Ribbon Wedge Bonding
07/01/2003
Machine Configuration and Tools
BY KEVIN SEUFERT
With the recent growth of optoelectronics assembly, the use of ribbon wire for interconnects has become more widely used. This article explores the basics of the ribbon bonding machine configuration and some of the finer points of ribbon wedge tools and wire.
Wedge vs. Ball Bonding
Wire bonding has been around since 1947. Today, it has evolved into a complex, mature manufacturing process with three bonding types: thermocompression (TC), ultrasonic (U/S) and thermosonic (TS). TC wire bonding involves deforming the wire under a bonding tool with force and high heat (>250°C). U/S wire bonding also involves deforming the wire under a bonding tool with force; however, without heat (normally room temperature) and with an ultrasonic movement of the wedge tool. TS wire bonding processing combines both TC and U/S, and involves deforming the wire under a bonding tool with force, heat (lower than in TC, typically ~ 150°C) and an ultrasonic movement of the wedge tool.
Wire bonding falls into two main classifications: ball and wedge bonding. Ball bonding is an omni-directional process. (i.e., the second bond can be at any 360° position from the ball), while ball bonding is used almost exclusively with gold wire and accounts for ~ 98 percent of all wire bonding done today. Ball bonding can be done with a TC or TS process and is faster than wedge bonding. Wedge bonding is an unidirectional process (i.e., the second bond must be aligned with the first bond) and can be done with aluminum, gold or copper wire. Wedge bonding is capable of finer pitch than ball bonding and can bond ribbon wire. Wedge bonding can be performed by TC or TS, and unlike ball bonding by the U/S process it can be performed at ambient room temperature. Some advantages of wedge bonding include finer bond pad pitch, lower loop profiles, controlled wire length and lower temperature when using U/S process.
Ribbon vs. Round Wire
Ribbon wire has a rectangular cross section as opposed to round, and is advantageous for devices using RF. Standard ribbon wire sizes range from 0.00025 x 0.0005" to 0.002 x 0.020", and contain different amounts of alloying elements from ribbon manufacturers with different processing methods, which could affect bonding quality.
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Ribbon Wedge Bonding. In the ribbon wedge bonding process, the ribbon wire is positioned under the bonding tool. The bond head of the machine lowers to the first bond, and after the bond is made, the head rises to form a loop and is positioned at the second bond. After the second bond is made, the ribbon is cut and fed back under the tool for the next cycle.
Machine Configuration. Ribbon and round wire wedge bonding machine configurations can have two different types of termination: clamp or table tear. Both manual and automatic bonders can use either type, and some bonders can switch between the two.
Clamp tear — On machines configured for clamp tear, the clamps are located behind the wedge tool and pivot away from the wedge to perform ribbon termination after the second bond is completed. This is the most common process and is reliable because the bonding tool rests on top of the second bond during the termination, keeping it from peeling up. The ribbon will break at the "heel" of the bond because this is the smallest cross-sectional area of the ribbon (Figure 1).
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The standard clamp's "jewel" face typically is made from sapphire or a highly polished metal so as not to damage the surface. For ribbon wire, this face material is changed to ceramic or another slightly rougher surface material to better grip the ribbon during the termination. This is especially critical when using the larger ribbon sizes.
The position of the clamps in relation to the wedge tool is ideal when the wire path through the clamps and the tool slot is at 30°. At this angle, the bottom of the clamps are close to the wedge tool tip and do not allow for clearance to adjacent parts or for getting into deep packages. The wedge tool and clamp position also can be configured at 38°, 45° or 60° feed angles for clearance of adjacent parts or the sidewalls of deep packages.
A problem with these steeper angles is that the ribbon is fed back under the wedge tool at that same steep angle. When the tool approaches the first bond on the next cycle, the ribbon may get "stuffed" back up the feed slot instead of bending under the bottom of the wedge tool. Generally, the tail of the first bond is only ~0.0005" long so any amount of "stuffing" will cause an improper tail length (Figure 2).
Some machines will allow the clamps to be moved up and away from the wedge tool, but still keep the feed angle at 30°. This allows clamp clearance of adjacent parts and keeps the feed angle at the preferred degree. The problem with having a large gap between the wedge tool and clamps is the ribbon can flex between the two, causing problems with tail length, looping and terminating.
On machines where the clamps open on the horizontal axis, the ribbon has to twist 90° so it is vertical when passing through the clamps and horizontal when passing through the wedge tool. If the clamp is too close to the wedge tool, a "kink" may form during the 90° twist and become jammed in the tool slot. If the machine's clamps open on a vertical axis, the ribbon does not have twist to align with the tool slot, and the clamp may be positioned closer to the wedge tool.
Golden Rule #1: On any wedge bonding machine (ribbon or round wire), the closer the clamps are to the feed slot/hole, the better the tail control, looping control and termination.
- Table tear — On some packages, the bonding is done next to adjacent devices or into deep packages, in which the clamp movement/pivot action during ribbon termination would cause interference. The major difference between table and clamp tear is with table tear, the ribbon clamps only open and close and do not pivot away from the tool for the termination step.
In the table tear process, after the second bond is finished, the wedge tool rises slightly off the second bond, and while the clamp is still open, the machine's X-Y table moves back from the second bond in the Y-axis only. This first table motion feeds out enough ribbon for the tail of the first bond on the next wire. The machine clamps then close and the table continues moving further back, and the ribbon is torn at the weakest point, which is the heel of the second bond.
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With the wedge tool/clamp set up as shown in Figure 3, the ribbon feed angle can be configured for 60° without the problem of "stuffing" the tail for the next bond cycle as the angle of the ribbon under the tool is a function of the programmed "Zup". This "Zup" should be set so the wedge tool just clears the second bond. If it is too low, the wedge tool will rip off the second bond and, if it is too high, the second bond may be peeled up during the last Y-table motion.
Some machines can be configured with a vertical wedge tool (Figure 4). This is similar to the ball bonding capillary in that the ribbon is fed down the center of the tool from the top, exits the bottom and passes through the feed slot, like on a standard ribbon tool. With this configuration, the ribbon clamps are above the tool and cannot interfere with anything. A problem with this configuration, however, is that there is a long distance between the clamps and the feed slot at the bottom of the tool (i.e., longer than the length of the wedge tool itself). This goes against the above-stated Golden Rule and may cause problems with tail control and ribbon termination.
Ribbon Wedge Tools. Titanium carbide is the preferred material, while tungsten carbide is only for aluminum ribbon. The two materials were selected for the properties of not welding (adhering) to the wire
ibbon in use and for manufacturability and low-wear resistance. Some wedge tool manufactures have designed tools where the tip is made from exotic materials such as osmium and cermet, which claim longer tool life and better ultrasonic energy transfer. Process engineers should perform an in-depth, production analysis of these exotic tools for each specific application. The benefits need to be justified because most of these exotic tools are expensive compared to standard titanium carbide tools.
- Tool Diameter — This depends on the transducer horn used on the specific machine. The most common diameter is 0.0625". Some of the larger diameter round wire machines use 0.125" tools.
- Tool Length — Some of the most commonly used lengths are 0.437, 0.625, 0.750, 0.828 and 1.078". Wedge tool length is critical to the ultrasonic horn design and wavelength for each transducer used.
- Tool Flat — The flat of the tool is used for proper alignment when clamped into the transducer and depends on the bonding machine transducer design. Most machines use a setscrew from the front, allowing the flat to be toward the front, while other designs have the alignment flat on the back.
 Figure 4. A vertical wedge tool configuration. |
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- Tool Foot — The foot or tip of the tool is the most critical part and is where the bonding takes place. It is called the "foot" because the front is the toe and the back is the heel. Ribbon tools are flat and Golden Rule #2 is to have a "cross groove" across the bottom at 90° to the ribbon path. The radius of the toe and heel is critical. The heel must have a large radius to prevent the first bond heel from cracking, but it also must have a small radius to help in termination after the second bond. The tool toe should be large enough for a smooth transition from the loop to the toe of the second bond.
- Tool Tip Configuration — For ribbon wire, it is best to use a tool with a "notch" between the tool foot and feed/guide slot. The "notch" gives some clearance so that when the tool rises to the loop, the ribbon bending at the heel is not overworked, causing heel cracks.
Conclusion
For high-frequency applications, ribbon wire bonding is the preferred method, and even with all the advantages over round wire, some challenges remain. The package access will determine if a clamp or table tear process is used. The ribbon wedge tool, ribbon wire and machine parameter selection may need a design of experiments to select the best tool dimensions, ribbon wire specifications and bonding machine parameters.
References
For a complete set of references, please contact the author.
KEVIN SEUFERT is a senior service engineer at ESEC (USA) Inc., and may be contacted at 1407 West Drivers Way, Tempe, AZ 85284; (480) 893-6990; Fax: (480) 893-6793; E-mail: [email protected].