Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
2001-05-24
2003-07-22
Picard, Leo (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S125000, C700S056000, C700S057000, C700S109000
Reexamination Certificate
active
06597963
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to the field of semiconductor devices and electronic systems and more specifically to reduced-error set-ups and operation of computer-controlled bonding machines used in integrated circuit assembly.
DESCRIPTION OF THE RELATED ART
In integrated circuit (IC) assembly, an IC chip is typically mounted on a leadframe and electrically connected to it by metallic segments. Commonly, the chip assembly is encapsulated in a protective package (for instance, ceramic package, or plastic package using molding process). Typically, the IC chip has a plurality of bond pads, which are often positioned around the chip perimeter; these bond pads have predetermined bonding area and spacing (bond pad pitch). The leadframe usually has a plurality of narrow “inner” leads for attachment to the segments and inclusion in the package, and a plurality of wider “outer” leads for attachment to other parts such as solder attachment to circuit boards.
The metallic segments used for electrical connection of the IC chip to the leadframe include wires and ribbons, and are attached by ball bonding, stitch bonding, or wedge bonding techniques. Wire bonding is a process in which a wire may be welded from a chip bond pad to the tip of an inner lead of the leadframe. As an example, in wire ball bonding the ball is attached to the chip bond pad and the stitch to the leadframe inner lead. For a given device type, there is a set of locations expressed in x and y coordinates which defines the bond locations on the chip and on the lead tips. These locations are generally stored collectively in a computer file, sometimes referred to as “Device Program”. Apart from the bond head, capable of providing x-y-z motion needed for bonding, a wire bonder has a material handling subsystem and the vision subsystem.
Conventional semiconductor computerized wire bonders use x-y tables to move the bonding capillary over the device for bonding between the chip and the leadframe. The x-y coordinate tables are driven by complex electrical and mechanical components that may convert rotary and linear motions of the axis drive motors to create the needed positioning. The bond head also carries several other components such as the z-axis drive motor, a camera and optics for vision functions, and further components required to control wire bonding. Specific features of the capillary and its alignment are described in a number of U.S. Patents and Patent Applications. Examples are: U.S. Pat. No. 5,934,543, issued on Aug. 10, 1999 (Koduri et al., “Wire Bonding Capillary having Alignment Features”); and application Ser. No. 08/993,638, filed on Dec. 18, 1997 (Koduri, “Wire Bonding with Capillary Realignment”). The interaction of capillary and vision system is illustrated, for example, by U.S. patent application Ser. No. 09/191,812, filed on Nov. 13, 1998 (Koduri et al., “Automation of Optics Offset Measurement on Wire Bonders”); Ser. No. 09/111,642, filed on Jul. 8, 1998 (Koduri et al., “An Efficient Hybrid Illuminator”); Ser. No. 09/111,977, filed on Jul. 8, 1998 (Koduri et al., “An Efficient Illumination System for Wire Bonders”).
The material handling system moves a leadframe so that each device can eventually be placed under the bond head for bonding. One or more devices may be placed under the bond head at a time to be bonded. The device may also be heated in a predetermined manner to establish reliable metallurgical bonding conditions. After a device has been bonded, the leadframe is step-moved such that the next unit can be bonded.
When a unit is indexed in by the material handling system for bonding, the position of the leadframe and the chip is not always the same because of variations in the handling and previous manufacturing (such as variable chip positioning during attachment to the leadframe). Without knowing accurately the target bonding locations, the bond head cannot place the bonds as expected. To aid this process, a machine vision system is employed. A typical vision system consists of a set of optics to provide the needed illumination and magnification of the device, a camera to capture the image provided by the optics and an image processing system to store and analyze the captured image.
Before bonding a device, it is essential to determine the device program with all the coordinate locations of the bonds that need to be created. With respect to a predetermined set of reference locations, those locations are often referred to as “homes”. A typical device may have one or more “homes”. Generally, the identification of homes needs to be done individually for each device to be bonded. It is common practice to use a three-step process to enable such identification.
In the step of “teaching”, the coordinate locations of the homes and all the needed bonds are identified and saved to create the “device program”. Once generated, a device program can be stored, copied and/or shared between multiple machines as needed.
In the step of “regeneration”, a human operator helps in locating the homes of the first device after loading in the information from the previously saved device program. At this point, the machine captures and saves a set of images, called “reference images” or “references” in the neighborhood of each home.
In the step of “bonding”, the machine indexes one unit or more at a time into the workstation under the bond head. At this point, the vision system, with the aid of a pattern recognition system, attempts to relocate the matching locations with the saved references. After finding the new coordinates of the matching references, the home and bond locations are re-computed for that specific unit from the device program data. The process of relocating the references and homes is normally referred to as “aligning” the device. Using the specific bond locations, the device can now be bonded. The process of indexing, aligning and bonding is repeated without any human intervention as long as nothing abnormal happens on the machine.
A typical alignment procedure may correct for a constant shift in x-y directions and/or a constant rotation of the device. In this context, it is important to understand the effects of variations in illumination settings across machines and the images formed using different levels of brightness. Large differences in intensity settings can reduce the ability of the pattern recognition system to locate the references accurately. It is very much desired to have a consistent level of brightness and image quality across all the machines used.
Problems in wire bonding techniques arise in part from the technology trends to increase the number of leads in a given package and to make IC chip packages smaller. As consequences, the bonding pads located on the chip receive smaller areas and are spaced closer together, and the inner leads of leadframes are made narrower and closer together. These trends demand tighter control of wire bond ball and stitch geometries and placements. For instance, even small bond placement errors may result in device loss.
For the bond machines, errors in x-y tables and motors need to be reduced. At the microscopic level, each axis of each table behaves differently with its own local variations within their usable regime. For instance, an axis might have a 0.5% error in its first half of working distance and a 0.8% error in its second half. Further, a x-y pair might have a global positioning error because of an error in the orthogonality between them; or the tables may exhibit a range of hysteresis errors. These variations become even more threatening as common device programs are shared due to quality enforcement and economic reasons.
The emerging technical problems for automated bond machines can be summarized as follows:
Accuracy: Small ball/fine pitch bonding requires a very accurate system to be able to place the ball completely on the bond pad. The current systems have difficulties in achieving this.
Large variations in illumination settings can lead to variations of the images as seen by the optics and the cam
Bon David J.
Koduri Sreenivasan K.
Masinick Michael D.
Picard Leo
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Wade James Brady III
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