I've searched this and many other sources on Info on decoupling capacitor placement, but couldn't find any info specific enough to my current problem.
I'm currently doing a PCB Layout for a tiny, ultra-fine-pitch WLCSP microcontroller. It's 3x3mm and has 49 Balls (7x7) at 0.4mm pitch, and I'm looking at another with 36 balls (2.1x1.9mm with 0.35mm pitch). There are a couple VDD and VSS pin pairs, and unfortunately not all on the outer balls. I've tried placing all the recommended decoupling caps as close as possible to the pin-pairs, and connecting them with the shortest traces possible, as everyone recommends. However, I'm running into routing problems, and have realized that trying to keep one Capacitor on each pair of supply pins actually leads to much longer traces than if I'd just connect some of the pins to each other, so some of the supply pins would share a single capacitor.
I get that each pair of supply pins ideally should have its own capacitor. I'm not 100% sure of all the reasons why, but my understanding is it's mainly for guaranteeing shortest traces-> lowest impendance, and therefore best (local) "stabilization" of the supply voltage. And of course for seperating Analog from Digital, but since my circuit is digital only, that's not MY worry at the moment.
Now I'm wondering if this still holds true for such tiny devices, where I could, for example place a single capacitor on the bottom side, right under my component, and have all my Vdd and GND traces reach it with a single via and less than 1mm trace. 1) Why would I want to put 2 or three there? 2) Would it be better or worse if there is absolutely no space there for say the third cap anymore, and my next best option would be to place the third Capacitor next to the chip on the top layer, but require two vias and 3mm trace length to reach it, compared to a single via and 1mm trace the first or second Capacitor already under the PCB?
The supplying LDO with a bigger Capacitor (2.2 or 4.7uF will be maybe 5mm further away...)
Thanks for any helpful Info.