Study of underfill corner cracks by the confocal-DIC and phantom-nodes methods

Abstract

In a flip chip package, due to the sharp square chip corner geometry and the loading from mismatches in the coefficients of thermal expansion between materials in the assembly, a stress concentration in the chip corner area is often thought to greatly impact reliability in thermal cyclic loading. Cracks often originate from this stress concentration, and may lead to the failure of the device when propagating into the chip or electrical connections. In order to better understand the mechanism of underfill cracking and predict the crack path, a method based on laser scanning confocal microscopy and digital image correlation (confocal-DIC) was used to measure the local deformation around a crack from a chip corner. A sample was fabricated by bonding a 6 × 6 × 0.55 mm3 silicon chip to a 20 × 12 × 0.4 mm3 quartz substrate, in a configuration that is similar to a flip-chip package. The SU8-2005 epoxy resin mixed with a mass fraction of 0.1 wt% alumina particle fillers was used as the underfill material for the purpose of measurements. Naturally initiated cracks were observed along the diagonal direction when the sample was subjected to −18/25 °C thermal cycles. Artificial cracks with lengths of 160 μm and 640 μm were fabricated from the chip corner by a laser, along the 45° diagonal direction, similarly to the naturally initiated cracks. The artificially cracked sample was imaged at 25 °C and 5 °C by a confocal microscope and the strain distribution around the crack was estimated by DIC. The maximum hoop strain and first principal strain were located at the crack front area for both the 160 μm and 640 μm cracks. These data were used to build a numerical model using the extended finite element method (XFEM) with the phantom-nodes approach. When the mesh size was decreased to 16 μm, the hoop strain obtained by simulation at the crack tip was 22.0% and 9.5% lower than that measured by confocal-DIC, for the 160 μm crack and the 640 μm crack, respectively. This difference decreased to 8.2% and 6.3%, respectively, when the distance was from 50 to 175 μm away from the crack tip along the diagonal direction. The distribution of hoop strain was in good agreement with the measured values, when the element size was small enough to capture the strong strain gradient near the crack tip. A propagation model was realized based on the phantom-nodes method. The crack started from the chip corner and evolved into a planar crack along the diagonal direction, which is in good agreement with the experimental observations.