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A micrograph of the dual-loop wire bonds taken before themolding operation is shown in Fig. 14. A typical transmission X-ray image of the finished package is presented in Fig. 15.While the results in Table II are considered to be well within the acceptable range, failure analysis was performed on theelectrical failures from lot 1 to determine if the failures were in any way related to the fine-pitch bonding design. The detailsof the analysis are not reported here; it suffices to say that electrical failures were found to be unrelated to the fine-pitchbonding design and are presumed to be defects at the die level (these defects escaped, since no wafer-level testing was doneon these lots), implying an effective 100% yield from the standpoint of fine-pitch bonding.Article 6December 1996 Hewlett-Packard Journal10Table IIAssembly Parameters and YieldsLot 1Lot 22971850.033 (0.0013)0.033 (0.0013)4.5 (0.177)4.5 (0.177)0.406 (0.016)/0.279 (0.011)0.406 (0.016)/0.279 (0.011)99.398.4Number of UnitsAssembledWire Diameter,mm (inch)Maximum WireLength, mm (inch)Nominal Loop Heightfor Inner/Outer Rows,mm (inch)Final Test Yield, %Fig.
14. Typical dual-loop wire trajectories before molding.Fig. 15. X-ray micrograph of a molded package(corner region).It should be noted that the wire sweep pattern depicted in Fig. 15 is typical of production packages and shows no evidenceof wire encroachment or overlapping. However, in view of the dual-loop structure, it could be argued that there will be noelectrical leakage despite any overlapping of adjacent wires, since the wires are in fact spaced in the z direction. This raisesthe interesting possibility of potentially circumventing the effects of wire sweep altogether, especially when the technologyhas to be extended to much finer pitches. Therefore, we studied the effects of wire overlapping as seen in transmission X-rayobservations on electrical leakage between the overlapping wires.
Fig. 16 is an X-ray micrograph of a package in whichsignificant wire movement has been induced, leading to overlapping of adjacent wires in the corner region. Electricalleakage measurements were made on ten sets of overlapping wires from three different packages. In all cases, no leakagewas detected up to a limit of 10 megohms.
It can be inferred from this preliminary evaluation that the 0.125 mm (0.005 inch)of z separation in the dual-loop design is perhaps adequate to prevent leakage in overlapped wires. A more detailed study isrequired to characterize this result fully.Extendability to Finer PitchesThe migration of radially staggered bonding technology to finer pitches (below the nominal 0.090-mm effective pitchdemonstrated in this study) is significantly easier with radially staggered bonding than with conventional inline bonding.This point is illustrated in Table III for the case of 0.070-mm effective pitch.Article 6December 1996 Hewlett-Packard Journal11Fig.
16. X-ray micrograph of a molded package with deliberately induced wire movementleading to the appearance of overlapped wires when viewed from above.Table IIIComparison of Wire Bonding Parametersfor Radially Staggered and Inline Configurationsfor 0.070-mm Effective PitchInlineRadiallyStaggeredPad-to-Pad Distance, mm0.0700.095Bond Pad Opening, mm0.0600.0750.025 (0.001)0.033 (0.0013)Free-Air Ball Size, mm0.0500.070Capillary Wall Thickness, mm0.0300.045Wire Diameter, mm (inch)It should be noted that reduction of the bond pad opening to 0.060 mm and the concomitant reduction in free-air ball size* to0.050 mm required for the inline case translate to ball size control and bond placement accuracy levels that are beyond thecapabilities of current wire bonding equipment, while the corresponding values for the radially staggered configuration arewell within the range of equipment capabilities. Additionally, it can be shown using finite element modeling that thereduction of wire diameter from 0.033 mm to 0.025 mm translates to an approximately twofold increase in wire sweep.Finally, the reduction in capillary wall thickness will reduce the capillary life by approximately a factor of two.
Thus it isclear that, in general, a finer effective pitch is achievable using the radially staggered configuration with considerablyrelaxed wire bonding design rules compared to the corresponding inline case.Despite these points, a few important limitations come into play when finer-pitch extensions are considered, even withthe radially staggered bonding configuration.
First, the reduced die sizes resulting from finer effective pitches lead toproportionately longer bonding wires. This point was illustrated in Table I. As an example, the wire length of 5.5 mm(0.216 inch) resulting from a presumed 0.050-mm effective pitch design is considered unfeasible to manufacture because ofwire sweep, based on current process specifications. It is possible that the dual-looping scheme may obviate this problem,but this needs to be evaluated. It is also possible to reduce the bond finger pitch of the leadframe (and thus the wire length)by using interposers,** but the cost and technical feasibility of this approach is so far unproven.A second limitation is that the routing requirements for interconnection of the bond pads to the I/O circuitry and ESDstructures are proportionately tighter at finer pitches and may impose some additional restrictions (see Article 7).* A typical thermosonic gold wire bond consists of a ball bond at one end and a stitch bond at the other end.
“Ball size” refers to the size of the ball thatmakes up the ball bond, after the ball is attached to the chip. “Free-air ball size” refers to the size of the ball as soon as it is formed, before it is attached tothe chip.** An interposer is a small circuit card that is placed between the die and the leadframe. It contains a pattern of traces that fan out from a fine pitch at thedie end to a coarse pitch at the leadframe end.Article 6December 1996 Hewlett-Packard Journal12A third limitation, as illustrated in Table III, is an overall tightening of wire bonding design rules dictated by reductionsin pitch. In addition to the burden that this imposes on the capability of wire bonding and molding equipment, it alsonecessitates the development of new metrology tools and bond quality standards.
Bond quality standards can only be setby a careful study of the impact of new wire bonding design rules on reliability. As an example, the smaller ball size, the finerwire diameter, and the presence of off-pad bonds may impact reliability. New specifications for these parameters can be setafter the effect on reliability is well-understood. This significant effort has been recently undertaken by SEMATECH, aconsortium of semiconductor companies.ConclusionsA new approach to the reduction of wire bonding pitch is presented that entails the use of two rows of radially staggeredbond pads on the die, as opposed to the conventional inline arrangement.
We have combined the radial staggeringmethodology with dual-loop bonding and a new leadframe design, forming an integrated solution for reducing the effectivewire bonding pitch and thus the die size of pad-limited IC devices. The approach has been qualified by extensive assemblyevaluations of a test device in the production environment. The approach is considered advantageous over the conventionalapproach because it is implemented with significantly relaxed design rules and therefore with minimal assembly costpenalty.AcknowledgmentsThe authors wish to thank Paul Van Loan, Chong Num-Kwee, Chong Chew-Wah, Steve Ratner, and Lynn Roylance for theirtechnical inputs, support, and encouragement throughout the course of this project.
We are also indebted to the techniciansand operators whose diligence made this work a reality.References1. R. Pendse, et al, “Die Size Reduction through Finer-Pitch Bonding,” Proceedings of the HP Design TechnologyConference, 1994, pp. 281-294.2. R. Pendse, “Algorithms for Optimal Design of High-Pin-Count Packages Using Radially StaggeredConfigurations,” paper in preparation.Article 6Go To Next ArticleGo To Table of ContentsGo To HP Journal Home PageDecember 1996 Hewlett-Packard Journal13.
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