Page 109 - Wire Bonding in Microelectronics

88 Cha pte r F o u r

10
Bond pull strength (gf) 8 6

4 Gold wire
Aluminum wire
2
1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Typical bond deformation (wire diameter)
FIGURE 4-5 Bond pull force versus bond deformation for 25 µm (1 mil)
diameter aluminum and gold wires, both having 13 gf breaking force. All
bonds were made on the same bonding machine using the same bonding tool.
Other bonding parameters were optimized for each metal to produce the best
overall pull force and the lowest standard deviation. All bonds were made on a
single level. The loop heights were approximately 0.3 mm (12 mil) and the bond-
to-bond spacing was 1 mm (40 mil). The loop-height-to-bond-spacing ratio is
much larger than that generally found in device production. Thus, a scaling
down of the bond-pull force axis by a factor of about two would be more typical
of values obtained from integrated circuits [4-9].

little decrease in the pull force. Unfortunately, dependence of the
pull force on bond deformation is not recognized in some specifica-
tions. MIL-STD-883G, Method 2017.8 [4-7], permits bond deforma-
tions of 3 wire diameters for both Au and Al ultrasonic wedge bonds
and 5 wire diameters for the crescent (wedge) bond following an Au
ball bond. This is not generally permitted by commercial (in-house)
specifications.

4.2.5 Effect of Wire Elongation on Bond Pull Force
(Primarily for Large-Diameter Al, but also
for Au Wire Used in Ball Bonding)[4-9]
As is evident from Eqs. (4-1) through (4-3), the bond pull force is
strongly dependent on the ratio of the loop height to bond spacing.
This loop height can increase during pulling if the wire elongates sig-
nificantly. Small-diameter wire made for ultrasonic wedge bonding,
either Au or Al, normally has an elongation of less than 2%, and this
has little effect on the measured pull force. However, small-diameter
Au wire, for TS ball bonding, or Al wire that has been annealed can have
elongations of approximately 5 to 10%, and large-diameter annealed
Al bonding wire, up to approximately 30%. During bond pulling,