The Fundamentals of Wire Bonding by Techstar Innovations Pte Ltd.

The Basic Gold Ball Wire Bonding Cycle

Centering of Free Air Ball Process
( inside the capillary chamfer)

The ball bonding process starts off with the wire clamp open and a free air ball at the end of the wire, which is protruding outside the capillary tip. The diameter of the free-air ball ranges from 1.2 times (for small ball ultra-fine pitch application) to 1.5 times (for large ball non-fine pitch application) of the wire diameter (WD) using the TechStar Vesta-S Electric Torch Upgrading kit.
To achieve consistency of the free air ball size, it reduces consistent tail length after second bond formation, and consistent electronic flame-off (EFO) firing.
A wire tensioner is used (an air column wire drag system) to ensure that the ball is up against the face of the capillary prior to the capillary being lowered onto the die bond. If the ball is not fully up against the face of the capillary and fully within the inside chamfer prior to bonding, a chance of producing a ball bond not centered on the bond pad will likely to result a "Golf Club Ball Bond".

Ball Bond Formation Process

The capillary is lowered and the free ball is centered within the inside chamfer area. The transducer's ultrasonic energy together with force, temperature and time deforms the ball to the shape of the inside chamfer and hole. Ball bonding into the bond pad occurs through a combination of plastic deformation and interfacial slip of the two materials due to force, temperature and ultrasonic energy applied.

Looping and Stitch Formation Process

After the ball is bonded to the die, the capillary raises, feeding out the wire through the capillary (commonly called the Loop) to begin to form enough wire length until if reaches above the second bond target point-necessary to make a connection to the second bond position. Sufficient wire is fed out above the ball bond to form the loop, and then the capillary lays the wire down in a natural parabolic or elliptical curvature must be supported during the stitch formation.
The looping profile is basically controlled by the wire bonder's software used and inputted by the operator. Achieving low-loop, long lead bonding is no problem because of the programmable looping algorithm that optimizes its formation for each different length.

Tail Formation Process

Once the capillary reaches the targeted second bond position, the stitch is then formed with similar factors applied during the first bond. The capillary deformed the wire against the lead or substrate producing a wedge-shaped impression.

Detach Formation Process

It is important to note that a certain amount of tail bond is left to allow pulling of the wire out of the capillary after the stitch bond formation in preparation for the next free air ball formation.

Free Air Ball Formation Process

The capillary lifts up with its tail protruding outside the capillary tip. This action would then enable the electronic flame off (EFO) to be activated and a free air ball is again formed to ready for the next bonding cycle.