How to decide whether to place a microcontroller on the top or bottom side in PCB layout?
I'm not a professional, but I try to answer for whatever I know:
Mostly components are put preferably on one side (top side). However, when you have limited space, you can use both sides.

If you plan to automatically pick&amp;place/assemble the components, mostly it is cheaper to use one side only. or if (due to space) you need both sides, and want to manually solder the bottom side yourself, use the most simple/easiest components to solder on the bottom side (which is mostly not an MCU).
Also, mostly a PCB is put on the bottom side of an enclosure. Since MCUs could get warm, the top side is mostly easier to release the heat.

But for functionality it does not matter on which side you place components.