Reliability of thermosonic bonded palladium wires in high temperature environments up to 350 °C

In this paper we present for the first time a detailed study of thermosonic bonded palladium (Pd) wire on gold (Au) thin film metallization after long term aging at temperatures up to 350 °C. The thickness of the upper Au layer is 0.8 µm. It belongs to a copper (Cu) / nickel (Ni) / gold (Au) stack electroplated on a sputtered Cu seed layer. A titanium/tungsten (Ti10%/W90%) layer beneath the Cu metallization is used as diffusion barrier. Palladium and gold wires of 25 µm diameter have been ball bonded to these pads and afterwards exposed to temperatures of 250 °C and 350 °C for up to 2500 hours. Finally the thermal evolution of the bond reliability was studied by wire pull tests, ball shear tests and cross sectional scanning electron microscopy. The comparison of the palladium test results with standard gold bonds shows a superior mechanic reliability of the palladium bonds.