Instructables

Step 4: Add flux to the PCB.

The balls in the BGA don't have flux so you *absolutely* have to put down flux on the board prior to doing the reflow. If you don't add flux then the oxide on the top of the pads will keep the balls from flowing and you'll end up with slightly squished balls that are not actually connected to the underlying PCB.
 
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pyper3 years ago
When doing SMD stuff at work I tend to use "tacky" flux which comes as a gel. It not only serves as a flux but makes positioning IC's a lot easier as you can sit them on top of a little blob of it and then fine tune their position with your tweezers (aligning TSSOP48's would be pretty much impossible without it IMHO) It also helps stop things getting blown about in the oven too!