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In recent years, package downsizing has become one of biggest trends in packaging technologies because of miniaturization and the high integration of electronic devices. As a result, the reliability of fatigue life has been prioritized as an important concern, since the thermal expansion difference between a package and printed circuit board causes thermal fatigue. But, the reliability of thermal fatigue life may be lowered by the dispersion of design factors such as the dimensions, shapes, and material properties of package systems. Also, the fracture modes (fatigue breakdown, frail destruction in the interface, and compound destruction, and so on) will be affected by these dispersion factors. Although this dispersion should be reduced in order to improve the reliability, reducing all dispersion is inefficient and time-consuming. So, the factors that greatly contribute to thermal fatigue life have to be reduced. In this study, evaluations of the influence of various design factors on the reliability of soldered joints of a Ball Grid Array (BGA) were carried out, and got each influence. With the results, design engineers can rate each factor's effect on reliability and assess the reliability of their design beginning at the concept design stage. Consequently, it will be possible to avoid almost all reliability problems from the beginning. Also, by rating the factors, the design period can be shortened.