Issue



Stacked Die & Multi-tier Applications


10/01/2005







New Capillary Addresses Looping Problems

BY YAIR ALCOBI

As the semiconductor moves from fine pitch to stacked die applications to meet the electrical demands of today’s electronic products, looping profiles used during wire bonding have become more complex. Materials and bonding tool manufacturing companies have developed new capillary technologies to address the looping challenges associated with today’s increasingly popular multi-tier applications. While older and more conventional bonding tools contribute little to the looping process, new and emerging capillaries are being designed and manufactured as more efficient tools that positively affect the looping response in challenging packages. When implemented 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. This article examines a newer capillary technology relating to the production benefits in addressing looping problems, as well as test results in actual applications.

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The trend within the semiconductor industry for finer-pitch applications is being replaced with a move to 3-D 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 package in multi-tier formation (Figure 1). Currently, there are stack-die applications with up to 7 different chip layers. The outcome of this configuration is better electrical performance, which meets the demands of various applications such as cell phones.


Figure 1. Two levels of stacked die.
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But with multi-layer stacking comes the challenge of more complex loop profiles. Wire loop length is elongated, which requires stronger loop process control to ensure robust loop formations 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. Often, that requires an unplanned capital expenditure.

While in the past, conventional bonding tools contributed little to the looping process; newer capillaries are being designed and manufactured as more efficient tools that positively affect the looping response in challenging packages. By supporting more stable and accurate looping outputs, these next-generation capillaries assist in improving wire bonding process production yield and productivity, when compared to conventional capillary designs.

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

Looping Challenges

Typically, 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 signal bar of BGA devices, where the wire tends to sink due to its own mass. This defect also may be a result of inconsistent 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 other wire. This may occur with any neighboring wire (adjacent, beneath, or above), such as in multi-tier or stacked-die package types.
  • Wire scratch - the wire is damaged 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, anywhere along the wire. The Sliver can be long enough to short between two adjacent wires. Slivers are difficult to trace, and can affect operations seriously.

Various factors, in multiple combinations, can contribute to the overall looping performance achievable in any given semiconductor package type. Factors that primarily contribute to the looping response quality include:

  • Bonder subassemblies (wire path assemblies, wire clamps, bond head, and XY table)
  • Gold wire properties (diameter, elongation factor, and 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, adjacent wire contact, and wire scratches. Reduction of these common wire looping-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 identifiable during the wire bonding operation, but at electrical testing after the molding process. At this point, however, it is too late to correct the problem. The unit is disqualified, as it can’t be reworked. Yields, therefore, suffer. Today’s newer capillaries with the correct engineered functionally supports yield improvements at this stage by preventing wire sways and other deficiencies that would disqualify devices during electrical testing.

Performance Tests

Tests were conducted on the new capillary 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 running products that were simulated by the engineering teams or conducted at major semiconductor assembly plants, following actual production process specifications. 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 usage 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 Diameter

The PBGA 45-µm BPP multi-tier package is one of the densest packages among the PBGA types. This simulation test was conducted at an applications center using high-performance bonders. In this application, S’ing and wire-scratch failure types were identified as major looping failures. Based on identical process conditions, the specific capillary performance was tested to demonstrate a reduction in S’ing and wire scratch failures compared to the conventional capillary-based process. All tests were conducted according to production requirements, while maintaining first- and second-bond responses within their specifications.

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

Note: 0% improvement represents the conventional capillary reference response.

Using the same bonding tool and equipment - Low- and high-tiers looping
Further 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% improvement represents the conventional capillary reference response.

Looping response stability - S’ing responses on a high-performance wire bonder
Using the new capillary, S’ing variation was significantly lower and distributes over a tighter range, as compared to results achieved with the conventional capillary.

Results
With 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.

Conclusion
As shown by the test results, the bonding tool minimized the S’ing, which was a major challenge. This new capillary functional design provides a more reliable and profitable process with a significant reduction in production-yield-loss risks.

Test Two: Multi-tier PBGA, 80-µm BPP, with 1.2-mil Wire Diameter

In another application, the live production process of a multi-tier package was evaluated on a high-capacity wire bonder (Figure 2). Both conventional and newer capillary designs were tested under identical wire bonding conditions. In this application, leaning failure was identified as the major challenge. Two different wire tiers of this package with various failures were tested under two different loop trajectories to improve overall response. All tests were conducted according to production requirements, while maintaining first- and second-bond responses within their specifications.


Figure 2. Multi-tier package design.
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At both wire tiers, substantial improvement was achieved with the new bonding tool, especially at the scratch and S’ing occurrences. Wire contact was not encountered at low tier wires, but was greatly improved at higher tiers.

Note: 0% improvement represents the conventional capillary reference response.

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Capillary Performance at BGA5 Loop Trajectory - Looping Failures
With BGA5 loop trajectory, S’ing occurrences were slightly less improved, and wire scratch received a minor aggravation of 1% at low tier wires. Leaning improvement rate was maintained at both tiers.

Looping response stability - Leaning responses at BGA5 and lateral worked loop
This newly designed capillary leaning variation is significantly lower, and distributes over a tigher range, as compared to results achieved with the conventional capillary.

Results
Comparing the two trajectories (BGA5 loop and lateral worked loop), results obtained with the bonding tool* on the lateral worked loop trajectory are preferable, as both low and high tiers are improved at higher percentages. On the high-tier wires group, this new capillary achieved a high percentage of wire contact phenomena reduction. Overall, this new capillary design surpassed the conventional capillary leaning failure variation response and proved its superiority.

Conclusion

These tests results and tool review examine how the next generation of wire bonding tools can help reduce yield losses and streamline manufacturing. There are many tool options and as each tool is phased out, another one will quickly take its place on the manufacturing line. Wire bonding 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. Companies are hoping that with this next generation of tools, a greater control in wire loop height and shape stability can be achieved. By supporting more stable and accurate looping outputs, these capillaries may assist in improving wire bonding process production yield and productivity, when compared to conventional capillary designs.

* 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.