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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 18 NO. 1

important packaging requirements. The demand for thin

and stacked-dice packages that are as thin as possible led

to the development of improved bond head control algo-

rithms andmanynewloopshapeoptions.Memorydevices

often have their bond pads located down the center of

the die surface rather than around their periphery. This

allows better signal and voltage distribution and results

in faster devices that command premium values. Figure

2(a) is a photo of low loop wires for memory. These loops

rise to a low height and then travel parallel to the edge of

the die, where they descend to the second bond. Stacked-

dice packages (Fig. 2b) often employ a hybrid bond called

a stand-off stitch (SOS). In an SOS bond, a ball is formed

and bondedwith thewire intentionally broken in theHAZ.

Another ball is formed and bonded to the substrate side of

the package. The stitch (second bond) side of the wire is

thenbondedon topof the original ball. Because it requires

the formation of three bonds rather than two, the SOS

bond is approximately 40%slower than a standard bond,

but it provides the lowest loop height available. Every

smart phone (more than1billionannually) has at least one

stacked-dice package. Stacked dice, because each die can

contain a separate technology (analog, digital, memory,

radio frequency), enable integration of the entire system

within the package. Earlier attempts to integrate all of

these technologies on the same chip proved costly and

decreased reliability. Joining the technologies by stacking

themwithin the package became the dominant method.

The secondbond is formedby adifferent portionof the

capillary tip than the ball bond. Figure 3 is an illustration

of a capillary tip and the portions of the tip that produce

the ball bond, the loop, and the second bond. In forming

the second bond, the capillary face and outer radius are

pressed on the top of a round wire. The combination of

ultrasonic energy, bond force, heat, and time deform the

round wire into the fishtail shape and form the initial

intermetallic bond.

The mechanical and other materials properties of the

ball and the wire are significantly different. The second

bond is more diffusion-controlled than the ball bond.

WIRE BOND FAILURE MECHANISMS

Semiconductor packagesmust normallypass abattery

Fig. 1

Wire bonding bond head for copper wire. Courtesy of Kulicke & Soffa Industries Inc.

Fig. 2

(a) Worked loop. (b) Stand-off stitch loop. Courtesy of Kulicke & Soffa Industries Inc.

(a)

(b)