Page 129 - Wire Bonding in Microelectronics

W ir e Bond Testing 107

4.3.6 Effect of Gold-Aluminum Intermetallics
on the Shear Force
When Au is thermosonically bonded to Al, intermetallic compounds
form in the interface. In fact, it has become a normal procedure to
evaluate the amount of weld formation by observing the amount
and distribution of those compounds as described above. Consider-
ing this, one may wonder what effect intermetallics have on the
shear test. In the as-made condition, these are thin and have no
effect on the shear force [4-26, 4-27] except that they are essential for
strong Au-Al welding. However, after thermal exposure (thermal
stress test, high-temperature environment life, etc.) the compounds
will grow. They may appear as spikes into the ball bond in poorly
welded interfaces (see Sec. 4.3.7) and have some effect on the shear
test.
There appears to be minimal data on either the tensile or the shear
strength of intermetallic compounds. However, Philofsky [4-29] made
estimates of the tensile strength as a result of tensile testing Kirkendall-
void-free Au-Al couples and concluded that all of the intermetallic com-
pounds are at least three times as strong as annealed Au or Al. (Also see
Table 5-1, Chap. 5, for many properties of these intermetallics.)
Considerations of the binding energy of the compounds would
suggest that these compounds could be 10 times as strong as either
Au or Al, and this is generally verified by hardness measurements
[4-30]. Even though such compounds are brittle, we conclude that
they should not result in lowering the ball-bond shear force as long as
the interface is void-free. As they spread laterally in the interface, they
can actually increase the shear force (equivalent to more complete
welding). Such an increase in strength (10% or more) during the early
part of high-temperature tests has often been observed [4-17], and
this explains why bonds made at relatively high temperature are
reported to be initially the strongest.
The formation of intermetallic compounds under a ball bond can
produce considerable stress on the silicon [4-31]. The added stress of
a ball-shear test can then result in silicon damage (cratering). See Sec. 8.1
for a discussion of this problem and App. 4A, Failure Mode 3, for an
example.

4.3.7 Pluck Test, Pry Test, Flip Test, etc.
(Failure Analysis Technique)
If an Au ball bond to Al is poorly welded and subsequently under-
goes thermal stress, then intermetallic spikes may form that will
extend into the Au and Al [4-32]. Figure 4-19 is a drawing of such a
bond. These spikes can add lateral strength to the bond when it under-
goes shear testing and yield a deceptively high shear force. A failure
analysis procedure to examine such bonds can be used to reveal this.