Singular stress fields at corners in flip-chip packages

An electronic device integrates diverse materials, and inevitably contains sharp features, such as interfaces and corners. When the device is subject to thermal and mechanical loads, the corners develop intense stress and are vulnerable sites to initiate failure. This paper analyzes stress fields at corners in flip-chip packages. The stress at a corner is a linear superposition of two modes of singular fields, with one mode being more singular than the other. The amplitudes of the two modes are represented by two stress intensity factors of dissimilar dimensions. To determine the stress intensity factors, we analyze the flip-chip structures under two loading conditions: stretching of the substrate and bending of the substrate. We show that the thermal loading of the flip-chip package is equivalent to the stretching of the substrate in generating the singular stress fields. We further show that the less singular mode may prevail over the more singular mode for some stretching-bending combinations. The relative significance of the two modes of stress fields also varies with materials, and with the substrate-to-chip thickness ratio.