Page 31 - Wire Bonding in Microelectronics

10 Cha pte r O n e

Kirkendall-type voids. As an example, Au bonds to Ag
(as discussed in Sec. 2.3.5) and may or may not be a problem,
depending upon the thermal environment of the device and
the type of metallurgical defects present (thick-film silver is
less reliable than electroplated silver for Au bonds). As another
example, one might consider Au-ball bonds to platinum. This
is a completely miscible metallurgical system, but because the
activation energy for interdiffusion is high, platinum is not
likely to diffuse into the Au. Data given by Hall (Chap. 6,
Fig. 6-9) indicates that temperatures in the range of 400°C
would be required for significant interdiffusion, whereas Ni
can diffuse into Au by grain boundary diffusion in the 100 to
200°C range (as discussed in Sec. 3.4.1).
5. Are the individual materials easily corroded? Does the bond
couple have a high probability of making a corrosion couple
with Cl? Halogens and sulfur compounds are omnipresent,
so look up their effect on the bare metals. Look up the elec-
trochemical series potentials as in Fig. 1-6 (which is a simpli-
fied version) [1-5]. If the most common reduction reactions
of the metals are widely separated, then corrosion is proba-
ble in the presence of halogens and moisture. As an example,
Al is strongly negative, −1.66, and Au positive, +1.69, making
a good battery that corrodes easily consuming the Al. Gold
and Ag, however, are both strongly positive; thus, one would
not expect corrosion on these bonds. Such corrosion has been
discussed in several chapters of the book. On the other hand,
most Ni reactions are negative except one, whose occurrence
is less probable. Thus, the Ni-Al couple has not been observed
to corrode. The electrochemical series must be used with
caution since the measurements are made under very spe-
cific conditions. However, such information can be useful in
prediction of possible problems in some new metallurgical
combination.

1.2.3 Some Unusual Uses of Wire Bonds
Bonds can be used for many different purposes other than simply
making electrical connection through wires. Figure 1-7 gives a cre-
ative application of ball bonding that forms an electrical connection
between conductors in two different planes. Soldering was not prac-
tical in this case [1-6]. Ball bonds are also used as a fab-less way of
bumping chips (frequently called stud bumps—see Chap. 9) for
flip-chip bonding, and every manufacturer of autobonders makes
specialized machines for that purpose, and even for whole wafer
bumping (see Chap. 9).