Tuesday, June 19, 2018
Publication Date: 02/1/2012
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Why Wedge Bond?
Common bonding processes with a comparison of their deformation as a function of initial wire diameter for an optimized process.

Wire bonding is an ultrasonic welding process that is accomplished by deformation of the wire and the substrate, forming them into an alloy of the two constituents. Ultrasonic energy enhances the process by lowering the flow stress and allowing easy slip mechanisms for dislocation movement — deformation occurs by the movement of dislocations. During deformation the materials flow together forming intermetallic compounds that grow by diffusion. Wedge bonding, because it deforms the wire without first forming a ball, is capable of producing a full strength weld that is smaller than a ball bond.

New generations of wedge bonding machinery have many features that provide enhanced capabilities. They are much faster than previously, now exceeding 6 wires/second. They have better looping capability and controls for more difficult applications and they have very large 12 x 16-in. (304 x 406mm) table travel for bonding large substrates.

Fine-Pitch Bonding
As device complexity (I/O) increases and die size shrinks, devices require more wires with tighter spacing, or pitch. To achieve finer pitch all bonders including ball and wedge, must use smaller diameter wire. However, as wire diameter is decreased, physical properties such as strength, stiffness, electrical and thermal conductivity are also decreased. The lower strength and stiffness of smaller diameter wire has a negative impact on performance and reliability. This is where wedge bonding has an advantage. Because it deforms the wire without first forming a ball it can achieve a full strength weld with less deformation at a smaller size. High quality welds can be produced with bond width 20-25 percent larger than the wire diameter. This represents a 33 percent reduction in bond size, so significantly smaller than the minimum size that a ball bonder can produce for the same wire diameter. Lower deformation benefits ultra-fine pitch bonding because for any pad width a wedge bonder can bond with larger wire diameter, thus providing greater strength and reliability.
Example of stacked die image where wedge bonding achieves a 20 percent reduction in height and a 32 percent reduction in cross sectional area compared to the best ball bonding capability.

As diameter is decreased, electrical and thermal conductivity also decrease, limiting current-carrying capacity. Many new devices require more current carrying capability. Safe current carrying capacity — defined as <83°C temperature rise — is a function of wire diameter and length. For wires longer than 2-3mm, which is common in today's packaging, safe current capacity is in the range of 0.25A for common 15-20µm diameter fine pitch bonding wires. By enabling the use of larger diameter wire, wedge bonding improves packaging performance.

Ribbon Bonding
Ribbon bonding uses a flattened, rectangular shaped wire, and is required for high-frequency devices. At high-frequencies a "skin effect" dominates conduction, where all conduction is through the wire "skin" or outer 0.5 µm of the surface. A 0.5 x 10 mil ribbon carries the same high frequency current as a 6 mil round wire. However, the ribbon is much easier to bond and has less than 33 percent the volume of the equivalent round wire resulting in a significant gold cost reduction.

Ribbon, because of its preferred rectangular cross section with a large width/thickness ratio, significantly lowers high-frequency impedance compared to round wire. This provides additional packaging performance in terms of speed.

Today's wedge bonders are capable of bonding both round and ribbon wire on the same platform and require only minor changeover downtime.

Better Looping
New wedge bonders, with enhanced computer control systems, have much better looping capability than in the past. New looping algorithms offer a menu of loop shapes previously unavailable. Better control algorithms have improved loop shape repeatability to achieve "best in industry" class performance.

Wedge bonders inherently achieve the lowest possible loop heights. Wedge bonds have a natural advantage for low loop applications, because the wire is bonded directly under the wedge foot — almost horizontally, depending on the wedge feed hole angle — and does not exit the top of the ball vertically as in a ball bond. In a recent wedge bonding production example, an 8-die stacked application using wedge bonding achieved a 20 percent reduction in height and a 32 percent reduction in cross sectional area compared to the best ball bonding capability. Wedge bonds can be bonded very close to the package, reducing the fan-out required by ball bonders. Wedge bond wires can have an almost vertical descent to second bond providing very close proximity to second bond for stacked die and Chip Scale Package (CSP) devices. The low height and close proximity reduce package dimensions, offering a low-cost alternative to Through Silicon Via (TSV) device designs. G. Dan Hutchenson recently stated that TSV interconnects currently are at least 20 times more costly than wire bonded devices.

Another wedge bond looping advantage is the ability to create the shortest flat loops between dies for low impedance requirements. Wedge bonders are also able to stitch multiple wires together in a continuous connector. When ball bonders are required to chain connections together they must form stand-off-stitch or multi-stitch bond. Wedge bonding is faster than either of these processes and produces lower loop heights with more reliability.
Ribbon is easier to bond and has less than 33 percent the volume of the equivalent round wire for a significant gold cost reduction.

Wedge bonders can create both constant loop height and constant loop length bonds. For constant loop height wires, the height is constant even though the distance between first and second bond varies. For constant wire length bonds, the amount of wire is fixed for constant impedance, however as the distance between first and second bond varies, the height varies so that a constant length of wire is always used.

Quality Control
High cost, high reliability devices used for military, medical and optoelectronic packages require a much higher level of quality assurance than lower cost commodity ICs. Some newer wedge bonders have real-time quality monitoring features that are not available on ball bonders. PiQC incorporates a patented sensor mounted directly on the ultrasonic transducer. During the bond cycle, the system monitors and captures both the normal ultrasonic control signals and unique signals reflected from the bond interface to the PiQC sensor. More than 5 individual signals are monitored and analyzed, including ultrasonic current, transducer impedance, ultrasonic frequency, tool displacement and bond interface friction. Each signal is compared to a standard and graded.
Example of a PiQC radar chart which provides a visual representation of bond quality based on 5 bond quality signals for every bond.

It has been found that different types of defects such as scratches, contamination and bond-off-pad trigger different responses from these indicators. A quality decision algorithm that combines all five response indicators determines the overall quality grade and maximizes quality discrimination. Acceptance limits can be specified that will either flag a poorly bonded device or stop the process for operator assistance. The PiQC system displays a graphic

representation of the quality tools for bond uniformity and a radar chart of the five signals simultaneously. A perfect pentagon indicates the best possible bond quality based on all signals. The process engineer can monitor production operations from his/her desk and be assured that the process is meeting requirements. Traceability for each device bond record assures that quality is maintained throughout the system and that it is well documented.

Copper for Gold
Substituting copper wire for gold has the most significant cost reduction potential available to our industry. Copper wedge bonding development efforts have begun. As copper bonding development proceeds it will provide significant benefits. In addition to lower costs copper has 25 percent better conductivity than gold and is twice as strong as gold.

Wedge bonding represents an excellent alternative wire bonding technology providing high reliability wire bonds with the most flexibility of any wire bonding process. New capabilities in fine pitch bonding, loop shapes, wire materials, and quality assurance tools provide valuable capabilities for semiconductor packaging.

Contact: Hesse & Knipps, Inc., 225 Hammond Avenue, Fremont, CA 94539 484-665-0219 fax: 484-231-3232 E-mail: bubel@hesse-knipps.us Web:

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