The Influence of Lid to the Stress of Underfill in Flip-Chip Package

In flip-chip package, in order to improve the reliability of solder joints, underfill (UF) are usually used. At present, the package used for evaluating UF reliability is lidless in most case, which is different to the actual flip-chip structure with metal lid. Thus, the influence of lid with different structures and material properties to the stress of underfill is a question worthy of study. In this paper, based on finite element analysis (FEA), the influence of CTE, elastic modulus and thickness of lid on package warpage and UF stress are analyzed in detail. We find that the lid will indeed significantly affect the deformation or warpage in package. For lid with different CTE, three different warpage modes are presented. However, it is worth noting that for stress of UF, larger lid CTE will lead to larger UF interface stress, regardless of the warpage in package. The mechanism of deformation and stress in package is explained in detail. In summary, we believe that the stress level of the UF-related interface will be significantly underestimated in lidless structure.