Page 27 - Wire Bonding in Microelectronics

6 Cha pte r O n e

This heat usually comes from the machine workholder (the base to
which the package is clamped), but in some cases, it can be a heated-
bonding tool or capillary. TS bonding usually takes place at greater
than or equal to 150°C, but there are many cases where lower or
higher temperatures are used. When the temperature approaches
300°C, a gold wire bond can often be made without any ultrasonic
energy. This is called thermocompression (TC) bonding. The bond-
ing sequence is the same as for TS bonding, Fig. 1-3, except that no
US energy is applied, a longer bond time is required, and the stage
temperature is higher (≥300°C). Thermocompression bonding is
rarely used today because the high temperatures may harm plastic
packaging materials, and also, the bonding process is more affected
by contamination than is TS bonding (see Chap. 7), and the TS bond
time (10–15 ms) is much shorter, a requirement of modern high-
speed autobonders. Animations of the tool motions are available
from the Web sites of several autobonder companies and impart
considerable understanding that still photographs cannot. See, Further
readings.

1.2 How to Approach Bonding Problems?

1.2.1 Which Metals Can Be Ultrasonically Bonded?
There may be cases when an unusual metal pad must be US bonded,
and it is not obvious whether this is possible. The Welding Handbook
[1-3] gives a figure, reproduced as Fig. 1-6, that shows which metal-
lurgical combinations have been ultrasonically welded (bonded) with
fine wire. The caveat is that there was no information on the reliability
or potential bond yield of such welded couples. Gold wire can be TS
bonded to Pd films (a recent high-volume process discussed in Chap. 5),
and Cu is also TS (ball) bonded to Al in volume IC production. Large-
diameter Al wire is readily bonded to stainless steel and Ni, as well
as directly to Si. Annealed Nb wire has also been bonded to Nb film
[1-4] for use in superconducting devices. Creativity abounds in the
area of US bonding with new materials.
Data in Table 1-1 is useful when approaching a new bonding situ-
ation such as often occurs in sensors, MEMS, high-temperature
devices, superconductors, and the like. This figure, developed by the
ASW, was not designed for microelectronics bonding, but rather US
welding in general. Many of these metals require special preparation,

such as in Ref. [1-4]. Some require temperatures above those accept-
able for microelectronics. In general, hard refractory metals cannot be
used as wire in microelectronics. However, the table shows the wide
range of materials that it is possible to bond.