Page 80 - Wire Bonding in Microelectronics

Bonding W ir e Metallur gy and Characteristics 59

with an elongation of −4 to 7% (for 25 µm diameter). Such a wire is
usually stabilized with copper and silver dopants, at less than 100 ppm
by weight. High-speed autobonders require a much stronger wire
with the (annealed) breaking loads in the 8 to 12 g range and elonga-
tions of 3 to 6%. This extra strength is necessary to prevent breakage
or stretching when the wire is rapidly pulled through the bonding
capillary, but especially to supply increased strength in the heat-
affected neck region (just above the ball), giving better loop forma-
tion and thermal cycle performance. The high strength also gives
added wire-sweep resistance for plastic encapsulation (see Sec. 8.1.7).
Many different dopants can be used to stabilize these wires. Such
original dopant was beryllium, introduced first in the general 10 to
100 ppm range but more recently used in the 5 to 8 ppm range [3-3]).
Later calcium (5–7 ppm) and then other dopants, usually in propri-
etary amounts, were added to improve the wire and neck character-
istics. As above, the total dopant concentration does not exceed
100 ppm, and all wires are specified as 99.99% Au. Recently more
impurities have been added to further strengthen the wire or the
neck above the ball and also to reduce intermetallic compound fail-
ures in fine pitch applications. In some cases (in 2008), the purity has
been lowered to 99.9% Au often with Pd or proprietary dopants).
Discussions of the wire neck region (its length, hardness, grain struc-
ture changes, etc.) have been given by several investigators [3-4,
3-5, 3-6]. Although stronger wires were designed to meet the needs
of high-speed autobonders, they can also be used quite well with
manual bonders.
Gold wires for ball bonding are supplied in the annealed condi-
tion. If the wire were left hard (as drawn), the portion immediately
above the ball would become annealed during wire melting and ball
formation. This zone would thus be much weaker than the rest of the
wire, bending sharply (like a hinge) above the ball and preventing
smooth loop formation. (This phenomenon, which allows easy break-
off at the neck, is used for ball-bumped flip chip applications and also
for uses requiring very low loops such as TSOPs, smart cards, and
stacked-chips, see Chap. 9.) Even with special additives to improve
the strength and minimize the length (38 to 100 µm, 1.5 to 4 mils) of
the neck, this zone is still the weakest part of the wire bonding system
[3-4]. It is called the HAZ (heat affected zone) and has a hardness
about 20% lower than the rest of the wire. A sketch of the grain struc-
ture in this zone, after ball formation, is shown in Fig. 3-6a, and an
actual wire section is in Fig. 3-6b. It should be noted that the HAZ has
a larger grain structure than the rest of the wire, making it the weak-
est part of the bond system. It usually breaks there in a pull test. There
is continuing research by the wire manufacturers to strengthen this
region in order to improve looping, and increase its resistance to
fatigue and wire sweep.