Page 73 - Wire Bonding in Microelectronics

52 Cha pte r T h ree

concepts.∗ Such understanding will be helpful in other parts of this
book, as well.
Ribbon wire applications are discussed in Chap. 2 and will not
be separately discussed here, with the exception of some fatigue data
in Sec. 3.8. It is assumed to be similar in metallurgical properties to
round wire of comparable size (99.99% Au—without dopants).

3.2 Stress-Strain Characteristics of Bonding Wires
Bonding wire is generally specified by its elongation and break-
ing load (sometimes incorrectly referred to as its tensile strength).
Presumably there are other properties that influence the bonding
process, but those are not understood and cannot be called out or
practically measured. They may be related to uniformity, surface
finish and hardness, crystal structure, heat treatment, etc. (e.g., a
similar specified Al or Au wire from a different manufacturer
often requires different bonding parameters to achieve equiva-
lent bonding).
The basic metallurgical stress-strain properties of typical bonding
wires are shown in the curves of Fig. 3-1. Such data are obtained by
following ASTM F 219 specifications (see App. 3A), and pulling
(applying force to) a 25.4 cm (10 in) length of wire while continuously
recording the elongation (stretching) and its actual breaking-load
(force). The stress axis represents the applied force (usually in gms or
mN), while the horizontal axis, the strain, represents the wire’s
response to the pulling stress/force and appears as the wire elongation
(stretching). Curves represent data from the same (large-diameter)
Al bonding wire in two different states of anneal. However, the curves
are generic for both large- and small-diameter wires made of either
Al or Au. Specific wires may yield slightly different shapes (e.g., be
flatter in region 3, etc.). Data for the stress ordinate are normalized
since the stress-relieved curve could be two or three times higher
(stronger) than the annealed one. Curve A is fully annealed and B is
stress-relieved (slightly annealed). Region 3 denotes the plastic defor-
mation, where the wire permanently stretches. The breaking load of
each wire is shown at point 4 and would be read from the vertical
stress (applied force) axis. Note that gold wire for use in thermosonic
and thermocompression bonding is annealed and would generally
have stress-strain characteristics nearer to those of curve A in Fig. 3-1.

∗See for instance, Understanding Materials Science: History, Properties, Applications
(Hardcover), by Rolf E. Hummel Springer-Verlag; 2d ed. (May 1998). Also see,
Fundamentals of Materials Science and Engineering: An Integrated Approach, 3d ed.
William D. Callister, Jr. and David G. Rethwisch, December 2007. For data, formulas,
etc. see CRC Materials Science and Engineering Handbook, by J. F. Shackelford and
W. Alexander (eds), 3d ed., 2001.