Page 119 - Wire Bonding in Microelectronics

98 Cha pte r F o u r

Friction rewelding

FIGURE 4-14 An example of a gold ball strongly bonded to gold metallization
that underwent friction rewelding to the pad. Note also the gross deformation
of the ball resulting from shear testing a strongly welded ball bond. This TS bond
was made from 25 µm (1 mil) diameter wire. (After Weiner [4-35]; © IEEE.)

Gold-to-gold friction rewelding: Shearing Au bonds on Au substrates
can lead to an unusual interference. Gold is capable of friction weld-
ing to Au surfaces at room temperatures. An SEM photograph of a
deformed Au ball and Au bonding pad showing the results of mul-
tiple friction rewelding is given in Fig. 4-13. Rewelding problems
may be eliminated by using a shear tool design that is slightly
ground-back from the forward edge. This will lift the ball, pre-
venting rewelding [4-20]. Friction rewelding seldom occurs while
shearing bonds on Al metallized IC pads, where the small size

of the pad, as well as the passivation surface film adjacent to the
pad, prevents it.
Interferences when shearing bonds on thick fi lms (and a method of
determining weak metallization adhesion problems): Several potential
interferences can occur when shearing ball bonds on thick-fi lm met-
allization. One would normally assume that balls of a given diam-
eter would yield shear forces somewhat lower when bonded to thick

fi lms than when bonded to thin films, since thick films contain pits

and voids, and, in some cases, glass or oxide occlusions on the sur-
face. There have been a number of shear test studies of ball bonds
on thick-fi lm metallization in the literature [4-21, 4-22, 4-23].
However, there was not enough information given on the
welded ball size, or the actual welded area, to compare directly
with the extensive laboratory data published on thin films. One
might expect to use the same experimental procedure in shearing
bonds made to thick films as for thin films. However, the thick films
themselves are often higher than the recommended vertical posi-
tion of the tool above the substrate when set for shearing bonds on