Page 91 - Wire Bonding in Microelectronics

70 Cha pte r T h ree

data on varying bare-wire length in the short length region less
than ~1 mm (where more heat is conducted out through the welds
2
on the end). Also, the shorter the wire, the less total I R heating is pro-
duced in that wire, as stated previously.
A graph of experimental data for DC-ramped-current wire-burn
out for both gold and aluminum wires having wedge bonds (or equiv-
alent) at each end is given in Fig. 3-9. Two short-wire-length data
points are included in the figure for comparison. The bond-length ver-
sus current transition is actually a continuous one. The data that are
available indicate that for 25-µm diameter bare gold wire, the burnout
current decreases continuously as the wire is lengthened (from ~1.8 A
at 1 mm length) and reaches approximate equilibrium for lengths
more than or equal to 5 mm (at ~0.6 A). The long-wire minimum burn-
out current is similar to that reported in [3-25, 3-30, 3-33]. See Fig. 3-10
for a calculated example of the burn-out current versus length for both
Au and Al wires, showing clearly the decrease in burn-out current
with increased length. Normally one may assume that wires contain-
ing the traditional <10 ppm impurities will have the same burn-out
properties as pure Au (calculated in Fig. 3-10 (up to 1%) which
increases the resistivity and can reduce the burn-out current for a
given situation. Note also that burn-out current can be much greater
when wires are plastic encapsulated.

Wire bond length (mils)
0 40 80 160 240 320 400

Calculated burn-out current in air for gold and
3 aluminum wires of 25 µm diameter at 20°C
–4
Burn-out current (l b ) (amps) 2 Au
(l of Au decreases ~ 2 × 10 A/°C for
b
range 20°–150°C)

1

0 Al
0 0.1 0.2 0.3 0.4 0.6 0.8 1.0
Wire bond length (cm)
FIGURE 3-10 Wire burnout calculated for pure Au and Al, 25 µm wires
using the resistivity of pure Al and Au. This compares adequately with data
in Fig. 3-9, considering that both measured material were doped, and the
diameters may vary by 3–5%, whereas the curves use pure metal data and
exactly 25 µm diameters.