Current vs. Trace Thickness and Temperature vs. Copper Density December 15, 2006
Regarding the PCB Trace Width Calculator , Regit asks:
(a.) What is the relationship between current and trace thickness? And, (b.) what is the relationship between temperature and copper density?
(a.) The relationship between current and trace thickness is shown in the IPC-2221 formulas under the trace width calculator . With a little algebra, it comes out that current is proportional to thickness raised to the power of c, where c = 0.725. So, per IPC-2221, if the thickness is doubled, the current capacity changes by a factor of 2^(0.725) = 1.65.
A common mistake is to assume that twice the trace thickness should allow twice the current, but it does not. Allowing twice the current for twice the thickness would mean allowing twice the power without any significant improvement in the heat path out of the trace. The heat path out of the trace would not significantly change since the trace still has the same surface area. (Also see comment 4 below.)
(b.) The relationship between temperature and copper density depends on what you mean by copper density. I assume you mean the amount of other copper traces and planes in the board and not the mass per volume of the copper itself. The formulas used in this trace width calculator are empirical relationships stated in IPC-2221.
According to , these formulas correlate to a PCB in Euro-Format (Lx=100 mm, Ly=160 mm, D=1.6 mm) made of pure FR4 (conductivity k=0.3 W/m-K, emissivity Îµ=0.9), with one copper trace of length L=100 mm and thickness t=35 Î¼m (=1 oz) on the top face and with a copper layer on the back plane (also of thickness 35 Î¼m, conductivity k=395 W/m-K and with a solder resist with emissivity Îµ=0.9). The ambient condition is that the PCB is suspended in still air by thermally nonconductive straps.
If your board has a different configuration than the above, the actual trace temperature rise and its current capacity will vary from the calculated values and you would have to conduct an analysis as done in  to find the relationships for your specific board configuration.
However, some generalizations can be made. Some things will decrease the temperature rise in the trace (and increase its current capacity) such as having more copper in the PCB, more air flow, or the inclusion of the heat path through the mounting hardware. Because of this, the current carrying estimates provided by the IPC-2221 standard are generally considered to be fairly conservative.
 PCB Trace Width Calculator
 New Correlations Between Electrical Current and Temperature Rise in PCB Traces, Johannes Adam
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- Author : Brad