EMC Question of the Week: July 27, 2026

The figure shows a cross-section of a 24-layer circuit board with GND planes on layers 2, 4, 7, 10, 12, 14, 17, 19, and 23. There are PWR planes on layers 5 and 15. Two decoupling capacitors are shown. C1 is on the backside of the board and shares via connections with a BGA. C2 is on the topside of the board and is 6.4 mm away from the BGA. Within the first 200 ps of a sudden demand for current to the BGA, where is most of the current coming from?

1. C1
2. C2
3. C1 and C2 equally
4. the plane capacitance

Answer

The best answer is “d.” The closely spaced power planes behave like a radial transmission line with a very low characteristic impedance. The current drawn from the planes ramps up as the wavefront moves outward. After 200 ps, the current is being drawn from a radius of about 28 mm. For a 0.25 mm plane spacing in an FR-4 dielectric, that represents a current of about 4 amperes per volt.

Neither of the decoupling capacitors in this example can respond that fast. C1 is 2.5 mm away and connected through two parallel vias. The vias form a transmission line with a characteristic impedance of about 100 ohms. A sudden 1-volt drop in the voltage at the BGA launches a depletion wave on the vias with a current equal to 1 volt divided by 100 ohms (10 mA). This travels to C1 in about 18 ps. However, even if C1 has infinite capacitance and zero inductance, it cannot immediately supply large amounts of current to the BGA.  When the 10-mA current wave hits the ideal capacitor, the impedance mismatch sets up a reflected current wave that is also 10 mA. After one round trip, the current at the BGA is 20 mA. Every successive roundtrip adds another 20 mA. After 200 ps, 5 round trips to C1 have been completed and the current is about 90 mA. This is far less than the 4000 mA being supplied by the planes.

The current supplied by the planes and the current supplied by C1 both increase (on average) linearly with time. An ideal C1 would never catch up to an infinite plane pair, but the plane pair is not infinite. Eventually, the depletion wave hits the plane boundary, so boards rely on the decoupling capacitors to replenish the charge stored in the planes. In this example, C2 and other capacitors similarly connected on the topside of the board are a little better at replenishing the charge in the upper plane pair due to their shorter via connections. At no time does C1 provide "most of the current." 

Getting charge (or energy) from point A to point B quickly is not simply a matter of distance. The characteristic impedance of the connection is also important. 

Note: For conducted and radiated emissions, the frequency domain behavior of the decoupling is generally more relevant than the time-domain behavior. In this example, C2 makes a better connection to the planes than C1 at the highest frequencies where the decoupling capacitors are effective. Nevertheless, it is generally a good idea to mount small decoupling capacitors on the backside of boards beneath BGA devices (like C1 in this example). The high density of vias under a BGA makes this area unusable for most other purposes.       

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