Page 56 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 35

FIGURE 2-16 Modern large-wire aluminum bonds left is 250 µm (~10 mil);
right ones are 150 µm (~6 mil), bonding frequency was 80 kHz. (Courtesy of
Orthodyne Electronics.)

considered to range from about 75 or 100 µm to over 0.5 mm in diam-
eter (3–30 mil). Figure 2-16 shows large-wire Al bonds. They typically
use either 99.99% Al or Al, 1% Si wire. A discussion of large-wire metal-
lurgy and burnout characteristics is included in Chap. 3. The current
large-wire technology includes both manual and autobonders. Earlier,
such bonders used manual or electrically activated scissors or a blade
to cut off the wire after the second bond. In some cases, that bond was
made by tweezer welding (see Sec. 2.7.2). However, the development
of a special bonding tool that both bonded and cut off the wire advanced
the field significantly so that both bonds and the cut-off could be made
ultrasonically. This increased the speed of the process and made auto-
bonders possible. Present-day bonding tools developed by Orthodyne
confine the wire in a parallel inverted V-groove during bonding, which
leaves the bond neck strong [2-59], see Fig. 2.9. This is quite different
from the relatively flat bonding tools used for fine-wire wedge bonding.
Even though large-diameter wires are usually fully annealed, they are
very stiff and the bonding forces are far greater than those required for
small-wire wedge bonding (up to ~1 kgf vs. 25–35 gf). The ultrasonic
energy required is equivalently higher also, up to ~25 W versus <1 W
for fine wire. Most power device packages are usually plated with Ni,
and large-diameter Al wire bonds very well to that metallization, if it is
free of oxides. Large-wire bonds can have the same reliability prob-
lems as small-diameter Al bonds; however, a well set-up process