1
APPARATUS AND METHODS FOR
SUBSTANTIAL PLANARIZATION OF
SOLDER BUMPS
This application is a divisional of U.S. patent application Ser. No. 09/370,498, filed Aug. 9, 1999, and issued as U.S. Pat. No. 6,267,650 on Jul. 31, 2001.
TECHNICAL FIELD
The present invention relates to apparatus and methods for substantial planarization of solder bumps for use in, for example, testing and fabrication of chip scale packages, bumped die, and other similar devices.
BACKGROUND OF THE INVENTION
The demand for smaller packaging of electronic components continues to drive the development of smaller chip scale packages (CSP's), bumped die, and other similar devices having solder bumps, ball grid arrays (BGA's), or the like. As a result, spacing (or "pitch") between adjacent solder balls on bumped devices has steadily decreased. Typical requirements for ball pitch have decreased from 1.27 mm to 0.5 mm or less, and the trend continues.
FIG. 1 is a side elevational view of a typical bumped device 10 (CSP, bumped die, etc.) mounted on, for example, a printed circuit board 20. The bumped device 10 includes a plurality of solder balls 12 attached to a plurality of ball pads (not shown) which are formed on a die 14. Each solder ball 12 has an outer edge 16 that aligns with a corresponding contact pad 18 on the printed circuit board 20. A conductive lead 22 is attached to each contact pad 18. Ideally, the outer edge 16 of each solder ball 12 contacts the corresponding contact pad 18 during assembly of the bumped device 10 with the printed circuit board 20, completing the electrical circuit between the conductive leads 22 and the die 14.
The height and width of the solder bumps 12 on the bumped device 10 are not precisely uniform. Variation of the solder bump height and width depends on several factors, including variation in size of the original unattached solder balls, variation in the sizes of the ball pads, and differences in the attachment process.
As the demand for smaller packaging continues, however, CSP reliability concerns arise. For example, using typical manufacturing methods and solders, the nominal variation between the tallest and shortest balls (shown as the distance d on FIG. 1) is presently about 60 microns (/an). Therefore, when the device 10 is placed on a flat surface resting on the solder balls, the three tallest balls or bumps define the seating plane of the device, and the smaller balls do not touch the corresponding contact pads of the printed circuit board or test interposer.
During assembly, and in some cases during testing, a moderate compression force may be applied to the bumped device 10 to drive the outer surfaces 16 of the solder balls 12 into contact with the contact pads 18 of the printed circuit board or test interposer 20. Typically, the compression force needed to bring the solder bumps into contact with the contact pads varies between 30 grams and 2000 grams depending upon the manufacturing or test process involved. The applied compression force should be kept to a minimum, however, because larger forces may damage the circuitry of the die 14, the CSP solder balls, or the test interposer.
One approach to the problem is to mount the contact pads 18 of the test interposer 20 on micro-springs. As the tallest
2
solder bumps engage the micro-spring mounted contact pads, the micro-springs are compressed, allowing the shorter solder balls to engage the corresponding contact pads. Numerous micro-spring contact pad models are available as
5 shown and described in Robert Crowley's article in Chip Scale Review published May 1998, p. 37, incorporated herein by reference. Although desirable results may be achieved with such devices, micro-spring mounted contact pads 18 are very expensive, relatively difficult to maintain,
10 and may excessively damage the solder ball itself.
During assembly of the bumped device 10 with the printed circuit board 20, some of the shorter solder balls may not solder to their associated contact pads during the reflow process. In the past, to increase the numbers of solder balls
15 making contact with the contact pads during reflow, the volume of the solder balls was increased. As packaging sizes and pitch requirements continue to decrease, however, the volume of the solder balls must be reduced accordingly, and thus, the percentage of balls that will not attach to the contact
20 pads during reflow increases. Again, if considerable force is applied during assembly, the CSP or the printed circuit board 20 may be damaged.
SUMMARY OF THE INVENTION
25
The present invention is directed toward apparatus and methods for substantial planarization of solder bumps for use in, for example, testing and fabrication of chip scale packages, bumped die, and other similar devices. In one
3Q embodiment, an apparatus in accordance with the invention includes a planarization member engageable with at least some of the plurality of outer surfaces, and a securing element engageable with the bumped device to securely position the bumped device during engagement with the
35 planarization member. During engagement with the at least some outer surfaces, the planarization member applies a planarization action on one or more of the outer surfaces to substantially planarize the plurality of outer surfaces. In one embodiment, the planarization member includes a cutting tool and the planarization action comprises a milling action. In another embodiment, the planarization member includes a heated platen and the planarization action comprises a thermo-mechanical deformation action. In yet another embodiment, the planarization member includes an abrasive
45 surface and the planarization action comprising a grinding action. Alternately, the planarization member includes a chemical solution and the planarization action comprises a chemical reaction. In yet another embodiment, the planarization member includes a solder deposition device and the
5Q planarization action comprises a solder deposition.
Alternately, an apparatus may include a planarization gauge that measures a planarization condition of the outer surfaces. The planarization gauge may measure the planarization condition before or after the planarization mem
55 ber is engaged with the outer surfaces.
In a further embodiment, an apparatus includes a load device engageable with at least one of the bumped device or the planarization member to urge the at least some outer surfaces of the bumped device into engagement with the
60 planarization member. The planarization member applies a planarization action on one or more of the plurality of outer surfaces to substantially planarize the plurality of outer surfaces.
In one embodiment, the planarization member includes a 65 substantially flat surface and the load device includes a mass having a weight that urges the at least some outer surfaces into engagement with the flat surface to mechanically flatten
the surfaces. In another embodiment, the load device includes a fixed surface and a pressurizable vessel, a pressure in the pressurizable vessel urging the bumped device away from the fixed surface and into engagement with the planarization member. In yet another embodiment, the load 5 device includes a press engageable with the bumped device. In still another embodiment, the load device includes a centrifuge engageable with the planarization member.
BRIEF DESCRIPTION OF THE DRAWINGS 10
FIG. 1 is a side elevational view of a bumped device engaged with a printed circuit board in accordance with the prior art.
FIG. 2 is a side elevational view of the bumped device of 15 FIG. 1 engaged with a planarization apparatus in accordance with an embodiment of the invention.
FIG. 3 is a side elevational view of the bumped device engaged with the printed circuit board of FIG. 1 following substantial planarization of the bumps in accordance with 20 the invention.
FIG. 4 is a side elevational view of the bumped device of FIG. 1 engaged with a device having protruding contacts.
FIG. 5 is a side elevational view of the bumped device of FIG. 1 engaged with a planarization apparatus in accordance 25 with an alternate embodiment of the invention.
FIG. 6 is a side elevational view of the bumped device of FIG. 1 engaged with an alternate embodiment of a planarization apparatus in accordance with the invention.
FIG. 7 is a side elevational view of an alternate embodi ment of a planarization apparatus in accordance with the invention.
FIG. 8 is a side elevational view of another alternate embodiment of a planarization apparatus in accordance with 35 the invention.
FIG. 9 is a side elevational view of yet another alternate embodiment of a planarization apparatus in accordance with the invention.
FIG. 10 shows a partial cross-sectional view of still 40 another embodiment of a planarization apparatus in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
The following description is generally directed toward apparatus and methods for substantial planarization of solder bumps for use in, for example, testing and fabrication of chip scale packages, bumped die, and other similar devices. 50 Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS. 2-10 to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or 55 that the present invention may be practiced without several of the details described in the following description.
FIG. 2 is a side elevational view of a bumped device 10 engaged with a planarization apparatus 100 in accordance with an embodiment of the invention. In this embodiment, 60 the planarization apparatus 100 includes a rotating cutting head 102 with a plurality of cutting blades 104. A securing element 106 having a recess 108 adapted to receive the bumped device 10 is engaged with the bumped device 10 to secure the bumped device 10 in position during engagement 65 of the solder balls 12 with the cutting head 102. A planarization gauge (or sensor) 109 is positioned proximate the
bumped device 10 to measure (or sense) a planarization condition of the outer surfaces 16 of the solder balls 12.
As used throughout the following discussion, the term "bumped device" refers not only to the bumped die depicted in FIG. 2, but also to a wide variety of microelectronics devices having solder bumps, including CSP's, flip-chips, ball grid array (BGA) packages, and micro-BGA packages. Furthermore, the term bumped device is intended herein to include multiples or combinations of bumped devices, such as an entire wafer of bumped dice prior to die singulation, or an entire handling tray containing multiple bumped packages.
In operation, the securing element 106 secures the bumped device 10 in position for engagement of the bumped device 10 with the cutting head 102. As the cutting head 102 is moved along the bumped device 10 (or vice versa), the cutting blades 104 rotate (as indicated by arrow w) and engage the outer surfaces 16 of the tallest solder bumps 12. The cutting blades 104 perform a planarization action (i.e. subtractive cutting or milling) on the outer surfaces 16 of one or more of the bumps 12. After engagement of the bumped device 10 with the planarization member 100, the gauge 109 may be used to check the outer surfaces 16, 16a to determine whether the solder balls 12 are all approximately the same height. If the outer surfaces are not planarized to the desired tolerance, the planarization apparatus 100 may be re-engaged with the outer surfaces 16, 16a one or more times until the balls are substantially planarized.
The terms "planarized" and "planarization" are used throughout this discussion to refer to the fact that the solder balls 12 are made to be approximately the same height—that is to say, the solder balls project from the bumped device by approximately the same distance or thickness. It is not intended to imply that the outer surfaces 16 of all of the solder balls 12 are made flat. As shown in FIG. 2, the planarization apparatus 100 need not engage all of the solder balls 12, and solder balls of different heights are engaged to different degrees. The actual number of solder balls engaged by the planarization apparatus 100 will depend upon the height variation of the balls of the array. Thus, some of the outer surfaces 16a of the solder balls 12 may be flattened, and some will remain rounded. Following application of the planarization action, the plurality of solder balls on the bumped device will be substantially (i.e. approximately) the same height, a condition referred to as "substantially planarized."
In an alternate embodiment, as shown in FIG. 2, a planarization member 100A includes a cutting tool 103 that is oriented approximately perpendicular to the die 14. The cutting tool 103 may be positioned on a controllably driven base (not shown) and may be sized to apply a planarization action on a single solder bump 12. Thus, rather than flattening a row or group of solder bumps 12 in a batch mode, the cutting tool 103 allows individual solder balls 12 to be selected for milling. The cutting tool 103 could also be used to remove most or all of a damaged ball. The ball could then be replaced by installation of a pre-formed ball, or by successive deposition of layers of solder using a solder deposition process, as described more fully below.
One may note that the planarization gauge 109 is depicted in FIG. 2 as being an optical device that senses the planarity of the outer surfaces 16,16a, such as the type of laser-based gauges disclosed in U.S. Pat. No. 5,663,797 to Sandhu for detecting the endpoint in chemical-mechanical polishing of semiconductor wafers. The gauge 109, however, may be of any type that is suitable for detecting the heights of the
|
