Page 57 - Wire Bonding in Microelectronics

36 Cha pte r T w o

experiences few such failures. Since such bonds have a high profile,
they can be tested with both the pull test and the shear test (see Chap. 4).

2.6.3 Thermosonic Ball and Wedge Bonding
Coucoulas was the first to combine ultrasonic energy with heat to
produce thermosonic bonding in 1970. As such, he is the father of
thermosonic bonding. He called it “Hot Work Ultrasonic Bonding”
[2-60]. Today, the vast majority of interconnections to integrated cir-
cuits are made with Au thermosonic (TS) ball bonding. It is also occa-
sionally used for Au wedge-wedge bonding. This bonding method
is a combination of ultrasonic and thermocompression welding that
optimizes the best qualities of each for microelectronic usage. TC weld-
ing usually requires interfacial temperatures in the order of 300°C. This
temperature can damage some modern die-attach plastics, packaging
materials, laminates, and some sensitive chips. However, in thermo-
sonic welding, the interface temperature can be much lower, typically
between 125 and 220°C (but may vary over a wide range), which
avoids such problems. Also the bonding time is much shorter than for
TS bonding, often <10 ms versus >100 ms. The ultrasonic energy helps
disperse contaminates during the early part of the bonding cycle and
helps mature the weld in combination with the thermal energy. This
combination also allows the US energy to be kept small enough to
minimize cratering damage to the semiconductor chip (see Chap. 8).
For ball bonding, the wire is threaded through a capillary-shaped tool,
and a spark melts the end of the wire forming a ball at the bottom of
the tool. The bond (weld) is formed when the tool under load presses
(deforms) the ball against the heated bonding pad (~150°C) and ultra-
sonic energy is applied completing the process as in Fig. 2-13.

2.6.4 Choosing a New/Different Wire-Bonding Technology
There may be times when one must choose a wire-bonding technology
for some new application or perhaps change an established technology
used on a current product. The three wire bonding methods—
thermocompression, ultrasonic, and thermosonic have many advan-
tages and disadvantages. These are compared in Table 2-1, which can
be used to help choose an appropriate bonding technology.
There are numerous options within the above bonding technolo-
gies. Some of these are to change the conventional bonding wire or
bond pad metallurgy, such as using Cu or Pd ball bonding, perhaps
Pd-plated pads, or possibly increasing the ultrasonic bonding fre-
quency (see above). Under some circumstances, round wire may be
replaced with ribbon (Sec. 2.7.1). In general, a currently used, well-
understood, high-yield process should not be changed unless there is
a compelling reason to do so. For example, if the device operates at
very high frequency (multi-GHz range) or very close chip placement
(tiling) is required, then a change from wire bonding to flip chip may