BGA Decoupling Capacitors Placement

[milos.stankovic]

Hi,
I am trying to place decoupling capacitors under BGA processor. If I understand strategy correctly - goal is to place one capacitor for each two power supply pins. I have a question regarding 1V5_DDR power supply on attached picture. There are 2 kind of capacitors: 220nF (smaller) and 22uF (bigger bulk capacitors). The arrow is pointing to 220nF decoupling capacitor which is to far away from any 1V5_DDR ball. Is that ok? I suppose that bigger bulk capacitor (22uF) doesn't have to be very close to power supply pins. My concern are 220nF decouple capacitors.

Best regards,
Milos

[robertferanec]

I suppose that bigger bulk capacitor (22uF) doesn't have to be very close to power supply pins

milos.stankovic Yes, you are right - the 22uF doesn't have to be directly under the pins, placing the 220nF directly under the pins is more important.

[milos.stankovic]

Maybe there are more 220nF caps than number_of_vcc_pins/2. I will check. What is the puprose of two 220nF caps adjacent to 22uF? The more is better in any case.

[robertferanec]

Maybe there are more 220nF caps than number_of_vcc_pins/2. I will check

Normally you may want to use 1 small CAP per 1 PIN.

What is the puprose of two 220nF caps adjacent to 22uF?

Smaller caps will react much faster than bigger caps.

[milos.stankovic]

I understand. Thanks