Measurements of the mechanical stress induced in flip chip dies by the underfill and simulation of the underlying phenomena of thermal-mechanical and chemical reactions

The stress sensing system, which has been developed recently, allows measuring the magnitudes and the distribution of mechanical stresses induced in the silicon dies during fabrication and testing of the electronic packages. The studies already presented in the last years focused on the effects of temperature cycling, 4-point-bending, moisture swelling, and molding. This paper reports the results of the latest investigation, in which the stress sensing system has been used to explore the chemo-thermo-mechanical effects of the epoxy underfill within a typical flip chip module. In-situ readings of all 60 measuring cells of the stress chip were performed cyclically during the entire underfill process. So, it was possible to clearly distinguish between the stresses curing of the underfill at the process temperature and the stresses induced by thermal shrinkage of the epoxy during the subsequent cooling - even as functions of process and design parameters. The results to be presented in the paper reveal the enormous curing stress and its dependency on the curing temperature as well as the influences of possible voids and geometric parameters like the stand-off height between chip to board on curing and thermal stresses. Furthermore, the paper presents a comprehensive multi-physics finite element analysis (FEA) on the induced stresses and leads to a better understanding of the underfilling process. In this simulation, the diffusion expansion material parameter has been used to model the curing stress that results from the chemical reaction while the thermal mismatch is captured by the coefficient of thermal expansion as usual. This way, the two phenomena could be addressed as separately as they appear in reality. This has further improved the validity and the quantitative accuracy of the FEA results.