Interface characterization of Cu Cu ball bonds by a fast shear fatigue method

Abstract

A highly accelerated shear fatigue testing method is presented to test the long-term reliability and reveal the bonded interface of thermosonic Cu[sbnd]Cu ball bonds. The method is an adaptation to a new industrial fatigue tester (BAMFIT) and can be conducted without an intricate specimen preparation. This method induces mechanical cyclic shear stresses to the Cu nailhead in order to initiate fatigue fracture until lift-off, revealing the actual bonded interface. This study compares the fatigue resistance of Cu wire bonded to coarse and fine grained Cu and Al metallization. The fatigue experiments are accompanied by nano indentation tests, shear tests and finite element analysis. The fatigue results showed the best performance for Cu bonds on coarse grained Cu pads (metallization), followed by those bonded on fine grained Cu while the Cu[sbnd]Al nailheads failed at least a decade earlier than Cu[sbnd]Cu bonds. Annealing the specimens prior to testing resulted in slight increases in the number of loading cycles to failure (Nf) for Cu bonds as well as for Cu[sbnd]Al bonds, while the scattering in Nf for Cu bonds increased. Nevertheless the calculated endurance limit of the fatigue data decreases with increasing annealing stages, due to a change in the fracture probability curve. With the ability to compare the fatigue behaviour of the bonded interface within minutes, this method is most suitable for rapid qualification at an early stage of development.