Page 48 - Wire Bonding in Microelectronics

Ultrasonic Bonding Systems and Technologies 27

6 6
Aluminum Aluminum
18°C 18°C
5 2 5
Stress (kg/mm 2 ) 4 18 W/cm 18°C 4 200°C

3
3
2
35 W/cm 2 2 400°C
1 18°C 1
50 W/cm 2
18°C 600°C
0 0
20 60 100 20 60 100
Elongation (%)

FIGURE 2-15 Stress versus elongation for Al single crystals. The left curve
indicates strain (elongation) during irradiation at 20 kHz ultrasonic energy
(dashed lines). The right-hand curves show comparable stress-strain behavior
resulting from heating alone. The solid curves indicate no applied ultrasonic
energy only the indicated temperature (from Langenecker [2-23]) (© IEEE).
(The author notes that this work has not been duplicated with modern
measurement equipment/techniques, and although believed to be correct, it
should be verifi ed, a good university thesis subject.)

15 kHz to 1 MHz, with most of his work being done at 20 kHz. He has
compared the similarity of the stress versus elongation in Al single
crystals (with 20 kHz ultrasonic energy at constant temperature) to
equivalent elongations resulting from heat. His results are repro-
duced in Fig. 2-15. It should be noted that Langenecker did not study
or even consider ultrasonic bonding or welding, only the ultrasonic
softening of metals, metal forming, and heating. However, the ultra-
sonic softening process that he described has been incorporated into
several explanations of the bonding process [2-1, 2-9, 2-21]. The stress
versus elongation is essentially equivalent to deformation under a
compressive load, such as occurs in metal forming and ultrasonic
welding. From this, it is clear that either ultrasound or heat can inde-
pendently cause equivalent deformations with a given stress. How-
ever, significant differences exist between the two types of excitation,
the major one being that the ultrasonic energy density required to
produce deformation in Al is about 10 million times less than is
required for an equivalent deformation resulting from thermal energy
alone, although some heating is a by-product of any such US process.
After US softening and deformation of the weldments occur, and the
US energy is removed, the metals are left work-hardened (acoustic
hardening), whereas equivalent thermal deformations leave the metal
permanently softer (annealed). Such work hardening in Al ultrasonic
wedge bonds has been experimentally verified by Coucoulas [2-24]
and on bonded Au balls by Pantaleon [2-25] and recently by Srikanth