Not BGA, but I have seen thin caps mounted under DIPs. The dielectric
material they used (Z5U or Y5V? I forget now) was not quite as good as the
better chip caps, so capacitance falls off faster at higher temperatures.
And they do get hotter since they are nearly in contact with the chip.
They were a space-saver, but we did no direct comparison to see how the net
result compared, noise-wise.
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