Dear guys! I want to ask a few question about ic soldering.

I brought a dummy (locked) i4s motherboard for testing and practicing.
I have stencils, flux, wire and any other thing to desoldering an ic (or atleast i think i have everything). I tried to desoldering the wifi ic, but it didnt come off… The air flow was about 300-350C. I dont know but i think i will buy an engraver to engrave the ics. Guys, help me. What did i do worng?