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Wire Bonding

More than 90% of the 15 trillion interconnects for integrated circuits, hybrids, Multi Chip Module and RF and Microwave modules are manufactured by wire bonding. There are two basic types of wire bonding processes.

Ball Bonding

The first type is ball bonding or sometimes called "ball and wedge bonding"(See Figure 1). The first bond is a gold ball, which is welded to the semiconductor die metallization and the second bond called a stitch or crescent is bonded to the substrate thick or thin film circuit metallization. Typically the gold wire is 25 µ in diameter but due to the high price of gold, copper wire is capturing a greater percentage of the total interconnects increasing to about 50% this year and will likely continue to about 80% in 2017. For Aerospace, military and Hi-reliability applications gold and aluminum wire still predominant.

 

Figure 1: Wire Bond Ball Bonding (photo courtesy of TJ Green Associates LLC)

In ball bonding, a "free air ball" is formed at the beginning of each bond sequence. (See Figure 2 step1). In the first versions of wire bond equipment, a hydrogen torch (See Figure 2 step7) was used to burn the end of the wire, which melted into a perfect ball shape at the end of the



Figure 2 Simplified Ball bonding Sequence

wire. Modern wire bonders use an electrode, which passes close to the wire with a high voltage arc to form the "air ball". For copper wire applications, the wire bonders must provide an inert atmospheric envelope by means of a forming gas (95% Nitrogen/5% Hydrogen) that surrounds the wire tip, thus preventing oxidation during bonding and enabling stronger bonds. The ball at the end of the wire prevents the wire from moving up the capillary with the clamps open.

The first bond (See Figure 2 step2) is achieved by bonding the ball to the pad. Typically today the bonding technique used is called thermosonic, which uses heat, force and ultrasonic energy. Ultrasonic vibration of the capillary welds the wire to the semiconductor-bonding pad (aluminum for silicon and gold for GaAs). The substrate temperature is held to ~1500C as compared to thermocompression bonding (heat, force and time) substrate temperature of 3000C and much higher forces. In Figure 2 step 3 the capillary raises to the loop height position. The clamps are open and wire is free to feed out the end of the capillary. The ball bonding technique allows the wire direction to move in any direction as compared to wedge bonding where the bonder can only move orthogonally. Figure 2 step 4-5 shows the second bond on the lead or substrate metallization of the device, which is positioned under the capillary and then lowered to the 2nd bond site. Wire is fed out of the end of the capillary, forming a loop. The capillary deforms the wire against the lead, producing a wedge-shaped bond, which has a gradual transition into the wire. In a thermosonic wire bond machine, ultrasonic vibration is then applied. In Figure 2 step 6 the capillary raises off the lead or substrate, leaving the stitch bond. At a pre-set height, the clamps are closed while the capillary is still rising with the bonding head. This prevents the wire from feeding out the capillary and pulls at the bond. The wire will break at the thinnest cross-section of the bond. The last step in the sequence is shown in Figure 2 step 7 a new ball is formed on the tail of wire, which extends from the end of the capillary. Typically an electronic spark is used to form the ball. The cycle is completed and ready for the next ball bond in Figure 2 step 1.

A fully automated computer controlled thermosonic high-speed, ball-and-stitch wire bonder capable of ball bumping, stud bumping, wafer bumping, chip bumping, security bonds (ball bump on substrate stitch) and customized looping profiles is shown in Figure 3.


Figure 3: Automatic Wire bonder

For High Frequency RF and Microwave circuits a standoff stitch (SOS) is used to reduce the wire length and loop height and associated inductance. A ball bump is placed on the die and the ball bonding process starts by placing a ball on the substrate and a wedge on the "ball bump" on the integrated circuit pad as shown in Figure 4a. The SOS can also be used to connect "die to die" pads for high frequency circuits as shown in Figure 4b.


Figure 4a: Standoff stitch bond


Figure 4b: Die to Die interconnect using SOS

Wedge Bonding

The second process for wire bonding is wedge bonding. The basic steps are the same for both which include forming the first bond, forming the wire loop and forming the second bond.

This is illustrated in Figure 5 step1 through step 7. The bonding wire typical used is 25Ãâ€Å¡Ãƒâ€šÃ‚µ diameter aluminum or gold wire. Aluminum can be wire bonded with ultrasonic energy ("Cold Bonding") only while gold requires both ultrasonic energy and a heated substrate temperature of ~1500C.


Figure 5 Simplified Wedge bonding Sequence

For High Frequency and Microwave circuits where the bonding pads (50Ãâ€Å¡Ãƒâ€šÃ‚µsquare) are small, 20Ãâ€Å¡Ãƒâ€šÃ‚µ diameter gold wire is used to bond MMIC die as shown in Figure 6. Ribbon wire sizes from 0.5 x 1.5mil to 1.0 x 2.0 mil are also used to decrease the inductance.


Figure 6:20 Ãâ€Å¡Ãƒâ€šÃ‚µ gold wedge bonding to MMIC

Aluminum wedge bonding is used for high current circuits with wire sizes from .002" to .025" diameter as shown in Figure 7 for Power hybrids manufactured by GED.



Figure 7: Large Diameter Aluminum Wedge wire bonder

COPPER BALL BONDING   

Copper is electrically more conductive (~26%) than gold wire and considerably less expensive.  It uses the same ball bonding process with the addition of a forming gas to provide an inert environment during free air ball (FAB) formation . Copper/aluminum (Cu/Al) intermetallics (IMC) have considerably slower inter-diffusion than gold/aluminum (Cu/Al) IMC, which prevents Kirkendall voiding, thereby ensuring better performance during high-temperature storage tests.  Heavy copper wire, greater than 50 microns in diameter, is already widely used in the industry today for power applications. 

As the copper wire is harder, the copper FAB is ~30 percent harder than a gold FAB. This difference in hardness, of copper is high enough to induce bond pad metal lift or worse silicon damage.  Recent developments in copper bonding, like the availability of soft Cu wires and the understanding of the effects of FAB parameters on ball hardness, have made it possible to overcome this challenge.

Copper Wirebonding References

 [1] M. Deley and L. Levine, "The Emergence of High Volume Copper Ball Bonding," KnS Technical Library, 2005

[2] M. Hundt, "Current Trends in Semiconductor Packaging," June 2003 

[3] KNS, "Copper and Gold Wire Technical Data Sheets," 2007

[4] F. Wulff, et al, "Further Characterization of Intermetallic Growth in Copper and Gold Ball bonds on Aluminum Metallization," Semicon Singapore 2005

[5] M.H. Hong, et al, "Investigation Factors Affecting Bonded Ball Hardness on Copper Bonding"

[ 6] Sheila Rima C et al, Advanced Interconnect Technologies, Singapore  "Copper as a viable solution for IC packaging" , circuitsassembly.com,  31 January 2008

[7] Tony Panczak, Amkor, "Copper Wirebonding OSAT Viewpoint", IMAPS May, 2011
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