which flux to use for replacing chips on logic board

Getting original Amtech flux used to be difficult. Amtech now sells direct (AMTECH VS-213-A-TF) so you can order it directly from the manufacturer. It works really well but I don't appreciate the smell.

Another alternative I like is the MG Chemicals 8341 (https://www.mgchemicals.com/products/sol...) It is very easy to source and works as well as the Amtech without the nasty odour :>).

Another flux that gets recommended is the ChipQuick SMD291. It works well enough but I don't like it because the flux tends to crystallize when heated making it very hard to clean off the board. That's okay on large robust, boards but on the high density iPhone logic board, sometimes with delicate micro-jumpers, it requires more work than it should.

Of course, they all do the job they're supposed to. The rest is all personal taste and every micro-solderer will have varying opinions. Try for yourself and see what works best for you.