Metal fusion bonding – With means to juxtapose and bond plural workpieces – Wire lead bonder
Reexamination Certificate
2001-05-07
2003-09-09
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
With means to juxtapose and bond plural workpieces
Wire lead bonder
C228S001100, C228S110100
Reexamination Certificate
active
06616030
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to machines and apparatus for making ultrasonic wire bonds on miniature workpieces such as microcircuits and read/write heads of the type used to read data from and write data to disk memories. More particularly, the invention relates to an automatic ultrasonic bonding machine that includes an orbital head rotatable by a motor drive, thus enabling an ultrasonic wedge-type wire bonding tool protruding from the head to be oriented in arbitrary azimuthal directions, thereby enabling wire to be paid out without twisting through a wire feed bore disposed diagonally through the bonding tool, from a first bond site to subsequent bond sites located at arbitrary compass directions from the first bond site, without requiring that the workpiece be rotated.
B. Description of Background Art
Miniature electronic circuits, or “micro-circuits,” are used in vast quantities, in a wide variety of consumer, commercial, industrial and military devices and equipment. The majority of such micro-circuits are of a type referred to as integrated circuits. Integrated circuits contain a number of active circuit elements such as transistors, and passive elements such as resistors and capacitors. In semiconductor integrated circuits, conductive paths between circuit elements on a semiconductor substrate are formed by selectively etching the substrate. In hybrid micro-circuits, circuit elements mounted on a ceramic substrate are usually interconnected by conductive ink paths on the substrate.
The functional portions of integrated circuits are typically in the form of very small, rectangular-shaped chips, ranging in size from 0.025 inch to 0.200 inch or more on a side. Input connections to integrated circuit chips are often made by bonding a very fine wire to conductive pads on the chips, the other end of each wire being bonded to a conductive terminal that is sufficiently large and robust to be inserted into a printed circuit board and soldered to conductors on the board. Wire bonding of this type utilizes ultrasonic energy and/or heat to form an intermetallic bond or weld between the wire and metallic bond site. Such wire bonds are also used to form interconnections between pads of integrated circuits, and to connect lead-out terminals to delicate read/write heads used in disk memories.
Typically, bonding wires used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip are made of aluminum or gold, and have a diameter of about 1 mil (0.001 inch). Those wires must be bonded to small, typically rectangular-shaped, integrated circuit pads a few mils wide.
The most common method of interconnecting wires between semiconductor chip pads and external terminals is to form an ultrasonic weld at each end of a conducting wire. To form such bonds, the free end of a length of bonding wire is placed in contact with a pad. Then the tip of an ultrasonic transducer is pressed against the wire, and energized with ultrasonic energy for a short time interval, welding an end of the wire to the pad. The free end of the bonded wire is then moved to other pads, and bonded thereto by the same process. After the last bond in a series of bonds has been thus formed, the wire is severed near the last bond.
In view of the very small size of the micro-circuit pads and bonding wire, it can be appreciated that ultrasonic bonding of connecting wires to integrated circuit pads must be performed using a tool mounted in a bonding machine that permits the tool to be manipulated to precisely controllable positions within a work area containing a workpiece.
Typical wire bonding machines used for ultrasonic welding of wires to micro-circuit pads includes an elongated, generally vertically disposed, force-applying member or “tool.” The tool is connected at the upper end thereof to a source of ultrasonic energy, such as a piezoelectric transducer connected to an electrical energy source alternating at an ultrasonic frequency. Usually, the tool is connected to the transducer through a tapered horn structure that matches the acoustic input impedance of the tool to the output impedance of the transducer, which typically has a larger diameter than the tool.
One type of ultrasonic bonding tool used to bond wires to micro-circuit pads is referred to as a wedge bonder and has a flat lower working face adapted to press a bonding wire into contact with a pad, while ultrasonic energy is applied through the tool to the wire to form an ultrasonic weld. This working face is usually quite small, typically having a rectangular shape only about a few mils on a side, to permit bonding wire to small micro-circuit pads, without contacting adjacent circuit elements. Typically, this is done by first viewing a particular workpiece pad and tool tip in a stereo microscope and video camera to align a workpiece relative to a bonding machine, and then using an automatic actuator system to position the tool tip at consecutive bond site locations on the workpiece, using a control system which employs pattern recognition logic.
In most wire bonding machines, the bonding tool is so constructed as to facilitate the positioning of bonding wire over a pad, prior to performing the bonding operation. Such bonding tools may include an upwardly angled lower face rearward of the working face, and a generally vertically disposed rear face. An angled bore or wire guide hole having an entrance aperture in the rear face and an exit aperture in the angled lower face of the tool enables bonding wire supplied from a reel mounted upwardly and rearwardly of the tool to be paid out through the exit aperture in the angled lower face of the tool. Typically, a remotely actuable clamp located rearward of the wire guide hole entrance and movable with the tool is used to feed bonding wire through the guide hole.
The clamp used to push wire through the guide hole of a bonding tool usually consists of a pair of jaws that may alternately be closed to grip the wire, and opened to allow free travel of the wire. Generally, such clamps may be moved toward and away from the guide hole entrance, typically on a line of movement which coincides with the axis of the guide hole. To feed wire through the guide hole, the jaws of the clamp are first opened, and the clamp then moved away from the guide opened, and the clamp then moved away from the guide hole. The jaws are then closed to grip the wire, and then moved towards the guide hole, thus feeding wire through the guide hole.
In wire bonding machines of the type just described, the machine is used to translate the bonding tool to the proper position to bond wire to a first bond site of a pair of bond sites, such as a pad on an integrated circuit die, feed wire out through the guide hole exit aperture, move the tool to a second bond site and form another bond. In this manner, any desired number of pads or other elements of a circuit can be connected together, in a procedure referred to a “stitch” bonding. After the last bond in a series of bonds has been made, the wire must be severed, to permit making bonds between other pairs of bond sites. Oftentimes, the bonding tool itself is utilized to sever the bonding wire.
In moving a wedge bonding tool from a first bond site to a second bond site, the tool must be translated rearward from, the first site to the second site, in a vertical plane containing both the longitudinal axis and wire-guide bore axis of the tool. This requirement results from the fact that wire paying out forwardly through the exit aperture of the bonding tool tip must remain in the plane containing the longitudinal and guide hole axes of the tool, to ensure that the wire will not bind on the exit aperture chamfer, or become twisted.
Because of the requirement for translating a wedge bonding tool from a first to subsequent bond sites in the plane of the bonding tool longitudinal axis and wire guide bore axis, existing wedge bonding methods require that a workpiece be rotated to align a direction vector between the two sites
Chapin William L.
Elve M. Alexandra
Tran Len
West Bond Inc.
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