Sure. Here's what you should do to apply repeatable amounts of flux on BGA during rework: * Squirt some gel flux on a plate or something like that that has a flat bottom and raised sides to corral the flux. * Cut notches into one edge of an old credit card. The number and depth of the notches control the flux thickness. We call it a comb. * With the BGA in one hand and the notched credit card in the other, swipe the notched edge of the credit card through the flux. * Dip the BGA into the flux as the card passes through, leaving a just perfect amount of flux on the solder balls of the BGA. * Place the BGA and get to soldering.
We get the impression from the way you posed the question that you're using a liquid flux. If that's true, it's not good practice to squirt liquid flux of any type on rework areas, because the liquid spreads and flux on the outside edge does not receive the proper amount heat for activation, leaving harmful resides.
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