Area array packaging technology is a common one at the present time largely because of its ability to provide a large number of interconnections. The analysis of an area-array package under thermal loading represents a challenging task primarily due to the non-linear, rate-dependent material behavior of the solder alloys. In a recent study [1], Deshpande and Subbarayan proposed a decomposed analysis procedure for area array packages that was shown to be substantially more efficient than a full three-dimensional finite element analysis without significant loss of accuracy. More importantly, the decomposed analysis procedure enabled re- usable modules akin to object-oriented paradigm of modern computer science. In this study, the decomposed procedure is modified and extended to underfilled area array packages. An approximation model based on the elastic foundation is formulated to capture the interaction between the solder joint and the underfill material. Foundation stiffness, in terms of five independent parameters, was defined using an empirically derived formula. A hypothetical 5×5 area array package was solved using developed decomposed procedure. Results show an average error of 6.4% in solder displacements and more than 100% saving in computation time. Moreover, by completely eliminating the interaction between solder joint and underfill, we were able to achieve the same level of accuracy when using decomposed analysis. This implies that in terms of solder energy dissipation, the direct interaction between the solder joints and the underfill has a very limited impact and therefore can be ignored. Underfill material affects solder's deformation mainly by acting as a computer between the substrate and the circuit board.
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