Investigation of Inter-Layer Dielectric (ILD) Failure by Hygroscopic Swelling

Hygro-mechanical stresses arise in an electronic package when the polymeric materials swell upon absorbing moisture while the adjacent non-polymeric materials, such as silicon chip, solder bump, and other metal interconnect do not experience swelling. The coefficient of hygroscopic swelling is a material property to characterize the dimensional change with moisture absorption. Current hygroscopic swelling characterization techniques use an averaged approach based on the averaged moisture content. However, the moisture distribution is not uniform across the test specimen during measurement. This introduces analysis errors in determining the hygroscopic material property. In the first part of this study, a locally accurate coefficient of hygroscopic swelling is obtained based on actual moisture distribution using diffusion law and the corresponding hygroscopic deformation. Results showed that the ratio of the averaged coefficient of hygroscopic swelling to the accurate coefficient of hygroscopic swelling by present method could be as large as 2.5. The mathematical formulation is then transformed to analyze the experimental data to obtain the true material hygroswelling property. Based on the locally accurate coefficient of hygroscopic swelling analysis, the second part of this paper investigates the impact of hygroscopic deformation on ILD structure under HAST condition using finite element analysis. FEA simulation results revealed the significance of contribution of hygroswelling-induced stresses under bump region. The major focus is on the impact of combined thermal and hygro-mechanical stresses from different underfills. Total of five kinds of underfills are evaluated. Experimental results and failure analysis show the ILD cracking/delamination during HAST. A strong correlation between the hygroscopic swelling and normal stresses exerted on ILD region was found. The moisture expansion by moisture absorption, i.e., the product of the coefficient of hygroscopic swelling and saturated moisture concentration, plays a key role in reliability performance. Finite element results give a good guideline on the underfill material selection, and also give an insight of the failure mechanism associated with moisture absorption