Void formation in flip chip solder bumps. II

For pt.I see L.N. Goenka and A. Achari, Eighteenth IEEE/CHMT Symposium, Austin, Texas (1995). Miniaturization and clever assembly of electronic components are bringing new challenges to their functional reliability. Manufacturing process improvements by a thorough understanding of the chemistry of solder reflow are important to interconnect reliability. One area of concern is the formation of voids in solder bumps during the solder reflow process. This can result in premature solder-joint failure under the application environment. In this study, a three-dimensional model which predicts the motion and coalescence of bubbles within a solder bump during reflow has been developed. The model assumes a recirculatory flow within the bump that is generated by a combination of buoyancy and thermocapillary effects. The results show that large voids are formed as a result of the coalescence of numerous smaller bubbles. This model is based upon an earlier study which utilized a two-dimensional analysis, but did not show the formation of large voids within the joint as observed in cross-sectional results. Additional refinements to the present model include incorporation of the “added-mass” effect in the bubble-motion calculations, as well as the inclusion of thermocapillary effects. A cross-sectional analysis of solder bumps is presented in support of this work