Of fundamental importance to enhance the reliability of flip chip on board (FCOB) packages is to avoid the initiation and propagation of various interfacial failures and, therefore, robust interfacial bonds between the underfill and other components are highly desired. In the present study, the interfacial bond strengths of both conventional and no-flow underfill resins with die passivation, eutectic solder and epoxy solder mask are measured using the button shear test. The surface characteristics of these substrates are analyzed using various techniques, including optical scanning interferometry, scanning electron microscopy and contact angle measurements. It is found that the interfacial bond strength of the underfill with the eutectic solder is far weaker than of other interfaces. The degradation of underfill bond strength with silicon nitride passivation, eutectic solder and polymeric solder mask surfaces is enhanced in the presence of solder flux, and cleaning the fluxed surface with a saponifier is an efficient means to restore the original interfacial adhesion. The necessity of post-solder reflow cleaning is shown by performing thermal cycle tests on FCOB packages with different extents of flux residue. Distinctive solder failure behaviors are observed for the packages with and without post-solder reflow cleaning from the cross-sectional analysis.