July 1998
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Radial Lead Bonding Offers Increased Flexibility and Process control
A newlead-bonding technology offers great potential maximizing assembly fexibility throughput, reliability and yield in chip-scale packages.By Joseph F. Lippincott Jr., Kulicke & Soffa Industries, Willow Grove, Pa
There are four families of CSPs: flex interposer, rigid interposer, lead-frame and wafer level. This article will focus on lead bonding flex-circuit interposer CSPs, in which the dice are interconnected to flexible polyimide tape by a tape automated bonding (TAB) process.
This category includes the patented Tessera mBGA® technology, which is currently licensed to numerous IC manufacturers and packaging foundries, as well as several other proprietary CSP designs.
An obvious design goal in all types of CSPs has been to accommodate the existing assembly equipment infrastructure. As the conversion to chip-scale packaging accelerates, however, increasing device complexity will create the need for improvements in lead bonding equipment capability.
Figure 1. CSP with orthogonally-oriented leads
Current high speed lead bonders, either TAB machines or automatic wire bonders, are limited to orthogonal operation, meaning the bondheads can only move in X, Y and Z directions. When these orthogonal machines are used to bond mTAB and mBGA packages, they are restricted to bonding leads that are at right angles. Multiple passes are required to bond any leads that are not at right angles. In this article, such leads will be referred to as diagonal or radial leads.
The use of orthogonal machines presents manufacturers considering conversion from an existing IC package design to a mBGA package with three options:
- Design the tape so that all the leads are orthogonally oriented, as shown in Figure 1. Although this is often the most straightforward solution, it requires availability of sufficient tape real estate for orthogonal orientation and can subsequently pose bonding tool setup challenges. Among them: the need for precise alignment of the features on a 4-6 mil-square bonding tool with the leads in both X and Y directions. This alignment must typically be done manually with the aid of the bonder's microscope and then verified by bonding one or more devices. Since subdegree increments of alignment may be involved, the process tends to be time consuming.
- If this orthogonal lead design cannot be achieved, multiple passes (or multiple bonders) will be required to attach the orthogonal and then the non-orthogonal leads. Compared to single-pass bonding, this approach reduces throughput and may compromise yield.
- If the orthogonal lead design cannot be achieved using the existing die, redesign of the die and requalification of the process will be required. Due to the time and cost involved, this option is seldom practical.
Figure 2. µBGA lead bonding sequence
Newly developed radial lead bonding technology will eliminate the need for manufacturers to configure the tape and/or die for the existing equipment infrastructure.
CSP Lead Bonding
Figure 2 shows the sequence used to create the intermetallic bond between the leads on the tape and the pads on a die during the mBGA lead- bonding process. The bonding tool applies downward force on the lead until it separates from the tape at a notch. The tool then forms the lead into a "Lazy S" shape and creates an intermetallic bond by thermosonically scrubbing the lead precisely onto a pad on the die.As the lead is separated from the tape, it forms around the bonding tool. This is integral to the bonder's ability to carry it through the Lazy S trajectory and to position the lead on the bond pad without slipping. The Lazy S also contributes to device reliability by compensating for the TCE variation between the die and the PC board to which the package will ultimately be attached. Optimization will be discussed in more detail later.
Radial Leads
The required mBGA motions are available in many existing orthogonal lead bonders. The more complex devices with radial leads, however, such as the new generation of mBGA packages currently being developed, frequently require multiple-pass bonding.In addition to reducing process efficiency, multiple-pass bonding can reduce yields by increasing material handling and the chance of damage. Process inconsistencies may occur when the bonding tool orientation in the transducer has to be changed to accommodate different lead angles or because of process variations between machines when multiple bonders and tool setups are used.
A new generation of rotary head lead bonding equipment is now available. The rotary head enables lead bonding at any angle, as in the device shown in Figure 3. Rotary head technology eliminates the need for multiple passes through several orthogonal bonders.
Manufacturing costs are reduced by eliminating the need to purchase multiple orthogonal bonders and by eliminating inefficient machine utilization and bond tool setup. The yield loss associated with material handling damage due to multiple passes is also avoided.
Rotary head technology also provides significant process benefits by enabling the bond tool to be positioned at the same angle as the lead, thereby creating a uniform ultrasonic scrubbing. It also provides additional bonding tool design flexibility, enabling finer pitch lead positioning and greater device reliability.
Equipment Characteristics
Figure 3. CSP with radial leads
Automatic ball bonding is currently the dominant method used for mBGA lead bonding. We have modified ball bonding machines to include software that enables the bondhead to make the looping motions required to form the Lazy S or other required lead trajectories. We have also designed lead bonding tools specifically to enhance this process. Together, the bonder and bond tool act as a system to separate the lead from the tape, form and position the lead, and finally, bond the lead to the die.
These machines can be equipped with enhanced vision systems to accommodate the large die-to-lead-height differentials found on most CSPs. These systems feature increased magnification and depth of field so that both the lead and pad are in focus at the same time. This results in more consistent pattern recognition system (PRS) and video lead locator (VLL) performance, which increases pad targeting accuracy for improved throughput and yield.
Figure 4. Rotary bondhead
Figure 5. The high strtain rate hardening of the 120 kHz process can reduce bondtime substantially compared to the 60 kHz process.
The primary improvement in the latest generation of lead bonding equipment is the rotary bond head driven by software algorithms that control movement in the X, Y, Z and Theta axes to allow radial lead bonding. The bondhead's 360Á rotation permits single-pass bonding of orthogonal and diagonal leads, greatly simplifying the conversion of more complex mBGA devices. Figure 4 shows the principal components of the rotary bondhead.
Other improvements include a linear encoder that improves table resolution to 0.1 mm and the addition of high-frequency ultrasonics to enable reductions in bonding time and temperature, while enhancing the bond characteristics.
High-Frequency Ultrasonics
As described by DeGrappo et al., high-frequency ultrasonics have been employed for several years in wedge bonding processes with a variety of wire bonded devices to enhance throughput and process parameters.1As shown in Figure 5, the high strain rate hardening of the 120 kHz high-frequency process can reduce bond time by nearly 50% compared to the 60 kHz process, resulting in bond cycle speeds as low as 85 msec.
As demonstrated by Ellis, temperature and force can be further reduced by increasing ultrasonic scrub energy.2 The extent of these reductions can be quantified by using the following equation to determine the amount of energy that is available and then maximizing the desired variable(s):
dG = VdP - SdT + uidni + ydA + 
dL
where
| dG | Gibbs free energy available for bonding (activation energy) |
| V | Volume |
| dP | Differential pressure |
| S | Entropy |
| dT | Differential temperature |
| ui | Chemical potential |
| dni | Differential atomic population |
| g | Surface energy |
| dA | Differential area |
| | Interfacial shear stress |
| dL | Scrub due to ultrasonics |
The use of higher frequency allows a substitution of thermal (heat) and force energy for ultrasonic energy to accomplish the bonding process. This ability to lower bonding temperature and/or force permits use of heat- and pressure-sensitive substrates based on less expensive polymers with lower molecular weight and glass transition temperature than normally required. This often allows for lower substrate cost, higher process adaptability across multiple substrate suppliers and less concern about lot-to-lot variations in substrate performance.
USG Orientation
With orthogonal bonders, the unidirectional scrub makes the bond wider on the X leads and longer on the Y leads. The rotary bondheadÍs ultrasonic orientation follows the direction of the lead, so the scrub always occurs in the optimum direction. This has been shown to improve tool impression consistency. Laboratory work is currently underway to quantify the contributions of directional scrubbing to process control, pull strength consistency and reliability.
Figure 6. Orthogonal lead bonding tool (top) allows bonding of right angle
leads during each pass through the machine. Unidirectional lead bonding tool
(bottom) can bond leads at any angle during a single pass.
High-frequency bonding also reduces bond deformation. During bond formation, the tool squashes and makes an impression into the lead. Lower frequency 60 kHz processes make an impression as deep as 9 mm, resulting in an 18-20 gram pull strength. High frequency 120 kHz bonding processes can be optimized to provide an impression only 2-4 mm deep without reducing the pull strength. We believe that reduced bond deformation will increase mBGA reliability by significantly reducing heel cracking and the pad lift caused by TCE variations.
Bonding Tool Design
Additional benefits provided by radial lead bonding equipment include simplified bonding tool design and setup. It also appears new radial bonding tool designs will enhance end product reliability and enable finer pitch lead bonding.Orthogonal bonding is limited to the machineÍs X and Y axes, and bonding tool features must be designed accordingly in order to shape and bond the lead. The orthogonal tool shown in Figure 6 (top), for example, requires precise alignment of its X or Y feature with each lead.
The size of orthogonal TAB bonding tools is dictated in part by face geometry, which is a result of lead orientation. The width of the face must be increased when multiple features are needed to accommodate four lead angles. As tool width increases, pitch capability is compromised.
Because radial lead bonding tools are directionally oriented by rotating the bondhead, their face geometry can be more easily enhanced. New designs, such as the unidirectional bonding tool shown in Figure 6 (bottom), requires only a single feature to shape and bond a lead in any direction. In addition to simplifying setup, a single radius permits the face width to be decreased to as little as 3 mils, permitting fine pitch bonding (currently less than 80 mm).
Unidirectional bonding tools also can be designed to reduce lead heel cracking, which is caused by the TCE variation between the board and the die as shown in Figure 7. It is possible to customize a unidirectional tool for improved control over Lazy S loop formation. Because only one feature is needed, more area is available on the tool face, permitting design of a larger radius on the tool. The resulting increase in the heel radius will significantly reduce heel cracking. The effect of the relationship between radii and reliability are currently being investigated.
Figure 7. This "lazy S" loop is an integral CSP reliability component.
Conclusion
Radial lead bonding shows great promise for maximizing assembly flexibility, throughput, reliability and yield in next-generation, radial-lead mBGA packages. Although this article has focused on flex interposer configurations, equal benefits may be realized when this new technology is used with any type of CSP or non-CSP application which uses lead bonding or TAB as the interconnect method. Rotary head lead bonding equipment is also well suited for fine pitch, high lead-count devices such as TBGA packages.References
- D. DeGrappo and A. Safabakhsh, "The Fast Track to Fine Pitch," Advanced Packaging, May/June 1996.
- T. Ellis, "Low-Temperature Bonding: from Theory to Production," TECHTalk (available from K&S), April/May 1997.
Contact Mr. Lippincott at 215.784.6053, fax 215.659.7588 or by e-mail at jlippincott@kns.com.
Mr. Lippincott is Product Manager - Chip-Scale Products for K&S. Since joining the company in 1979, he has worked in the engineering and marketing groups. He earned a bachelor's degree in mechanical engineering from Temple University, Philadelphia, and is enrolled in the MBA program at LaSalle University, Chicago.