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As the semiconductor moves from fine pitch to stacked die applications to meet the electrical de-mands of today’s electronic products, looping pro-files used during wire bonding have become more

complex. Materials and bonding tool manufacturing com-panies have developed new capillary technologies to address the looping challenges associated with today’s increasingly popular multi-tier applications. While older and more con-ventional bonding tools contribute little to the looping pro-cess, new and emerging capillaries are being designed and manufactured as a more efficient tool that positively affect the looping response in challenging packages. When imple-mented in the wire bonding process, these unique bonding tools provide a greater control in wire loop height and shape stability, and significantly reduce looping failures typically found on wires formed with a conventional capillary. For example, a new capillary tool has been introduced. This ar-ticle examines a newer capillary technology relating to the production benefits in addressing looping problems as well as test results in actual applications.

The trend within the semiconductor industry for finer pitch applications is being replaced with a move to three-dimensional packaging solutions. Instead of shrinking die on pad pitch, IC chip manufacturers now are starting to stack die or add more I/Os on a given size chip and pack-age in multi-tier formation (Figure 1). Currently, there are stack-die applications with up to seven different layers of chip. The outcome of this configuration is better electri-cal performance, which meet the demands of applications such as cell phones.

But with multi-layer stacking comes the challenge of more complex loop profiles. Wire loop length is now elongated, which requires stronger loop process control to ensure ro-bust loop formation void of looping failures. To address these complicated loops, some manufacturers are slowing down the looping process speed to gain better control at the cost of throughput. Others are turning to new state-of-the-art wire bonders, which support multi-tier applications. Of-ten, that requires an unplanned capital expenditure.

While in the past, conventional bonding tools contrib-

BY YAIR ALCOBI

NEW CAPILLARY ADDRESSES

LOOPING PROBLEMS

Multi-tier ApplicationsStacked Die&

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uted little to the looping process; newer capillaries are being designed and manu-factured as more efficient tools that posi-tively affect the looping response in chal-lenging packages. By supporting more stable and accurate looping outputs, these next-generation capillaries assist in im-proving wire bonding process production yield and productivity, when compared to conventional capillary designs.

One newer capillary* provides for more robust loop formation as the loop-ing process does not have to be slowed down to ensure defect-free results. When compared with the performance of con-ventional capillaries, this newly released bonding tool has proven to assist in pro-ducing higher yield and less scrap, while still achieving tolerances for complex looping in stacked die, multi-tier, and other complex, tight tolerance devices.

Looping ChallengesTypically, wire loops on a bonded package suffer random height variations or defects, in various levels of frequency and severity, depending upon the process complexity and the manufacturing capabilities. A 2% to 3% yield loss can result from shorts in stacked-die applications. Typical types of looping failures include:• S’ing — the undesired “S” shape of a

wire loop occurring in several types of looping failures, such as:Wire kink. A mechanical kink anywhere along the wire loop severe enough to cause adjacent wires to contact each other;Wire sagging. Commonly found on wires bonded to the power/ground sig-nal bar of BGA devices, where the wire tends to sink due to its own mass. This defect also may be a result of inconsis-tent wire payout ;Wire sway. Commonly observed on wires exiting from the corner of a bonded package;Wire sweep. Commonly observed on relatively long wires, particularly after molding encapsulation process.

• Wire leaning — describes cases when the wire is not vertical to the ball bond at the initial part of the wire loop. This can easily cause adjacent wires to come in contact with each other, as this is where the distance between loops is the smallest along the wire.

• Wire contact — when one wire is in actual contact with any oth-er wire. This may occur with any neighboring wire (adjacent, beneath, or above), such as in multi-tier or stacked-die pack-age types.

• Wire scratch — the wire is dam-aged as a result of mechanical friction with any of the parts along the wire path (diverters, tubes, wire clamps, capillary).

• Wire slivers — this failure could result from a wire scratch case. The “sliver” is generally a tiny filament of gold created during the scratch, any-where along the wire. The Sliver can be long enough to short between two adja-cent wires. Slivers are difficult to trace, and can affect operations seriously.Various factors, in multiple combina-

tions, can contribute to the overall loop-ing performance achievable in any giv-en semiconductor package type. Factors that primarily contribute to the looping response quality include: • Bonder subassemblies (wire path as-

semblies, wire clamps, bond head, and XY table)

• Gold wire properties (diameter, elon-gation factor, stiffness)

• Package density • Architecture complexity

Today’s next-generation capillaries can improve the bonding process from a capillary viewpoint. When tested in actual applications, a bonding tool* showed a reduction in looping-related issues such as wire S’ing, leaning, ad-jacent wire contact, and wire scratches. Reduction of these common wire loop-ing-related failures, along the capillary life, will enable a reduction of looping failures occurrences.

This new capillary design has solved many of the issues that are difficult to catch during the looping process. For example, wire sways are often not iden-tifiable during the wire bonding opera-tion, but at electrical testing, after the

molding process. At this point, howev-er, it is too late to correct the problem. The unit is disqualified, as it can’t be re-worked. Yields, therefore, suffer. Today’s newer capillaries with the correct engi-neered functionally supports yield im-provements at this stage by preventing wire sways and other deficiencies that would disqualify devices during electri-cal testing.

Performance TestsTests were conducted on the new cap-illary according to leading assembly houses specifications in the industry and validated on several package types by quantifying the looping response. All applications results were based on run-ning products that were simulated by the engineering teams or conducted at ma-jor semiconductor assembly plants, fol-lowing actual production process speci-fications. Tests covered various package architectures, with fine and large bond pad pitch, wire bonded with small and regular gold wire diameter.

Various wire bonders, offering different levels of capacity, were used throughout the different tests to check the effect of us-age on different machines and to identify any wide variances. Prior to each test, a conventional capillary-based process and results were established as the reference point, and later used to compare to the specific capillary-based process.

Test One: Multi-tier PBGA, 45-µm BPP, with 0.8-mil Wire Di-ameterThe PBGA 45-µm BPP multi-tier package is one of the densest pack-ages among the PBGA types. This simulation test was conducted at an Figure 1. Two levels of stacked die.

Figure 2. Multi-tier package design.

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applications center using high-perfor-mance bonders. In this application, S’ing and wire-scratch failure types were iden-tified as major looping failures. Based on identical process conditions, the specific capillary performance was tested to dem-onstrate a reduction in S’ing and Wire Scratch failures compared to the con-ventional capillary-based process. All tests were conducted according to pro-duction requirements, while maintaining first- and second-bond responses within their specifications.

Relative improvement is achieved with this particular capillary as compared to the conventional capillary for four dif-ferent failure types, at both low- and high-tier loop groups. Leaning was not encountered, wire contact was com-pletely eliminated, and improvements were achieved for S’ing and Scratch fail-ure types.

Note: 0% improvement represents the conventional capillary reference re-sponse

Using the same bonding tool and equip-ment — Low- and high-tiers loopingFurther improvement was achieved by combining the wire bonders with the

capillary*, at both low- and high-tier loop groups. Here, some minor leaning improvement was achieved on high-tier loops with no wire contact. Significant improvement was achieved for S’ing and scratch failure types. Note: 0% improve-ment represents the conventional capil-lary reference response.

Looping response stability —S’ing responses on a high-performance wire bonderThe following plots compare the variance of S’ing response, quantified by ANOVA model, between the two capillary types, at 95% confidence level.Using the new capillary, S’ing variation was significantly lower and distributes over a tigher range, as compared to results achieved with the conventional capillary.

ResultsWith both bonder types, results obtained with this capillary at S’ing response are clearly preferable. This capillary helps contain the phenomena by tightening its range of occurrences.

ConclusionAs shown by the test results, the bond-ing tool minimized the S’ing, which was a major challenge. This new capillary func-tional design provides a more reliable and profitable process with a significant re-duction in production yield loss risks.

Test Two: Multi-tier PBGA, 80-µm BPP, with 1.2-mil Wire DiameterIn another application, the live produc-tion process of a multi-tier package was evaluated on a high-capacity wire bond-er (Figure 2). Both conventional and the newer capillary designs were tested un-der identical wire bonding conditions. In this application, leaning failure was iden-tified as the major challenge. Two differ-ent wire tiers of this package with various failures were tested under two different loop trajectories to improve overall re-sponse. All tests were conducted accord-ing to production requirements, while maintaining first- and second-bond re-sponses within their specifications.

At both wire tiers, substantial improve-ment was achieved with the new bonding tool, especially at the Scratch and S’ing occurrences. Wire contact was not en-countered at low tier wires, but was great-ly improved at higher tiers.

Note: 0% improvement represents the Conventional capillary reference re-sponse

Capillary Performance at BGA5 Loop Trajectory —Looping FailuresWith BGA5 loop trajectory, S’ing occur-rences were slightly less improved, and wire scratch received a minor aggravation of 1% at low tier wires. Leaning improve-ment rate was maintained at both tiers.

Looping Response Stability —Leaning Responses at BGA5 and Lateral Worked LoopThe following plots compare the vari-ance of Leaning response, quantified by ANOVA model, between the two capil-lary types, at 95% confidence level.This newly designed capillary leaning varia-tion is significantly lower, and distributes over a tigher range, as compared to results achieved with the conventional capillary.

ResultsComparing the two trajectories (BGA5 loop and lateral worked loop), results ob-tained with the bonding tool* on the lat-eral worked loop trajectory are prefera-ble, as both low & high tiers are improved

Application Conditions

Wire bonder type: ***

Application Conditions

Bonder type: ***

Package BPP: 45-µm linear

Package type: PBGA test device

Wire type and diameter: AW99, 0.8 mil

Low-tier wire: 3-mil high, 140-mil long

High-tier wire: 12-mil high, 250-mil long

Loop profile: BGA2 LOOP

Test Volume

Eight conventional capillaries vs. eight capillaries* per bonder model

Three inspected devices per capillary type and per loop tier

200 wires per device

Result Analysis —

Capillary performance on high-capac-ity wire bonders with low- and high-tiers looping

Package BPP: 80 µm

Package type: Multi-tier PBGA

Wire diameter: AW88, 1.2 mil

Loop height/ Low-tier at 10/180 length: mil and high-tier at

15/220 mil

Loop profiles: Lateral worked LOOP and BGA5 LOOP

Test Volume

10 conventional capillaries vs. 10 specific capillaries

Six devices inspected per capillary type

650 wires per device and per tier

Result Analysis —

Capillary Performance at Lateral Worked Loop Trajectory Looping Failures

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at higher percentages. On the high-tier wires group, this new capillary achieved a high percentage of wire contact phenom-ena reduction. Overall, this new capillary design surpassed the conventional cap-illary leaning failure variation response and proved its superiority.

Quantification Method — Flow-chart for any Looping Failure TypeThe flowchart describes the simplified recommended procedure by which a con-ventional capillary-based looping pro-cess can be quantified and compared to a next-generation capillary. For each set of conventional capillaries** compared, the same cycle can be repeated.

ConclusionThese tests results and tool review ex-amine how the next generation of wire bonding tools can help reduce yield loss-es and streamline manufacturing. There are many tool options to choose from and as each tool is phased out, another one will quickly take its place on the man-ufacturing line. Let’s face it, wire bond-ing failures happen and a 2% to 3% yield loss may not seem significant, but it is especially when manufacturing has been slowed to reduce looping mishaps. Com-panies are hoping that with this next gen-eration of tools, a greater control in wire loop height and shape stability can be achieved. By supporting more stable and accurate looping outputs, these capillar-ies may assist in improving wire bonding process production yield and productiv-ity, when compared to conventional cap-illary designs. AP

* Kulicke & Soffa

** Kulicke & Soffa’s ARCUS

*** Kulicke & Soffa’s Maxµmplus and MaxµmUltra

YAIR ALCOBI, bonding tools director of marketing,

may be contacted at Kulicke & Soffa, 2101 Blair Mill

Road, Willow Grove, PA; e-mail: yalcobi@kns.com.