Direct Measurements of Underfill Local Strain Using Confocal Microscopy and Digital Image Correlation

In order to better understand the initiation of crack defects and predict the thermo-mechanical failure of underfill in microelectronic packages, a confocal microscopy-based digital image correlation (confocal-DIC) method was developed to measure the underfill local strain directly. A special underfill, consisting of transparent epoxy with Al₂O₃ particle fillers, was applied to meet the requirement of confocal imaging inside the resin. A preliminary validation was accomplished on two samples with a simple structure: (a) non-constrained sample for isotropic dilatation and (b) a thin-layer sample for strain gradients in the resin on a glass substrate. Results from both samples were in good agreement with the calculation from the coefficient of thermal expansion (CTE) or the numerical simulation from finite element method (FEM). Furthermore, we applied this technique to measure the strain distribution in the underfill at the chip corner area when the assembly was under thermal loading at 60 °C. The results showed that the maximum strain value appeared exactly at the chip corner area, which is consistent to the simulation results. The measured maximum first principle strain reached around 0.9 %, while the strain on the sidewalls was approximately 0.5 %. Due to the imperfections of the real corner resulting from dicing effects, the measured strain at the corner was lower than the FEM result. In general, the good agreement between measurements and calculations demonstrates the accuracy of our methodology for measuring the underfill local strain in microelectronic packaging assemblies.