Thermomechanical description of interface formation in aluminum ultrasound (US)-wedge/wedge-wirebond contacts

Wire-bonding is and will stay one of the essential interconnection methods in electronic packaging. Physics-based modeling of interconnects between the wire material and the substrate metallization has been constantly advanced over the last five years. Due to the variety and complexity of the mechanisms occurring during bonding it is required to clarify some open issues for a thorough interpretation of the metallurgical processes that take place at the interface of the weld and in the wire. One of these issues relates to the thermo-mechanical processes at or in the vicinity in the interface between the wire and the substrate during an ultrasonic wedge/wedge wire-bonding. These processes shall be qualified by a micro-thermomechanical model and also incorporated quantitatively in the bonding process. In this paper a thermomechanical analysis of ultrasonic wire bonding is performed by means of 3D finite element (FE) simulations. On the wire the bonding force is applied as well as ultrasonic vibration, which causes shear stresses in the wire and a frictional movement between the wire and the pad. The change of temperature in the interface between the aluminum wire and the gold metallization of a copper-nickel pad is studied. It is shown that a rise of temperature is obtained due to the plastic deformation of the wire caused by the bonding force, the shear stresses and by the friction with the pad. It is also shown that the maximum temperature is still lower than the temperature required for a proper material transport, proofing that this temperature rise cannot be alone responsible for ultrasonic wire bonding.