Last Week's Question
The DC resistance of a half-ounce copper plane in a circuit board is approximately,
- 1 mΩ per square
- 100 mΩ per square
- 1 Ω per square
- 100 Ω per square
Answer
The correct answer is "a". This can be confirmed by solving for the resistance using the equation R = 1/σt or by using one of the many online resistance calculators, such as the one here. If you're an EMC engineer, or if you're regularly involved in the EMC design or review of circuit boards, you shouldn't require a calculator. This is one of those numbers that people working in EMC should be able to recall instantly. It is important when deciding whether or not to allow the current return paths of two signals to share the same copper plane on a circuit board.
At frequencies below ~100 kHz, currents returning on a circuit board plane spread out. Remembering that the DC resistance of a half-ounce copper plane is approximately 1 mΩ per square allows one to quickly calculate maximum crosstalk due to common impedance coupling. Amps of current returning in a half-ounce copper plane induce millivolts of voltage across the plane. Some (or all) of this voltage will be superimposed on all of the signals using the same plane for their own return currents. If this coupling is too high, at least one of the coupled circuits will require an isolated return path.
Note: When an isolated return path is required, it is usually better to provide it on a separate layer. Splitting or gapping a solid return plane is almost never a good idea. Multiple "grounds" make it more difficult to control radiated emissions and often make systems more susceptible to electromagnetic interference.
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