Ball bonding is a common method of wire bonding. Gold is preferred because its oxidation tolerance is much greater than that of aluminum. In comparison to copper, gold is also preferred due to the harshness of copper and therefore potential damage to the surface of die. Wire bonding requires a near perfect environment - making the bond with the best of materials is prudent. In high-reliability applications, this is even moreso the case.
Using heat, pressure and ultrasonic energy, a "ball" is made at each end of the wire. The capillary of the wire bonder fires off an electrical charge that heats the outgoing wire and, through a melting process, forms it into a ball. The capillary is applied with ultrasonic energy via an attached transducer. Depending on the application, transducers ranging from 60Khz to 152Khz can be integrated into the bonder.
Ball bonding should not be confused with ball bumping.