With the tacky flux process, joined balls and solder ball bridging defects are typically caused by bridging flux, flux flooding or decreasing flux viscosity between solder balls combined with ball movement. Ball movement can be caused by transport vibrations from the ball placement machine to the reflow oven as well as by hand or automatic conveyor systems.
Furthermore, as the flux slumps due to the reflow temperatures, small vibrations of the reflow conveyor belt and uneven airflow in convection ovens can cause the balls to move and wick towards one another. The warpage of the package will also magnify this effect because it has been found that larger FC package sizes are found to be more prone to solder ball movement than smaller ones as demonstrated by lower first pass yields with these packages.
