Page 66 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 45

TAB offers the advantage of testability and burn-in of chips before
committing them to expensive SIP or other multichip packages.
This advantage has diminished as known-good-die or tested chip-
sized-packages became more available. In general, TAB tape is expen-
sive and inflexible, requiring new tape (and masks) for every small
change in chip or package pad location. As with flip chips, to be eco-
nomical, TAB requires either very high-volume or very high-cost
chips, where packaging costs are insignificant. An example of the latter
could be in flat-panel displays. TAB is a niche technology and will con-
tinue to find such uses, but not in mainstream chip packaging.

2.9 Wire-Bonding Technology: A Comparison and
Future Directions
Wire bonding is the most flexible of all IC interconnection technolo-
gies. If the die undergoes a downsizing or has other pad-dimensional
changes, the autobonder can be reprogrammed (taught) in a short
time. Once taught and recorded, the same die can be wire-bonded by
the same machine simply by inserting the appropriate floppy disk.
Short wire bonds have acceptable low inductance (~1 nH/mm), and,
by surrounding signal leads with ground-power leads, they can
approximate a transmission line in some cases. Nevertheless, induc-
tance and crosstalk at high frequencies are the main drawbacks for
using wire bonds on high-performance chips. Wire bonds are cost-
effective for both low- and high-device volumes. Chips are bonded
face up, so heat can be conducted out from the back through the die-
attach material into the substrate. There are many combinations of
basic wire-bond technology with other interconnection methods as
described above; for example, ball-bumped TAB and a variation used
for applying Au or Cu FC bumps (generically called stud bumps),
TAB leads bonded to chips with TS wire bonders, and wire bonds on
top of cut-off TAB leads for package interconnections after TAB was
used on the die for KGD testing. Thus, even if other interconnection
technologies are used, the knowledge and equipment of wire bond-
ing are often required to make the interconnections. A comparison of
the three technologies is given in Table 2-2.
The ITRS packaging roadmap (2007) predicts that ball bonds will
have bond pad pitches ~20 µm by 2013 [2-76]. For wedge bonds, the
pitch is expected to decrease to 20 µm by 2009. Over 10 wire bonds are
13
expected to be made in 2008. This number has increased by 12 to 14%
per year in recent years. A huge materials and equipment infrastructure
for wire bonds is in place worldwide, with many thousands of workers
already trained. As a result, one should expect momentum to keep wire
bonds the dominant interconnection method for many years.
Considering the above interconnection technologies (and the
many possible variations of each), it is likely that both wire and flip-
chip technologies will be used, often side-by-side in the same facility