Page 138 - Wire Bonding in Microelectronics

116 Cha pte r F o u r

Thus, adequate standards are available to implement the ball-
bond shear test. The recommended values (below) do need to be
eventually modified in the future. The top curve in Fig. 4-23 [5.5 g/
2
mil (84 MPa)] should be chosen as the minimum individual shear
reading for Au ball bonds on Al pads, when long-term reliability
(1000 h at 175°C or equivalent) is required [4-48]. This is typically a
required test for some important industries.

4.4 Evaluating Both the Ball and the Wedge Bond
on a Single Wire
The wedge (or crescent) bond of a ball-wedge bonded wire, as with a
wedge-wedge bonded wire, is best evaluated with a pull test. One
study found that there was only negligible degradation of the ball-
shear force after bond pulling and concluded that the pull test can
therefore be done first [4-39]. Similar statements are made in ASTM F
1269-06. Thus, the ball can be sheared after the loop is pull tested,
allowing data to be obtained from both bonds of a single wire and
minimizing the required sample size.

4.5 Thermal Stress Test for Au-Al Wire Bond Reliability
Gold bonds on Al pads (or the reverse) have long been observed to
fail beyond some level of thermal stress (see Chap. 5, Intermetallics).
However, Horsting [4-49] (see Chap. 6 for Al bonds on Au) found
that, if the bond is well made and there are no impurities present in
the bond interface, the bond will remain strong even after long times
at high temperatures. If impurities are in the interface or the bond is
poorly welded, then the bond-strength may degrade rapidly during
such stress. To reveal potential problems in a new gold-plated pack-
age lot, Horsting applied a stress test that consisted of a 390°C bake
for 1 h, followed by a pull test. If the bonds lifted (interface separa-
tion) in a pull test, the entire package lot was rejected. Ebel [4-50]
introduced an entire bake schedule as a screening procedure to
reveal similar potential bond failures for hybrids. Later, MIL-STD-
883, Method 5008 for hybrids, specified a similar, though less severe,
test of 30°C for 1 h and a pull-test value specified as greater than or
equal to 1.5 gf (14.7 mN)—post seal. Currently (2008), this is in MIL-
PRF-38534. The time, temperature, and other conditions of these
various stress tests are given in Table 4-2. It should be emphasized
that this stress test is only useful for bimetallic bonds that diffuse
and react readily (e.g., Au-Au and noble metal bonds improve with
temperature, and Al-Al bonds stay about the same—see Chap. 5,
Sec. 5.3.7).