Page 132 - Wire Bonding in Microelectronics

110 Cha pte r F o u r

this technology. It was used for much early work that is still part of the
basis of our understanding of bonding and was essential in developing
the ball-shear test. Directly or indirectly, the majority of published work
on the ball-shear test has resulted in improving the setup parameters of
bonding machines. Generally, for TC bonding, the machine is set up for
a bond interface temperature of 300°C, a bonding time of 0.2 s, and a
bonding force of 100 to 125 gf to obtain strong thermocompression ball
bonds from 25 µm (1 mil) diameter gold-wire on either Al or Au metal-
lizations. These parameters offer good shear strength, even in the pres-
ence of a moderate amount of organic contamination [4-14, 4-16]. Typi-
cally, the higher the temperature the greater the bond strength if some
contamination is present, see Chap. 7.
Bonding Machine Setup Parameters, Thermosonic Bonding
Jellison [4-21] studied thermosonic bonding characteristics of both
thin and thick Al films and is further discussed in Cleaning, Chap. 7.
In general, early workers found that the shear force improved sig-
nificantly with increased ultrasonic power and temperature. Weiner
[4-35] studied TS bonding the effect of US power on ball bonding
setup with both Al and Au metallizations, using manual bonders. His
data is plotted in Fig. 4-21. It shows greater US power sensitivity of
the shear force for Al metallization than for Au.

Effect of first bond power on
shear strength
50

40
Gold metallization
Shear strength (gf) 30

20

10 Aluminum metallization

0
0 2.3 2.5 2.7 2.9 3.1
First bond power setting (arbitrary units)
FIGURE 4-21 The effect of ultrasonic power on ball-shear force. Bonds made
with 25 µm (1 mil) diameter gold wire. The stage temperature for these
measurements was 125°C, and the bonding force was 30 gf. This may be
appropriate for one set of conditions/bonder and is given as an example.
(After Weiner [4-35]; © IEEE.)