To remove flux residue, water must be able to penetrate underneath a flip chip – this is no easy task with low standoff bump designs. There are some strategies to help with water cleaning.
Higher water temperatures and higher pressures may be the obvious solution, although tuning the water jets to the right angle may be more efficient since it doesn’t consume more power like the previous methods. Although the Fluid Dynamics courses we attended in college may give you a good idea where to focus the stream of water – don’t stop there. Try a few different angles at the same temperature and pressure and compare the results. With a little trial-and-error you should be able to increase the water-only cleaning capability of your equipment without increasing process time, temperature, or flow rate.