A Mechanistic Study of Underfill Cracks by the Confocal-DIC Method

Due to the geometry of the rectangular chip and the mismatch of the coefficient of thermal expansion between chip and substrate, a stress concentration usually appears in the chip corner area in the underfill, when a flip-chip package is subjected to thermal loading. A high level of stress concentration can generate cracks from the chip corner that may lead to the failure of the device. In order to better study the mechanism of underfill cracking, a method based on laser scanning confocal microscopy and digital image correlation (confocal-DIC) was developed to measure the local deformation around a crack from a chip corner. A sample with a similar geometry to a flip-chip package was fabricated by bonding a 6x6x0.5 mm3 silicon die to a 20x14x0.4 mm3 quartz substrate. A transparent epoxy resin mixed with 0.1 wt% alumina particle fillers was used as underfill for the purpose of measurements. Artificial cracks were fabricated from the chip corner by a 355 nm laser, with lengths of 160 pm and 640 pm, along the 45° diagonal direction. The direction of artificial cracks was in good agreements with the naturally initiated cracks at the chip corner, which were observed when the chip was subjected to -18/25°C thermal cycles. The artificially cracked sample was observed at 25°C and 5°C by a confocal microscope and the strain distribution around the crack was estimated by image processing. The maximum value of the first principal strain was located at the crack front for both the 160 pm and 640 pm cracks. A numerical model was built using the extended finite element method (XFEM) with phantom-nodes in ANSYS. Thanks to the enrichment nodes by XFEM, no refinement meshing was required for the crack neighborhood area. The distribution of hoop strain was in a general agreement with the experimental results, but deviations were observed. We conclude from the comparison with confocal- DIC measurements that the XFEM simulation provides an effective methodology for the crack modeling in the flip-chip package, but that further work is required to correctly model all aspects of the complex strain distribution around actual underfilled chip corners.