or 0.5 milliohm electrical resistance per square, whether 1mm square or 10mm square

I'd grab a pad of gridded paper, and start sketching.

schematic

simulate this circuit – Schematic created using CircuitLab

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