The Influence of Properties of Solder Joint on the Stress of Underfill in Flip Chip Package

Underfill (UF) is an essential important material for releasing internal stress in the package during the operation life of a chip, significantly improving the reliability of the chip and box. In most cases, to thoroughly investigate the impact of UF on the package structure, it is necessary to carry out a series of complex experiments for reliability evaluation, which costs a lot of time and money. At present, numerical simulation, such as finite element analysis (FEA), is an effective method to help to analyze the failure-related problem in the package. However, several weak points still exist on accuracy and efficiency in FEA. The establishment of a reliable and straightforward FEA model will play a crucial role in evaluating the interfacial stress of UF. For example, the viscoelasticity of UF has been declared to be essential for the precise values of pressure and investigated in detail in our previous research. Meanwhile, the material constitutive of solder joints is also very important for evaluating the stress in the package, even if for the interfacial stress of UF. However, the related research is still lacking. In this paper, when the viscoelastic property has been applied for UF, the influences of the different material properties of solder joints on the interface stress of UF are carefully investigated. The most suitable solder joint constitutive has been proposed.