US 20050251282 A1
A method and apparatus for applying packaging material to workpieces, with a high degree of precision, and resulting articles. A machine vision system including at least one camera is operably associated with a computer controlling a system for application of material so that the system may recognize the position and orientation of workpieces proximate to which the material is to be applied. The requirement for precise mechanical workpiece alignment is eliminated, and the ability of the machine vision system to recognize size, configuration and topography of different workpieces affords greater manufacturing flexibility. The method includes application of flowable material in separate volumes and draws formation of the material to at least a semisolid state for packaging electronic components, and the electronic components so packaged are also part of the invention.
1. A method of fabrication of articles, comprising:
placing a plurality of workpieces at a common horizontal plane;
recognizing a presence, location and orientation of each workpiece of the plurality; and
using the recognized location and orientation of each workpiece of the plurality, mutually adjoined forming a structure comprising a plurality of segrments of at least semisolid material by selectively transforming separate volumes of a curable flowable material to at least a semisolid state at locations proximate each of the plurality of workpieces.
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
inverting the plurality of workpieces and placing the plurality of workpieces at a common horizontal plane;
recognizing a location and orientation of each inverted workpiece of the plurality; and
forming another structure comprising at least one segment of at least semisolid material abutting each of the plurality of workpieces.
8. The method of
9. The method of
10. The method of
11. The method of
12. The method of
13. An apparatus for fabrication of articles, comprising:
a material disposition system structured for selective transformation of portions of material in a flowable state to at least a semisolid state;
a machine vision system in operable communication with the material disposition system including at least one camera oriented for detecting objects within the vision field;
a computer in operable communication with both the material disposition system and the machine vision system, the computer being programmed to respond to input from the machine vision system indicative of a presence, location and orientation of at least one workpiece in the vision field and to initiate and control the material disposition system to selectively transform portions of material in a flowable state to at least a semisolid state responsive to the input and relative to the presence, location and orientation of the at least one workpiece to form at least one structure comprising a plurality of segments of at least semisolid material proximate the at least one workpiece.
14. The apparatus of
15. The apparatus of
16. The apparatus of
17. The apparatus of
18. The apparatus of
This application is a continuation of application Ser. No. 10/293,160, filed Nov. 12, 2002, now U.S. Pat. No. 6,909,929, issued Jun. 21, 2005, which is a divisional of application Ser. No. 09/259,142, filed Feb. 26, 1999, now U.S. Pat. No. 6,549,821, issued Apr. 15, 2003.
1. Field of the Invention
The present invention relates generally to stereolithography and, more specifically, to the use of stereolithography in the packaging of electronic components. Use of a machine vision system such as a pattern recognition system to facilitate application of stereolithographic techniques to fabrication of electronic components and other products is encompassed in the invention.
2. State of the Art
In the past decade, a manufacturing technique termed “stereolithography,” also known as “layered manufacturing,” has evolved to a degree where it is employed in many industries.
Essentially, stereolithography, as conventionally practiced, involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or “sliced” into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and non-metallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries, followed by selective consolidation or fixation of the material to at least a semisolid state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer. The unconsolidated material employed to build an object may be supplied in particulate or liquid form, and the material itself may be consolidated or fixed or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size, whereas when a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer which can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials, and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques.
However, to the inventors' knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to otherwise assuring precise, repeatable placement of components.
In the electronics industry, state-of-the-art packaging of semiconductor dice is an extremely capital-intensive proposition. In many cases, semiconductor dice carried on, and electrically connected to, lead frames are individually packaged with a filled-polymer material in a transfer molding process. A transfer molding apparatus is extremely expensive, costing at least hundreds of thousands of dollars in addition to the multi-hundred thousand dollar cost of the actual transfer molding dies in which strips of lead frames bearing semiconductor dice are disposed for encapsulation.
So that the reader may more fully understand the present invention in the context of the prior art, it seems appropriate to provide a brief description of a transfer apparatus and method for forming a plastic package about an LOC die assembly. The term “transfer” molding is descriptive of this process as the molding compound, once melted, is transferred under pressure to a plurality of remotely-located mold cavities containing die assemblies to be encapsulated.
In operation, a heated pellet of resin mold compound 430 is disposed beneath ram or plunger 432 in pot 434. The plunger descends, melting the pellet and forcing the melted encapsulant down through sprue 436 and into primary runner 438, from which it travels to transversely-oriented secondary runners 440 and across gates 442 into and through the mold cavities 444, wherein die assemblies 500 comprising dice 100 with attached lead frames 502 are disposed (usually in strips so that a strip of six lead frames, for example, would be cut and placed in and across the six cavities 444 shown in
Encapsulant flow in the mold cavities 444 is demonstrably non-uniform. The presence of the die assembly 500 comprising a die 100 with lead frame 502 disposed across the mid-section of a cavity 444 splits the viscous encapsulant flow front into upper and lower components. Further, the presence of the (relatively) large die 100 with its relatively lower temperature in the middle of a cavity 444 permits the flow front on each side of the die 100 to advance ahead of the front which passes over and under the die 100.
Encapsulant filler particles may become lodged between lead ends and the underlying die surfaces. The non-uniform flow characteristics of the viscous encapsulant flow may cause particles to be more forcefully driven between the lead ends and the die 100 and wedged or jammed in place in low-clearance areas. As the encapsulant flow front advances and the mold operation is completed by packing the cavities, pressure in substantially all portions of the mold cavities reaches hydrostatic. With LOC arrangements where lead ends extending over the active surface of a die 100 are bonded thereto by adhesive-coated tape or an adhesive material patterned on the active surface, the relative inflexibility of the tightly-constrained (adhered) lead ends maintains the point stresses of any particles trapped under the lead ends. These residual stresses are carried forward in the fabrication process to post-cure and beyond. When mechanical, thermal or electrical stresses attendant to post-encapsulation processing are added to the residual point stresses associated with the lodged filler particles, cracking or perforation of the die coat may occur, with the adverse effects previously noted. It has been observed that filler particle-induced damage occurs more frequently in close proximity to the adhesive, where lead flexure potential is at its minimum. In addition to damage by filler particles, transfer molding also results in the problem of bond wire sweep, wherein bond wires may be damaged, broken, loosened from their connections to bond pads or lead ends or swept into shorting contact with an adjacent bond wire under the impetus of the flow front of molten resin encapsulant as it flows through a mold cavity.
In addition to end-product deficiencies as noted above due to the phenomena of particulate die coat penetration and bond wire sweep, the capital-intensive nature of the transfer molding apparatus, including the requirement for different, multi-hundred thousand dollar molds for each die and lead frame arrangement as well as the high cost of the encapsulant resin and waste of same which is not used in the mold cavities, renders the transfer molding process an extremely expensive one. Mold damage and refurbishment is an additional, ongoing cost. Further, the elevated temperatures used in the molding process as well as in the post cure of the resin encapsulant is detrimental to the circuitry of the die as well as to the electrical connections to the lead ends.
The present invention provides a method of applying material to a preformed structure such as an electronic component with a high degree of precision to create a package therefor. For example, a semiconductor die may be provided with a protective structure in the form of a layer of dielectric material having a controlled thickness or depth over or adjacent one or more surfaces thereof. As used herein, the term “package” as employed with reference to electrical components includes partial as well as full covering of a given semiconductor die surface with a dielectric material, and specifically includes a semiconductor die configured in a so-called “chip scale” package, wherein the package itself, including the die, is of substantially the same dimensions as, or only slightly larger than, the die itself.
The packaging method of the present invention may be applied, by way of example and not limitation, to a die mounted to a lead frame (having a die mounting paddle or in a paddle-less leads-over-chip (LOC) or in a leads-under-chip (LUC) configuration), mounted to a carrier substrate in a chip-on-board (COB) or board-on-chip (BOC) arrangement, or in other packaging designs, as desired.
The present invention employs computer-controlled, 3-D CAD initiated, stereolithographic techniques to form structures comprising one or more layers of material abutting a workpiece such as an electronic component and, more specifically, a semiconductor die. A dielectric layer, or layer segments, may be formed over or adjacent a single die or substantially simultaneously over or adjacent a large number of dice or die locations on a semiconductor wafer or other large-scale semiconductor substrate, individual dice or groups of dice then being singulated therefrom. As used herein, the term “semiconductor die” may be taken to encompass all of the aforementioned semiconductor substrate-based elements and the term “electronic component” may be taken in its broadest sense to encompass both active and passive components, combinations thereof and assemblies of components as well as individual components.
Precise mechanical alignment of workpieces, including singulated semiconductor dice or larger semiconductor substrates having multiple die locations, is not required to practice the method of the present invention, which includes the use of machine vision to locate workpieces, their orientation and features. Specifically, and in a preferred embodiment, semiconductor dice, dimensions thereof and features or other components thereon or associated therewith (such as lead frames, bond wires, solder bumps, etc.), or features on a larger semiconductor substrate, may be identified by a machine vision system for alignment and material disposition purposes by an associated stereolithographic apparatus.
In a preferred embodiment, packaging for electronic components according to the invention is fabricated using precisely focused electromagnetic radiation in the form of an ultraviolet (UV) wavelength laser under control of a computer and responsive to input from a machine vision system such as a pattern recognition system to fix or cure a liquid material in the form of a photopolymer.
It should be understood that the invention is not so limited to stereolithographic techniques employing a UV-curable photopolymer, but may be employed with other techniques employing alternative materials. Furthermore, the apparatus of the present invention, insofar as it employs a machine vision system, encompasses any and all stereolithographic apparatus and the application of any and all materials thereby, including both metallic and non-metallic materials applied in any state and cured or otherwise fixed to at least a semisolid state to define a three-dimensional structure having identifiable boundaries.
With reference again to
The data is preferably formatted in an STL (for StereoLithography) file, STL being a standardized format employed by a majority of manufacturers of stereolithography equipment. Fortunately, the format has been adopted for use in many solid-modeling CAD programs, so often translation from another internal geometric database format is unnecessary. In an STL file, the boundary surfaces of an object are defined as a mesh of interconnected triangles.
Apparatus 10 also includes a reservoir 14 (which may comprise a removable reservoir interchangeable with others containing different materials) of liquid material 16 to be employed in fabricating the intended object. In the currently preferred embodiment, the liquid is a photo-curable polymer (hereinafter “photopolymer”) responsive to light in the UV wavelength range. The surface level 18 of the liquid material 16 is automatically maintained at an extremely precise, constant magnitude by devices known in the art responsive to the output of sensors within the apparatus and preferably under control of computer 12. A support platform or elevator 20, precisely vertically movable in fine, repeatable increments responsive to control of computer 12, is located for movement downward into and upward out of liquid material 16 in reservoir 14. A UV wavelength range laser plus associated optics and galvanometers (collectively identified as laser 22) for controlling the scan of laser beam 26 in the X-Y plane across platform 20 has associated therewith mirror 24 to reflect beam 26 downwardly as beam 28 toward surface 30 of platform 20. Beam 28 is traversed in a selected pattern in the X-Y plane, that is to say, in a plane parallel to surface 30, by initiation of the galvanometers under control of computer 12 to at least partially cure, by impingement thereon, selected portions of liquid material 16 disposed over surface 30 to at least a semisolid state. The use of mirror 24 lengthens the path of the laser beam, effectively doubling same, and provides a more vertical beam 28 than would be possible if the laser 22 itself were mounted directly above platform surface 30, thus enhancing resolution.
Data from the STL files resident in computer 12 is manipulated to build an object 50 one layer at a time. Accordingly, the data mathematically representing object 50 is divided into subsets, each subset representing a slice or layer of object 50. This is effected by mathematically sectioning the 3-D CAD model into a plurality of horizontal layers, a “stack” of such layers representing object 50. Each slice or layer may be from about 0.0001 to 0.0300 inch thick. As mentioned previously, a thinner slice promotes higher resolution by enabling better reproduction of fine vertical surface features of object 50. In some instances, a base support or supports 52 for an object 50 may also be programmed as a separate STL file, such supports 52 being fabricated before the overlying object 50 in the same manner, and facilitating fabrication of an object 50 with reference to a perfectly horizontal plane and removal of object 50 from surface 30 of elevator 20. Where a “recoater” blade 32 is employed as described below, the interposition of base supports 52 precludes inadvertent contact of blade 32 with surface 30.
Before fabrication of object 50 is initiated with apparatus 10, the primary STL file for object 50 and the file for base support(s) 52 are merged. It should be recognized that, while reference has been made to a single object 50, multiple objects may be concurrently fabricated on surface 30 of platform 20. In such an instance, the STL files for the various objects and supports, if any, are merged. Operational parameters for apparatus 10 are then set, for example, to adjust the size (diameter, if circular) of the laser light beam used to cure material 16.
Before initiation of a first layer for a support 52 or object 50 is commenced, computer 10 automatically checks and, if necessary, adjusts by means known in the art the surface level 18 of liquid material 16 in reservoir 14 to maintain same at an appropriate focal length for laser beam 28. U.S. Pat. No. 5,174,931, referenced above and previously incorporated herein by reference, discloses one suitable level control system. Alternatively, the height of mirror 24 may be adjusted responsive to a detected surface level 18 to cause the focal point of laser beam 28 to be located precisely at the surface of liquid material 16 at surface level 18 if level 18 is permitted to vary, although this approach is somewhat more complex. The platform 20 may then be submerged in liquid material 16 in reservoir 14 to a depth equal to the thickness of one layer or slice of the object 50, and the liquid surface level 18 readjusted as required to accommodate liquid material 16 displaced by submergence of platform 20. Laser 22 is then activated so that laser beam 28 will scan liquid material 16 over surface 30 of platform 20 to at least partially cure (e.g., at least partially polymerize) liquid material 16 at selective locations, defining the boundaries of a first layer 60 (of object 50 or support 52, as the case may be) and filling in solid portions thereof. Platform 20 is then lowered by a distance equal to the thickness of a layer 60, and the laser beam 28 scanned to define and fill in the second layer 60 while simultaneously bonding the second layer to the first. The process is then repeated, layer by layer, until object 50 is completed.
If a recoater blade 32 is employed, the process sequence is somewhat different. In this instance, the surface 30 of platform 20 is lowered into liquid material 16 below surface level 18, then raised thereabove until it is precisely one layer's thickness below blade 32. Blade 32 then sweeps horizontally over surface 30, or (to save time) at least over a portion thereof on which object 50 is to be fabricated, to remove excess liquid material 16 and leave a film thereof of the precise, desired thickness on surface 30. Platform 20 is then lowered so that the surface of the film and material level 18 are coplanar and the surface of the material 16 is still. Laser 22 is then initiated to scan with laser beam 28 and define the first layer 60. The process is repeated, layer by layer, to define each succeeding layer 60 and simultaneously bond same to the next lower layer 60 until object 50 is completed. A more detailed discussion of this sequence and apparatus for performing same is disclosed in U.S. Pat. No. 5,174,931, previously incorporated herein by reference.
As an alternative to the above approach to preparing a layer of liquid material 16 for scanning with laser beam 28, a layer of liquid material 16 may be formed on surface 30 by lowering platform 20 to flood material over surface 30 or over the highest completed layer 60 of object 50, then raising platform 20 and horizontally traversing a so-called “meniscus” blade across the platform (or just the formed portion of object 50) one layer thickness thereabove, followed by initiation of laser 22 and scanning of beam 28 to define the next higher layer 60.
Another alternative to layer preparation of liquid material 16 is to merely lower platform 20 to a depth equal to that of a layer of liquid material 16 to be scanned and then traverse a combination flood bar and meniscus bar assembly horizontally over platform 20 (or merely over object 50) to substantially concurrently flood liquid material 16 over platform 20 and define a precise layer thickness of liquid material 16 for scanning.
All of the foregoing approaches to liquid material flooding and layer definition and apparatus for initiation thereof are known in the art and are not material to practice of the present invention, so no further details relating thereto will be provided herein.
Each layer 60 of object 50 is preferably built by first defining any internal and external object boundaries of that layer with laser beam 28, then hatching solid areas of object 50 with laser beam 28. If a particular part of a particular layer 60 is to form a boundary of a void in the object above or below that layer 60, then the laser beam 28 is scanned in a series of closely-spaced, parallel vectors so as to develop a continuous surface, or skin, with improved strength and resolution. The time it takes to form each layer 60 depends upon its geometry, surface tension and viscosity of material, and thickness of the layer.
Once object 50 is completed, platform 20 is elevated above surface level 18 of liquid material 16, and the platform 20 with object 50 may be removed from apparatus 10. Excess, uncured liquid material 16 on the surface of object 50 may be manually removed, and object 50 then solvent-cleaned and removed from platform 20, usually by cutting it free of base supports 52. Object 50 may then require postcuring, as material 16 may be only partially polymerized and exhibit only a portion (typically 40% to 60%) of its fully cured strength. Postcuring to completely harden object 50 may be effected in another apparatus projecting UV radiation in a continuous manner over object 50 and/or by thermal completion of the initial, UV-initiated partial cure.
In practicing the present invention, a commercially available stereolithography apparatus operating generally in the manner as that described with respect to apparatus 10 of
Referring again to
It is noted that a variety of machine vision systems are in existence, examples of which and their various structures and uses are described, without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437; 4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174; 5,463,227; 5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and 5,644,245. The disclosure of each of the immediately foregoing patents is hereby incorporated by this reference.
In order to facilitate practice of the present invention with apparatus 10, a data file representative of at least one physical parameter, such as (for example) the size, configuration, thickness and surface topography of, for example, a particular type and design of semiconductor die 100 to be packaged, is placed in the memory of computer 12. If the die 100 is to be packaged with a lead frame, data representative of the die with attached and electrically connected lead frame is provided. If packaging material in the form of the aforementioned photopolymer is to be applied only to an upper surface of a die 100, or to the upper surface and portions or all of the side surfaces of a die 100, a large plurality of such dice 100 may be placed on surface 30 of platform 20 for packaging, as depicted in
Continuing with reference to
Laser 22 is then activated and scanned to direct beam 28, under control of computer 12, about the periphery of each die 100 to effect the aforementioned partial cure of material 16 to form a first layer 60. The platform 20 is then lowered into reservoir 14 and raised to another sidewall layer thickness-equaling depth increment and the laser 22 activated to add another sidewall layer 60. This sequence continues, layer 60 by layer 60, until the package sidewalls 102 are built up about dice 100. As noted below with regard to
For example, as shown in
An alternative approach, shown in
Yet another approach, shown in
It is also notable that the method depicted and described with respect to
Referring now to
In yet another, board-on-chip (BOC) embodiment of the present invention shown in
Referring now to
It is notable that the method of the present invention, in addition to eliminating the capital equipment expense of transfer molding processes, is extremely frugal in its use of dielectric encapsulant material 16, since all such material in which cure is not initiated by laser beam 26 remains in a liquid state in reservoir 14 for use in packaging the next plurality of dice 100 or modules 1000. Further, since it is no longer necessary to encapsulate dice with packaging of sufficient wall thickness to accommodate relatively large dimensional variations such as those which may be exhibited by wire bond loop heights, the overall volume of packaging material may be smaller in some cases. Also, surprisingly, the package dimensional tolerances achievable through use of the present invention are more precise, e.g., three times more precise, than those of which a transfer molding system is capable, and there is no need for an inclined mold sidewall (and thus extra packaging material) to provide a release angle to facilitate removal of a packaged die from a mold cavity. Moreover, there is no potential for mold damage, mold wear, or requirement for mold refurbishment. Finally, the extended cure times at elevated temperatures, on the order of, for example, four hours at 175° C., required after removal of batches of dice from the transfer mold cavities are eliminated. Post-cure of die packages formed according to the present invention may be effected with broad-source UV radiation emanating from, for example, flood lights in a chamber through which dice are moved on a conveyor, or in large batches. Additionally, if some portion of a package is shadowed by a portion of a die or lead frame, cure of material 16 in that area may be completed in an oven at a relatively low temperature such as, for example, 160° C.
It should also be noted that the packaging method of the present invention is conducted at substantially ambient temperature, the small beam spot size and rapid traverse of laser beam 28 around and over the semiconductor dice 100 resulting in negligible thermal stress thereon. Physical stress on the semiconductor dice and associated lead frames and bond wires is also significantly reduced, in that material 16 is fixed in place and not moved over the dice in a viscous, high-pressure wave front as in transfer molding, followed by cooling-induced stressing of the package. Bond wire sweep is eliminated, as is any tendency to drive particulates in the polymer encapsulant between lead fingers and an underlying portion of the active surface of the die with consequent damage to the integrity of the active surface.
It should be specifically noted that packaging electronic components in accordance with the invention may be effected with the use of a liquid material, such as the aforementioned polymers, which is “filled” with particulates of silicon or other materials. By such an approach, the cost of the liquid material may be lowered in appropriate instances where the filler does not adversely alter the coefficient of thermal expansion (CTE) of the packaging material. Further, the CTE of the packaging material may be tailored to be similar to, or even closely match in some instances, the CTE of the substrate onto, or adjacent, which the packaging material is applied by appropriate selection of the volume and type of filler material. Thus, whether the substrate to which the packaging material is applied comprises a plastic, a ceramic, or silicon (or other material), the packaging filler mixture may be adjusted as desired or required.
While the present invention has been disclosed in terms of certain preferred embodiments, those of ordinary skill in the art will recognize and appreciate that the invention is not so limited. Additions, deletions and modifications to the disclosed embodiments may be effected without departing from the scope of the invention as claimed herein. Similarly, features from one embodiment may be combined with those of another while remaining within the scope of the invention.