Page 82 - Wire Bonding in Microelectronics

Bonding W ir e Metallur gy and Characteristics 61

Problems Possible Solutions
Clubbed ball Increase tail length, lower EFO position,
avoid, capillary or machine sideways motion
Wire grain growth Shorten EFO spark time, use improved
(weakness in HAZ) (doped) wire
Neckdown above ball, tie Use reverse looping
bar severance
Wire scratching or marks Clean clamps, replace/clean capillary

TABLE 3-1 Gold Ball Bonding Problems/Solutions

3.5 Aluminum Wire for Ultrasonic Wedge Bonding
Small-diameter Al wire for ultrasonic (US) wedge bonding normally
has 1% Si added to increase its strength (pure Al is too soft to draw to
small-wire dimensions). This alloy was adopted in the 1960s as having
the right mechanical properties and has proved satisfactory ever since.
It is not greatly changed today, although it is much more uniform and
reproducible. Silicon is not in solid solution below 500°C and thus it
appears in these wires as finely divided particles. These particles can
grow with heat treatments, and large particles can serve as stress risers,
initiating cracks and causing the wire to break during device thermal
cycling (see Sec. 8.2). However, in practice Al, 1% Si wire has proven
quite reliable in billions of devices. Its ASTM standard (F-487) was
upgraded in 2006. Aluminum alloys, containing 1 or 0.5% Mg, are in
uniform solid solution at room temperature. As such they might have
been a better choice for the industry. However, it has dropped out of
use for fine wire bonding, and that ASTM standard (F-638) was discon-
tinued in 2006. We note that at the 0.5% level, Mg is still in limited use
for some large-diameter wire interconnections, up to 250 µm (10 mil)
diameter.
Small-diameter aluminum wire for ultrasonic wedge bonding
has quite different mechanical properties from similar diameter gold
wire for ball bonding. The former is generally supplied in the stress-
relieved condition, meaning that it is not fully annealed (only par-
tially so), see Fig. 3-2. Wire with breaking loads as high as 21 and as
low as 12 g (for 25-µm diameter Al wires) have been used for US
bonding. Fully annealed Al, 1% Si wire would be in the range of ~4 to
7 g BL with the elongation about 10%. This is too soft to be (usefully)
US wedge bonded to normal Al IC metallization. Typically, 25-µm
diameter Al wire is specified as having a breaking load of 14 to 16 g
and an elongation in the range of ~0.5 to 2%, the same specification as
used 25 years ago. The low elongation is needed to allow clamp-pull-
breakoff of the wire after the second bond. High-elongation wire