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The influence of an underfill's engineering properties on the reliability of Sn/Ag/Cu-bumped flip chips was systematically investigated using a homologous series of experimental underfills. In all cases, "bump-fading" was found to be the prominent failure mechanism after thermal cycling. This is characterized by the progressive disappearance of the solder bump contrast image, as observed under scanning acoustic microscopy. The bump-fading phenomena also correlates to the development of cracks at the interface of the bulk metallic solder and inter-metallic compounds, located in the region where the solder ball is attached to the under bump metallization. The rate of solder bump fading and crack development is inversely proportional to the underfill's glass transition temperature Tg and modulus E, in accord with theoretical predictions of the changes in solder joint shear strain during thermal cycling. In particular, high Tg, high modulus underfills are more effective in coupling the misfit thermal strains of the flip chip die and substrate, converting shear strains in the xy plane into z-axis bending strains. The implications of increased package bending as the price for reductions in Sn/Ag/Cu solder joint shear strain, and the accompanying danger of increased die cracking and low-k interlayer delamination, are discussed.