Fairchild Camera and Instrument Corp.

Fairchild Semiconductor Corporation

Donald E Cousens

John A Kurtz

Mark D Dufour

An apparatus is described for maintaining and delivering a slack reserve length of lead wire between a spool or other source and the wire bonding tool of a lead wire bonding machine. A slack chamber or wind chamber comprised of a housing enclosure, an inlet guide on one side for guiding lead wire into the slack chamber from a spool, an outlet guide on the other side for guiding lead wire out of the slack chamber towards the wire bonding tool maintains the reserve length of lead wire in untangled condition. A source of pressurized dry air or other gas directs a gaseous flow into the slack chamber so that the lead wire is maintained suspended in the gaseous flow in an offset configuration. Wire sensors are operatively positioned in the slack chamber for sensing the offset of lead wire in the wind stream. The wire sensors are coupled to sensor and control logic for controlling the delivery and feeding of lead wire from a spool into the slack chamber. The wire sensors define inner and outer limits of offset thereby maintaining the reserve length within a desired range. Additional elements for back-up control of the lead wire and for handling the spool are described.

Apparatus for feeding bonding wire

A lead wire bonding machine is described for ball bonding the end of a lead wire held in a bonding tool to a die pad of an integrated circuit chip and for wedge bonding a segment of the lead wire spaced from the ball bond to a lead frame finger during successive ball bond wedge bond cycles. The bonding machine includes a variable linear drive such as a solenoid or small linear motor coupled to the bonding head for applying the first bond force to the bonding tool during ball bonding and the second bond force to the bonding tool during wedge bonding. A control circuit coupled to the solenoid or other variable linear drive delivers a first current having a desired profile or amplitude wave envelope for applying the first bond force with a first force profile during ball bonding to die pads and by delivering a second current having a desired profile or amplitude wave form for applying the second bond force with a second force profile during wedge bonding to lead frame fingers. A wave form generator generates the wave form envelopes for achieving the desired force profile so that the bond force may be varied according to the force profile during bonding at a particular site. Thus, the bond force may be varied and modulated according to the welding requirements at different die pads or lead frame fingers and according to the composition of the constituent metals of the lead wire and bonding substrate.

Variation and control of bond force

An improved lead wire ball bonding machine for bonding wire leads between an integrated circuit chip and the lead frame on which the chip is mounted is provided with a bonding tool position sensor coupled to receive the Z-motion velocity waveform signal to the servo motor which drives the bonding head and bonding tool. This sensor detects the signal level and direction of change or polarity of the Z-motion velocity waveform signal for determining the location of the bonding head and bonding tool. The bonding tool position sensor is coupled and adjusted for generating a first output signal corresponding to a first location of the bonding head and bonding tool during motion downward to the die pad of an integrated circuit chip prior to contact by the bonding tool and lead wire for ball bonding. The position sensor also provides a second output signal corresponding to a second location of the bonding head and bonding tool during motion downward toward a lead frame finger prior to contact of the bonding tool and lead wire for wedge bonding. The first and second output signals from the sensor are coupled to the ultrasonic generator of the bonding machine for application of ultrasonic bonding energy to the bonding tool prior to contact thereby increasing the bonding surface area between the lead wire and substrate. According to an alternative embodiment, an electrical sensor is provided for electrically sensing initial contact of the bonding tool and lead wire with the die pad or lead frame finger for actuating the ultrasonic generator and applying ultrasonic bonding energy to the bonding tool substantially upon initial contact with minimal delay thereby also optimizing the bonding surface area of the ball bond and wedge bond. Both the apparatus and method for lead wire bonding with increased bonding surface area are described.

Lead wire bonding with increased bonding surface area

A method and circuits are described for sensing and detecting bond attempts and weld attempts during bonding and welding of lead wire. The method and circuitry are particularly applicable for detecting missed ball bonds and missed wedge bonds during bonding of lead wire between the die pad of a microcircuit chip and the lead frame on which the chip is mounted. A sensor (30) or sensing circuit (42) senses the different characteristic electrical condition of the lead wire (11) following a ball bond attempt and following a wedge bond attempt. A bond attempt indicator (45) indicates high resistance in the lead wire following a missed ball bond while weld attempt indicator (46) indicates low resistance in the lead wire (11) following a missed wedge bond. The lead wire (11) is isolated from uncontrolled contacts with ground potential while the lead wire is held in the bonding tool and bonding machine. Switching circuit (38) electrically switches and couples the lead wire to a controlled ground coupling (27), (52) during ball formation or to a ground isolating voltage source (28 ), (54) for delivering current to the lead wire and establishing a bond sensing and detecting mode following the ball bonding operating mode and wedge bonding operating mode of the bonding machine. The switching circuit (38) is actuated by a control signal (40) from the control signal circuitry of the bonding machine or from independent control circuitry. Time sequence circuitry may also be provided for the control signals.

Lead wire bond attempt detection

Apparatus for maintaining reserve bonding wire

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.

Pulse-width control of bonding ball formation

Apparatus and method are described for rapid ball formation at the end of lead wire retained in the capillary wire holding tool of a ball bonding machine. A circuit is coupled between the cover gas shroud and lead wire for establishing controlled electrical discharge between the end of the bonding wire and the shroud, for melting and forming a ball. The circuit includes a DC power supply for delivering a positive polarity to the shroud and negative polarity to the lead wire for drawing a discharge of electrons from the end of the lead wire to the shroud. A capacitor is coupled in series with the DC power supply for receiving the electrical discharge. An impedance is also coupled in series for limiting the electrical discharge current. Charging of the capacitor limits and shapes the electrical arc discharge to a controlled pulse profile of short duration for rapid ball formation. An electronic gate is also used to control pulse duration. The rapid and controlled ball formation avoids necking or thinning of the lead wire which often occurs at the stem above the bonding ball.

Controlled bonding wire ball formation

A method and apparatus is disclosed for forming a ball at the end of bonding wire or lead wire held in a capillary wire holding and bonding tool for ball bonding of the lead wire to an integrated circuit chip. The method of ball formation is of the type in which the end of the bonding wire is enclosed in a shroud or shield and the shield and the end of the bonding wire are flooded with an inert cover gas. Ball formation is accomplished by electrically discharging an arc between the bonding wire and the shroud for melting and forming the ball at the end of the wire. A passageway is provided for delivering and mixing hydrogen gas into the inert cover gas delivery line at a location upstream from the shroud sufficient for complete mixing. The rate of flow of hydrogen gas is metered and controlled for adjusting the percent by volume of hydrogen in the cover gas mixture to a desired range.

Cover gas control of bonding ball formation

In a method for welding a lead wire or bonding wire from a microcircuit chip mounted on a lead frame to a lead frame finger, the lead frame finger is preheated prior to any substantial electrical or thermal coupling between the lead frame finger and chip. Intense but controlled energy is applied to the lead frame finger at levels which might otherwise damage the IC chip. In one embodiment the lead frame finger is preheated to a temperature below the melting point of the metal comprising the lead frame. Enhanced bonding is thereafter effected by thermocompression bonding etc. In another embodiment the preheating step comprises melting a portion of the surface of the lead frame finger, forming a molten pool or puddle in the surface. Bonding of the lead wire is effected by immersing a section of the wire in the molten pool or puddle. In order to preheat the lead frame finger a controlled pulse train is delivered for arc discharge at the bonding location. Bonding apparatus and circuitry for preforming the method are described.

Lead frame wire bonding by preheating

Method and apparatus for forming a ball at the end of bonding wire or lead wire in a capillary wire holding and bonding tool is described suitable for ball bonding of copper and aluminum lead wire to integrated circuit chips. A ball is formed by substantially enclosing the end of the bonding wire in a shroud or shield, flooding the shroud or shield and the end of the bonding wire with an inert gas, and generating a controlled pulse train of a preset count of electrical pulses for establishing arc discharge between the wire and the shroud or shield. The method permits precise control and metering of energy delivered by controlling the parameters of the pulses of the pulse train for melting and forming a ball of uniform quality without oxidation of the metal. Corresponding apparatus and circuitry are described which may be retrofitted into stock ball bonding machines or provide new machines.

Growing community of inventors

Saco, ME, United States of America

Donald E Cousens