Display Your Know How: Decoupling Capacitor

Are these caps Mirrored as in on the bottom layer of the board under a BGA. ?

If that's the case then Picture C looks the best.

Best Regards,

That's correct, excellon1. The components are indeed placed underneath the IC, specifically on the bottom layer of the board.

That's correct, excellon1. The components are indeed placed underneath the IC, specifically on the bottom layer of the board.

I have had SI engineers request the the routing go from the pin to the cap then to the via:

...

if I remember correctly they thought this reduces Ground Bounce type of issues

Hi PCBTech.

So on the Pictures A, B ,C are OK. pic D has etch that is too small for the trace. Some considerations. The best connection off the pin to the pad of the cap should be as wide as possible so as to minimize inductance. The other consideration is how much power that pin is drawing. One would want to take into consideration the current. Typically the vias will connect to a plane or micro plane if they are used for power. Configure the vias so that they are direct connect to the plane. No waggon wheels so as to minimize inductance.

On PIC B, That via is too close to the BGA pin, A, C look better. Depending on space under the BGA one may have to make a choice between perfect and good enough !. Depending on the pitch of the BGA one can also rotate the caps so they are at 45 degrees which can allow to get the cap very close to the pin if needed.

Because the distances are so small between the BGA Pin and the cap, via connections,  the dominating factor electrically will be the capacitor used and how effective that capacitor really is at reducing noise at the BGA Pin.

Last thing, personal preference is to use rounded rectangles for the capacitor pads. Not rectangles. There are 2 combinations not shown in the pictures, wonder what they could might be ?.

Best Regards.

Same engineers here.

For me,  pin->capacitor->via is usefull for analog circuit.

Tip: for numeric, ideal is to have loop's area between power and ground the smaller as possible.For bga, unfortunately no choice but to plca capacitor on opposite side of bga. For others, i take into account the pcb thickness (already use 5mm thick!)  to know the real loop area.

Thank you, DavidJHutchins, for sharing your expertise. I completely concur with your point.

Decoupling capacitors, strategically placed between the power supply pins and ground pins of an IC, provide a low-impedance pathway for current during switching transients. This effectively mitigates voltage fluctuations on both the power supply and ground lines.

Do you have any additional thoughts or insights to share on this topic?