Analysis of the Wire Bonding Joints of an IGBT Module

IGBT modules used in electric and hybrid vehicles are assembled by connecting approximately 500 thick Al wires ( ϕ 400 μ m), requiring the largest scale wire bonding of any automobile part. It is accepted that the probability of cracks occurring within the IGBT chip due to damage during wire bonding is about 1 in 1,000,000. Toyota has been conducting research to clarify the cause and generation mechanism of this problem. Other companies who have also conducted investigations have reported that the cause of the problem is Si nodules resulting from Si components within the Al electrode of the chip. However, characteristics of the generation mechanism, such as the influence of surface convexity of the chip and the path by which stress sufficient to generate cracks is exerted, have not been clearly explained. In this article, the generation mechanism is examined through detailed observations of damage within the chip and analysis of stress using simulations. From these investigations, the following conclusions were reached: (1) the simulations and analyses suggest that bonding damage occurs at the bonding terminal joints from the concentration of tensile stress during bonding, (2) convexity of the chip surface causes stress to concentrate in the convex portions, (3) the extent of the damage is made worse by the shape of the convex portions and the Si nodules, and (4) the stress that concentrates on the bonding surface is variable and depends on the shape of the wire loop.