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Flux is necessary to promote wetting between solder and metallized surfaces during flip chip bonding process. However, flux residues are unavoidable and may have detrimental effects on the properties of the underfill materials. In this study, the effects of flux residues from a commercial no-clean flux on two types of underfills were systematically investigated using a contact angle goniometer, thermal mechanical analyzer (TMA), thermogravimetric analyzer (TGA) and dynamic mechanical analyzer (DMA). It was found that the presence of flux residue reduced the glass transition temperatures (Tg) of both cured underfills studied. Although α1 (CTE below Tg) of cured underfills were slightly affected, α2 (CTE above Tg) increased significantly. The DMA tanδ curves of flux residue contaminated samples were found to be broader than those of pure underfills. In addition, the rubbery storage moduli of underfills contaminated with flux residues also decreased significantly. The existence of flux residues was found to increase the moisture absorption of the cured underfills.