That goes without saying But the way you accomplish the impedance getting low enough is the million dollar question. At some point you simply run out of room to add more 10uf capacitors around a BGA package (not to mention leaving room for traces). So what to do then?
...You do nothing at all, of course.
What more could you do, anyway?
Probably the most graphic examples are those ancient, giant CPU hybrids -- IBM, and... SPARC??, among others?? -- where the power supply interconnect was multilayer PCB absolutely studded with ceramics to essentially make a loaded transmission line of extremely low impedance.
And those were only, what, 100s of MHz? Very hungry, 3.3 (or even 5?) V, huge chips, butting into the limits of the relatively coarse process node they had at the time.
Many of those modules had wide body and even LGA ceramic chip caps on them, too, for the same reason we put them on interposers (or even die directly) today of course. But they did it on metal-ceramic monster backplates, because PCBs couldn't possibly handle the number of signals and planes, impedances, tempco (expansion rate), etc. that these beasts demanded.
The trickery doesn't end there; PDN design extends quite far on-chip, to the extent that logic itself is designed carefully so that limited populations are switching all at once, in any given region, so that the say 90% of inactive transistors that cycle
can act as bypass capacitors for the ones that are switching. Chip designers aren't dummies, obviously; in particular, they know the limitations of their systems and environments. You can make estimates for the characteristic impedance or stray inductance of wire bonding, or die attach (flip chip), the interposer, balls, etc. You will find it's not simply difficult, but outright impossible to get low supply impedances, at the PCB, much beyond 100MHz or thereabouts.
You can certainly have effective bypassing higher than 100MHz -- indeed bypasses need not even be explicit components at the highest frequencies, but that's for RF amplifiers, where much higher impedances are seen (say 50 ohm ballpark), and the inductance of the return paths is integrated as part of the resonant circuit design. No such advantage is had for extremely low impedance CPU supplies, which also need it low across the board, and the only way they can do that is with a staged bypass scheme, from PSU to chip-adjacent to onboard to within-die.
What I don't know, is whether embedded stuff generally does this, onboard (interposer, stacked-die, or sufficient as bare die), or how close it must be placed on PCB. Datasheets
never give impedance and frequency requirements, so we are all in the dark here, bickering about long shadows cast upon our walls.
That still leaves "best practice", which aims for low and stable impedance, which is easiest done with the larger values, small chips, and avoiding certain parallel combinations, staggered sizes, etc. as discussed in part above. But don't be a damned fool about it. Plenty of newbies throw together boards with poor bypassing, or even lacking planes entirely, and manage to do work with them. They might be an EMC disaster, but only the worst are so bad they're entirely nonfunctional.
Personally, I suspect that, by combination of onboard capacitance and modest power consumption, most embedded MCUs (of more than modest frequency i.e. >= 100MHz; say, STM32F4 to F7 and similar) aren't very demanding, whether by way of a relatively high impedance limit (imagine, bypassing an MCU with a couple high-ESR electrolytics instead... but, that should indeed be possible with some parts!), or a relatively low frequency limit (say, onboard handles >50MHz so you only have to worry up til then). The noise and jitter requirements are also rather banal, except for certain specific peripherals, which likely have isolated / separately filtered IO or VCCA domains anyway (ADC most obviously, but also Ethernet, USB, core clock (PLL), etc. may apply).
Even the most trivial specs, manufacturers never provide -- consider the capacitance of VDD to GND alone! This is so easily measured, you can do it yourself; I'd be willing to bet you'll find many large-ish MCUs in the ~uF range. Now
that would be some interesting (and useful) context for PDN design, wouldn't it?
Related note: microSD cards are permitted up to 8.2uF onboard capacitance, good to know for hot plugging purposes. Where's that capacitance coming from? Integrated chip caps? Or the chip itself? Who knows...
Tim