Page 74 - Wire Bonding in Microelectronics

Bonding W ir e Metallur gy and Characteristics 53

4
3
4
2
Stress (arbitrary units) 2 A 1 B
3

1

0 5 10 15 0 2
Strain (% elongation)

FIGURE 3-1 Typical stress (pulling force) versus strain (elongation) curves for
aluminum bonding wire in two states of hardness: wire (A) is annealed and is
typical of large-diameter aluminum bonding wire, but also similar to those of Au
used in TS bonding; (B) is stress-relieved (partially annealed). Its characteristics
are similar to those used for small-diameter ultrasonic wedge bonding (either Al
or Au wire). In order to display both curves on the same chart, the stress axis
was made arbitrary [the breaking load of (A) was approximately one-half that
of (B)]. On both curves (1) is the elastic region where the stress is proportional
to the strain, (2) is the proportional or elastic limit, (3) is the region of inelastic
or plastic deformation, and (4) is the breaking load of the wire. The elongation
at the breaking point is 15% for (A) and 1.5% for (B).

If the wire is nondestructively pulled (NDPT), the force must not
exceed the elastic limit (point 2 in both curves). This produces irre-
versible metallurgical changes in the wire, which by definition can-
not be nondestructive (see Sec. 4.3).

3.3 The Shelf-Life Aging of Bonding Wires
High-volume manufacturers receive their wire on “just-in-time”
delivery, and use wire within a week or month of receiving shipment,
and they are not concerned about aging properties. However, small
organizations and those that only occasionally use a particular type/
size of wire need to be concerned about the long-term storage (aging)
properties of many types of bonding wire. Some discard wire after an
arbitrary period, such as 3 or 6 months. They are not willing to risk a
change in its metallurgical properties which could affect yield from
an existing bonding machine setup. In the mid-1980s, ASTM (Com-
mittee F 1.07) had several wire manufacturers systematically study
the actual aging properties of both Au and Al, 25 µm (1 mil) diameter