The present invention relates to wire and ribbon bonding operations, and more particularly, to support and retaining structures for semiconductor devices used in connection with wire and ribbon bonding operations.
In the processing and packaging of semiconductor devices, wire and ribbon bonding continue to be a widely used method of electrical interconnection between two locations within a package (e.g., between a die pad of a semiconductor die and a lead of a leadframe). For example, wire bonding machines (or ribbon bonding machines) are used to form wire loops (or ribbon interconnections) between respective locations to be electrically interconnected.
Semiconductor die are commonly supported by leadframes to transport them through various stages of the assembly process including ultrasonic bonding processes. A continuous trend in the semiconductor industry is that global markets demand smaller semiconductor devices at lower costs. One exemplary cost reduction strategy involves using less material in the devices, for example, using less copper material in the leadframe support structure which supports the semiconductor die. This strategy tends to lead to the creation of highly populated leadframes through the manufacturing process. Such highly populated leadframes tend to contain many rows and columns of semiconductor die and other components, where the leadframe portions are connected to the leadframe matrix by connecting portions such as small and thin tie bars. The density and small sizes of the leadframe components make properly constraining the portions of a semiconductor device (including leadframe portions, die portions, etc.) during ultrasonic wire or ribbon bonding processes very difficult.
In ultrasonic bonding, for example, a transducer drives a bonding tool to a predetermined vibratory frequency so that the bonding tool tip scrubs the bonding site to facilitate bonding. Since ultrasonic bonding is highly dynamic and energetic for large wire and ribbon bonding, a semiconductor device can be driven to high velocities similar in amplitude to the bonding tool's tip velocity during the bonding process. That is, the semiconductor device (including the die supported by the leadframe), may move (at least partially) with the vibrating bonding tool. When this occurs, the relative displacement between the tip of the bonding tool and the semiconductor die is decreased which may lead to poor quality bonds. Conventional structures and methods use clamping materials with low rates of wear under repeated ultrasonic bonding to increase their useful life. Such materials generally exhibit relatively low resistance to the velocity of the tool during bonding.
As illustrated in
Thus, it would be desirable to provide improved bonding support systems to minimize or eliminate movement of semiconductor devices relative to the support systems during bonding to improve bond quality.
According to an exemplary embodiment of the present invention, a support system for a semiconductor device during a wire or ribbon bonding operation is provided. The support system includes a body portion defining an upper surface that includes an upper surface contact region configured to support at least a portion of a lower surface of a semiconductor device at a lower surface contact region during the wire or ribbon bonding operation. The support system also includes a plurality of protrusions on the upper surface contact region.
According to another exemplary embodiment of the present invention, a support system for a semiconductor device during a wire or ribbon bonding operation is provided. The support system includes a lower body portion defining an upper surface, the upper surface being configured to support at least a portion of a bottom surface of a semiconductor device during the wire or ribbon bonding operation. The support system also includes an upper body portion defining a lower surface configured to contact at least a portion of a top surface of the semiconductor device at a contact region during the wire or ribbon bonding operation. The support system further includes a plurality of protrusions on the lower surface.
According to yet another exemplary embodiment of the present invention, a method of supporting a semiconductor device during a wire or ribbon bonding operation is provided. The method includes the step of providing a body portion that defines an upper surface including an upper surface contact region having a plurality of protrusions. The upper surface contact region is configured to support at least a portion of a lower surface of the semiconductor device at a lower surface contact region during the wire or ribbon bonding operation. The method further includes the step of supporting at least the portion of the lower surface of the semiconductor device at the lower surface contact region with the upper surface contact region such that the lower surface contact region is deformed by the plurality of protrusions.
According to yet another exemplary embodiment of the present invention, a method for supporting a semiconductor device during a wire or ribbon bonding operation is provided. The method includes the step of providing a lower body portion that defines an upper surface configured to support at least a portion of a bottom surface of the semiconductor device during the wire or ribbon bonding operation. The method also includes the step of providing an upper body portion that defines a lower surface configured to contact at least a portion of a top surface of the semiconductor device at a contact region during the wire or ribbon bonding operation. The lower surface includes a plurality of protrusions. The method further includes the step of deforming the top surface with the plurality of protrusions during the wire or ribbon bonding operation.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
The terms “wire”, “ribbon”, and “conductive material” are used herein to generically describe the material bonded by a wire bonding system. It is understood that a wire bonding system may bond a wire material, a ribbon material, etc., as is desired in the given application. Thus, it is understood that these terms are used interchangeably and are not intended to be limiting with respect to each other.
“Plastic shearing” refers to a deformation when parallel surfaces slide past one another, for example, the surface structures and the leadframe surface contacting them as described herein. Such plastic shearing may be irreversible and, as such, markings on the leadframe surface are visible after the bonding operation.
Bonding tool 210 is engaged in a transducer (e.g., an ultrasonic transducer, not shown) of wire bonding system 218. The transducer causes lateral vibratory movement 214 of bonding tool 210 in, for example, the X-direction or the Y-direction. Bonding tool 210 is pressed against wire or ribbon 212 with a downward force 216. The transducer is activated to cause bonding tool 210 to vibrate at 214 to assist in bonding conductive material 212 to the bonding location on die 202 (or a bonding location on substrate 200).
If desired, the force of vibration may be in the same order of magnitude as downward force 216 (e.g., where exemplary ranges for the force are: between about 0.01 to 4.0 N; between about 1.0 to 30.0 N; and between about 1.0 to 100.0 N) for ribbon bonding by bonding tool 210. This vibratory loading depends upon the material properties and the frictional coupling at the interface between bonding tool 210 and semiconductor device 206. Exemplary ranges for lateral vibration 214 are about 0.5 to 20 μm, and about 0.5 to 6.0 μm.
According to various exemplary embodiments of the present invention, a support system/structure (and method) is provided to reduce movement of a semiconductor device (e.g., a die supported by a leadframe) relative to a support and/or clamping structure during ultrasonic bonding. Surface features may be formed/provided on the support structure and/or clamping structure(s) that contact the semiconductor device in certain regions to resist vibratory movement induced by the bonding tool. For example, surface features (e.g., pyramidal structures, pointed features, etc.) may be formed on/into the support/clamping structures by machining, electrical discharge machining, laser ablation, etc. In another example, such surface features may consist of particles embedded in a coating on the support/clamping structures (e.g., diamond particles embedded in a nickel coating). Other example surface features and their methods of formation will be noted hereafter.
An upper support body portion, such as clamp fingers 428 (one is shown for simplicity) may then be moved downwardly by, for example, a finger cam (not shown) so that lower surface 444a of finger clamp 428 contacts, and applies pressure to, upper surface portion 430a of leadframe 400.
It is noted that surface structures (like structures 480a, 480b) may instead be on lower surface 444a of finger clamps 428, which may result in a simplified design of body portion 408. Also, additional surface structures (e.g., having a structure similar to surface structures 480a, 480b) may also be located on upper surface 440 of anvil/support structure 408 that may or may not be directly under the area(s) to be bonded. For example, additional surface structures may be placed directly under where the bonding tool will be pressed against leadframe 400/semiconductor device 406 with a downward force or load normal to upper surface 440 of leadframe 400. As one skilled in the art would appreciate, the bonding tool force would create a compression force to at least partially embed (or further embed) such surface structures, and/or to cause plastic deformation of leadframe 400 at such localized area(s) (see below).
An upper support body portion, such as clamp fingers 528 (one is shown for simplicity), may then be moved downwardly by, for example, a finger cam (not shown) so that lower surface 544a of finger clamp 528 contacts upper surface portion 530a of leadframe 500 and applies pressure at each finger clamp 528.
While
Window clamp 660 may be lowered (e.g., by a window clamp cam) to contact upper surface 650 of leadframe 600 of semiconductor device 606. Window clamp surface structures 680 in portion 664a of lower surface 664 contact leadframe portion 650a on leadframe upper surface 650. Then, anvil 608 may be raised (e.g., by an anvil cam) until hard plate 674 (e.g., stainless steel plate 674), contacts bottom surface 640 of leadframe 600. Stiff base 670 may then be further raised upwardly to compress complaint layer 672 so that a substantially even pressure may be applied against leadframe bottom surface 640 in the areas being clamped.
It is noted that surface structures 480a, 480b, 580, 580′, 680, 780 illustrated and described in the exemplary embodiments may collectively comprise, without limitation, abrasive particles, a series of machined structures, a series of electrical discharge machined (EDM) structures in conductive materials, a series of laser ablation structures, a series of pyramidal structures, a series of pointed structures, etc.
In one example, the surface structures may be machined directly into the anvil, clamp fingers and/or window clamp (e.g., see
Exemplary sizes (e.g., D1, D2, and D3 in
For each exemplary embodiment, surface structures 480a, 480b; 580; 580′; 680; 780; 880, may be designed such that the contact stress between the leadframe lower surface portion and the upper surface of the anvil portion may be elevated in that localized area. With the additional contact stress caused by, for example, the contact pressure of finger clamps essentially directly above the leadframe contact portion (or by a window clamp), compression may occur causing the leading portion of the surface structures to rest beyond the datum surface defined by the lower surface of the leadframe/semiconductor device. For the exemplary embodiment having surface structures 680 on the lower surface of a window clamp or the possible use of surface structures 680 in the lower surface of finger clamp(s), the combined compression of the leadframe against the lower anvil similarly causes the leading portion of surface structures 680 to rest beyond the datum surface defined by the upper surface of the leadframe/semiconductor device. Thus, the greater the contact stress at the surface feature region(s), the greater the penetration of the surface features into the surface of the semiconductor device/leadframe and the greater the resistance to the velocity of the bonding tool. That is, at least a portion of the leading surfaces of the surface structures may be embedded into the leadframe/semiconductor device at the leadframe contact portion (e.g., by a small amount, such as from about 1 to 10 um).
Placement of the selected surface structures may depend upon, for example, the leadframe and package structure, manufacturing requirements, clamp placement and bond placement. When the surface structures of the present invention are placed on the anvil, the resistance to movement/velocity may be achieved, for example, in areas of clamp finger contact, directly under the bond locations during bonding, or other areas as desired. When the surface structures of the present invention are located on the bottom surface of a window clamp, their location may be limited to a specific area; however, in certain applications it may be desired that the surface structures be widely distributed on the bottom surface of a window clamp to engage varying types of devices (e.g., leadframe devices). The surface features may also be located at, for example, areas close to positions of ultrasonic bonding to achieve maximum plastic shearing (see below).
When an ultrasonic bonding tool is engaged with a leadframe/semiconductor device to form a bond, the leadframe contact portion of the lower or upper surface of the leadframe/device may move, or attempt to move, relative to the anvil, window clamp and/or finger clamps having the surface features. The geometry of such surface structures, in conjunction with the elevated contract stress, may cause the leadframe/device to resist the velocity caused by the ultrasonic excitation in the localized area of the bonding tool. This may result in dissipation of energy and plastic shearing of the leadframe material at the interface between the surface structures and the leadframe contact portion. During continued application of such ultrasonic excitation during the bonding process, more material may be sheared and the surface features may penetrate further into the surface of the device/leadframe. A pattern, marking, or marring on the leadframe at the leadframe contact portion also may occur where such plastic shearing results from ultrasonic bonding.
Careful selection of the materials used to clamp/retain the semiconductor device during a bonding operation may result in high rates of resistance to the velocity induced by a bonding tool as well as low rates of wear under the ultrasonic loading of the tool. This may allow for longer clamp/retaining structure life and better resistance to movement/velocity of the device during ultrasonic bonding which may lead to a simplification of the clamp/retaining structure design and a more robust and stable bonding process.
Thus, the amount of contact stress required to cause plastic shearing (e.g., to dissipate energy and resist the velocity of the bonding tool) may be dictated by the properties of the leadframe. Leadframes may be made of work hardened copper-iron alloys. The approximate yield strength of exemplary leadframe materials ranges from about 300 to 600 N/m2. Therefore, the contact stress may need to exceed this yield strength to cause static penetration (embedding) into the leadframe. The degree of such static penetration may be dictated by the hardness of the material and the impact energy imparted on the leadframe during a clamping cycle, as well as the actual geometry of the surface (penetrating/embedding) features. An exemplary range of static penetration (embedding) is between 1 um to 10 um, although other ranges are contemplated.
Further, the amount of pressure applied to the semiconductor device/leadframe during a clamping operation may be up to and slightly beyond the yield strength of the material from which the device/leadframe is composed. However, an operator may generally apply as little pressure as is necessary to achieve stable bonding, for example, so as to extend the life of the clamp tooling. Many leadframes are made of copper, copper with a nickel plating, copper alloys (such as copper iron alloys), etc. The yield strength of 99.9% pure copper is about 70 MPa, and the yield strength of an example copper iron alloy is about 140 MPa range. For an exemplary clamping device employing 2 to 20 clamp fingers per device, each clamp finger may exert a force between about 4 to 60N and an example bonding tool may exert a force from about 2 to 37N. Of course, such force parameters may vary widely. Regarding the bonding tool force, this may vary based on whether the bonding tool is bonding a small wire, a large wire, or a conductive ribbon. Exemplary ranges for the force applied by the bonding tool are: between about 0.01 to 4.0 N; between about 1.0 to 30.0 N; and between about 1.0 to 100.0 N.
Generally, the higher the contact stress, the greater the resistance to movement/velocity in the localized area of surface feature contact as the surface feature(s) tends to penetrate further into the leadframe or device surface. However, as the contact area is reduced so as to increase the contract stress within the localized area, wear life may decrease, potentially causing a greater wear rate which will reduce the useful life of the design.
As will be appreciated by those skilled in the art, any of the surface structures/protrusions described herein (e.g., surface structures 480a, 480b; 580; 580′; 680; 780) may be formed to be part of a unitary piece of material (e.g., a support structure such as an anvil for supporting a leadframe portion during bonding, an upper layer of the support structure, etc.). Further, the surface structures/protrusions may be separate structures integrated into another structure (e.g., integrated into a support structure for supporting a leadframe portion during bonding).
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
This application claims the benefit of U.S. Provisional Application No. 61/324,053 filed on Apr. 14, 2010, the content of which is incorporated herein by reference.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US11/32028 | 4/12/2011 | WO | 00 | 8/7/2012 |
Number | Date | Country | |
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61324053 | Apr 2010 | US |