Characterization and modeling of hygroscopic swelling and its impact on failures of a flip chip package with no-flow underfill

Moisture plays an important role in the integrity and reliability of plastic electronic packaging. While the basic procedures for characterizing the hygroscopic swelling properties has been detailed in a number of papers, the characterization of packaging materials presents unique challenges that are not adequately addressed in these papers. One of those challenges is to investigate the impact of non-uniform moisture distribution across the specimen using currently available metrologies. In this paper, an analytical coefficient of hygroscopic swelling was derived based on three dimensional actual moisture distribution using diffusion law and the corresponding hygroscopic deformation. The mathematical formulation was then transformed to analyze the experimental data to obtain the accurate material property of hygroscopic swelling. The theoretical predication showed a remarkable agreement with the experiment results. Detailed discussions are given on the effect of test specimen aspect ratio, diffusivity, and time range in collecting the test data. A practical guideline is proposed in conducting the experiments. Based on the accurate coefficient of hygroscopic swelling, the second part of this paper assessed the reliability of a flip-chip ball grid array package with no-flow underfill under the pressure cooker test (PCT) conditions. Finite element modeling implementation was used to analyze the moisture distribution, hygroscopic swelling behavior, and thermomechanical stress. The magnitude of tensile hygroswelling stress acting on UBM is found to be greater than the compressive thermal stress, and may cause the UBM opening failure during the PCT. Finite element results give an insight of the failure mechanism associated with moisture absorption