Improved Modeling and Analysis Strategies for Plastic Ball Grid Array Package Assemblies Subjected to Thermal Cycling
Type of DegreePhD Dissertation
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Solder joint fatigue is one of the primary reliability issues in the electronic packaging area. Solder joints typically experience cyclic loading, yielding, and highly nonlinear material behavior when subjected to temperature changes. It is well-known that experimental fatigue life testing is an extremely time-consuming task. Thus, reliability predictions using the method of design for reliability (DFR) using finite element analysis can provide a more effective way for performing solder joint fatigue life studies, often reducing investigations from several months in length to a few days or weeks. In the initial portion of this research, several different FEA approaches for Plastic Ball Grid Array (PBGA) assemblies have been compared, and their advantages and disadvantages have been discussed, especially in the areas of simulation accuracy and efficiency. The examined approaches included different types of 2D and 3D models (2D plane strain, 3D slice, and 3D quarter model), use of Multi-Point Constraints (MPCs), and use of Submodeling. The general assumptions used in all models included use of the Anand viscoplastic model for the solder constitutive behavior, and the Darveaux volume-weighted averaging energy-based fatigue criterion. For each approach, the predicted inelastic work density per cycle was calculated in the critical solder joint, and comparisons were made to distinguish the advantages and disadvantages of each of the evaluated models. In the remainder of this work, several new approaches have been developed to improve the finite element modeling of PBGA assemblies including new techniques for (1) continuous meshing methods, (2) discontinuous meshing methods, and (3) global/local (submodeling) methods. For the continuous meshing models, an improved fan-out meshing approach has been developed for modeling PBGA assemblies. In the investigated approach, a dome-like (hemispherical) brick element fan-out meshing scheme was used for the transitions from the circular interfaces at the top and bottom of the solder joints to the rectangular geometries found in the BT laminate and PCB substrate. This approach improved the accuracy of the simulations while simultaneously reducing the element count and simulation times up to 66%. In the study of MPCs, a contacting element ratio was proposed for meshing mismatched contacting surfaces to adjust the balance between the required solution quality and simulation time. The master-slave relationship of the MPC technique has also been evaluated. Considering the developed master-slave relationship and the suggested location of the MPC contact pairs, the modified MPC-based analysis strategies presented great reductions in the computational times without sacrificing solution quality. In the study of global/local submodeling approaches, it was found that the solution accuracy was dominated by the mesh quality and the step size for the local model. Furthermore, for the local model, the minimum required volumetric size has been determined for accurate simulations. Finally, the use of simplified geometry shapes for the solder joints in the global model has been studied. In this dissertation, the most popular FE approaches for analyze the PBGA assemblies were evaluated and improved upon. Several new modeling and analysis strategies were developed to improve computational efficiency, simulation accuracy, and solution consistency between different approaches. Thus, with the proposed strategies and guidelines, confusions can be reduced and a more reliable solution can be calculated with fewer computer resources.