The effect of underfill on various thermo-mechanical reliability issues in super ball grid array (SBGA) packages is studied in this paper. Nonlinear finite element models with underfill and no underfill are developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and direction-dependent, as appropriate. The stress/strain variations in the package due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, timedependent creep strain and total inelastic strain in solder balls. The effect of copper core on the solder ball strains is presented. The possibility of delamination at the interposer-underfill interface as well as substrate-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA packages. The predicted number of thermal cycles to solder joint fatigue are compared with the existing experimental data on similar nonunderfilled BGA packages.
The accumulated equivalent inelastic strain per cycle and the maximum strain energy density over one cycle have been used as damage metrics to map the solder fatigue damage during Field-Cycling (FC) and Accelerated Thermal Cycling (ATC) simulations. The objective of this work is to develop accelerated thermal cycling guidelines for flip-chip on board and flip-chip chip-scale electronic packages used in, for example, automotive applications. The percentage contributions of plastic and creep strains to the total inelastic strain and contributions within the accelerated cycling are used as a basis for developing modified accelerated thermal cycles. Different temperature regimes are explored to match the contributions of plastic and creep strains to total inelastic strain during field-cycling and accelerated thermal cycling and to reduce the time required for accelerated thermal cycling. The process mechanics of component assembly, time-and temperature-dependent material behavior and critical geometric features of the assembly are taken into consideration while developing the comprehensive virtual accelerated thermal cycling methodology.
The effect of underfill on various thermo-mechanical reliability issues in SBGA (Super Ball Grid Array) packages is studied in this paper. Non-linear finite element models with underfill and no underfill have been developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperaturedependent, while other materials are modeled temperature and direction-dependent, as appropriate.The stress/strain variations in solder joints due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, timedependent creep strain and total inelastic strain in solder balls. The possibility of delamination at the interposer-underfill interface as well as board-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA solder balls. IntroductionWith the increasing need for higher performance, smaller size, and higher pin count, ball grid array (BGA) and chipscale packages (CSP) are being increasingly used in many applications. Burnette et al.[2] have studied the board-level reliability of underfilled ceramic BGA packages. In flip-chip on board (FCOB) assemblies, underfill is often used to enhance the reliability of solder joints. However, in the case of BGAs and CSPs, underfilling is done under select instances, typically to enhance reliability and/or to provide environmental protection to the solder balls.The Super Ball Grid Array (SBGA) is a BGA package developed for higher thermal and electrical performance. In an ongoing research program at Georgia Tech, test vehicles with SBGA packages are being studied for military aircraft applications. When SBGA packages are used in military aircraft applications, the second-level solder balls are often encapsulated with an underfill for preventing possible moisture condensation around the solder balls. Although the underfill is used for such environmental protection purposes, the effect of underfilling SBGA packages on thermomechanical reliability is not adequately studied. In the ongoing research program, computational models are developed to understand the reliability of SBGA packages with various underfills.
Various constitutive and fatigue-life predictive models for lead-tin solders in SBGA (Super Ball Grid Array) packages are studied and compared with the results from experimental data. Two solder compositions, 62Sn/36Pb/2Ag and 63Sn/37Pb are studied in this work. The fatigue life of 62Sn/36Pb/2Ag solder is studied using different constitutive models that take into consideration both the time-independent and time-dependent behavior of the solder. The fatigue life of 62Sn/36Pb/2Ag solder is predicted using an energy-based predictive model and compared with the experimental data. The choice of various predictive models on the solder joint life is studied using 63Sn/37Pb solder. Various predictive models, available in the literature, for eutectic and near eutectic solder compositions are studied to predict the fatigue life. Guidelines are provided for selecting constitutive and predictive models with appropriate damage metrics.
The effect of underfill on various thermomechanical reliability issues in SBGA (Super Ball Grid Array) packages is studied in this paper. Non-linear finite element models with underfill and no underfill have been developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and directiondependent, as appropriate. The stresdstrain variations in solder joints due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The possibility of delamination at the interposer-underfill interface as well as board-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA solder balls.
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