Bonding head

Abstract: A bonding head 18 for an automatic wire bonding machine, comprises a flame-off electrode 2 which is adapted to be lifted away from an operative position in which it is used to form a ball 36 on the end of the bonding wire 38 and a capillary 34 through which the bonding wire 38 is supplied to the flame-off electrode 2. The capillary 34 is also adapted to apply pressure and ultrasonic energy to the wire 38 when it is in contact with the surface to which it is to be bonded. The flame-off electrode 2 is adapted to be lifted away from the operative position by movement of the bond head 18 from a flame-off position to a bonding position.

Description: This invention relates to automatic wire bonding machines for use in ball bonding. One known type of such machine comprises a flame-off electrode which is used to form a ball on the free end of a bonding wire fed through a capillary, which electrode is lifted away from its ball forming or operating position, before the capillary is lowered into position for bonding. The European Patent Application EP-A-0 043 692 discloses an automatic wire bonding machine by which a flame-off electrode is moved to an operative position to form a ball on the end of a bonding wire by means of a cam mechanism. A capillary supplies the bonding wire to the flame-off electrode and then to a bonding position on a surface. The capillary is moved by another cam mechanism, driven independently of the cam drive mechanism for the flame-off electrode. Generally in a wire bonding machine of this type, the flame-off electrode is lifted away from its operating position by means of a solenoid, before the capillary is lowered into its bonding position. There are two principle disadvantages in this system involving the use of a solenoid. The first is that the speed of operation of the solenoid is slow, which limits the overall bonding time, and the machine throughput. The second disadvantage is that synchronisation between movement of the flame-off electrode and movement of the bond head can also cause problems, leading to a further limitation on the speed of operation of the machine. In order to avoid damage to either the flame-off electrode, or the capillary of the bond head, through which the wire is supplied, an additional margin for safety has to be built into the operation of the machine to attempt to ensure that contact between these two components is avoided. It is an object of the present invention to provide a bonding head for an automatic wire bonding machine in which the above disadvantages are reduced or substantially obviated. The invention provides a bonding head for an automatic wire bonding machine as defined in claim 1. A method of wire bonding is also provided as defined in claim 3. In a preferred embodiment of the invention, the flame-off electrode is provided with a substantially cylindrical hole extending along the length of the electrode, said hole being open at the end of the electrode which is adjacent to the capillary in the operating position and adapted to be connected at the other end to a supply of gas. In this embodiment, a gas such as forming gas may be fed through the electrode to ensure that the ball is formed on the end of the bonding wire under an atmosphere of the gas such as forming gas, in order to prevent oxidation. Where the ball formation is carried out under forming gas, it is possible to use copper wire as bonding wire. Embodiments of the invention will now be further described with reference to the accompanying drawings, in which Figure 1 is a side view of the flame-off electrode, in the operating position; Figure 2 is a schematic side view of the flame-off electrode in the operating position, and the capillary in the flame-off position; and Figure 3 is a schematic side view of the flame-off electrode in the non-operating position, and the capillary in the bonding position. As can be seen in Figure 1, a flame-off electrode 2 fitted with a gold plated electrode foot 4, comprises a cylindrical hole extending along the length of the electrode. The electrode 2 is mounted on a support 6 attached by means of two adjustment screws 8, 10 to a lever arm 12 pivoted on a bearing 14. The arm 12 is connected by means of a connecting pin 16 to the bond head 18. The bond head 18 is provided with an eccentric 20 for height adjustment of the capillary (not shown in Figure 1). Electric current is supplied to the flame-off electrode 2 by means of a wire connected at 22. The electrode assembly is adapted to rock about the bearing 14. Rotation of the assembly in the clockwise direction is limited by a limiting stop 24, the height of which can be adjusted by means of a nut 26. The flame-off electrode 2 is hollow, forming a tube 28 through which gas can be blown from the end 30 remote from the spark gap to the open end 32 adjacent to the electrode foot 4. The flame-off electrode 2 is movable between a first, operative position, as shown in Figures 1 and 2, and a second inoperative position, as shown in Figure 3. In the first, operative position, the foot 4 of the electrode 2 is situated vertically beneath the capillary 34, the capillary 34 being in the flame-off position. A ball 36 is formed on the end of the bonding wire 38 supplied through the capillary 34, by creation of a spark between the electrode 2 and the wire 38, and the electrode is then moved from the operative to the inoperative position by moving the capillary 34 from the flame-off to the bonding position. The electrode 2 moves out from the operative position at the same time as the capillary 34 is lowered, as a result of the mechanical connection between the bond head 18 and the flame-off electrode 2. The flame off position is adjustable mechanically and electronically with respect to its height above the bonding position. The wire 38 is supplied from a supply spool (not shown) through a vacuum feed tube 40. While in the operating position, the wire 38 is held by a wire clamp 42 which moves with the bond head 18. The vacuum feed tube 40 is fixed and does not move during the bonding cycle. The electrode assembly is prevented from moving too far by the limiting stop 24 as discussed above.

Claims: A bonding head for an automatic wire bonding machine comprising a flame-off electrode (2) which is adapted to be moved from an operative position in which it is used to form a ball (36) on the end of the bonding wire (38) and a capillary (34) through which the bonding wire (38) is supplied to the flame-off electrode (2), which capillary (34) is also adapted to apply pressure and ultrasonic energy to the wire (38) when it is in contact with the surface to which it is to be bonded, characterized in that the flame-off electrode (2) is adapted to be moved away from said operative position by movement of the capillary (34) from a flame-off position to a bonding position. The bonding head for an automatic wire bonding machine according to claim 1, characterized in that the flame-off electrode (2) is provided with a substantially cylindrical hole (28) extending along the length of the electrode (2), the hole (28) being adapted to be connected at the end (30) remote from the spark gap to a Supply of gas, and open at the other end (32). A method of bonding a wire (38) to a component or to a substrate, using the bonding head according to claim 1, the method comprising the steps of: supplying the wire (38) through a capillary (34) to the flame-off electrode (2) in the operative position; forming a ball (36) on the end of the wire (38) under an atmosphere of forming gas; characterized by moving the flame-off electrode (2) from said operative position by moving the capillary (34) from a flame-off position to a bonding position.

Inventors: FARASSAT FARHAD, FARASSAT, FARHAD
Applicants: F&K DELVOTEC BONDTECHNIK GMBH, F & K DELVOTEC BONDTECHNIK GMBH
Application year: 1991
Main classification: B23K20
IPC codes: H01L21, B23K20