Investigation of bump crack and deformation on Pb-free flip chip packages

The demand for die to package interconnects free of Pb in the next generation flip chip packages requires a flux and underfill solution that meets package reliability requirements. Bump cracks and bump deformation were observed during temperature cycling on large body, full Pb-free ceramic flip chip BGA packages during initial package development. The phenomenon is considered unique in terms of its nature and failure mechanism. Traditional bump crack issues are concentrated at or near the silicon and underfill interfaces. In this case, cracks occurred within the bulk solder away from either silicon or substrate interface. Failed bumps also showed severe deformation. In addition, morphology differences in the underfill material surrounding the affected bumps provided important clues as to the nature of the failure mechanism. An extensive investigation to understand the root cause of the unique bump crack issue in the bulk solder, which covered both process and material-related factors, resulted in a clear understanding of the failure mechanism and the implementation of an effective solution to the problem. This manuscript describes the relationship of the flux residue-underfill interaction, the localized change in underfill properties due to the flux residues, and eventually the impact of this change on the bump integrity during package stressing. These findings made it possible to establish a good flux-underfill selection methodology to achieve a robust Pb-free package solution that is being implemented in next generation flip chip products.