During gold ball bonding, a gold ball is first formed by melting the end of the wire (which is held by a bonding tool known as a capillary) through electronic flame-off (EFO). This free-air ball (Fig. 1a) has a diameter ranging from 1.5 to 2.5 times the wire diameter.  Free air ball size consistency, controlled by the EFO and the tail length, is critical in good bonding.  For a discussion on how the melting of the wire affects grain size distribution and wire strength, see: Grain Size Distribution in Gold Ball Bonds.

The free-air ball is then brought into contact with the bond pad. Adequate amounts of pressure, heat, and ultrasonic forces are then applied to the ball for a specific amount of time, forming the initial metallurgical weld between the ball and the bond pad as well as deforming the ball bond itself into its final shape (Fig. 2).

The wire is then run to the corresponding finger of the leadframe, forming a gradual arc or "loop" (Fig. 1b) between the bond pad and the leadfinger. Pressure and ultrasonic forces are applied to the wire to form the second bond (known as a wedge bond, stitch bond, or fishtail bond and shown in Fig. 3), this time with the leadfinger. The wirebonding machine or wirebonder (see Fig. 5) breaks the wire in preparation for the next wirebond cycle by clamping the wire and raising the capillary.

During aluminum wedge bonding, a clamped wire is brought in contact with the bond pad. Ultrasonic energy is then applied to the wire for a specific duration while being held down by a specific amount of force, forming the first wedge bond (Fig. 4) between the wire and the bond pad. The wire is then run to the corresponding lead finger, against which it is again pressed. The second bond is again formed by applying ultrasonic energy to the wire. The wire is then broken off by clamping and movement of the wire. 

Gold ball bonding is faster than wedge bonding because it is non-directional. However, gold ball bonding can not be used in hermetic packages because gold will melt at sealing temperatures.  Unlike Al wedge bonding, Au thermosonic bonding requires heat to match the hardness of Al to Au.