Page 130 - Wire Bonding in Microelectronics

108 Cha pte r F o u r

Blade
Aluminum
bond pad
Shear
Gold ball bond force

Die
Au-Al
intermetallic

FIGURE 4-19 Schematic of a ball bond with isolated intermetallic growths.
These growths can offer considerable resistance to a ball-shear probe, but can
often be “pried” up with a scalpel or will lift with low force in a pull test [4-32].

A fine scalpel blade is used to pry or “flip” the bond up, leaving the
intermetallic spikes on the pad and on the lifted ball for examina-
tion. This was called the “pluck test.” Weak, “as-made,” bonds
could more easily have been revealed by a shear test during pro-
duction (before any spikes were thermally generated), and the
problems of poor bonding machine setup or contamination could
have been solved at that time. But when it does occur, failure anal-
ysis may be required. Thermal stress experiments on intentionally
weak bonds (shear force <50% of optimum) made on clean pads
by Harman (unpublished) indicate that, when the shear force has
decreased to about half its original value (<25% of normal), the
described mechanism becomes significant. In some cases, bonds
with shear forces of 10 to 15 gf will lift in a pull test at 3 to 5 gf.
This happened frequently enough to require application of the
nondestructive pull test at various stages of the ball-shear, ther-
mal-stress experiments to remove bonds with this failure mecha-
nism. It should be noted that strongly welded Au ball bonds that
are thermally stressed result in relatively uniform intermetallic
formation and have not been observed to fail by this mechanism.
An example of this phenomenon would be similar to that shown
in Fig. 5A-1 in Chap. 5, App. 5A.
Such failure analysis methods have not been applied to fine pitch
balls, because of the difficulty of inserting a probe under them. Almost
as much information can be obtained from etching the Al pad out
from the ball with 20% potassium hydroxide and examining the
“turned over” ball. However since the pull test is often applied to
these bonds, one occasionally sees spikes remaining in the pad after
thermal stress test lift-offs.