A new approach in free air ball formation process parameters analysis

The shape and size of gold wire ball formation deeply affects the quality of wire bonding. It not only affects the bond-ability of the first bond (ball bond), but also affects the possibility of processing low loop height bonding for thin packaging [such as thin small outline package (TSOP) and thin quad flat package (TQFP)] and high input/output (I/O) fine pitch packaging such as ball grid array. The parameters which affect the gold wire ball formation include: 1) tail length left after second bond; 2) type and shape of capillaries used; 3) material characteristics of gold wire; 4) supplied voltage, current, and time of electrical flame-off (EFO) unit; 5) gap between tail and electrode plate; and 6) relative position between capillary and electrode plate. In this paper, experiments were conducted to find the effect of these parameters on ball formation. Taguchi method together with neural network is applied in this research to find the best parameters setting for gold wire ball formation. It can then be used for wire bonding process parameter adjustment and process monitoring. It can also be used as reference for the development of wire bonders.