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Current vs. Trace Thickness and Temperature vs. Copper Density December 15, 2006

Regarding the PCB Trace Width Calculator [1], Regit asks:

(a.) What is the relationship between current and trace thickness? And, (b.) what is the relationship between temperature and copper density?

Answers: (more…)

Reducing EMI in PCBs December 6, 2006

I got the following question by email:

When a data sheet says “minimize the trace area for lowest EMI”, do they mean thin trace? Does higher reactance minimize EMI?

It is a good question, so I though I would post something on this.

Minimizing EMI requires minimizing the electric and magnetic fields around the trace. If a trace has a lot of voltage that is changing fast, it will tend to emit a strong electric field. In such a case, making the trace have less surface area can help limit the emissions. Better yet, shielding the noisy trace with a trace that lets the energy return to where it came from is very effective - the shorter the return path, the better. In fact, if the noisy trace is completely surrounded by the shield, the electric field lines won’t get out and the noise will be contained. Once shielded, the area of the trace is not so critical in terms of noise, however more area also means more capacitance, which your circuit may not like. Therefore, yes, it is usually a good idea to minimize the area of an E-field emitting trace, but it must be able to carry the needed current of course.

Next is the magnetic field, which comes from a changing current. The higher the current and the faster it changes, the worse the EMI can be. The magnetic field can be somewhat canceled by routing the returning current as close as possible to the outgoing current. In other words, the loop area traveled by the current needs to be small. We are talking about AC current here, so it can be contained to a small area by effective use of decoupling capacitors. These should be placed near the source of the noise current.

Ground planes can help contain both electric and magnetic fields. They give the electric field lines a place to terminate, and those high frequency return currents will automatically flow near the outgoing currents due to an imaging effect where the ground plane will actually mirror the trace currents with currents flowing in the opposite direction.