Page 54 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 33

deformation. In each case, the bond-monitoring systems can con-
tribute to improved quality control. However, a complete under-
standing of the US welding process is necessary before a true in-process
bond monitor can be made, although the empirical ones do con-
tribute to improved quality control. There have been a number of
ultrasonic bonding mechanism studies as discussed above, but there
is no accepted and validated mathematical model to guide a designer
of bond-monitoring equipment.
Currently, wire bonds can be produced at very high yields using
the means described in Chaps. 4, 7, and 9, such as molecular cleaning
and carefully optimizing bonding parameters with DOE. By follow-
ing these procedures, it is currently possible to bond with ≤25 ppm
defectives [2-52] in medium-to-high-volume production. Under such
conditions, it could be hard to justify an expensive system added onto
each autobonder if it slowed the bonding process, or only reduced
those defect numbers by a few parts per million. However, low-
volume hybrids, SIPSs, and other technologies that use multiple chips
from different sources seldom achieve such high yields. For those
and other cases in which the defects can range from several hundred
to several thousand ppm, a currently available bond monitor could
pay for itself. Low-production volumes make it difficult to establish
that there is a real decrease in defectives into the <50 or 100 parts
per million range. To prove such requires a better understanding of
small-sample statistics, or by making 100,000 setup and test bonds
(see Chap. 9). Thus, incorporating an expensive bond monitor in
each autobonder might not be practical for all uses, but certainly can
enhance many.

2.6 Wire-Bonding Technologies

2.6.1 Thermocompression Bonding
Thermocompression (TC) wedge and ball bonding for microelectron-
ics was developed by Bell Laboratories in 1957 [2-53]. This method
was used until US wedge bonding largely replaced it in the mid-1960s.
Although Au-Au TC welds can be made in high vacuum at room
temperature, such welds require a high-force and a high-interface
temperature to take place in a normal manufacturing environment.
TC bonding is a type of solid-phase welding that combines heat and
force to plastically deform the weldments, sweeping aside surface
contaminants (air, carbonaceous impurities, oxides, etc.), resulting in
intimate contact between cleaned surfaces. At this point, short-range
interatomic forces with heat supplying the metal-metal activation
energy result in a metallic welded bond. The primary process vari-
ables (time, temperature, and deformation) follow an Arrhenius
relationship, and their activation energies have been studied [2-32].