Page 139 - Wire Bonding in Microelectronics

W ir e Bond Testing 117

Time (h) T °C Pull Test Failure Criteria Reference
1 390° Weak or lifts@ [4-49]
1 350 Less than one [4-50]
4 300 half the minimum
24 250 acceptable post
200 200 seal pull force of
3000 150 (MIL-STD 883, Method 2011)
1 300°C <1.5 gm for 1 mil 25 µm wire MIL-PRF-38534F
<1 gm for smaller wire App. C [4-7]

TABLE 4-2 Various Thermal Stress Tests for Au-Al Bond Reliability Assessment

4.6 Future Issues in Wire Bond Testing
Bond testing in the future will be influenced by the increasingly finer
pitch of IC bond pads and the corresponding decreasing size of the
bonds. For the pull test, as package sizes and loop heights decrease, it
has become increasingly difficult to get the hook under the wire. The
problems described above of pulling wires in multiple-tier packages
applies to decreasing pitch and loop height as well. This further compli-
cates application of the destructive pull test. The normal procedure for
multiple-tiered packages is to pull test each tier, from the top down, but
some loops may be too low to insert a hook un derneath them. The NDPT
becomes impossible to apply.
The ball-shear test will be applied to smaller diameter balls, as
well as ones that are not as high. This requires flatter substrates, more
precise shear-tool height adjustment, and narrower shear blades
(which will wear out more rapidly). Modern shear testers have the
capability of shearing with the required precision. However, in many
cases similar test methods can be used, but with more care. One is
substituting SPC as an alternative to extensive destructive pull test-
ing. SPC must be applied as the pitch decreases. High volume auto-
bonders make bonds very uniformly and in general require less test-
ing, as long as DOE (Chap. 8, App. 8B, by Lee Levine) is used for
setting them up.
An important testing change has occurred as ball-bond pitch has
decreased below 50 µm and the shear test becomes impractical. ∗ The

∗ ITRS predictions are that both ball bonds and wedge bonds will decrease in size
to 20 µm pitch.