Delamination-induced stitch crack of copper wires

Abstract

Stitch crack is a recurring failure mechanism for many years in semiconductor packaging. Currently, the combination of highly filled mold compounds, and copper wire increases the risk for stitch cracks after temperature cycling (TC). Firstly, highly filled mold compounds generally have a Coefficient of Thermal Expansion (CTE) that is much lower than that of copper, and a much higher elastic modulus compared to traditional compounds, which results in a higher stress on the bond wire compared to traditional compounds having a CTE that matches the CTE of copper. Secondly, the copper wire experiences large plastic deformation while forming the stitch during wire bonding at elevated temperature, which leads to local ‘embrittlement’. The combination of both effects, i.e., increased stress, and reduced resistance to fatigue, leads to a higher risk of cracking. However, in general the stitch is sufficiently strong to withstand the increased stress. In this work, we will show that delamination is a prerequisite for stitch cracking to occur because it has a pronounced effect on the stress in the stitch, as it reduces the support by the lead, resulting in more force being exerted onto the stitch. Simulations reveal increased stress for a stitch where the mold compound is delaminated compared to a fully adherent compound. The results of the simulations match the results of TC reliability tests, and scanning acoustic tomography to identify delaminated regions within the package.