Electrical – Difference between VDD-VSS and VDDQ-VSSQ pin pairs for SDRAM ICs

You do not need to place the decoupling capacitors as close as possible (or even directly) to the power pins. Instead, be sure to use a VDD and VSS plane. Connect the microcontroller power pins as direct as possible to these planes using vias. Then, place the decoupling capacitors to a place nearby where it suits you best. Connect these decoupling capacitors directly to the planes using vias, again.

Why does this work? At the frequencies the controller is working, the power supply impedance should be low. Capacitors are low impedance in the range of approx. 100 kHz to 20 MHz. (Btw, at about some tens of MHz, the plane capacitance is slowly taking over.) But this only works if the caps are connected via a low inductance path. The idea behind placing the decoupling capacitors next to the power pins is to reduce the inductance between them and the power pins. Now, the plane has almost no inductance. Capacitor placement has almost no impact if you are using planes. We have successfully been using well defined "capacitor isles", instead of capacitor scattering, on relatively large PCBs.

Another tip: Try to place the VDD and VSS vias of the capacitor close to each other, as this reduces the inductance from capacitor to plane.

As a chip maker, it's easy for me to explain the cause of the imbalance. It's that there are several different rings of VDD in the IC for different purposes, but only a single ground. The different VDD rings can be in different voltages, but the ground is always at zero volts.

So for the ground, there is a solid copper rectangle in the leadframe (that's what the IC pins extend from) under the die for the ground. Internally, there can be dozens of pads that are all down-bonded to the ground copper. This way the ground can be quite solid across different parts of the IC, which minimizes substrate currents - current flowing through the copper will not cause problems such as latch-up in the IC, unlike strong substrate currents that cause latch-up conditions.

So, internal to the plastic casing in the IC, there are, more or less, the GND/VCC pairs that you mention in your question. But as for the ground, due to the ground pad in the leadframe, not every GND pin needs to extend out from the IC package - the ground copper inside the IC package is strong enough.