Page 55 - Wire Bonding in Microelectronics

34 Cha pte r T w o

This activation energy is supplied by the high-interface temperature,
which for most TC bonding is around 300°C. The temperature is usu-
ally supplied by heating the entire device, that is, placing it in contact
with the heated work holder (WH). However, variations that heat the
bonding tool alone (≥400°C) or in combination with WH heating, have
also been used. Ball-bonding technologies normally feed the wire
through a capillary, and the tool can move in any direction after making
the ball bond. This results in current autoball-bonding machines
being several times faster than ones designed for wedge bonding.
A majority of the TC bonding-mechanism studies were done at
Bell Laboratories and Western Electric. [2-32, 2-54 to 2-56]. However,
important work was also done by others [2-57, 2-58]. Thermocom-
pression bonding is more sensitive to surface contaminants than any
other bonding method (see Sec. 7.2), the bonding time is much longer,
and the interface temperature is higher. Because of these, this process
is seldom used in microelec tronics today, having been replaced by
thermosonic bonding.

2.6.2 Ultrasonic Wedge Bonding (Small- and
Large-Diameter Wires)
Ultrasonic wedge bonding was introduced to the microelectronics
industry in about 1960 and became dominant in device production,
until gold-ball thermosonic autobonders took over. Ultrasonic wedge
bonding is normally done at room temperature (if heated, it is also
called thermosonic bonding). It is primarily used to bond Al wire to
either Au or Al bond pads, although it can bond Au wire with special
“gold bonding” tools (cross-grooved or roughened). Large-wire Al US
bonding is the dominant method of interconnecting power devices.
The US weld is formed by the application of ultrasonic energy through
a resonating transducer-tool combination while applying a clamping
force. An example of the transducer and bonding tool configuration
and vibration modes were given in Figs. 2-1 and 2-2 and the bonding
sequence in Chap. 1, Fig. 1-2. The wedge-bonding system (transducer-
tool combination) must be oriented in approximately a straight line
from the first bond to the second, before the first bond is made. This
is a disadvantage for autobonders, since it requires mechanically align-
ing the package or the transducer for each wire. This slows down the
wire bonding process by more than 50%, requiring several times more
autobonders (and their overhead costs) to bond the same number of
devices as thermosonic ball bonders.
Large-diameter Al wire bonding is used primarily in power devices
and hybrids that require more than several amperes per wire. Such
wire bonding is addressed in various parts of this book in appropriate
context, rather than in a separate section. Large-Al wires are bonded by
cold ultrasonic welding methods, using 60 or 80 kHz US power sup-
plies, although in earlier times some used 25 kHz. Large wire is usually