Analysis of Cu-wire pull and shear test failure modes under ageing cycles and finite element modelling of Si-crack propagation

In microelectronic packaging, wire bonding is the predominant method for making electrical connections. Copper is increasingly substituting gold as interconnection material since it is a much cheaper alternative and it also offers several physical advantages. Adequate and reliable mechanical integrity of the connection is usually checked by process controls based onto ‘‘wire pull’’ and ‘‘ball bond shear’’ tests. In this paper the two methods are compared in terms of sensitiveness in detecting a latent weakness of the bond-pad structure, either induced by inappropriate wire bonding process or cumulated during reliability ageing. The failure modes (in terms of frequency and maximum test load) observed at the ball bond interface have been investigated on two different batches of a same chip, obtained from different wire-bonding recipes and including both unstressed and aged units. Cross-sections of the samples, submitted to pull and shear both in destructive and non-destructive tests, have allowed us to investigate the relationship between the bond morphological characteristics (metal deformation and potential micro-damages induced by copper bonding) and the weak points for fracture propagation inside the bond-pad inner layers and the silicon substrate. Besides the experimental activities, fracture mechanics and the finite element method have been employed to model the pull and shear tests. The aims of the finite element modelling have been to predict the reduction of test maximum load in defective ball bonds and the crack growth angle adopting a mixed- mode criterion. Good results have been obtained by the numerical fracture analysis, which can then sup- port the reliability characterization and mechanical improvement of the bond.