Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2001-12-31
2003-12-23
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S527000, C324S537000, C324S511000, C228S104000
Reexamination Certificate
active
06667625
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to ultrasonic bonding machines of the type used to bond interconnecting wires to miniature electronic devices such as integrated circuits and magnetic read/write heads used in disk drive memories. More particularly, the invention relates to a method and apparatus for detecting absence of a bonding wire within the bore of an ultrasonic bonding tool, because of inadvertent breakage or miss-feeding of a bonding wire being used to make interconnections between bond sites.
2. 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 mounted on a substrate. Semiconductor integrated circuits consist of a small monolithic chip made of a semiconducting material such as silicon having discrete areas into which impurities are diffused to form circuit elements and having conductive paths between circuit elements on the chip formed by selectively etching metallized layers of the chip. In hybrid micro-circuits, circuit elements mounted on a ceramic substrate are usually interconnected by conductive ink paths on the substrate.
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 soldered to conductors on a circuit 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 used to form interconnections between pads of an integrated circuit chip and terminals of a package used to enclose and protect the chip, and are also used to connect lead-out terminals to delicate read/write heads used in disk memories.
Bonding wires used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip are generally made of aluminum or gold, and have a diameter of about 1 mil (0.001 inch) or less. Each interconnecting wire must be bonded to the upper surface of a small, typically rectangular-shaped, integrated circuit pad a few mils wide at one end of the wire to form a first bond site, and to a similarly shaped, larger package terminal comprising a second bond site.
The most common method of interconnecting wires between semiconductor chip pads and external terminals uses ultrasonic energy to form a welded bond 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. The combination of a normally downwardly directed pressure applied by the tool to the contact region between the lower surface of the wire and the upper surface of the pad, combined with a rapid lateral scrubbing action of the tool tip in a horizontal direction parallel to the pad, causes an inter-molecular diffusion bond, sometimes referred to as a “weld,” to be formed between the wire and pad. The free end of the bonded wire is then moved to another pad or terminal, and bonded thereto by the same process. After the last bond in a series of bonds has been thus formed, the wire is severed at the last bond site.
In view of the very small sizes of both 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 include an elongated, generally cylindrically shaped force-applying member or “tool.” The tool is usually vertically disposed, and has a shank fastened at the upper end thereof to a source of ultrasonic energy, such as a piezoelectric transducer which is 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 shank.
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. The bonding process typically includes the steps of 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 respect to 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 or clamp blades 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 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 a second bond. In this manner, any desired number of pads or other elements of a circuit can be connected together, in a procedure referred to as “stitch” bonding. After the second or 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. In one method of severing the wire, the clamp blades are then closed upon wire rearward of the bonding tool, and the clamp is translated rearwards from a second bond site to exert tension on the bonding wire sufficient to sever the wire. The clamp is then fed forward to feed a new length of wire from the tool. Alternatively,
Chapin William L.
Deb Anjan K.
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