Pulse-width control of bonding ball formation

Abstract

A method, timing control circuit, and power supply are described for initiating arc discharge between the cover gas delivery shroud and lead wire held in the bonding tool of a lead wire bonding machine for melting and forming a ball at the end of the lead wire. An arc discharge timing control pulse controls duration of the arc discharge within an empirically determined time window between the shortest and longest durations of arc discharge which result in optimal ball formation of a substantially spherical ball at the end of the lead wire without necking of the lead wire above the formed ball. The timing control circuit also provides an initial cover gas movement delay before ball formation for displacing oxygen from the shield and the end of the lead wire, and a subsequent cooling delay for solidifying and cooling the formed ball in the cover gas stream prior to ball bonding. The timing control pulses are applied to the ball bonding machine and are also coupled to the transistor electronic switch of a relatively high voltage power supply circuit for applying the arc discharge voltage through a resistance to the shield and lead wire when the electronic switch is conducting. Optical coupling is used between the low voltage timing control circuit and high voltage power supply circuit for electrically isolating the two voltage levels. The duration of arc discharge is controlled to fall within the critical time window between longest and shortest durations of arc discharge which result in optimal ball formation. Initially, the lead wire tail length extending below the bonding tool is selected to provide a ball of specified size; the voltage is selected according to tail length to form the mass of tail length metal into a substantially spherical ball; and a neck of desired thickness is selected by adjusting the duration of the arc discharge timing control pulse within the time window.