Don't use solder paste as flux.

No. Solder paste is not a substitute for flux when reballing RAM chips, and it won’t make the connection more reliable. It will probably make a mess.

Flux is just the chemical cleaner and wetting agent. Solder paste has solder balls suspended in flux. During reballing, you already have your solder balls arranged on the BGA. Adding paste introduces extra, uncontrolled solder balls that will merge, bridge, or shift your alignment. You end up with a lumpy, uneven joint array.

If reliability is the goal, use tacky flux (like Amtech or ChipQuik). It holds balls in place, cleans oxidation, and wicks into joints cleanly. Paste is for applying new solder, not for reflowing existing balls.

Stick to flux for reballing. Paste is a shortcut that makes your life harder.