Automated ribbon bonding
07/01/2001
Electrical and mechanical benefits offer high-performance solutions
BY LAURIE S. ROTH, IVY W. QIN AND ROBERT E. WERNER
The extent of the commercial marketplace for high-frequency devices requires high-quality, high-volume ribbon bonding. The ability to automatically bond ribbon wire with consistency is important for producing the required electrical and mechanical performance.
Market Conditions
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The increasing need for bandwidth, driven by the strong growth of data communications, is pushing devices into higher frequencies for applications such as microwave, millimeterwave and optoelectronics. This, in turn, is presenting new challenges in packaging technologies. Although some of these technologies have been used in more mature applications, such as defense electronics, there was no need to produce the quantities of devices that are now being forecast. Today's challenge is to master this complex packaging technology in a high-volume, low-cost environment. Automatic ribbon bonding technology has been developed to meet this challenge.
Wireless applications: Ribbon bonding first came into use in the defense electronics sector, where it was the first-level interconnect of choice for gallium arsenide millimeterwave monolithic integrated circuits (GaAs MMICs) in millimeterwave radar. Recently, governments around the world have been auctioning off licenses in higher frequencies to satisfy demand for bandwidth in the fixed and mobile wireless markets.
 Figure 1. Optoelectronic hermetic package requiring deep access. |
Telematics (communications to and from a stationary or moving vehicle) have had the 77 GHz band allocated worldwide for collision warning systems and adaptive cruise control.
The traditional technologies that could handle production runs consisting of hundreds of devices cannot make the transition to true high-volume manufacturing that is measured in thousands or even millions of devices.
 Figure 2. Ball bonder second bond. |
Optoelectronic applications: Optoelectronic applications run at higher frequencies than most wireless devices. In addition, many of these devices - optical transceivers in particular - are in hermetic packages that require deep access clearance for the bonding tool (Figure 1). Temperature-sensitive materials that are often found in these packages, such as LiNbO3, can shatter if not handled properly. Manufacturing automation is one of the critical requirements for the economic rollout of high bandwidth fiber optic networks.
Other applications: Ribbon bonding also offers an effective alternative to heavy aluminum wire bonding. Its use is growing in medical electronics, especially in pacemakers and defibrillators, as well as for many power amplifier applications in defense electronics and industrial sensors and controls. Ribbon bonding is also an effective alternative to tape automated bonding (TAB) for burn-in and testing applications.1
Ball Bonding vs. Wedge Bonding
Today, more than 90 percent of chip interconnects are performed with gold wire ball bonding, because ball bonding is much faster than wedge bonding on a similar machine platform. This is primarily because ball bonding only requires three axes of movement (XYZ), whereas wedge bonding requires four (XYZq), with the theta move increasing process time. So why would anyone choose wedge bonding?
 Figure 3. Wedge bonder second bond. |
Pull strength: In ball bonding, the first bond is made by connecting a ball formed by flaming off the gold wire that is threaded through a capillary bond tool. The second bond is a crescent shape formed by the imprint of the capillary (Figure 2). With wedge bonding, both the first and second bonds look the same because the bonding occurs under the foot of the wedge tool in both instances (Figure 3).
The second bond of the ball bond has less surface contact than that of the wedge bond. Therefore, the wedge bond exhibits higher pull strength for the same size wire diameter. This gives wedge-bonded devices higher reliability and yields (Figure 4). Die-to-die bonding in multi-chip modules is easier, because wedge bonding does not require a stand-off stitch (bump) for the second bond.
 Figure 4. Ball and wedge bond wire pull strengths. |
Deep access: Another advantage of wedge bonding is deeper package access. Powdered ceramic technology used in manufacturing ball bonding tools has length and diameter limitations. Wedge bonding tools can be longer because they can be manufactured using an extrusion process. Fine wire or ribbon wedge tools can easily be made 25 mm or longer, and heavy wire or ribbon wedge tools can be 50 mm or more in length. Ball bonding capillaries are limited to about 17 mm in length (Figure 5).
Low, short loops: In high-frequency devices, lower, shorter wire loops provide improved electrical performance. For the same package distance between first and second bond, wire length is shorter in a wedge bond because the loops can be lower. As the distance between bond pads increases, the ratio of wire payout between wedge bonding and ball bonding diminishes. In RF packages, short wire lengths are critical, making wedge bonding the preferred interconnect (Figure 6).
Fine pitch: High-frequency devices can have large numbers of input/output (I/O) pads, placed as close to each other as possible, or very small individual bond pads. Wedge bonding can achieve finer pad pitch geometries than ball bonding with the same wire diameter because of the smaller amount of bond "squash," or wire deformation. This attribute also allows wedge bonding to smaller bond pads without the need to decrease wire diameter (Figure 7).
 Figure 5. Wedge (left) and ball bond (right) package access. |
Copper wire applications: Copper wire is often used for its electrical performance and low cost. During ball bonding, the device must be heated and a cover gas, typically nitrogen, is required to prevent oxidation of the copper wire. Wedge bonding of copper can be performed at ambient temperature with parameters similar to those used to bond aluminum wire, thereby eliminating the need for heat and a cover gas.
Other Benefits of Ribbon Bonding
Skin effect: For direct current (zero frequency), the electron flow is uniform throughout a conductor. If the signal is alternating current, as the frequency range increases, the electron flow begins to move, by magnetic effect, toward the conductor surface. At high frequencies, almost all of the flow is on the surface. This is called "skin effect."
When skin effect is strong, most of the conductor cross-section is wasted as far as electrical conduction is concerned.2 For example, for the same wire length to have the same surface area as a ribbon wire of 25 x 125 µm, an equivalent round wire would have to be nearly 100 µm in diameter (Figure 8.). It is a much easier process to bond a 25-µm thick flat gold wire than a 100-µm thick round gold wire.
 Figure 6. Ball and wedge bond loop shapes and wire lengths. |
Other benefits: Additional benefits provided by the use of ribbon vs. round wire include increased reliability, because there is a larger cross-section at the heel of the bond, and less heel cracking because the loop is not worked as much due to the lower loop profile, which reduces stress during thermal cycling.3 Less cratering occurs because the bond force and ultrasonics are distributed over a larger area, and there is virtually no wire sway, because of the structural rigidity of flat wire. Additionally, longer wire spans with less sag are possible for the same reason, and it is easier to make compound (stacked) bonds because the bonding surface is flat rather than round.
Automated Ribbon Bonding Process
A well-controlled ribbon bonding process requires strong bonds for package reliability, consistent looping for impedance control and fast wire cycles for high-volume production. As with round wire processes, several factors should be defined and optimized, including wire properties, bonding wedge geometry and material, machine configuration and bonding parameters.
 Figure 7. Ball and wedge bond pad pitch and geometries. |
Wire: Gold is most frequently chosen as ribbon wire because of its superior bonding properties. There is a small amount of aluminum, silver and palladium ribbon in use, and copper wire processes are in development. The majority of ribbon sizes range from 6 to 50 µm thick by 50 to 500 µm wide, preferably with the highest ratio of width to thickness that the application can support. Other ribbon properties that affect the bonding process include elongation, tensile strength and composition of the wire.
Wedge: Wedge tools for ribbon wire are similar to round wire wedge tools, except for a rectangular wire feed slot. A variety of bondfoot styles are available, such as cross-groove, linear-groove or linear-cross groove. The wedge is a vertical feed style to accommodate the 90-degree wire feed system on the ribbon bonder. Wire is fed vertically from the top of the wedge and exits near the end of the wedge tip, and is then fed back through a feed hole at the wedge foot.
 Figure 8. Surface area comparison of ribbon and round wire. |
Machine requirements: Because ribbon bonding has been used for some time in low-volume applications, the state of the art of equipment has not evolved tremendously since the 1980s. Most ribbon bonding has been done with manual machines,4 which are no longer adequate to meet the production quantities sought today. Older automatic wedge bonders can also accommodate ribbon wire. Typically, wire cycle time is around one wire per second for the lighter ribbon sizes. For larger ribbon, the process is even slower. Many of these machines have limitations in accuracy, which affect their ability to stay within very small (50-µm diameter) bond pads.5 Additionally, most older machine platforms use lower frequency ultrasonics, less than 120 kHz, which restrict their ability to successfully bond at lower temperatures.
An advanced automatic ribbon bonder has been developed to overcome these shortcomings. The machine is equipped with a high-frequency transducer (120 kHz), which provides superior bonding results at reduced bonding temperature and bonding time.6 Process testing of different sized ribbon wires with different looping profiles has shown that this machine can meet the requirements of low temperatures, short and low loops, minimum bond squash, good accuracy and good productivity. AP
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References
- Norman Stockham, telephone conversion, Microtechnology Centre, The Welding Institute, Cambridge, UK, January 2000.
- Lawrence Skarin, "Mad Scientist," Electrical Engineering, Monroe Community College, www.madsci.org, 1998.
- David C. Guidici, "Ribbon Wire Versus Round Wire Reliability for Hybrid Microcircuits," IEEE Transactions on Parts, Hybrids and Packaging, June 1975, pp. 159-163.
- Christina M. Conway and Nicole L. Cavanah, "Gold Ribbon Bonding for Microelectronic Interconnection: A Designed Experiment to Evaluate Process Opportunities," 2000 International Symposium on Microelectronics, pp 681-687, 2000.
- Keith I. Johnson and Stephen T. Riches, "Microjoining in Europe," 5th Annual Symposium of the Japan Welding Society, Tokyo, April 1990, pp. 239-244.
- Y. Shirai et al, "High Reliability Wire Bonding by the 120 KHz Frequency of Ultrasonic," ICEMM Proceedings, pp. 366-375, 1993.
Laurie S. Roth, director of strategic marketing, Ivy W. Qin, process engineering manager, and Robert E. Werner, senior process engineer, can be contacted at Kulicke & Soffa Industries Inc., 2101 Blair Mill Road, Willow Grove, PA 19090; 215-784-6440; Fax: 215-784-6221; E-mail: [email protected].