Lead-free flux used in lead-free solder ball attachment process has been found to play a vital role in improving lead-free solder joint robustness & performance. In this study, two water soluble lead-free solder ball attach fluxes, flux A & flux B, each from different supplier, were compared in terms of the following characteristics: time variation of flux properties, flux stability vs. temperature change, flux wetting through Ni/Au reflow coupon test, and flux performance during & after ball attach process. It was also found that ball attach reflow profile has to be optimized for individual flux type according to the flux properties. Result shows that lead-free flux B performs better than flux A in terms of flux stability over time & temperature, wetting ability, cold ball pull (CBP) & ball shear result with lower brittle fracture rate.