Page 59 - Wire Bonding in Microelectronics

38 Cha pte r T w o

be justified. However, remember that any technology change (even
within the three wire-bonding technologies) requires new equipment,
operator training, long learning curve, and extensive (expensive)
requalification of the existing product. Some non-wire-bond tech-
nology options are briefly described in Table 2-2.

2.7 Variations of Fine-Wire Bonding Technology
Although the vast majority of all wire bonding is done by TS, or cold
US methods using round wire, there is a smaller amount of specialized
bonding using US Al and thermosonically bonded Au ribbon wire.
There is also some electrical-discharge parallel-gap (split-electrode)
welding of wires of intermediate to large diameter, especially for Pt and
other wires that do not bond readily by conventional ultrasonic means.
Such wires have been bonded with focused lasers, but this usually
involves melting and is not a part of solid-phase and US welding, as
discussed in this book.

2.7.1 Ribbon Wire Bonding
Ribbon wire has been used in hybrid microcircuits, mostly as cross-
overs, for decades. It is also used in microwave circuits, since its
larger, rectangular perimeter results in lower losses. Initially, all rib-
bon wire was Au and was bonded by thermocompression or parallel
gap welding (see below). However, in 1969, Kessler [2-11, 2-61] and
later others [2-62, 2-63] investigated both Al and Au US ribbon wedge
bonding. Figure 2-17 is an SEM photograph of US wedge-bonded Al

FIGURE 2-17 An SEM photograph of three US wedge-bonded Al, 1% Si ribbon
wires, 12.5 × 38 µm (0.5 × 1.5 mil). Note the low bond deformation.