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The risk of pre-solder crack and bump crack after thermal cycling test is often observed in large die flip chip ball grid array (fcBGA) packages to affect the corresponding reliability. The reason always comes from the singularity of geometry as well as the mismatch of material properties, which cause the critical stress in intermetallic compounds on the copper pad layer and solder bump. Failure phenomena, such as polyimide and extreme low-k (ELK) delamination, solder crack and die crack have often been investigated by using incompatible and unsuitable underfill materials during the packaging process. Choosing the right underfill material for every package is essential in solving these potential failure risks. However, the resulting mechanical behaviors are more complicated to analyze because of the complex properties of under-fill materials. For the purpose of realizing the thermal-mechanical stress distributions and effects of under-fill materials in large die fcBGA, the reliability of the thermal cycling test with several commercial under-fill materials is evaluated. The statistical and simulation result achieved by adopting 3-D finite element analysis (FEA) is presented to validate the reliability result, which are well aligned with each other. To capture the most important mechanical properties in 28-nm ELK large die fcBGA, such as the parameters of packaging geometry, material properties of Young's modulus, coefficient of thermal expansion and glass transition temperature to comprehend the corresponding warpage and stress responses, a comprehensive simulation study is also utilized by FEA. Through these results, not only can the proper parameters to have better stress and warpage reduction be selected, but also the significant factors to provide the best reliability in 28-nm ELK large die fcBGA can be determined. This paper can be a good reference, and can effectively serve as design guidelines in cases where under-fill selection analyses and optimum mechani- al solutions for 28-nm ELK large die fcBGA and other critical packages are required. It is helpful to prevent the critical failure issues that are caused by improper geometry and material selection with enabling reliable simulations and analyses.