We use no flux when preparing a rework site, after BGA removal, and all pads
look and rework fine. We ocassionaly replace and reflow BGA's without solder
paste using our choice of paste, not liquid, flux. This too works fine and
X-Rays (always used after BGA rework) show no voiding.

Mostly, we replace and reflow BGA's using solder paste. We also use high temp
solder balls in each of four corners to effect real solder fillets much as
with column grid arrays as our data shows them to be more reliable.

And on it goes.

Earl Moon