Bonding of lead wires to a microcircuit chip or die mounted on a lead frame for coupling to external circuitry is generally accomplished by ball bonding. According to this technique, a fine lead wire or bonding wire is held in a capillary tool so that the lead wire projects beyond the end of the tool. The capillary tool forms part of a ball bonding machine in which the tool is appropriately mounted and positioned over the metalized die pad of an integrated circuit chip. Examples of such ball bonding machines include the Model 478, High Speed Tailless Thermocompression Ball Bonder, manufactured by Kulicke & Soffa Industries, Inc. (K&S), Horsham, Pa. 19044, described in U.S. Pat. No. 3,643,321; the K&S Model 1418/1419, Automatic High-Speed Wire Bonder with Digital Bonding Head; K&S Model 1482, Automatic Wire Bonder; and the ball bonding machines of The Welding Institute, Abington, England, described in U.S. Pat. Nos. 4,323,759 and 4,098,447.
A ball is formed at the end of the lead wire or bonding wire by, for example, arc discharge between the bonding wire and another electrode. A melted portion of the metal at the end of the lead wire forms a spherical ball under surface tension. After solidification, the metal ball at the end of the lead wire is brought into intimate contact with the metalized die pad and a bond is formed typically by thermocompression or ultrasonic bonding.
Prior art methods of ball formation for bonding lead wires generally suffer from lack of control over the ball formation process with consequent variation in the size, shape, overall strength and quality, and oxidation of the bonding ball. In particular, one of the problems associated with prior art methods of ball formation is the thinning and weakening of the lead wire which often occurs above the bonding ball. Such thinning and weakening of the lead wire at the stem adjacent to the ball is referred to as "necking" and presents the risk of failure or fracture of the lead wire at the ball bonding site. Further disadvantages of prior art ball bonding machines and methods are discussed in the "Background Art" section of U.S. Pat. No. 4,390,770 entitled "Bonding Wire Ball Forming Method and Apparatus", assigned to the assignee of the present invention.
According to the improved apparatus and method for bonding wire ball formation described in U.S. Pat. No. 4,390,771, a ball is formed at the end of bonding wire extending from a capillary tool 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 electric arc discharge between the lead wire and the shroud or shield which function as complementary electrodes. According to the method disclosed, a controlled pulse train of electrical pulses is generated and delivered for arc discharge between the wire and shroud so that the quantity of energy delivered by the pulse train may precisely match the energy requirements for melting a ball of desired dimensions.
Applicants have found, however, that control over ball size and quality may be achieved while at the same time eliminating the problem of "necking" by delivering the ball formation energy with even greater rapidity in a shorter time and with improved timing control over the arc discharge energy in the context of the related cooperative operations of the ball bonding machine.
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