I have designed some PCBs using ATmega328p TQFN IC's.
I have the circuit breadboarded and working using a ATmega328p but the DIP version.
Couple of questions before I go any further. It complies 'just' in the DIP breadboard version using UNO as the target board (97% usage - not ideal I know).
But, if I try to compile for a Pro Mini, it says that the program is too big at 103%.... why the difference? I thought they were identical IC's?
I cannot find any bare chips in the UK that are not either on unknown backorder or mentally expensive.
So, it's looking like an AliExpress order for the 328p TQFN IC's.
My other question is, surely I may as well buy Pro Mini's so that the bootloader is already programmed (I can just lift it off with my heat gun), rather than bare processors and have to faff around loading the bootloader.
The Uno has a 512 byte bootloader. The Pro Mini has a 2K bootloader. So does the Nano. That's why the Uno has room for your code, but the Pro Mini does not. If you buy Pro Minis and desolder the chips, they will have the 2K bootloader, so you'll have to reflash them anyway as Unos to make room for your code.
I should also warn that counterfeit 328Ps have been found on Pro Minis, but not on Nanos as far as I've heard. So if you're going to buy something to scavange processors from, you might want to go with Nanos.
Edit: I'm talking about clones, which I assume is what you are looking at buying.
Sorry.... TQFP.
I have a store on AliExpress that have been pretty good on Arduino boards.
It's actually only a few pence more for the Uno's, so I might grab a couple of those instead
Desoldering and resolder is every time a risc. SMD µC are not designed to multiple soldering actions. The ATmega has a max temperature of 260°C for 12 seconds. See figure 1-2 at page 2.