Page 87 - Wire Bonding in Microelectronics

66 Cha pte r T h ree

Wire diameter (µm)
25 51 102 250
20
DC burnout current of
wires 0.4" (1 cm) long
10
8
6 4 Gold

Burnout current (amps) 2 1.4 mm

Au wire

1.3 mm
1 Al, 1% Si
0.8
0.6 Al, 1% Si

0.4

0.2
0.8 1 2 4 6 8 10
Wire diameter (mils)
FIGURE 3-9 Wire burnout for DC-ramped current measured for 1 cm lengths
of Au and Al wire of indicated diameter, except for shorter lengths shown by
black data points.

During high-temperature exposure, Al, 1% Si wire may grow large
silicon aggregates and develop a weak bamboo structure. The Si aggre-
gates can serve as stress risers (causing cracks) and shortening the
fatigue life. Because of this, the T-Cy life of such Si-doped Al wire could
be shorter than its expected life without such long high-temperature
exposure. Thus, as with Au wire, an accurate value of wire fatigue life
in a real device is difficult to estimate. A study of fatigue life of several
specified Al wire alloys was made by Ravi [3-15]. His data are repro-
duced as Fig. 3-9 and indicate that Al, 1% Mg wire has the superior CF
fatigue life. (Temperature cycle fatigue was not measured.) He also
compares the deflection against the actual measured value from a
working device. Aluminum wire alloys have changed minimally
since his work, so the comparison should still be valid, but should be