The thesis investigates the thermo-mechanical deformation and stress of a flip-chip package (FCBGA) via both experiment and simulation. First, Shadow Moiré is used to evaluate the warpage of a package at elevated temperature. Then we adopt the finite element method incorporated with the software ANSYS to simulate the warpage of a package and compare the obtained results with experiment at data. Then, the material properties of underfill, the thickness of die and the substrate are considered as important parameters. Their effects on stress and strain fields of package are studied. In case of FCBGA with and without underfill, we find that FCBGA with underfill can reduce stress concentration and increase warpage of a package in comparion with FCBGA without underfill. As for FCBGA with and without heat slug, it is observed that the warpage of FCBGA with heat slug is smaller than that of FCBGA without heat slug. Both stress and strain in the packages of above two cases are similar. The parametric study about the underfill, we find that smaller modulus and CTEs of underfill can reduce the stress and strain of package. However in the consideration of thicknesses of both die and substrate, it is shown that thinner die can reduce stress and strain of package, but thinner substrate does not. So it is suggested that thicknesses of die are the thinner the better.

This work discusses the deformations of various unsymmetric composite panels due to thermal and mechanical loads. Chapter 2 focuses on the warpage of large unsymmetric curved composite panels due manufacturing anomalies. These panels are subjected to a temperature change of -280°F to simulate the cooling from the autoclave cure temperature. Sixteen layer quasi-isotropic, axial-stiff, and circumferentially-stiff laminates are considered. These panels are intended to be symmetric laminates, but are slightly unsymmetric due to the manufacturing anomalies. Rayleigh-Ritz and finite-element models are developed to predict the deformations. Initially, to serve as a basis for comparison, warpage effects due to orthotropic thermal expansion properties in perfect panels are investigated and are found to produce deformations not captured in two-dimensional theories. This is followed by the investigation of the effects of ply misalignments. Ply misalignments of 5° are incorporated into the laminate, one layer at a time, to produce unsymmetric laminates. It is found that ply misalignments produce warpages much larger than those induced by orthotropic thermal expansion properties. Next, unsymmetric laminates resulting from ply thickness variations are investigated. Layers 10% thicker than nominal are incorporated into the laminate, one layer at a time, while the remaining layers are of uniform thickness. Due to the change in fiber volume fraction of the thicker layers, corresponding material properties are modified to reflect this change. The results show that ply thickness variations cause warpages of about 25-50% of those induced by ply misalignments. Finally, warpage of panels due to nonuniform cooling due to inplane thermal gradients during cure is investigated. A thermal gradient of 0.1°F/in. is used to construct six inplane distributions. It is found that the warpages induced by thermal gradients are very small. The warpages are negligible with respect to those induced by ply thickness variations or ply misalignments. Deformations induced by thermal gradients depend primarily on the magnitude of the thermal gradient, but not on the pattern of distribution. Overall, ply misalignments cause the most warpage, followed by ply thickness variations. Important variables for these imperfections are, the through-thickness location of the imperfections, the orientation of the layer containing the imperfections, and the lamination sequence. All cases show that geometric nonlinearities are important to accurately predict the deformations induced by these imperfections. Chapter 3 discusses the deformations of composite plates that are intentionally fabricated to be unsymmetric. Such plates, if flat, might be considered in applications where bending-stretching coupling effects can be used to advantage. It is assumed the laminates are cured at an elevated temperature and then cooled 280°F. Significant deformations result because of the high level of asymmetry in the laminate construction. Accordingly, geometric nonlinearities are included in the models. Four cross-ply laminates and three angle-ply laminates are considered. Four-term and 14-term Rayleigh-Ritz models are developed, together with finite-element models to model the deformations. Actual specimens were constructed and the deformations measured to compare with predictions. The results show that agreement between predictions and the experimental results are good. The 14-term Rayleigh-Ritz model is found to be the most useful due to its ability to find multiple solutions, its physical basis, and computational efficiency. Chapter 4 discusses the deformations of initially flat aluminum, symmetric, and unsymmetric composite plates due to axial endshortening under various boundary conditions, the aluminum and symmetric plates serving as a baseline. Seven plates are considered, each with three boundary condition combinations, namely, clamped ends and sides (CL-CL), clamped ends with simply-supported sides (CL-SS), and simply-supported ends and sides (SS-SS). Generally, the boundary conditions play a key role in the deformation characteristics of the plates. The aluminum and symmetric cross-ply plates have no out-of-plane deformations until classic buckling, or primary instability, then each exhibits two stable solutions. Each also exhibits secondary instability that results in two stable solutions. The symmetric laminates show less of a dependence on the boundary conditions compared to the unsymmetric laminates. Unsymmetric laminates show a mixture of characteristics. Some cases exhibit primary instability, other cases do not. Some cases exhibit secondary instability, while some case do not. The unsymmetric cross-ply laminates have only one stable solution after secondary buckling, while most other laminates and boundary condition combinations have two stable solutions. It is interesting to note that for the unbalanced unsymmetric [302/90/0]2T laminate, the boundary conditions controlled the sign of the out-of-plane deflection from the onset of axial endshortening. Generally speaking, the CL-CL cases carry the most load, followed by the CL-SS, and then the SS-SS cases. Like all the problems discussed in Chapter 2 and 3, geometric nonlinearities are found to be important for this case as well. / Ph. D.

The main objective of this research is to studying the moisture effect on warpage of PBGA via IR-reflow process by utilizing Shadow Moirè method. The relationship between precondition variations and moisture content of PBGA is also studied.

To ensure the shelf life of a semiconductor, the integrated circuits are usually embedded in an epoxy molding compound (EMC) to shield it from stress and corrosion. The EMC is a viscoelastic material and is categorized as a thermoset. Working with viscoelastic material appose a challenge due to the cure-induced warpage. Warpage is a deformation that reduces stress resistance and can also make for assembly issues. Being able to predict the warpage accurately is an essential part of the electronics industry. To simulate the warpage, the finite element method is usually used. The Research Institutes of Sweden (RISE) in Piteå, which focuses on composites, have created a solver and corresponding material definition for simulating the curing process of a viscoelastic material. In this thesis, the solver created by RISE was investigated to see how well it could predict the warpage during the curing of an EMC. The investigation consisted of reproducing the simulating conducted by Lin et al. in the article "Modeling and Characterization of Cure-Dependent Viscoelasticityof Molded Underfill in Ultrathin Packages" where the curing of an ultrathin flip-shipChip-Scale Package (fcCSP) was simulated. The result will be reproduced using the simulating program LS-Dyna and both the RISE material definition and an build-in material definition will be simulated to see the difference. From the investigation, we could conclude that although the result from the article was unobtainable, the RISE model could predict the warpage more accurately which shows the importance of the cure shift factor. The investigation also found that the RISE model has some implementation errors for the stress-strain calculations in the RISE model. In conclusion, the RISE model was able to predict the warpage in a desired way, but more studies need to be created to ensure the model’s accuracy for the correct warpage magnitude.

The Plastic Ball Grid Array (PBGA) package and flip-chip technology have been widely used in the microelectronics industry. However, due to the effect of hygroscopic and thermal stresses, the reliability is still of concern during manufacturing and operation, especially for the thermal-mechanical behavior of its corresponding interfaces. Influences of the storage conditions and reflow parameters on the warpage of the PBGA package are investigated in this study first. As the results, the warpage reflected the interaction of the extent of moisture absorption and the change in reflow parameters significantly. Furthermore, a critical relative moisture absorption between 0.25% and 0.30% is found for a considerable warpage response. Next, this study presents an experimental investigation of the adhesion strength of epoxy-based encapsulant material to solder mask coated FR-4 substrate under thermal cycling. Effects of the number of thermal cycles on the interfacial strength are investigated by using button shear test. The relationship between the interfacial strength and thickness of solder mask is also examined. Moreover, to characterize the degradation and fracture behavior, the morphologies of fractured surfaces of the test specimens are analyzed by scanning electron microscopy. The results of this experiment show that the interfacial strength of the epoxy-based encapsulant/solder mask/substrate joint is apparently reduced by thermal fatigue. And, the test specimen with larger solder mask thickness has higher interfacial strength. Finally, the single-lap joint test, nonlinear finite element analysis and Moiré interferometry are employed to obtain strain/stress distributions on the interface of solder mask and substrate. The effects of solder mask thickness and overlap length are then determined, separately. The results of this study can afford important information for characterizing the features of moisture absorption, warpage and interfacial adhesion of PBGA packages. Furthermore, it can be helpful to identify improvements required in reliability of the package design.

Abstract¡G The main aim of this paper is to utilizing the Shadow Moirè method to study the moisture effect on the warpage of PBGA IC package under the IR-reflow process. An environmental controlled box is designed and built. Three combinations of different moisture and temperature status are chosen for the purpose of study. The result shows that the moisture and temperature change do yielding the change of the warpage of the IC package.

The main aim of this study is to extending the holographic interferometry technique to measure the effect of the temperature ramp-up rate on the warpage of IC packages in the IR-reflow process . It is noted that both the warpage and the ambient temperature change can cause image fringes. Therefore, an auxiliary sphere is used to identify the fringe numbers caused by the ambient temperature change during the experiment. Then, the Taguchi method will be deduced to study the effect of the temperature ramp-up rate and peak waiting temperature on the warpage of PBGA package in the IR-reflow process.

The current packaging trend toward to smaller and thinner has pushed the manufacturing technology to the limits. During the assembly processes of IC packages, delamination at interfaces and mechanical breakage of components are common mode of failure. The induced thermal stress within the package is one of the major contributions to these failures. According to the disparity of the coefficient of thermal expansion (CTE) between different components, internal thermal stress and warpage will be induced when the package undergoes temperature excursion. In this paper, the Die attach epoxy curing and encapsulation curing process induced warpage and thermal stress were studied by finite element software Marc & Mentat. As comparison, two kinds of Molding materials of the package considered, and the result will compared with the experimental data. Finally, studied the effect of the material thickness and the impact significance of each design variable on the design objective will also be discussed.

The main objective of this paper is to utilizing the Shadow Moiré method to study the effect of IR-reflow Condition Variations on the warpage of PBGA Packages. Two combinations of different moisture and temperature status are also chosen for the purpose of the study. The result shows that the first peak temperature is the most effective variable on the warpage of PBGA packages under the IR-reflow process.

The main aim of this paper is to study for the warpage and von Mises stress of PBGA package caused by the variations of mechanical properties of materials in IR-reflow process. At first, taking advantage of the package symmetry only one quarter of the package was modeled by MSC. Marc. The influence of heat transfer was considered by comparing the results of Coupled mechanical-heat transfer analysis and Mechanical analysis. In the second part, the coefficient of thermal expansions and elastic modulus of molding compound and substrate are selected as the four control factors, and the influence of the four control factors to the warpage and von Mises stress were observed. At last, emperical formulas to predicted warpage and von Mises stress values were obtained.