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We propose a kinetic model to describe a pancake-type void propagation in flip chip solder joints due to current crowding in electromigration. The divergence of the vacancy fluxes at the interface between the solder and Cu6Sn5 leads to void formation and propagation along the interface between them. Based on the continuity condition, the void growth velocity is calculated. The theoretical calculations are in reasonable agreement with the experimental results.

Articles you may be interested inBroad ion beam milling of focused ion beam prepared transmission electron microscopy cross sections for high resolution electron microscopy J. Vac. Sci. Technol. A 19, 982 (2001); 10.1116/1.1368198 Cantilever technique for the preparation of cross sections for transmission electron microscopy using a focused ion beam workstation Proposals for exact-point transmission-electron microscopy using focused ion beam specimen-preparation technique J.Transmission electron microscopy observation of thin foil specimens prepared by means of a focused ion beam Precision transmission electron microscopy sample preparation using a focused ion beam by extraction methodIn surface mount technology of electronic packaging, the lead frames are finished with a layer of Pb-free solder. A large number of Sn whiskers are found on the surface of the finish, especially that of eutectic SnCu or pure Sn. The whiskers have a faster growth rate on an eutectic SnCu finish than on a pure Sn finish. Some of the whiskers on SnCu are long enough to short neighboring legs of the lead frame. We report here the study of spontaneous growth of Sn whiskers on these finishes using focused ion beam imaging and transmission electron microscopy. Cross-sectional samples, both normal and parallel to the growth direction of a whisker, were prepared. Precipitates of Cu 6 Sn 5 in the grain boundaries of the finishes have been found. The growth of these grain boundary precipitates, due to the chemical reaction between Cu and Sn at room temperature, provides the driving force for spontaneous Sn whisker growth. Many more of these grain boundary precipitates exist in the SnCu finish than in the pure Sn finish. This is the main reason why Sn whiskers grow faster on the SnCu finish.

The flow characteristics of a number of underfills were evaluated with quartz dies of different patterns and pitches bonded on different substrate surfaces. Perimeter, mixed array, and full array patterns were tested. Observations on the flow front uniformity, streaking, voiding, and filler segregation were collected. The information was compared with the results predicted by a new simulation code, plastic integrated circuit encapsulationcomputer aided design (PLICE-CAD) under DARPA-funded development. The two-phase model of the combined resin and air takes into account geometrical factors such as bumps and die edges, together with boundary conditions in order to track accurately the propagation of the flow fronts. The two-phase flow field is based on the volume-of-fluid (VOF) methodology embedded in a general-purpose three-dimensional (3-D) flow solver. Index Terms-Capillary flow, filler settling, flip chip, flow simulation, flow streaking, full array pattern, mixed array pattern, organic laminates, peripheral pattern, quartz dies, underfill flow.