We always apply solder paste to either the PCB pads or the BGA balls. I have found that only using liquid or tacky flux does not always ensure proper wetting of the pads. Also The BGA sits closer to the PCB making cleaning and endoscopic inspection more difficult. Appying solder paste facilitates the wetting and breaks the surface tension of the solder ball allowing the solder ball and the paste to completely reflow together and create a good solder joint. I have seen many times BGAs placed with just flux where the BGA balls reflow and colapse but do not form a joint with the pad. Applying solder paste to the PCB pads can be an art... thats why we apply paste to the BGA balls 95% of the time. Appying paste to the balls also lets us use paste when there is very little or no real estate around the BGA site.
I have read in several component data shhets that recomend between 3-5 heat cycles as the maximum. So that means 1 to place the BGA, 1 to remove it, 1 to reball it, and one to place it back again. Thats four heat cycles for one rework cycle. I know people do more all the time with success. I think the key is to make sure the BGA is "dry" before any rework is attempted by baking the BGA according to the coponent data sheets recomended baking procedure.
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